Flat Rock Creek Final Report - edocs.willoughby.nsw.gov.au

Job No:ar181 File: Flat Rock.doc

Date: 29 March 2006 Rev No: 3.0

Principal: BWL Author: BWL

FLAT ROCK CREEK

FLOOD STUDY

MARCH 2006 Prepared by: Lyall & Associates Consulting Engineers Level 1, 26 Ridge Street North Sydney NSW 2060 Tel: (02) 9929 4466 Fax: (02) 9929 4458 Email: [email protected]

Flat Rock Creek Flood Study

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FOREWORD The State Government’s Flood Policy is directed at providing solutions to existing flooding problems in developed areas and to ensuring that new development is compatible with the flood hazard and does not create additional flooding problems in other areas. Under the Policy, the management of flood liable land remains the responsibility of local government. The State subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist councils in the discharge of their floodplain management responsibilities. The Policy provides for technical and financial support by the Government through the following four sequential stages:

1. Flood Study Determines the nature and extent of flooding.

2. Floodplain Risk Management Study Evaluates management options for the floodplain in respect of both existing and proposed development.

3. Floodplain Risk Management Plan Involves formal adoption by Council of a plan of management for the floodplain.

4. Implementation of the Plan Construction of flood mitigation works to protect existing development. Use of Local Environmental Plans to ensure new development is compatible with the flood hazard.

The Flat Rock Creek Flood Study is jointly funded by Willoughby City Council and Department of Natural Resources. The Flood Study constitutes the first stage of the Floodplain Management process for this area and has been prepared for Willoughby City Council to define flood behaviour under current conditions.


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TABLE OF CONTENTS

Page No.

1 SYNOPSIS........................................................................................................................ 1 1.1 Study Background................................................................................................. 1 1.1 Approach to Flood Modelling ................................................................................ 2

1.1.1. Hydrologic Modelling ................................................................................ 2 1.1.2. Hydraulic modelling .................................................................................. 3

1.2 Model Development and Testing........................................................................... 4 1.3 Design Flood Estimation ....................................................................................... 4 1.4 Study Tasks .......................................................................................................... 5 1.5 Layout of Report ................................................................................................... 5

2 FLAT ROCK CREEK CATCHMENT AND ITS DRAINAGE SYSTEM............................... 7 2.1 Catchment Description.......................................................................................... 7

3 HYDROLOGIC MODELLING OF FLAT ROCK CREEK CATCHMENT .......................... 10 3.1 Selection of Hydrologic Model............................................................................. 10 3.2 Model Setup and Layout ..................................................................................... 10 3.3 Model Testing Procedure and Results ................................................................ 11 3.4 DRAINS Model Parameters ................................................................................ 13 3.5 Sensitivity of Model Results ................................................................................ 14

4 DESIGN FLOOD ESTIMATION ...................................................................................... 15

4.1 Rainfall intensity.................................................................................................. 15 4.1.1. Areal Reduction Factors ......................................................................... 15 4.1.2. Temporal Patterns .................................................................................. 15

4.2 Design Discharges.............................................................................................. 15 4.3 Probable Maximum Flood ................................................................................... 17

5 HYDRAULIC MODELLING OF FLAT ROCK CREEK AND SOUTHERN TRIBUTARY .. 18

5.1 Requirements for Hydraulic Model ...................................................................... 18 5.2 Brief Review of HEC-RAS Modelling Approach................................................... 18

5.2.1. Structure of Models................................................................................. 18 5.2.2. Boundary Conditions .............................................................................. 19

5.3 Testing Hydraulic Models.................................................................................... 19 5.3.1. General................................................................................................... 19 5.3.2. Roughness Values for Stream Channels ................................................ 20

5.4 Hydraulics of Willoughby Road Bridge ................................................................ 21 5.4.1. General................................................................................................... 21 5.4.2. Assessment of Hydraulic Capacity ......................................................... 22

6 HYDRAULIC MODELLING OF DESIGN FLOODS ......................................................... 23

6.1 Presentation of Results – Flat Rock Creek Channel ........................................... 23 6.2 Discussion of Results – Flat Rock Creek Channel .............................................. 23

6.2.1. Flood Levels and Flow Patterns.............................................................. 23 6.2.2. Impacts on Existing Development Bordering Flat Rock Creek ................ 24

6.3 Sensitivity Studies – Flood Levels in Flat Rock Creek Channel .......................... 25 6.3.1. Potential Blockage of Willoughby Road Culvert ...................................... 25


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6.4 Presentation of Results - Southern Tributary of Flat Rock Creek........................ 26 6.5 Use of Model Data to Assess Flood Levels......................................................... 27 6.6 Flood Hazard Areas and Floodways ................................................................... 28

6.6.1. Provisional Flood Hazard........................................................................ 28 6.6.2. Floodways .............................................................................................. 28

7 SUMMARY...................................................................................................................... 31

8 REFERENCES................................................................................................................ 33

APPENDICES

A Historic Floods and Model Testing B Review of Previous Flood Studies on Flat Rock Creek C Tabulations Flood Level, Flow and Velocity Distribution - Design Floods

LIST OF FIGURES 2.1 Location Plan 2.2 Willoughby Road Culvert Layout 3.1 DRAINS Sub-Catchment Plan 3.2 Stage – Storage Curve Artarmon Reserve Detention Basin 3.3 Stage – Storage Curve Floodplain Storage Upstream of Willoughby Road 3.4 DRAINS Sub-catchments Southern Tributary

4.1 Discharge Hydrographs 100 yr ARI

5.1 Rating Curves - Willoughby Road Bridge (Culvert at Full Hydraulic Capacity) 5.2 Rating Curves - Willoughby Road Bridge (Bridge Arch Partly Blocked by Debris over

Cycleway) 5.3 Rating Curves - Willoughby Road Bridge (25 % Blockage at Tapered Inlet) 6.1 Main Arm Flat Rock Creek. Design Water Surface Profiles 5, 20, 100, 200 year ARI and

PMF 6.2 Main Arm Flat Rock Creek Extents of Inundation – 5, 20, 100, year ARI and PMF 6.3 Main Arm Flat Rock Creek Provisional Flood Hazard Diagram – 100 year ARI 6.4 Main Arm Flat Rock Creek Provisional Flood Hazard Diagram – 20 year ARI 6.5 Main Arm Flat Rock Creek Floodway Delineation Diagram – 100 year ARI


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6.6 Southern Tributary Design Water Surface Profiles 5, 20, 100, 200 Year ARI and PMF 6.7 Southern Tributary Extents of Inundation – 5, 20, 100 year ARI and PMF 6.8 Southern Tributary Provisional Flood Hazard Diagram – 100 year ARI 6.9 Southern Tributary Floodway Delineation Diagram – 100 year ARI


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NOTE ON FLOOD FREQUENCY The frequency of floods is generally referred to in terms of their Annual Exceedance Probability (AEP) or Average Recurrence Interval (ARI). For example, for a flood magnitude having 5% AEP, there is a 5% probability that there will be floods of equal or greater magnitude each year. As another example, for a flood having a 5 year ARI, there will be floods of equal or greater magnitude once in 5 years on average. The approximate correspondence between these two systems is:

ANNUAL EXCEEDANCE PROBABILITY

(AEP) %

AVERAGE RECURRENCE INTERVAL

(ARI) YEARS

0.5 1 5

20(1)

200 100 20 5

(1) Approximate

The report also refers to the Probable Maximum Flood (PMF). This flood occurs as a result of the probable maximum precipitation (PMP). The PMP is the result of the optimum combination of the available moisture in the atmosphere and the efficiency of the storm mechanism as regards rainfall production. The PMP is used to estimate PMF discharges using a model which simulates the conversion of rainfall to runoff. The PMF is defined as the limiting value of floods that could reasonably be expected to occur.


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ABBREVIATIONS AEP Annual Exceedance Probability (%) AHD Australian Height Datum ARI Average Recurrence Interval (years) ARR Australian Rainfall and Runoff, 1998 Edition BOM Bureau of Meteorology DNR Department of Natural Resources (formerly, the Department of Infrastructure

Planning and Natural Resources and previously, the Department of Land and Water Conservation)


Flat Rock.doc Page 1 Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

1 SYNOPSIS 1.1 Study Background A comprehensive floodplain risk management plan (FRMP) is to be prepared for Flat Rock Creek as part of a Government program to mitigate the impacts of major floods and reduce the hazards in the floodplain. An important first step in the process of preparing an FRMP is the undertaking of a Flood Study for the stream. The Flood Study is the formal starting process of defining management measures for flood liable land and represents a detailed technical investigation of flood behaviour. For the Flood Study, mathematical models of the catchment and the floodplain were developed using detailed field surveys and interpreted to present a comprehensive picture of flooding under present day conditions. The study objective was to define flood behaviour in the streams in terms of flows, levels and flooding behaviour for floods ranging between 5 and 200 years average recurrence interval (ARI), as well as for the Probable Maximum Flood (PMF). Figure 2.1 shows the Study Area. The investigation involved hydraulic computer modelling to assess flood levels and flooding patterns in the channel of Flat Rock Creek between the North Shore Railway and Willoughby Road and the channel of the Southern Tributary between Waters Road and the junction with Flat Rock Creek. Upstream of the North Shore Railway and Waters Road the respective catchments are drained by a piped stormwater system. Hydrologic computer modelling was undertaken to define discharge hydrographs entering the open channels. The section of the drainage system to be subject to hydraulic modelling was nominated in Council’s Brief and reflects the nature of the drainage systems i.e. open channel versus fully piped, the types of development at risk and the severity of the flooding problems in the catchments. The main land use in the Flat Rock Creek catchment upstream of the Railway comprises industrial and commercial developments, with the Gore Hill Freeway running along the line of the original creek system. There are pockets of remnant residential areas in the vicinity of Artarmon Station and on the fringes of the catchment, but they are well above flood level. This portion of the catchment is drained by an underground trunk drainage system. Willoughby City Council decided that in view of the high capacity of the trunk drainage system which runs beneath the Gore Hill Freeway upstream of the railway and the nature of development, it would be more cost-effective to focus on the open channel section of the creek between the railway and Willoughby Road bridge, where there is residential development bordering the southern side of the channel and where flooding problems have been experienced in the past. Similarly on the Southern Tributary, which joins the main arm at Willoughby Road, the study focussed on the open channel section downstream of Waters Road, which also has residential development bordering both sides of that creek.



The Northern Tributary catchment has an influence on flooding because it contributes flows to the open channel section of the main arm of Flat Rock Creek. Flows from that catchment are controlled by a large detention basin located in Artarmon Reserve near the junction of the two streams just upstream of Chelmsford Avenue. The detention basin storage characteristics have been included in the hydrologic model. However, Council advised that no hydraulic modelling of the drainage system on the Northern Tributary upstream of the basin was required. This decision was made due to limitations in the budget for the study and also because there do not appear to be historic flooding problems on the Northern Tributary. 1.1 Approach to Flood Modelling Flood behaviour was defined using a computer based hydrologic model of the catchments and hydraulic models of the stream channels and floodplains of Flat Rock Creek and the Southern Tributary.

1.1.1. Hydrologic Modelling For the hydrologic modelling of the Flat Rock Creek catchment, there were four models which could have been selected. There was a RORB model which had been developed by Lyall and Macoun Consulting Engineers, (LMCE, 1987) and used for the design of the Gore Hill Freeway trunk drainage system and a RAFTS model which had been developed by Snowy Mountains Engineering Corporation, (SMEC,1995) in their Flood Study prepared for Willoughby City Council. Both of these models pre-date the current upgrading of the freeway and its drainage system associated with the Lane Cove Tunnel project. More recently, a DRAINS model was prepared by Hyder Consulting for the preparation of the Environmental Impact Statement for the Lane Cove Tunnel project. Most recently in 2004, Parsons Brinkerhoff (PB) had prepared a DRAINS model of the catchment for the design of the freeway pavement drainage system and also for the purposes of demonstrating that the upgrading of the freeway would not exacerbate flooding conditions in the channel upstream of Willoughby Road. (PB were consultants for the Lane Cove Tunnel Joint Venture, the organisation responsible for the construction of the project). The present Consultants became conversant with the PB model when engaged by RTA to undertake a detailed review of the Flood Study report which had been prepared by PB to support their design of the pavement drainage system for the Freeway upgrading (PB, 2004). The PB model included a detailed representation of the Flat Rock Creek catchment, incorporating the trunk drainage, the pavement drainage and the lateral drainage system contributing flows from the local catchments bordering the Freeway. The PB model was backed by a detailed survey of the catchment which had been undertaken by the Joint Venture. This survey included a re-survey of the Artarmon Reserve and the crest of the embankment of the detention basin. (These survey data were supplied by the Tunnel Joint Venture and used in the present investigation.) Importantly, this survey identified the fact that the crest of the basin embankment had been constructed about 0.5 – 0.7 m lower than envisaged in the design which the present Consultants had prepared for RTA in 1990 at the time of construction of the Gore Hill



Freeway. This situation was rectified in 2006 by the contractors undertaking the Lane Cove Tunnel works, who raised the embankment to recapture the design storage and reduced the size of the pipe controlling outflows. Having undertaken the review of the PB DRAINS model, the present Consultants came to the conclusion that the model adequately represented post-Freeway conditions on the Flat Rock Creek and that implementation of yet another model would not be cost-effective use of the Flood Study budget. The upgraded basin in Artarmon Reserve was adopted in the hydrologic modelling undertaken for the present Flood Study as representing present day conditions on the catchment. The Southern Tributary catchment was less well defined in the PB model and was refined for the present Flood Study by additional definition of the sub-catchments to improve its accuracy.

1.1.2. Hydraulic modelling A one-dimensional model based on the HEC-RAS system was adopted for the hydraulic analysis to model flood levels and flow patterns in the main arm of Flat Rock Creek and the Southern Tributary. The model allowed for the interaction of flows between the channel and the floodplain, flow through culverts and flow over control structures such as the decks of road bridges. For the hydraulic analysis, there were a number of one-dimensional models of the Flat Rock Creek channel which had been developed in previous studies. There was a HEC-2 model which had been prepared by LMCE, 1987 and a MIKE 11 model had been prepared by SMEC, 1995. Hyder Consultants had prepared a HEC-RAS model for analyses undertaken for the EIS of the Lane Cove Tunnel project. However, the survey used to prepare that model was rather limited, as is often the case with EIS investigations. The detailed surveys undertaken for the upgrading of the Freeway by the Tunnel Joint Venture allowed PB to develop a HEC-RAS model of Flat Rock Creek based on cross sections taken at 20 m spacing. This was by far and away the most comprehensive model available and also incorporated two recent features in the channel which potentially could impact on flooding conditions: namely a Gross Pollutant Trap located near Chelmsford Avenue and a footbridge upstream of Willoughby Road. The footprint of recent residential development on the southern side of the channel had also been incorporated in the model. PB’s hydraulic model was adopted for the present investigation with some modifications. However, the weak point in PB’s modelling appeared to the present Consultants to be their analysis of the hydraulics of the Willoughby Road bridge, which controls flood levels for several hundred metres upstream of the bridge. Considerable time was devoted in the present study to analysing the hydraulics of this structure and preparing a rating curve (flood stage versus discharge curve) which was used as the downstream boundary of the hydraulic model. The channel of the Southern Tributary terminates a short distance to the west of Willoughby Road. From that point flows are conveyed in a box culvert which joins the low



level culvert conveying Flat Rock Creek flows a short distance downstream of Willoughby Road. (Figure 2.2 shows the layout of the drainage system at Willoughby Road.) The Southern Tributary culvert has a limited capacity and when its inlet surcharges, flows are conveyed as overland flow to join the Flat Rock Creek channel upstream of Willoughby Road. The rating curve is therefore based on the total flow which enters the western (upstream) face of the sandstone arch bridge waterway. The total flow is represented by the flow conveyed along the Flat Rock Creek channel from the direction of the railway, together with overland flow surcharging the culvert on the Southern Tributary. In the case of the Southern Tributary, there was no existing hydraulic model of the open channel section. Surveyors were engaged to take sections and measure up the bridges between Waters Road and the entrance to the culvert. 1.2 Model Development and Testing There are no stream flow data available on the Flat Rock Creek catchment. Several historic flood marks and rainfall data had been identified during previous flood investigations by LMCE, 1987; SMEC, 1995. Other flood marks for a more recent flood which occurred in April 1998 were identified as a result of the distribution of a Community Newsletter for this present investigation. Rainfalls for these historic storms which were recorded at the Chatswood Bowling Club, located about two kilometres to the north of the Flat Rock Creek catchment, were applied to the DRAINS model to estimate flows. The resulting flows were applied to the HEC-RAS model and the computed water surface profiles compared with the recorded flood marks. It was anticipated that the testing of the models would lead to the derivation of model parameters, which could then be used in the design flood estimation. However, as discussed later in the report, the results of this model testing were inconclusive, due to uncertainties in the data and conditions on the catchment and its stormwater drainage system applying at the times of the historic floods. Accordingly, the selection of model parameters for design purposes was also based on experience of the study team with similar investigations on urbanised catchments, as well as the engineering literature. 1.3 Design Flood Estimation Design storms were derived from Australian Rainfall and Runoff (ARR), 1998 and then applied to the DRAINS model to generate discharge hydrographs within the study area. Peak flows from those hydrographs constituted the upstream boundary and tributary inflow inputs to the hydraulic model. The hydraulic model was then used to derive water surface profiles for the design flood flows, as well as provide an assessment of the flow distribution and average velocities of flow for the design events.



1.4 Study Tasks The Flood Study had three main components: (1) Review of available hydrologic and hydraulic data and previous investigations. As mentioned, a Brief was prepared for a cross sectional survey of the Southern

Tributary to fill in gaps in the survey data. Historic flood marks identified by the Community Newsletter were also levelled during the creek survey. The Bureau of Meteorology and Sydney Water supplied rainfall data for the historic storms identified in previous investigations on flooding on Flat Rock Creek and by the Community Newsletter.

(2) The hydrologic component, which included refining and testing of the hydrologic model of the catchment, estimation of design storm rainfalls and their application to the model to assess flows.

(3) The hydraulic component, which comprised the testing of the hydraulic model of the

open channel section of Flat Rock Creek and the preparation of the Southern Tributary model and the definition of the water surface profiles, flows and velocities for the design floods.

1.5 Layout of Report Section 2 contains background information including a description of the catchment, a brief review of the data base available for the study and a discussion on the history of flooding in the catchment. Testing of the models based on the historic flood data is described in Appendix A. Section 2 also lists the previous investigations carried out over the past 15 years to define flooding conditions in the catchment. The results of these investigations are outlined and compared with those of the present study in Appendix B. Sections 3 and 4 deal with the hydrology of the Flat Rock Creek catchment and the results of the DRAINS modelling undertaken to assess flood flows on the catchment. These sections describe the set up of the model, the determination of design storm rainfall depths over the catchments for a range of storm durations and conversion of the rainfall hyetographs to discharge hydrographs. Section 5 deals with the development of the HEC-RAS hydraulic models. This section also includes a detailed investigation of the hydraulics of the Willoughby Road crossing, which controls flood levels in the lower reaches of the creek. Section 6 details the results of the hydraulic modelling of the design floods using HEC-RAS. Results are presented as water surface profiles and plans showing indicative extents of inundation for each of the design flood events. A provisional assessment of flood hazard is also presented. (The assessment of flood hazard according to hydraulic criteria such as velocity and depth of floodwaters is necessarily “provisional”, pending the more detailed assessment of other flood related criteria which would be undertaken during the Floodplain Risk Management Study.) The flood study investigation also included an assessment of the hydraulic capacity of the bridge opening in the event of partial blockage by debris.



Section 7 contains a list of references. Supplementary details are given in the Appendices. Appendix C contains tabulations of flood level, discharge and velocity data for design storm events between 5 and 200 year ARI, as well as the PMF.



2 FLAT ROCK CREEK CATCHMENT AND ITS DRAINAGE SYSTEM 2.1 Catchment Description The valley drained by Flat Rock Creek has a total catchment area of about 5 km2 and extends eastwards from the Pacific Highway to Long Bay, a distance of 4 km (Figure 2.1). The catchment is completely urbanised and the natural drainage characteristics have been altered by residential and commercial development and by the Gore Hill Freeway, which was constructed in 1991 along the route of the original creek and displaced the natural drainage system and its flood storage characteristics. Pacific Highway to North Shore Railway A new trunk drainage system was constructed in conjunction with the Freeway in 1991, from the Pacific Highway to the North Shore railway crossing and comprised the following main elements:

The Northern Trunk Drain which runs along the northern side of the freeway and intercepts runoff from the residential catchment between its upstream end at Baden Powell Street and Simpson Street.

The Southern Trunk Drain which runs along the southern side of the freeway and

intercepts runoff between McLachlan Avenue and its junction with the NTD at Simpson Street.

The Combined Trunk Drain which conveys runoff along the northern side of the

freeway from Simpson Street to a large energy dissipator on the eastern side of Hampden Road.

A short section of open channel which conveys runoff along the original course of Flat

Rock Creek to the stone arch culvert running beneath the North Shore Railway embankment.

North Shore Railway to Willoughby Road The trunk drainage system downstream of the North Shore Railway culvert was constructed in the 1930’s and was not altered by the freeway, which consists of sections of embankment and viaduct. The trunk drainage system along this reach comprises the following elements:

Between the railway and Chelmsford Avenue, the trunk drainage comprises a low level conduit running beneath a vegetated floodway which caters for overflows.

A conduit which conveys runoff from the Northern Tributary catchment and joins the

low flow culvert of Flat Rock Creek about 50 m upstream of the freeway viaduct at Chelmsford Avenue. This tributary has a catchment which extends northwards to Mowbray Road and has a total area of 1.45 km2. It flows near Sydney Street and under Artarmon Road as a covered concrete lined drain and then runs beneath Artarmon Reserve to the junction with Flat Rock Creek.



As part of the trunk drainage for the Gore Hill Freeway, Artarmon Oval was converted

to a dual purpose playing field – retarding basin. The objective was to reduce the peaks of stormwater flows originating from the Northern Tributary, in order to offset the increase in peak flows generated on Flat Rock Creek by the road construction. The design of the basin was undertaken by Lyall and Macoun Consulting Engineers (LMCE, 1988). The basin embankment on the southern side of the Reserve, which controls ponding levels within the storage, was constructed lower than “as designed”. The resulting storage available for detention of floodwaters was significantly smaller than the as designed value and would not have retarded the flow to the extent envisaged in the design. As part of the widening of the Gore Hill Freeway associated with the Lane Cove Tunnel, the Tunnel Joint Venture raised the embankment elevation by about 500 mm and reduced the capacity of the outlet works. These modifications which were constructed in 2006 had the effect of bringing the operation of the basin more into line with the intent of the design.

An open concrete invert and brick sided stormwater drain comprises the main arm of Flat Rock Creek and runs from Chelmsford Avenue to Willoughby Road. This drain is trapezoidal in cross section with a waterway area of 6.5 – 7.3 m2 and is 580 m long. Due to the smooth lining and the steep bed slope, which ranges between 0.7 and 1 per cent, flow is in the transition range between the subcritical and supercritical regimes and velocities up to 6 m/s are experienced over this reach.

Willoughby Road Crossing to Flat Rock Drive At Willoughby Road, flows are conveyed through a stone arch bridge. The total waterway area of the bridge is 23 m2, its length is 16.5 m and its bed has a grade of 2.2 per cent. Figure 2.2 shows details of the waterway at Willoughby Road. This figure is an extract from a flood study of the Flat Rock Creek catchment prepared by LMCE, 1987 and was also reproduced by SMEC, 1995. During major flood events, the Willoughby Road bridge conveys flows derived from the Flat Rock catchment, as well as surcharges from the Southern Tributary culvert which join the channel upstream of the bridge as overland flow. The section of bridge waterway extending from its upstream face to the junction with the Southern Tributary culvert acts as the hydraulic control and the resulting upstream ponding results in a hydraulic jump in the channel as the flows pass from the supercritical to the subcritical regime. A major box culvert commences at the downstream face of the bridge and runs beneath Hallstrom Park before discharging to an open channel 280 m to the east of Flat Rock Drive. The culvert near Willoughby Road changes in dimensions from a 4.9 m wide by 3.05 m high rectangle at the inlet to a 3.05 m wide by 2.64 m high box section. This box section continues for a distance of 155 m, after which the size progressively increases to a maximum of 4.26 m by 4.38 m over the last 180 m. The culvert beneath Hallstrom Park is laid at an average gradient of 3.6 per cent, resulting in flow velocities up to 12 m/s. It has a hydraulic capacity in excess of the 100 year ARI discharge and does not influence flood levels at Willoughby Road. The steep gradient and the relatively flat terrain within the Park result in a depth of cover of 26 m as the culvert passes beneath Flat Rock Drive.



Additional runoff enters the Flat Rock Creek culvert about 410 m downstream of Willoughby Road via a 1200 mm diameter pipe which drains part of Hallstrom Park and the urban catchment on the northern side of Small Street. The natural surface along the southern boundary of Hallstrom Park from the bridge to Flat Rock Drive, acts as an overland flow path for runoff generated within the park as well as for surcharges of the culvert inlet at Willoughby Road. These overland flows are conveyed beneath the underpass at Flat Rock Drive to Flat Rock Creek which becomes a natural channel downstream of that road. Downstream of Flat Rock Drive, stormwater is conveyed in the natural rock lined channel of Flat Rock Creek to the Northbridge Suspension bridge (Strathallan Avenue). A multi-celled box drain then conveys flows beneath Munro and Tunks Parks to Long Bay. The reach below Willoughby Road is about 2 km in length and is tidal over the last kilometre. Southern Tributary The Southern Tributary catchment has an area of 1.02 km2. It drains the southern portion of the valley, which includes the St Leonards and Gore Hill area west of the North Shore Railway and extends as far west as Carlotta Street. Between the North Shore Railway at St Leonards station to the commencement of the box culvert near Willoughby Road, flows derived from the Southern Tributary catchment are conveyed along a concrete invert and brick lined stormwater drain. The drain has steep bed slopes up to 3 per cent grade, which result in sections of supercritical flow followed by hydraulic jumps upstream of the various local road crossings. At the downstream end of the channel below Ruth Street, flows enter a box culvert which runs beneath the western side of Willoughby Road and joins the culvert of Flat Rock Creek on the eastern (downstream) side of the bridge.

PENKIVAL ST

COBAR ST

WALTER ST

PARK

RD

DALLEYS RD

WATE

RS

RD

NO

RTH

CO

TEST

ARTARMON RD

OLYMPIA RD

DA

RC

AN

RUTH

SMALLST

MC

LAC

HLA

N

AV

E

CARLOTTAST

RE

SE

RV

ER

D

GOREHILL PARK

ROYALNORTH SHORE

HOSPITAL

HA

MP

DE

NR

D

SIMPSONST

ARTARMONRESERVE

(DETENTION BASIN)FLAT ROCK CREEK

(CHANNEL)

SOUTHERN TRIBUTARY(CHANNEL)

LITTLE FLAT ROCK CREEK(PIPED)

NORTHERNTRIBUTARY

SY

DN

EY

RD

FLA

TR

OC

KD

RIV

E

TUNKSPARK

HALLSTROMPARK

CHELMSFORD AVE

FLAT ROCK CREEK FLOOD STUDY

Figure 2.1

LOCATION PLAN

P

P

N

0 500 1000m

Scale



3 HYDROLOGIC MODELLING OF FLAT ROCK CREEK CATCHMENT 3.1 Selection of Hydrologic Model Consideration was given to the hydrologic modelling approach for the investigation. With the construction of the freeway in 1991, the natural creek upstream of the North Shore Railway was replaced by large, fast flowing concrete culverts with a 100 year ARI capacity. The DRAINS rainfall-runoff modelling software adopted for the flood study is a more suitable approach to modelling such a system than the RAFTS and RORB software used in previous investigations, which do not explicitly model the piped component of the main drainage systems. DRAINS is specifically designed to model urban catchments drained by piped drainage systems such as Flat Rock Creek. Rainfall on each sub-catchment is adjusted to allow for infiltration and other losses. The resulting sub-area rainfall-excess is converted into a hydrograph and assumed to enter the drainage system, subject to constraints imposed by the entrance and conveyance capacity of the system. There, it is added to any existing flow in the system and the combined flow is routed through the system to the outlet. DRAINS allows for features which control the capacity of the piped system such as pit entry capacity and localised storage areas, assesses the capacity of the piped system using a Hydraulic Grade Line analysis, models gutter flows and routes overland flows along the street system to downstream areas via defined flow paths. Overall, by accounting for the various elements of the constructed drainage system, DRAINS allowed a more realistic routing of flows through the drainage system than approaches which adopts a more lumped approach to routing flows through the model sub-catchments and do not specifically model piped systems. 3.2 Model Setup and Layout A catchment plan of the Flat Rock Creek catchment external to the Gore Hill Freeway was initially developed by PB, 2004 from 1:2000 scale topographic maps and Council data and RTA plans of the existing drainage systems. PB, 2004 defined sub-catchments to facilitate the assessment of cross drainage systems interlinked with the Freeway pavement drainage system. Catchment boundaries were later confirmed during site inspections. PB calculated percentages of impervious area using aerial photos and cadastral boundary data. Drainage systems and sub-catchment areas within the Freeway corridor were identified using RTA documentation. The sub-catchment areas, pits, conduits, overland flow paths, open channels and storage data were used to develop a DRAINS model representing the actual drainage system. Modelling the Freeway drainage system included definition of the main trunk drains (northern and southern) with junctions to pipe linkages to the external catchments. Six such cross drainage systems were identified on the northern side of the freeway, and eight on the southern side. The carriageway catchment areas were simplified by lumping areas



encompassing several inlet drainage pits. Figure 3.1 shows the sub-division of the catchment developed by PB, 2004. On the southern side of the Freeway between Hampden Road and the North Shore Railway, two 1800 mm diameter culverts convey flows under the Freeway embankment to join Flat Rock Creek a short distance upstream of the railway. These culverts were combined into a single equivalent 1800 mm high box culvert. There are several local ponding areas within the catchment which have been identified by PB, 2004. They were additional to the detention basin in Artarmon Reserve which controls runoff from the Northern Tributary. These storages were also incorporated in the DRAINS model. Figure 3.2 shows the storage characteristics of the Artarmon Reserve basin following its upgrading. At the spillway level of RL 58.15 m AHD, the basin storage capacity is about 42,000 m3

. The culvert conveying flows from the Northern Tributary runs beneath Artarmon Reserve and joins Flat Rock Creek on the southern side of the embankment. The basin is “offline” to the piped drainage system. When the culvert beneath the reserve surcharges, a gully on the upstream (northern) side of the reserve fills and commences to inundate the storage area. A 1050 mm diameter pipe set in the basin embankment controls outflows which are directed along a drainage line into Flat Rock Creek. Figure 3.2 also shows peak storage levels in the basin in the event of the design flood events. The pondage in Flat Rock Creek and its overbank areas upstream of Willoughby Road behaves like a detention basin and results in a reduction in the peak discharge of the approaching flow. These storage effects have been included in the DRAINS model by incorporating a stage-storage volume relationship derived using the survey model of the creek. Figure 3.3 shows the storage characteristics and peak flood levels on the upstream side of the bridge opening. At the level of the bridge soffit, the storage amounts to about 25,000 m3. For the present flood study, the PB, 2004 DRAINS model shown in Figure 3.1 was refined with additional sub-areas to provide a more accurate assessment of the increase in peak flows between the upstream end of the channel at Waters Road and the culvert below Ruth Street. Figure 3.4 shows the catchment sub-division. 3.3 Model Testing Procedure and Results The procedure adopted for testing the DRAINS model, in situations where historic flood data are available, involves the collection and analysis of rainfall data to ascertain the temporal and areal distribution of rainfall over the catchment. These rainfalls are input to the model to generate flows within the catchment. In situations where there is a stream gauging station located on the catchment, the modelled discharge hydrograph is then compared with historic hydrographs and model parameters varied until a fit is achieved. Similarly, when sufficient data is available on historic flood levels along the channel it is possible to use the known discharges and adjust the parameters of the hydraulic model to achieve a fit between recorded and modelled levels. Thus it is possible to achieve independent calibration of each of the models (hydrologic and hydraulic) in turn. However, in most situations the streams are not gauged



and information is usually limited to some isolated flood marks along the stream, plus some recorded rainfall data. Under those circumstances, independent “calibration” of the models cannot be achieved. The usual procedure adopted is to use “realistic” values of parameters for the hydrologic model, which are adopted from experience and the engineering literature, in conjunction with recorded rainfall data to estimate flows and to vary the parameters of the hydraulic model to achieve a reasonable agreement with recorded flood levels. Sometimes the recorded flood marks or levels recorded at structures are used in conjunction with uniform flow or culvert formulae to estimate historic flood flows to assist with the selection of model parameters. However, in the absence of recorded stream flow data, the overall process as outlined above can at best be termed “model tuning” or ”model testing” rather than calibration. In the case of Flat Rock Creek there were several historic storms, including significant events in April 1998 and August 1986, as well as a lesser storm in March 1994 for which there were several recorded flood marks bordering the lower reaches of the channel upstream of Willoughby Road. Unfortunately, the Community Questionnaire distributed at the commencement of the flood study did not uncover much quantitative information on historic flooding. This was due to the fact that over the last 10 years considerable re-development of the area bordering the creeks has taken place and the long term residents with experience of flooding have moved out. Pluviographic data for the historic storms were recorded at the Chatswood Bowling Club and Sydney Observatory, as well as at a daily-read rain gauge at Northbridge. Recorded rainfalls were applied to the DRAINS model to estimate flows, which were then applied to the HEC-RAS model of the Flat Rock Creek channel. The procedure and results are discussed in Appendix A and summarised below. August 1986 Flood The August 1986 storm occurred prior to the construction of the Gore Hill Freeway when the drainage system in the middle to upper reaches of the catchment upstream of the North Shore Railway was of much less capacity than the present day system. (There is insufficient data available on the nature of the original drainage system to convert the DRAINS model to represent pre–freeway conditions). However, it would be expected that the increase in flow velocities and the loss of flood storage upstream of the railway associated with the freeway construction would result in an increase in downstream flood peaks, so that if this storm had occurred under present day conditions, peak flows downstream of the railway would have been higher than historic values. The fact that the detention basin on the Northern Tributary was not constructed until 1991 also makes it difficult to use the August 1986 flood for testing the models. The SMEC, 1995 study had assessed the peak discharge of the August 1986 flood in the channel as 60 m3/s near Chelmsford Avenue. This estimate was based on recorded flood marks and uniform flow calculations. Reasonable correspondence was achieved in Appendix A between the water surface profile computed by the HEC-RAS model for this estimate of discharge and the flood marks which had been levelled for the SMEC, 1995



study. The DRAINS model based on 2006 catchment conditions assessed the peak discharge at this location as 77 m3/s. March 1994 Flood The March 1994 flood occurred under post-freeway conditions and therefore offered the opportunity for model testing with the catchment in its present day conditions. (The Artarmon Reserve basin was at a lower level than in 2006 conditions, but it appears that the basin did not flood in March 1994.) Unfortunately, this storm was a relatively minor event and did not cause significant overbank flooding in the open channel section of Flat Rock Creek. The peak flow at Willoughby Road estimated by DRAINS was 24 m3/s. The results presented in Appendix A show quite good correspondence between the modelled water surface profile and recorded flood marks. April 1998 Flood In the case of the April 1998 flood, rainfalls at the Chatswood pluviometer for the 1 hour to 90 minute durations likely to maximise peak flows in the lower reaches of the catchment were around the 50 to 100 year ARI intensities. The modelled peak flow at Willoughby Road was 83 m3/s. However, the elevations of the recorded flood marks along the channel are well below those experienced in August 1986, when recorded rainfall intensities at Chatswood were less and, as mentioned previously, the drainage system upstream of the North Shore Railway was less efficient. It is also understood that in April 1998, the detention basin in Artarmon Reserve did not store any significant flows resulting from surcharging of the stormwater system of the Northern Tributary, even though the hydrologic analyses for this present study predicted that the basin would commence to operate for design storms between 5 and 10 year ARI. The conclusion reached from analysis of the April 1998 storm event was that rainfalls actually experienced over the Flat Rock Creek catchment were less than intensities recorded at the respective rain gauges. (Unfortunately, there are no rain gauges located within the catchment boundary to test this theory.) 3.4 DRAINS Model Parameters After consideration of the results of the DRAINS and HEC-RAS modelling testing described in Appendix A, the following parameters have been adopted for the design flood estimation described in Section 4. Rainfall Losses Soil Type = 2.5 (assessment of a soil’s rate of infiltration.) AMC = 3.0 (Antecedent Moisture Condition – assessment of a catchment’s wetness

at the start of storm event).



Paved area depression storage = 2.0 mm. Supplementary area depression storage = 1.0 mm. Grassed area depression storage = 10.0 mm. Pipe and Pit Data In addition, the roughness k for the pipes was assumed to be 0.3, as recommended in ARR (1998) and 0.06 for the Freeway trunk drainage box culverts. Pit loss coefficients were assigned with values adopted in accordance with Missouri Charts, the DRAINS manual and various technical papers. Cross drainage sub-catchments were simplified to have a single inlet pit and pipe which did not limit the entry of flow into the Freeway drainage system. This assumption is in accordance with the prototype drainage system which was designed by LMCE with a 100 year capacity. Travel Times Information contained in ARR, 1998 suggests that for large commercial and industrial buildings, which are typical of the commercial and industrial areas in the catchment particularly on the southern side of the freeway upstream of the North Shore Railway culvert, the response time of the allotments to rainfall would be in the range 5 to 15 minutes. For design purposes, DRAINS modelling was carried out with the response time in the commercial and industrial sub-areas of 10 minutes and 5 minutes in residential sub-catchments. In addition, the path of travel of runoff was adjusted to follow the pattern of the street system. The resulting flow length and slope were then used by DRAINS to assess the travel time of the floodwave. 3.5 Sensitivity of Model Results The impact on modelled peak flows associated with various assumptions for travel times is presented in the discussion on previous flood investigations in Appendix B. Several sensitivity runs were reported by PB, 2004 for their DRAINS model which are also valid for the model used in the present flood study (see Appendix B).


Figure 3.1

DRAINS SUB-CATCHMENT PLAN

P

P

0 200 400m

ScaleN

Northern

Tributary

Catchment

Southern

Tributary

Catchment

Main Arm

Catchment

Source: PB,2004

CTD

MO

WBRAY

ROAD

PACIFIC

HIG

HW

AY

0 20,000 40,000 60,000 80,000

Storage Volume (m3)

54

55

56

57

58

59

RL

- m

AH

D

FLAT ROCK CREEK FLOOD STUDYFigure 3.2

STAGE - STORAGE CURVEARTARMON RESERVE DETENTION BASIN

SPILLWAY LEVELRL 58.15m

100 YR

50 YR

5 YR

42,000 m3

PEAK STORAGE LEVELIN BASIN

0 20,000 40,000 60,000 80,000

Storage Volume m3

42

44

46

48

50

52

RL

- m A

HD


STAGE-STORAGE CURVEFLOODPLAIN STORAGE UPSTREAM OF WILLOUGHBY ROAD

SOFFIT OF BRIDGE ARCH

RL 47.7 m

100 YR

50 YR

5 YR

PEAK STORAGE LEVELU/S WILLOUGHBY ROAD BRIDGE

25,000 m3

WILLO

UG

HBY

RO

AD

RUTH ST

NO

RTH

CO

TE

ST

DA

RG

AN

ST

DALLEYS RD

MITCHELL

ST

CH

RIS

TIE

ST

PACIFIC HWY

WESTBOURNE ST

HE

RB

ER

TS

T

FL

AT

RO

CK

CR

EE

KF

LO

OD

ST

UD

Y

Fig

ure

3.4

DR

AIN

SS

UB

-CA

TC

HM

EN

TS

SO

UT

HE

RN

TR

IBU

TA

RY

PP

N

250

500

m0

Scale

SOUTHERN TRIBUTARYCHANNEL

Legend

SU

B-C

AT

CH

ME

NT

BO

UN

DA

RY



4 DESIGN FLOOD ESTIMATION 4.1 Rainfall intensity The procedures used to obtain temporally and spatially accurate and consistent intensity-frequency-duration (IFD) design rainfall curves for the Flat Rock Creek catchment are presented in Chapter 2 of ARR (1998). Design storms for frequencies of 5, 20, 50, 100 and 200 year ARI were derived for storm durations ranging between 1 hr and 6 hrs. The procedure adopted was to generate IFD data for each catchment by using the relevant charts in Volume 2 of ARR (1998). These charts included design rainfall isopleths, regional skewness and geographical factors.

4.1.1. Areal Reduction Factors The rainfalls derived using the processes outlined in ARR (1998) are applicable strictly to a point. In the case of a large catchment of over tens of square kilometres, it is not realistic to assume that the same rainfall intensity can be maintained over a large area, an areal reduction factor is typically applied to obtain an intensity that is applicable over the entire area. However, as the area of the Flat Rock Creek catchment is only 5 km2, the reduction in rainfall intensities would be quite small and accordingly, no reduction in point rainfalls was made for this study.

4.1.2. Temporal Patterns Temporal patterns for various zones in Australia are presented in ARR (1998). These patterns are used in the conversion of a design rainfall depth with a specific ARI into a design flood of the same frequency. Patterns of average variability are assumed to provide the desired conversion. The patterns may be used for ARIs up to 500 years where the design rainfall data is extrapolated to this ARI. The derivation of temporal patterns for design storms is discussed in Chapter 3 of ARR (1998) and separate patterns are presented in Volume 2 for ARI < 30 years and ARI > 30 years. The second pattern is intended for use for rainfalls with ARIs up to 100 years, and to 500 years in those cases where the design rainfall data in Chapter 2 of ARR (1998) are extrapolated to this ARI. 4.2 Design Discharges The DRAINS model was run with the parameters presented in Section 3.4 to obtain flows for input to the hydraulic model. Peak flows at the model outlets for the critical storm durations, which ranged between 25 minutes and 2 hours depending on location and flood frequency, are shown on Table 4.1. These analyses were carried out with the embankment of the retarding basin in Artarmon Reserve raised by 500 mm to its design level. The analyses showed that the raised basin will contain flows up to the 100 year ARI without overtopping the spillway. In the event of a 200 year ARI flood, the spillway crest would be overtopped by about 150 mm.



Discharge hydrographs for the 100 year ARI storm of 2 hours duration are plotted on Figure 4.1. The results show the “flash flooding” nature of the catchment. Flows derived from the upper portion of the catchment would reach a peak at the North Shore Railway culvert 40 minutes after the commencement of the storm and the peak flood level at Willoughby Road would be experienced about 10 minutes later. The results also show the importance of the Artarmon Reserve basin in attenuating flood peaks. The peak 100 year ARI overland flow entering the storage of 32.7 m3/s would be reduced to 4.3 m3/s.

TABLE 4.1

DESIGN FLOOD ESTIMATION PEAK FLOWS IN FLAT ROCK CREEK CATCHMENT

(m3/s)

Average Recurrence Interval - years Location

5 10 20 50 100 200 PMF

Flat Rock Creek

North Shore Railway Culvert

31 36 43 49 57 62 184

Chelmsford Avenue d/s Northern Tributary

46 53 60 66 73 83 394

Willoughby Road at Bridge (Flat Rock Ck. plus overland flow from Southern Tributary)

45 57 63 73 82 89 329

Northern Tributary

Artarmon Reserve Detention Basin

Peak Storage Level in Reserve RL m AHD

56.2 56.6 57.2 57.6 58.0 58.3 59.0

Inflow to basin (as overland flow onto Reserve)

4.5 9.1 17 24.4 32.7 37.6 160

Outflow ( via pipe in basin embankment and spillway)

2.9 3.3 3.7 4.0 4.3 11.8 160

Flow via Low Level Culvert under Reserve

12.1 13.4 13.4 13.4 13.4 13.4 13.4

Southern Tributary

u/s Mitchell Road 10 13 15 17 20 23 79

Waters Road 14 17 19 22 26 29 94

at Willoughby Road Bridge comprising:

17 20 24 28 33 38 119

- culvert joining d/s Bridge. 17 18 18 18 19 20 27

- o’land flow to Flat Rock Ck.

- 2 6 10 14 18 92



4.3 Probable Maximum Flood Estimates of probable maximum precipitation were made using the Generalised Short Duration Method (GSDM) as described in the Bureau of Meteorology’s Bulletin 53 (BOM, 2003). This method is appropriate for estimating extreme rainfall depths for catchments up to 1000 km2 in area and storm durations up to 6 hours. The steps involved in assessing PMP for the Flat Rock Creek catchment are briefly as follows:

Calculate PMP for a given duration and catchment area using depth-duration-area envelope curves derived from the highest recorded US and Australian rainfalls.

Adjust the PMP estimate according to the percentages of the catchment which are

meteorologically rough and smooth, and also according to elevation adjustment and moisture adjustment factors.

Assess the design spatial distribution of rainfall using the distribution for convective

storms based on US and world data, but modified in the light of Australian experience.

Derive storm hyetographs using the temporal distribution contained in Bulletin 53,

which is based on pluviographic traces recorded in major Australian storms. Design storms were derived for durations ranging between 1 and 6 hours and applied to the model using the linear model parameters. One in 100 year ARI rainfall losses were adopted for the PMF. The 30 minutes storm was found to be critical. Peak flows are shown in Table 4.1 and are around four to five times the magnitude of the 100 year ARI peaks. These multiples are generally in agreement with the results of other investigations on small urbanised catchments. The reduction in the magnitude of peak discharge on the main arm between Chelmsford Avenue and Willoughby Road is due to the flood storage between these two locations, which functions as a detention basin.

0 40 80 120 160 200

Time after Commencement of Storm - minutes

0

20

40

60

80

100

Dis

char

ge -

m3 /s

LEGENDFLAT ROCK CREEKNorth Shore Railway CulvertWilloughby RdNORTHERN TRIBUTARYOverland Flow onto Artarmon BasinLow Level Culvert Beneath Reserve1050mm Dia. Basin OutletSOUTHERN TRIBUTARYVia Culvert to Willoughby RoadOverland Flow to Flat Rock CK


DISCHARGE HYDROGRAPHS - 100YR ARI



5 HYDRAULIC MODELLING OF FLAT ROCK CREEK AND SOUTHERN TRIBUTARY

5.1 Requirements for Hydraulic Model A model was required which could produce flows, velocities and water surface elevations at nominated locations in the channels. The model was to be capable of analysing hydraulic conditions at culvert and bridge crossings, and capable of adjustment so that it could analyse the effects of possible modifications such as levees, channel enlargement, adjustments to bridge waterways or future land use changes on the floodplain, all of which could influence flooding behaviour. Few commercially available hydrodynamic models contain all the features required for this present study. One however, HEC-RAS, has the required capabilities and is readily available to all potential model users at minimal cost. On the technical side, HEC-RAS is capable of undertaking single model runs of “mixed flow” where the flow is a mix of the sub-critical and super-critical flow regimes, such as is the case in the Flat Rock Creek and Southern Tributary channels. Previous investigations have shown that mixed flow occurs on Flat Rock Creek, with a hydraulic jump forming upstream of the influence of the Willoughby Road Bridge. The MIKE 11 approach used by SMEC, 1995 was not capable of simultaneously assessing mixed flow in a single run. In that investigation, separate runs were carried out with and without the bridge in place and the location of the hydraulic jump was approximated, with a correspondingly less accurate solution than would be achieved by HEC-RAS which accurately models the location and magnitude of the hydraulic jump. 5.2 Brief Review of HEC-RAS Modelling Approach HEC-RAS is a one-dimensional hydraulic modelling package developed by the Hydrologic Engineering Centre of the US Army Corps of Engineers and has seen widespread application in Australia in recent years. The momentum equation of open channel flow is solved numerically between user defined grid arrangements (more typically, cross section locations) for given boundary conditions. Typically, a peak discharge comprises the upstream boundary and the downstream boundary is either a rating curve (stage versus discharge relationship) or the assumption of uniform flow (friction slope equals the bed slope of the stream). In the present flood study, the DRAINS model took into account the effects of flood storage in the lower reaches of Flat Rock Creek and gave estimates of flow along the length of the channel. Accordingly, the HEC-RAS software was used in its “steady state” mode, as unsteady effects are not significant. Similarly, a steady state analysis was adopted for the Southern Tributary channel, where storage and unsteady effects are not significant.

5.2.1. Structure of Models The Flat Rock Creek model consisted of cross sections derived from ground survey and are about 20 m apart along the length of the channel of Flat Rock Creek. Their locations are shown on the exhibits showing extents of inundation presented in Section 6. The



spacing of the sections is quite dense and accurately represents features on the floodplain which influence hydraulic behaviour (e.g. bridge constrictions, changes in channel and floodplain dimensions, footprints of residential developments). The Southern Tributary model is based on surveyed cross sections between Waters Road and the entrance to the culvert joining the Flat Rock Creek culvert at Willoughby Road. Four local street crossings at Mitchell, Dalleys, Dargan and Ruth Streets are also incorporated in the model. The total length of stream modelled was 0.75 km.

5.2.2. Boundary Conditions Flat Rock Creek Peak flows derived from DRAINS provided the boundary conditions at the upstream end of the model. The flow was adjusted along the modelled reach to account for the effects of flood storage, runoff from the lateral sub-catchments bordering the channel and the overland flow from surcharges of the culvert on the Southern Tributary which occurs for storms in excess of 10 year ARI. A rating curve, i.e. stage versus total discharge relationship, defining the hydraulic characteristics of the Willoughby Road bridge comprised the downstream boundary condition. Southern Tributary Peak flows derived from DRAINS provided the boundary conditions at the upstream end of the model. The flow was increased along the modelled reach to account for the effects of runoff from the lateral sub-catchments bordering the channel. A rating curve defining flow conditions within the Flat Rock Creek culvert downstream of Willoughby Road comprised the downstream boundary condition. 5.3 Testing Hydraulic Models

5.3.1. General The main physical parameter for HEC-RAS is hydraulic roughness. There are other parameters such as contraction and expansion head loss coefficients which are of a hydraulic nature and need to be estimated. There are very limited historic flood level data available to assist with calibration of the model. Accordingly, roughness was estimated from site inspection, past experience and values contained in the engineering literature (Arcement and Schneider, 1984; Cowan, 1956; Barnes, 1967).



5.3.2. Roughness Values for Stream Channels Although several factors affect the selection of an “n” value for the channel, the most important factors are the type and size of the materials that compose the bed and banks of the channel as well as its shape. Cowan, 1956 developed a procedure for estimating the effects of these factors. In this procedure, the value of n may be computed by the following equation:

n = (nb + n1 + n2 + n3 + n4) m ……… 5.1

where nb = a base value of n for a straight, uniform, smooth channel in natural materials

n1 = a value added to correct for the effects of surface irregularities n2 = a value for variations in shape and size of the channel cross

section n3 = a value for obstructions to flow n4 = a value for vegetation and flow conditions and m = a correction factor for meandering of the channel It is usually necessary to determine roughness values for channels and floodplains separately. The fabric of a floodplain can be quite different from that of a channel. The physical shape of a floodplain is different and the vegetation and obstructions located on the floodplain are typically different from those found in a channel.

On Flat Rock Creek, flows which surcharge the channel during major flood events are retained on the northern overbank by the batter of the Gore Hill Freeway over most of the reach between Chelmsford Avenue and Willoughby Road. The creek runs along the rear of residential allotments fronting Olympia Road and Park Road, which are located on the southern side of the creek. Most of these allotments are separated by paling fences, which run normal to the creek. Experience with historic flooding indicates that the hydrodynamic forces associated with overbank flows would result in overturning of the fences near the occurrence of the peak of the flood. Residents supplied photographs showing failure of the fences in the lower reaches of the creek in the April 1998 flood. This effect would best be simulated in the hydraulic modelling by assuming that the southern overbank was effective for the conveyance of flow, but with a relatively high value of hydraulic roughness representing the additional hydraulic resistance on flow imposed by the upturned fences. For the major and extreme flood events, the extent of flooding would reach the buildings on the southern overbank. To model this eventuality, the portion of the overbank occupied by the footprint of the buildings was excised from the effective waterway area. On Flat Rock Creek there is a Gross Pollutant Trap located within the channel in its upper reaches near Chelmsford Avenue. The footprint of the GPT was also excised from the



effective waterway area of the channel. A footbridge crosses the creek upstream of Willoughby Road which was also incorporated in the model. The Southern Tributary runs along the rear of residential allotments located on both sides of the channel. Over most of the reach, paling fences are located both parallel with and normal to the channel. As for Flat Rock Creek, high values of hydraulic roughness were assigned to the overbank to account for upturning of the fences during major floods, with lesser values applying on the left overbank in the upper reaches where there are no fences. Table 5.1 summarises the adopted hydraulic roughness values.

TABLE 5.1 “BEST ESTIMATE” OF HYDRAULIC ROUGHNESS VALUES

FLAT ROCK CREEK AND SOUTHERN TRIBUTARY

Channel Left Floodplain Right Floodplain

Flat Rock Creek 0.015 0.045 0.045-0.09

Southern Tributary 0.015 0.045-0.1 0.06-0.1 5.4 Hydraulics of Willoughby Road Bridge

5.4.1. General The main uncertainty in the hydraulic modelling is the downstream boundary condition adopted for the HEC-RAS modelling at Willoughby Road and its influence on the upstream water surface levels. The present investigation, as well as all of the previous investigations on flooding on the Flat Rock Creek catchment, are in agreement that there is a backwater upstream of the Willoughby Road bridge which extends for a distance of around 300 m upstream. Over this reach the flow is in the subcritical mode. Further upstream flow is supercritical, with a hydraulic jump occurring at the junction of the two flow regimes. Initially, on the rising limb of the flood hydrograph, flows are conveyed in the supercritical regime through the culvert running beneath Hallstrom Park on the eastern side of Willoughby Road (Figure 2.2). As flow increases, the water surface elevation rises to the level of the soffit of the 15 m long tapered portion of culvert at the entrance to the underground drain, which thereafter functions as an orifice. For higher flows, the water surface elevation within the bridge surcharges the kerb at the top of the headwall of the culvert which has an elevation of RL 44.9 m AHD. The flow is



therefore conveyed as a combination of orifice flow through the low level culvert and weir flow over the headwall. As the flow is conveyed from the channel into the waterway of the arch bridge, there would be a drawdown in the water surface elevation resulting from contraction losses and the Bernouilli effect as the velocity of flow increases. Eventually, with increasing discharge the water surface level would rise to touch the underside of the arch at RL 47.7 m AHD. At this level it is likely that there would be some instability in the water surface, as levels alternate between orifice and free surface conditions. With further increases in flow, orifice conditions would stabilise and the hydraulic control would move upstream from the low level culvert to the upstream face of the sandstone arch of the bridge.

5.4.2. Assessment of Hydraulic Capacity For this investigation an assessment was made of the hydraulics of the Willoughby Road bridge and associated culvert. The assessment was based on the hydraulic formulae presented in the HEC-RAS manual, but recognised that the solution adopted by HEC-RAS is based on gradually varied one-dimensional flow and does not allow for the drawdown effect which occurs at the entrance to waterway openings as the flow accelerates from the upstream channel. As shown on Figure 5.14 of the Austroads Waterway Design, the drawdown is a two-dimensional effect, which in the present case would commence around 4 m upstream of the bridge and may, depending on the magnitude of discharge, allow the flow to be conveyed into the arch as free surface flow rather than impacting on its underside. The hydraulic analysis was based on the culvert and weir equations and discharge coefficients given in Chapter 5 of the HEC-RAS manual. Contraction losses at the entrance to the arch bridge were based on a contraction head loss coefficient of 0.6, which is recommended therein for an abrupt loss. Capacity with No Restriction on Bridge Capacity Due to Blockage The rating curves describing the relationship between water surface elevation and flow, both within the bridge and upstream of the bridge opening, are shown on Figure 5.1. The lower line is the rating curve within the bridge arch (incorporating the Bernouilli effect) and the upper line is the rating curve transferred to the location on the upstream side of the bridge opening, outside the influence of the bridge constriction. The upper line has been adopted as the downstream boundary condition of the HEC-RAS model at “river station” (i.e. cross section) 1340, just upstream of the bridge. These curves indicate that the arch of the bridge would flow free for the potential range of flows up to 82 m3/s (ie the 100 year ARI flood peak). There would be hydraulic losses, with a corresponding reduction in water surface elevation, as the flow contracts from the channel into the arch section. However, within the arch, the water surface would be below the level of the arch soffit for the 100 year ARI flood. The hydraulic control would be located at the downstream side of the arch, where the flow is conveyed through and over the culvert running beneath Hallstrom Park. Rating curves for two assumptions of partial blockage of the bridge are shown on Figures 5.2 and 5.3. The derivation of these figures is discussed later in Section 6.3.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Discharge (m3/s)

42

42.5

43

43.5

44

44.5

45

45.5

46

46.5

47

47.5

48

48.5

49

Wat

er S

urfa

ce E

leva

tion

(m A

HD

)

LegendRating Curve Upstream of Bridge Opening Rating Curve Within Bridge Arch

SOFFIT OFBRIDGE ARCH

RL 47.7

FLAT R

OC

K C

REEK

FLOO

D STU

DY

Figure 5.1R

ATIN

G C

UR

VES

WILLO

UG

HB

Y R

OA

D B

RID

GE

(CU

LVER

T AT FU

LL HY

DR

AU

LIC C

APA

CITY

)

TOP OF KERB

RL 44.9

CULVERT SOFFIT

RL 44.38

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125

Discharge (m3/s)

42

42.5

43

43.5

44

44.5

45

45.5

46

46.5

47

47.5

48

48.5

49

49.5

Wat

er S

urfa

ce E

leva

tion

(m A

HD

)



RL 47.7

FLAT R

OC

K C

REEK

FLOO

D STU

DY

Figure 5.2R

ATIN

G C

UR

VES

WILLO

UG

HBY

RO

AD

BR

IDG

E(B

RID

GE

AR

CH

PA

RTLY

BLO

CK

ED

BY D

EBR

IS O

VER

CY

CLE

WA

Y)

TOP OF KERB

RL 44.9

TOP OF KERB

RL 44.38

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Discharge (m3/s)

42

42.5

43

43.5

44

44.5

45

45.5

46

46.5

47

47.5

48

48.5

49

Wat

er S

urfa

ce E

leva

tion

(m A

HD

)



RL 47.7

FLAT R

OC

K C

REEK

FLOO

D STU

DY

Figure 5.3R

ATIN

G C

UR

VE

SW

ILLOU

GH

BY R

OA

D BR

IDG

E(25%

BLO

CK

AG

E AT TA

PE

RE

D IN

LET)

TOP OF KERBRL 44.9

CULVERT SOFFITRL 44.38



6 HYDRAULIC MODELLING OF DESIGN FLOODS 6.1 Presentation of Results – Flat Rock Creek Channel Water surface profiles for the 5, 20, 100 and 200 year ARI and the PMF design events on the channel of Flat Rock Creek are shown in Figure 6.1. The locations of the cross sections are shown at the bottom of the diagram. Each cross section is denoted as a river station “RS” in the hydraulic model. Figure 6.2 shows the indicative extents of inundation for the 5, 20 and 100 year ARI floods, as well as the PMF. The extent of inundation of each flood event is necessarily indicative only. It is based on flood levels derived at the model cross sections. Whilst the flood level and velocity data derived from the analyses are accurate at the sections comprising the model, the flood extent diagrams should not be used to give a precise determination of flood affectation in individual allotments. In Figures 6.3 and 6.4 the floodplain is divided into provisional “high” and “low” hazard zones for the 100 and 20 year ARI floods respectively. Figure 6.5 shows the “floodway” zone for the 100 year ARI flood. The significance of these terms in floodplain management planning is discussed later in Section 6.5. Peak water surface elevations and the average flow and velocity distributions for the full range of flood events are tabulated in Appendix C. Uncertainties associated with numerical hydraulic modelling are such that water levels are usually rounded off to the nearest 100 mm. However, in the present study water surface profiles along the steeper reaches of the creek do not show large differences in elevation for floods up to the 200 year ARI, indicating that large increases in flow result in relatively small increases in water level. Consequently, the results have generally been presented to two decimal places (i.e. to the nearest 10 mm), to highlight differences in the model results for the various floods. 6.2 Discussion of Results – Flat Rock Creek Channel

6.2.1. Flood Levels and Flow Patterns Minor floods with peak flows up to 45 m3/s, slightly less than the 5 year ARI flood, are conveyed smoothly through the bridge which functions as an extension of the channel, without significant backwater effects. For the larger events which exceed the capacity of the culvert, some of the flow is conveyed overland across Hallstrom Park and the top kerb of the culvert headwall at RL 44.9 m AHD acts as a broad crested weir. The ponding levels upstream of the bridge rise and the hydraulic jump becomes progressively more pronounced and moves further upstream with increasing flow. For the 100 year ARI flood, the jump is located about 300 m upstream of Willoughby Road. Significant overland flows are likely to occur across Hallstrom Park due to surcharging the culvert headwall in the event of a 20 year ARI flood. Flow velocities in the channel are in the range 5 to 6 m/s in the supercritical reach and reduce to about 3 m/s downstream of the hydraulic jump. On the floodplain the flow velocity



would be 0.8 to 1 m/s on the northern side reflecting the smoother hydraulic conditions, reducing to 0.5 m/s in the backyards of the allotments on the southern side of the channel, where the hydraulic roughness would be higher due to the restrictions on flow imposed by the inter-allotment fences. As shown on Figure 6.1, there are local perturbations in water levels at the Gross Pollutant Trap near the upstream end of the model at RS 1880 and at the footbridge further downstream at RS 1437. There is also a local increase in flood levels in the reach between RS 1740 and 1780. Due to the constrictions on flow imposed by buildings in this reach, there is a localised conversion from the supercritical to subcritical flow regime which is responsible for the rise in flood level in the vicinity of RS 1740. There is also a zone between RS 1700 and RS 1660 in which 200 year ARI peak water levels are lower than corresponding levels for the lesser flood events. The reason for this apparent anomaly is that the 200 year ARI flow in this reach is in the supercritical regime, returning to the subcritical regime at RS 1660, whereas the flows remain in the subcritical regime for the other events. In this reach it is recommended that for design purposes, a level 100 mm above the 100 year ARI level be adopted for the 200 year ARI flood to remove the conflict in comparative flood levels. The flood level data shown in Appendix C have been adjusted to conform with this recommendation. However, the cross sections comprising the hydraulic model are closely spaced at 20 m apart. Consequently, it would be expected that the modelled variations in the computed flow profiles would be replicated in the prototype channel. In addition, the Froude Number of the flow along the channel in the supercritical zone is in the range 1.1 and 1.5, where considerable wave action would be expected. The effects of wave action are not incorporated in the numerical hydraulic modelling, but may be allowed for via the freeboard allowance which is added to the computed flood level when fixing the floor levels of new development. It is usual practice to allow 500 mm for freeboard, but in the present case a larger allowance could be considered in the future Floodplain Risk Management Study. A larger than normal freeboard allowance may also be justified to account for the increase in design flood levels resulting from possible partial blockage of the bridge at Willoughby Road, which is discussed in the following section.

6.2.2. Impacts on Existing Development Bordering Flat Rock Creek The Lane Cove Tunnel Joint Venture surveyed the floor levels of many of the buildings on the southern side of the channel. By inspection of these data, none of the existing residences bordering the channel would have their floor levels inundated at the 100 year ARI level of flooding. The minimum freeboard would be about 70 mm and there are 12 residences with freeboard less than 500 mm. In the event of the 200 year ARI flood the floors of 2 residences would be inundated by up to 270 mm. In the event of the PMF, Willoughby Road would be overtopped by about 2 m and the reach between the bridge and Chelmsford Avenue would function as a level pool. All of the residences would be flooded, with most experiencing 3.5 m of inundation above floor level. In the Floodplain Risk Management Study it will be necessary to quantify the economic impacts of flooding over the full range of flood events. In view of the fact that significant



property damages are only initiated at the 200 year ARI level of flooding, it will be necessary in that study to model several floods between the 200 year ARI and the PMF (say the 500 year ARI flood) in order to accurately assess the shape of the residential flood damages versus frequency curve. 6.3 Sensitivity Studies – Flood Levels in Flat Rock Creek Channel Hydraulic modelling showed that the results are not particularly sensitive to variations in hydraulic roughness. The middle to lower reaches of the creek are within the zone of influence of Willoughby Road and the water surface level is relatively flat, with comparatively low velocities of flow. The main factor influencing flood levels in this zone is the hydraulic capacity of the bridge waterway. Accordingly, consideration is given below to the impacts of blockage on hydraulic capacity.

6.3.1. Potential Blockage of Willoughby Road Culvert Blockage Scenario and Impacts The likely impacts of two blockage scenarios have been assessed below. The resulting water surface levels at the upstream side of Willoughby Road are shown on Table 6.1. a) Scenario A – Assuming that the projected area of the bicycle track and its

supporting structure was blocked to the level of the handrail, RL 45.45 m AHD (see Figure 2.2). Based on the cross sectional areas normal to the direction of flow, the blockage factor would reach a maximum of around 20 per cent at the elevation of the top of the railing, but for higher elevations would reduce somewhat. Below the soffit of the arch, RL 47.7 m AHD, the blocked area would represent about 13 per cent of the total unblocked waterway area of 23.6 m2.

The average velocity of flow through the bridge waterway would increase from 4 m/s (unblocked) to 5 m/s (partly blocked). This increase in velocity would increase the water level at the upstream face of the bridge to RL 47.9 m AHD for a discharge of 82 m3/s, the estimated peak of the 100 year ARI flood. Figure 5.2 shows the rating curve. There would be a drawdown as flow velocities increase from an average of 2 m/s in the ponding area upstream of the bridge to 5 m/s within the arch. The drawdown would allow the flow to enter the bridge without impacting on the top of the arch and initiating orifice flow conditions.

b) Scenario B – The second blockage scenario involved a 25 per cent reduction in the

area in the throat of the tapered culvert at the entrance to the underground drain on the eastern side of the bridge. The throat of the tapered culvert has dimensions of 2.64 m wide and 3.05 m high and is more prone to blockage than the entrance to the bridge.

For a discharge of 82 m3/s, the water surface elevation upstream of the bridge would rise from RL 47.6 m (unblocked) to RL 48.1 m AHD due to the 25 per cent blockage. (Figure



5.3) This level is about 400 mm above the soffit of the bridge arch. Assuming that the drawdown in the water level approaching the bridge commences about 4 m upstream of the bridge, a reduction of the water surface to RL 47.7 m AHD would require a minimum water surface slope of around 1V:10H. It may be possible to convey 82 m3/s under free flow conditions. However, confirmation of free flow conditions would require a physical model study. In the event of a 25 per cent blockage, flood levels in the pool upstream of the bridge would be raised by 500 mm compared with the unblocked case. According to the available floor level data, the blockage would result in the inundation of five residences in Park and Olympia Roads in the event of a 100 year ARI flood.

TABLE 6.1 ESTIMATES OF 100 YEAR ARI PEAK FLOOD LEVELS

AT WILLOUGHBY ROAD BRIDGE BRIDGE PARTLY BLOCKED

m-AHD

Bridge Unblocked Bicycle Track and Supports Blocked

Throat of Tapered Culvert Blocked by 25 per cent

(1) (2) (3) 47.6 47.9 48.1

(1) These levels apply for the 100 year ARI discharge of 82 m3/s at the bridge. 6.4 Presentation of Results - Southern Tributary of Flat Rock Creek Water surface profiles for the 5, 20, 100 and 200 year ARI and PMF design events on the channel of the Southern Tributary are shown in Figure 6.6 and Figure 6.7 shows the corresponding indicative extents of inundation for 5, 20 and 100 year ARI and PMF events. For the 5 year ARI flows are conveyed within the channel and through the culverts without surcharging. Larger floods exceed the capacity of the culverts and a portion of the flow is conveyed over the roads. The pattern of flow in the channel is quite complex with reaches of supercritical and subcritical flow and bridge crossings where the model is called upon to apportion the total discharge to flow through the culvert and flow over the top of the road acting as a broad crested weir. There are several locations where there are inconsistencies in the modelling results due to water surface profiles for the various flows “crossing over”. For example, upstream of Mitchell Street, the modelled 20 year ARI flow profile changes from the supercritical to the subcritical regime upstream of the bridge and has a higher water surface elevation than the 100 and 200 year ARI floods, which remain in the supercritical regime until the bridge structure is reached. A similar situation occurs for the PMF, where immediately upstream of the bridge, computed water surface levels are lower than the 20 year ARI flood, even though the value of the PMF peak flow is four times the 20 year ARI magnitude.



Fortunately these effects are localised. In most locations along the channel, there is a progressive increase in peak flood level with increasing discharge. Flow velocities in the reaches of the channel where supercritical flow occurs are in the range 5 to 6.5 m/s, reducing to 1.5 to 2 m/s as the flow regime changes to subcritical. For the purposes of design, it is recommended that in the localised zone upstream of Mitchell Street, the modelled 100 and 200 year ARI levels be raised to 50 and 100 mm respectively above the 20 year ARI level to provide a consistent increase in levels with increasing ARI. It is also recommended that the PMF profile be adjusted by joining a line between the predicted levels on the downstream side of the bridge and RS 690. Similarly at Dargan Street it is recommended that the modelled PMF profile be adjusted with a line joining the computed levels at the bridge and RS 314. The flood level data in Appendix C have been adjusted to conform with this recommendation. About 75 m downstream of Ruth Street, flows enter an 80 m long culvert, which joins the main culvert of Flat Rock Creek on the eastern side of Willoughby Road. Flows which surcharge the culvert at about the 10 year ARI level of flooding are conveyed overland to join the open channel of Flat Rock Creek upstream of the bridge crossing. The pathway on the eastern side of the bridge therefore functions as an overland flow path. Peak water surface elevations and the average flow and velocity distributions for the full range of flood events are tabulated in Appendix C. There are no floor level data available for the Southern Tributary. Residential development borders both sides of the channel over the modelled reach and by inspection, it appears that residential floor levels would be above flood level. However, several garages and buildings close to the creek would be inundated, particularly those in the vicinity of the road crossings where the flow spreads out to typically a 20 to 30 m width. The economic consequences of flooding, including definition of flood prone property, would be assessed in the future Floodplain Risk Management Study. 6.5 Use of Model Data to Assess Flood Levels Consideration was given to presenting the model results as contours of peak water surface levels for the various floods. However, this approach was not appropriate due to the variations in water levels in the unstable flow regime due to the impacts of channel bed slope, structures in and across the channel and the impacts of building footprints for the major flood events. Accordingly, the following approach is suggested for using the flood data when assessing peak flood levels within the study area.

• Mark the location for which flood information is required on Figures 6.2 or 6.7. This diagram will give an initial (but not necessarily final) estimate on whether or not that particular location is flood prone. Note on the Southern Tributary whether the site is upstream or downstream of any adjacent bridge crossing. This is important because of the considerable water level drop across most of the bridges.



• Consult the appropriate water surface profile (ie Figures 6.1 or 6.6), locate the position of the site on the reach and obtain a first estimate of peak flood level for the various frequencies by scaling.

• Consult the tabulations of flood data in Appendix C to refine the estimate of flood levels and obtain further information on the local distribution of flows and velocities.

Note that as mentioned previously, the above procedure will only yield the flood level at the cross section adjacent to the point of interest. A detailed site survey would be required to confirm the extent of flood affectation. 6.6 Flood Hazard Areas and Floodways

6.6.1. Provisional Flood Hazard Flood hazard categories may be assigned to flood affected areas in accordance with the procedures outlined in the Floodplain Development Manual, 2005. Flood prone areas may be provisionally categorised into Low Hazard and High Hazard areas depending on the depth of inundation and flow velocity. Flood depths as high as 0.8 m in the absence of any significant flow velocity represent Low Hazard conditions. Similarly, areas of flow velocities up to 2.0 m/s but with minimal flood depth also represent Low Hazard conditions. Following a review of the modelled distribution of flows and velocities at the various model cross sections a depth of 0.5 to 0.8 m was adopted, depending on the velocity in the overbank areas, as the boundary between provisional Low and High Hazard zones. Provisional hazard diagrams for the 100 and 20 year ARI floods on Flat Rock Creek are presented in Figures 6.3 and 6.4 respectively. The hazard diagram for the 100 year ARI flood for the Southern Tributary is shown on Figure 6.8. The Flood Hazard assessment presented herein is based on considerations of depth and velocity of flow and is provisional only. As noted in the Floodplain Development Manual, other considerations such as rate of rise of floodwaters and access to high ground for evacuation from the floodplain should also be taken into consideration before a final determination of Flood Hazard can be made. These factors are normally taken into account in the Floodplain Risk Management Study for the catchment, which is the next stage in the flood management process for the area.

6.6.2. Floodways According to the Floodplain Development Manual (NSW Government, 2005), the floodplain may be subdivided into the following zones: • Floodways; • Flood storage; and • Flood fringe



Floodways are those areas where a significant volume of water flows during floods and are often aligned with obvious natural channels. They are areas that, even if partially blocked, would cause a significant increase in flood level and/or a significant redistribution of flow, which may in turn adversely affect other areas. They are often, but not necessarily, areas with deeper flow of areas where higher velocities occur. Flood Storage areas are those parts of the floodplain that are important for the temporary storage of floodwaters during the passage of a flood. If the capacity of a flood storage area is substantially reduced by, for example, the construction of levees or by landfill, flood levels in nearby areas may rise and the peak discharge downstream may be increased. Substantial reduction of the capacity of a flood storage area can also cause a significant redistribution of flood flows. Flood Fringe is the remaining area of land affected by flooding, after floodway and flood storage areas have been defined. Development in flood fringe areas would not have any significant effect on the pattern of flood flows and/or flood levels (NSW Government, 2005). The notion of hydraulic categories is subjective, and to a large degree can reflect the opinion of the assessor, particularly with what is considered to be a significant impact. A procedure in common use for the definition of the floodway for a particular flood event is to adopt the extent of inundation of a lesser flood event as its floodway extent. For example, the 100 year ARI floodway may be defined as the extent of inundation reached by the 20 year ARI flood, provided that most of the 100 year ARI flood flow is conveyed within that extent. The remaining flooded area between the extent of inundation of the 20 year ARI flood and that of the 100 year ARI event may then be adopted as the flood storage and flood fringe areas. This pragmatic categorisation of the hydraulic areas of the floodplain has considerable merit and is easily understood. The hydraulic model may also be used to provide guidance on the floodway determination. The procedure is to progressively encroach across either floodplain towards the channel until the modelled flood level has increased by a significant amount (usually 0.1 m) above the existing (un-encroached) flood levels. This indicates the limits of the hydraulic floodway since any further encroachment will intrude into that part of the floodplain necessary for the free flow of flood waters – that is, into the floodway. The HEC-RAS software has the capability to determine the stations at each cross section which define the hydraulic floodway. It computes the encroachment stations so that the conveyance within the encroachment cross section (at some higher level) is equal to the conveyance of the natural cross section at the natural water level. This higher water level is specified as a fixed amount above the unconstricted flood profile. Experiments were carried out with HEC-RAS used in the encroachment mode to assist with floodway delineation. The extents of the 100 year ARI floodway adopted on the various streams are shown on Figures 6.5 and 6.9 for Flat Rock Creek and the Southern Tributary respectively.



On Flat Rock Creek the 100 year ARI floodway is based on the encroachments determined from the HEC-RAS analysis between Willoughby Road and RS 1600. Upstream of RS 1600 the analysis showed that the 20 year ARI flood extent was a reasonable representation of the 100 year ARI floodway. The remaining zone between the floodway and extent of inundation is defined as a flood storage/flood fringe area. On the Southern Tributary the HEC-RAS analysis showed that all of the zone inundated by the 100 year ARI flood is important for the conveyance of floodwaters and that significant constrictions in the flow would result in excessive afflux. Accordingly, the extent of inundation has been adopted as the 100 year ARI floodway.


MAIN ARM FLAT ROCK CREEKDESIGN WATER SURFACE PROFILES

5,20,100,200 YEAR ARI AND PMF

CH

ELM

SFO

RD

AVE

NU

E

GR

OS

S P

OLL

UTA

NT

TRA

PTR

AP R

S186

0-18

80

FOO

TBR

IDG

E

WIL

LOU

GH

BY R

OA

D

0 100 200 300 400 500 60042

44

46

48

50

52

54

56 Legend

WS PMF

WS 200year ARI

WS 100 year ARI

WS 20year ARI

WS 5year ARI

Ground

1...

13...

140.

..

1420

1430

1437

....

1460

1480

1500

1520

1530

.*15

40

1555

.*

1580

1600

1620

1640

1655

.*

1680

1700

1710

1720

1740

1760

1780

1800

1820

1840

1850

1860

1870

1880

1890

1899

Wat

er S

urfa

ce E

leva

tion

(m A

HD

)

Main Channel Distance (m)

NOTE: 200 YEAR ARI FLOOD LEVELS PRESENTED IN APPENDIX C HAVE BEEN INCREASED TO 100mm ABOVE 100 YEAR ARI LEVELS BETWEEN RS1660 AND 1720

SEE NOTE


Figure 6.2

MAIN ARM FLAT ROCK CREEKEXTENTS OF INUNDATION

5,20,100 YEAR ARI AND PMF

P

P

N

LEGEND

PMF

100 YEAR ARI1000

Scale

50 200m

20 YEAR ARI

5 YEAR ARI

NOTE

THE EXTENTS OF FLOODING SHOWN WEREDETERMINED FROM SURVEYED CROSS SECTIONSOF THE CREEK AND FLOODPLAIN AND AVAILABLECONTOUR DATA AND ARE APPROXIMATE ONLY.THE EXTENT OF INUNDATION OF INDIVIDUALALLOTMENTS NEAR THE FLOOD FRINGE MUSTBE CONFIRMED BY SITE SPECIFIC SURVEY.


Figure 6.3

MAIN ARM FLAT ROCK CREEKPROVISIONAL FLOOD HAZARD DIAGRAM

100 YEAR ARI

P

P

1.36.30.5

1.2

5.5

0.7

0.83.40.4

0.7

3.3

0.4

0.3

2.6

0.8

1000

Scale

200 m50

N

LEGEND

5.5 m /s AVERAGE VELOCITY OF FLOW

HIGH HAZARD

LOW HAZARD

NOTE



Figure 6.4

MAIN ARM FLAT ROCK CREEKPROVISIONAL FLOOD HAZARD DIAGRAM

20 YEAR ARI

P

P

N

1000

Scale

200 m50

1.26.10.5

1.0

5.3

0.6

0.83.80.4

0.7

3.7

0.5

0.3

2.8

0.8

NOTE


LEGEND

5.5 m/s AVERAGE VELOCITY OF FLOW

HIGH HAZARD

LOW HAZARD


Figure 6.5

MAIN ARM FLAT ROCK CREEKFLOODWAY DELINEATION DIAGRAM

100 YEAR ARI

p

p

N

1000

Scale

200 m50

NOTE


LEGEND

FLOODWAY

FLOOD STORAGE/ FLOOD FRINGE


SOUTHERN TRIBUTARYDESIGN WATER SURFACE PROFILES

5,20,100,200 YEAR ARI AND PMF

0 200 400 600 800 100035

40

45

50

55

60

65 Legend

WS PMF

WS 200yr ARI

WS 100 year ARI

WS 20 year ARI

WS 5 year ARI

Ground

-40

-2 34 72 79 128.

7

161.

3

194

213.

2523

2.5

249

275.

5

300

314

369

424

479

494

593

609

650

690

745

765

Wat

er S

urfa

ce E

leva

tion

(m A

HD

)


MIT

CH

ELL

STR

EE

T

DA

LLE

YS R

OA

D

DA

RG

AN

STR

EET

RU

TH S

TREE

T

WIL

LOU

GH

BY

RO

AD

NOTE: 100, 200 YEAR ARI AND PMF FLOOD LEVELS PRESENTED IN APPENDIX C HAVE BEEN INCREASED SO AS TO BE ABOVE 20 YEAR ARI LEVELS U/S DARGAN AND MITCHELL STREET CROSSINGS.

SEE NOTE

SEE NOTE


Figure 6.7

SOUTHERN TRIBUTARYEXTENTS OF INUNDATION

5,20,100 YEAR ARI AND PMF

P

P

N

LEGEND

PMF

100 YEAR ARI1000

Scale

50 200m

20 YEAR ARI

5 YEAR ARI

NOTE



Figure 6.8

SOUTHERN TRIBUTARYPROVISIONAL FLOOD HAZARD DIAGRAM

100 YEAR ARI

P

P

N1000

Scale

25 200m

NOTE


LEGEND

HIGH HAZARD

LOW HAZARD

5.5 m /s AVERAGE VELOCITY OF FLOW

0.2

2.6

0.2

0.1

0.1

5.8

0.3

0.0

6.30.2

1.60.1

7.3

0.0

0.0


Figure 6.9

SOUTHERN TRIBUTARYFLOODWAY DELINEATION DIAGRAM

100 YEAR ARI

p

p

N

1000

Scale

200 m50

NOTE


LEGEND

FLOODWAY



7 SUMMARY The flood study investigation involved the computer modelling of the channel of Flat Rock Creek between the North Shore Railway and Willoughby Road and the channel of the Southern Tributary between Waters Road and Willoughby Road. Figure 2.1 shows the Study Area. The study objective was to define flood behaviour in the streams in terms of flows, levels and flooding behaviour for floods ranging between 5 and 200 years average recurrence interval (ARI), as well as for the Probable Maximum Flood (PMF). Flood behaviour was defined using computer based hydrologic models of the catchments and a hydraulic model of the stream channels and floodplains. The hydrologic modelling approach was based on the DRAINS rainfall-runoff software. One-dimensional models based on the HEC-RAS system were adopted for the hydraulic analysis to model flood levels in the main arm of Flat Rock Creek and the Southern Tributary, which joins the main arm culvert downstream of Willoughby Road. There is a considerable volume of temporary flood storage within the section of Flat Rock Creek between Chelmsford Avenue and Willoughby Road Bridge. This storage has the effect of reducing, the peak flow as it travels downstream. This storage was incorporated in the DRAINS model as a conceptual storage. DRAINS therefore gave an estimate of the reduction in peak flows along the channel. The resulting peak flows were applied to HEC-RAS to give estimates of peak water surface profiles. Based on data relating to bridge hydraulics contained in the engineering literature a rating curve (relationship between water surface elevation and discharge) was computed for the Willoughby Road bridge which was used as the downstream boundary condition for the hydraulic model. Unfortunately, due to a lack of historic flood data, it has not been possible to calibrate the models. However, they give results for the design flood events which are consistent with those derived from previous flood studies on the catchment. The derived levels and flows are consistent with expected results and, as far as can be ascertained with historic flooding patterns. Upstream of the face of the Willoughby Road Bridge the comparative estimates of the peak 100 year ARI level are RL 47.6 m AHD compared with RL 47.0 m AHD (SMEC, 1995). These estimates of flood levels apply with the bridge operating at its full, unblocked capacity. The hydraulics of the bridge and underground and overland flow paths are complex and would best be analysed by a physical hydraulic model as opposed to the mathematical modelling used in the various investigations. The impacts of a partial blockage for two blockage scenarios were assessed for the present investigation. If the bicycle pathway and its supports were blocked by debris, the best estimate is that the 100 year ARI flood level at the bridge would be RL 47.9 m AHD. Alternatively if the throat of the low level culvert on the eastern side of the bridge (dimensions 2.64 m x 3.06 m) were blocked by 25 per cent, flood levels would rise to a



minimum of RL 48.1 m AHD and could be substantially higher if pressure flow conditions are initiated by the water surface impacting on the underside of the bridge arch. In 2006, the crest of the embankment of the retarding basin in Artarmon Reserve was raised by between 0.46 and 0.71 m, and the piped outlet was reduced from 1500 to 1050 mm diameter. These measures would mitigate the impacts of the widening of the Gore Hill Freeway associated with the Lane Cove Tunnel Project and conform with the original design of the basin embankment.



8 REFERENCES Arcement and Schneider (1984), “Guide for Selecting Manning’s Roughness Coefficients of Natural Channels and Floodplains”. US Department Transportation. FHA. Bureau of Meteorology, (2003). “The Estimation of Probable Maximum Precipitation in Australia: Generalised Short-Duration Method.” Lyall and Macoun Consulting Engineers (1987) “ Gore Link Road Drainage Study” Lyall and Macoun Consulting Engineers (1988) “ Design of Gore Link Road Drainage ” New South Wales Government, (2005). “Floodplain Development Manual – The Management of Flood Liable Land” The Institution of Engineers, Australia, (1998). “Australian Rainfall and Runoff – A Guide to Flood Estimation”, Volumes 1 and 2. Austroads (1994). “Waterway Design. A Guide to the Hydraulic Design of Bridges, Culverts and Floodways”. Parsons Brinkerhoff (2004). “Lane Cove Tunnel Technical Memorandum. Technical Memo 007: Flat Rock Creek Flood Assessment”. Roads and Traffic Authority (2001). “Lane Cove Tunnel and Associated Road Improvements. EIS Working Paper Twelve Hydrology and Hydraulics”. Snowy Mountains Engineering Corporation (1995). “Flat Rock Creek Flood Study. Final Report Volumes I and II”. US Army Corps of Engineers (2001). “HEC-RAS River Analysis System. Hydraulic Reference Manual”.

Flat Rock Creek Flood Study Appendix A

Flat Rock Appendix A.doc Page i Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

APPENDIX A

HISTORIC FLOODS AND MODEL TESTING


Flat Rock Appendix A.doc Page ii Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

APPENDIX A

TABLE OF CONTENTS

Page No.

1 INTRODUCTION.......................................................................................................1

2 COMMUNITY NEWSLETTER...................................................................................2

3 RAINFALL DATA .....................................................................................................3

4 TESTING HYDROLOGIC AND HYDRAULIC MODELS ...........................................4 4.1 DRAINS Model .............................................................................................4 4.2 DRAINS Model Parameters ..........................................................................4 4.3 DRAINS Model Results for Historic Floods..................................................5 4.4 HEC-RAS Model Results for Historic Floods ................................................5 4.5 Selection of Model Parameters for Design....................................................6

4.5.1. Sensitivity of Model Results ............................................................................. 6 4.5.2. Design Model Parameters ............................................................................... 7

LIST OF FIGURES

1.1 Rainfalls August 1986, March 1994 and April 1998 4.1 Main Arm Flat Rock Creek Water Surface Profiles Historic Floods


Flat Rock Appendix A.doc Page 1 Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

1 INTRODUCTION The procedure adopted for testing the DRAINS model of Flat Rock Creek, in situations where historic flood data are available, would involve the collection and analysis of rainfall data to ascertain the temporal and areal distribution of rainfall over the catchment. These rainfalls would then be applied to the model to generate flows within the catchment. In situations where there was a stream gauging station located on the catchment, the modelled discharge hydrograph would then be compared with historic hydrographs and model parameters varied until a fit was achieved. Similarly, when sufficient data are available on historic flood levels along the channel it is possible to use the known discharges and adjust the parameters of the hydraulic model to achieve a fit between recorded and modelled levels. Thus it would be possible to achieve independent calibration of each of the models (hydrologic and hydraulic) in turn. However, in most situations the streams are not gauged and data is usually limited to some isolated flood marks along the stream plus some recorded rainfall data. Under those circumstances, independent “calibration” of the models cannot be achieved. The usual procedure adopted is to use realistic values of the hydrologic model, adopted from experience and the engineering literature, in conjunction with recorded rainfall data to estimate flows and to vary the parameters of the hydraulic model to achieve a reasonable agreement with recorded flood levels. Sometimes the recorded flood marks or levels recorded at structures are used in conjunction with uniform flow or culvert formulae to estimate historic flood flows to assist with the selection of model parameters. However, in the absence of recorded stream flow data, the overall process as outlined above can at best be termed “model tuning” or ”model testing” rather than calibration. In the case of Flat Rock Creek there were several historic storms, including significant events in April 1998 and August 1986, as well as a lesser storm in March 1994 for which there were several recorded flood marks bordering the lower reaches of the channel upstream of Willoughby Road. Pluviographic data for the historic storms were recorded at the Chatswood Bowling Club and Sydney Observatory, as well as at a daily-read rain gauge at Northbridge. Recorded rainfalls were applied to the DRAINS model to estimate flows, which were then applied to the HEC-RAS model of the Flat Rock Creek channel. The procedure and results are summarised in Section 4 below.

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FLAT ROCK CREEK FLOOD STUDYAPPENDIX A

Figure 1.1RAINFALLS AUGUST 1986, MARCH 1994

AND APRIL 1998

Hours

Hours

Hours



2 COMMUNITY NEWSLETTER A Community Newsletter was prepared and distributed to residents bordering the creeks to gain knowledge of flood behaviour in the study area. A total of 150 Newsletters were distributed and 12 responses were received of which 9 were located along the Southern Tributary and 3 on the channel of Flat Rock Creek. One long term resident on the Southern Tributary identified the occurrences of significant flows in February 1984, October 1994 and April 1998. In February 1984, floodwaters overtopped the channel and entered backyards. Unfortunately the resident did not supply any contact details, which would have allowed a follow up visit by the Consultants. On Flat Rock Creek, a respondent noted that in the April 1998 flood, water entered backyards in the lower reaches of the channel and removed side fences of several allotments in Olympia and Park Roads. However, this flood does not appear to have resulted in inundation of residences or sheds in the backyards. Another resident identified the location of the extent of the flood and also, noted that the creek had broken its banks in March 1994. In all, two flood marks were identified and later levelled for the April 1998 flood. None of the respondents identified the occurrence of the August 1986 flood, which appears from previous investigations (LMCE, 1987; SMEC, 1995) to have been the largest flood in the past 20 to 30 years. The Olympia Road area has been extensively redeveloped since the construction of the Gore Hill Freeway and a large number of new residents with little experience of flooding have moved into the area. At the time of the previous investigations, a number of then residents had a good recollection of historic floods. Flood level data were collected for events which occurred on 7 March 1994 and 10 August 1986. These flood level data are tabulated in SMEC, 1995 and were used to test the models developed for the present flood study.



3 RAINFALL DATA Australian Water Technologies (AWT) supplied rainfall intensity data for the pluviometer at the Chatswood Bowling Club, which is located on the Pacific Highway about 2 km from the centroid of the catchment. The Bureau of Meteorology sponsors a daily rain gauge at the Northbridge Bowling Club, situated to the east of the catchment. These data were used to assess the temporal pattern of rainfall experienced on the Flat Rock Creek catchment for the August 1986, March 1994 and April 1998 storms. Figure 1.1 shows cumulative depths of rainfall recorded at the Chatswood Bowling Club for each of these storm events. The 10 April 1998 storm had the most intense rainfalls, with the most intense burst occurring over the 30 minute period from 11:50am to 12:20pm when 72.5 mm fell. Over the one to two hours durations which maximise flows in the Flat Rock Creek catchment, the rainfall intensities approximated a 50 year ARI storm. The 5 August 1986 storm was a longer duration event, with about 300 mm of rainfall being experienced at Chatswood over the 24 hour period from 00 to 2400 hours. Rainfall intensities were less than in April 1998 and were around 20 year ARI for the critical 1 to 2 hours duration on Flat Rock Creek. The 7 March 1994 storm was essentially a one hour and comparatively minor event, when about 50 mm of rainfall were recorded.



4 TESTING HYDROLOGIC AND HYDRAULIC MODELS 4.1 DRAINS Model Pluviographic data for the three historic storms identified in Section 3, as recorded at the Chatswood Bowling Club, were applied to the DRAINS model developed for the present investigation. 4.2 DRAINS Model Parameters Initial model testing was undertaken with the following parameters: Soil Type = 2.5 (assessment of a soil’s rate of infiltration.) AMC = 3.0 (Antecedent Moisture Condition – assessment of a catchment’s wetness

at the start of storm event). Paved area depression storage = 2.0 mm. Supplementary area depression storage = 1.0 mm. Grassed area depression storage = 10.0 mm. In addition, the roughness k for the pipes was assumed to be 0.3, as recommended in ARR (1998) and 0.06 for the Freeway trunk drainage box culverts. Pit loss coefficients were assigned with values adopted in accordance with Missouri Charts, the DRAINS manual and various technical papers. Cross drainage sub-catchments were simplified to have a single inlet pit and pipe which did not limit the entry of flow into the Freeway drainage system. This assumption is in accordance with the prototype drainage system which was designed by LMCE with a 100 year capacity. A response time of 10 minutes was adopted in the commercial and industrial sub-areas and 5 minutes in the residential areas. In addition, the path of travel of runoff was adjusted to closely follow the pattern of the street system. The resulting flow length and slope was then used by DRAINS to assess the travel time of the floodwave.



4.3 DRAINS Model Results for Historic Floods

TABLE 4.1

HISTORIC STORMS MODELLED PEAK FLOWS

(m3/s)

Location April 1998 August 1986 March 1994

Flat Rock Creek

North Shore Railway Culvert 74 72 23


80 77 24

Willoughby Road u/s Bridge

83 73 24

4.4 HEC-RAS Model Results for Historic Floods Water surface profiles as modelled by the HEC-RAS model developed for the present study are shown on Figure 4.1 for the April 1998, August 1986 and March 1994 floods. The August 1986 storm occurred prior to the construction of the Gore Hill Freeway when the drainage system in the middle to upper reaches of the catchment was of much lower capacity than the present day system and flows were conveyed at relatively low velocities in the then unlined channel. (There are insufficient data available on the nature of the original creek channel and drainage system to convert the DRAINS model to a pre–freeway model). The peak discharge of this flood was of sufficient magnitude to result in overtopping of the headwall of the culvert on the eastern side of the Willoughby Road bridge and flow across the overland flow path within Hallstrom Park to Flat Rock Drive. It would be expected that the increase in the velocity of flow in the main drainage system and the loss of flood storage upstream of the railway associated with the freeway construction would result in an increase in downstream flood peaks, so that if this storm had occurred under present day conditions, peak flows downstream of the railway would have been higher than historic values. The fact that the detention basin on the Northern Tributary was not constructed until 1991 also makes it difficult to use the August 1986 flood for testing the models. In the SMEC, 1995 study, the peak discharge for the August 1986 flood was estimated at 60 m3/s in the reach between Chelmsford Avenue and Willoughby Road. This discharge was applied to the hydraulic model and the resulting water surface profile is shown on Figure 4.1.



The results are in reasonable agreement with the recorded flood levels in the lower to middle reaches of the channel. The recorded flood level of RL 47.73 m AHD at RS 1740 could not be replicated by the model. Over the past 20 years there has been considerable encroachment of development and fences into the southern overbank of the channel. The model of the channel in its present day conditions predicts that a hydraulic jump would occur in this vicinity due to the resulting obstruction to flows, with a considerable local increase in water level. The March 1994 flood occurred under post-freeway conditions and therefore offers the opportunity for model testing. This storm was a relatively minor event and did not cause overbank flooding in the open channel section of Flat Rock Creek. The computed water surface profile is in reasonable agreement with the recorded flood levels. In the case of the April 1998 flood, rainfall intensities at the Chatswood pluviometer for the 1 hour to 90 minute durations likely to maximise peak flows in the lower reaches of the catchment were around the 50 year ARI. However, the elevations of the recorded flood marks along the channel are well below those experienced in August 1986, when recorded rainfall intensities at Chatswood were less and, as mentioned previously, the upstream drainage system was less efficient. It is also understood that in April 1998, the detention basin in Artarmon Reserve did not store any significant flows resulting from surcharging of the stormwater system of the Northern Tributary, even though this basin is designed to operate for storms in excess of 10 year ARI. The conclusion reached from analysis of this storm event was that rainfall intensities actually experienced over the Flat Rock creek catchment were less than intensities recorded at the respective rain gauges. Unfortunately, there are no rain gauges located within the catchment boundary to test this theory. 4.5 Selection of Model Parameters for Design

4.5.1. Sensitivity of Model Results Sensitivity analysis is commonly carried out to assess the impact of model parameter assumptions on results. Several sensitivity runs were reported in PB, 2004 for the DRAINS model. Changes were made to the sub-catchment travel times, pressure loss coefficients (Ku) and pipe roughness (K), as described below. Travel times which were previously calculated based on a 2 m/s pipe velocity were reduced to 1.5 m/s, pressure loss coefficients were reduced by 25% for pits along the main trunk main, and pipe roughness was doubled. The results of the sensitivity analysis showed that by changing the model parameters the maximum flows and water levels at key locations varied as follows:

Increasing the stormwater travel time by reducing the assumed water velocity from 2 m/s to 1.5 m/s results in a reduction in the maximum flow. The 100 year ARI flows reduced by 5% at the North Shore Railway culvert and by 2% at the Willoughby Road culvert.



Reducing the pressure loss coefficient (Ku) by 25% increased the maximum flow by

1% at the North Shore Railway culvert and by 3% at the Willoughby Road culvert.

By doubling the pipe and culvert roughness, the maximum flows are decreased by 0.5% at the North Shore Railway culvert and by 0.5% at the Willoughby Road Culvert.

4.5.2. Design Model Parameters Model parameters set out in Section 4.2 were adopted for design flood estimation.

FLAT ROCK CREEK FLOOD STUDYAPPENDIX A

Figure 4.1MAIN ARM FLAT ROCK CREEK

WATER SURFACE PROFILES HISTORIC FLOODS

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RIVER STATION No.

Flat Rock Creek Flood Study Appendix B

Flat RockAppendix B.doc Page i Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

APPENDIX B

REVIEW OF PREVIOUS FLOOD STUDIES ON FLAT ROCK CREEK


Flat RockAppendix B.doc Page ii Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

APPENDIX B

TABLE OF CONTENTS

Page No.

1 PREVIOUS INVESTIGATIONS.................................................................................... 1

2 COMPARISON OF HYDROLOGIC MODELLING RESULTS ...................................... 2 2.1 Peak Flows .......................................................................................................... 2 2.2 Comparison PB, 2004 and WP12 ........................................................................ 3 2.3 Comparison Present Flood Study, 2006 and PB, 2004 ...................................... 3

3 FLOOD LEVELS AT WILLOUGHBY ROAD................................................................ 5 3.1 Comparison of Results......................................................................................... 5 3.2 Discussion ........................................................................................................... 6

4 OPTIONS FOR MITIGATING FLOODING ................................................................... 7



1 PREVIOUS INVESTIGATIONS Several flood investigations have been undertaken on the Flat Rock Creek catchment, since 1988, namely:

Lane Cove Tunnel Technical Memorandum. Technical Memo 007: Flat Rock Creek Flood Assessment prepared by Parsons Brinkerhoff for the Thiess John Holland Joint Venture for the Lane Cove Tunnel project in August 2004, (PB, 2004).

This investigation which is denoted PB, 2004 in the following discussion was prepared to meet the Scope of Works and Technical Criteria and the Minister’s Conditions of Approval 234. The investigation comprised an assessment of flows and water levels along Flat Rock Creek to quantify the potential increases in flows that could occur as a result of the widening of the Freeway associated with the Lane Cove Tunnel Project. The study was undertaken using the DRAINS rainfall-runoff software to assess flows and the HEC-RAS hydraulic modelling software to assess water surface profiles in the lower reaches of Flat Rock Creek between Chelmsford Avenue and Willoughby Road bridge.

Working Paper Twelve - Hydrology and Hydraulics (WP12, 2001) prepared for the

EIS of the Lane Cove Tunnel and Associated Road Improvements in October 2001.

This investigation included a flood study of the Flat Rock Creek catchment to assess the impacts of the proposed extension of the Gore Hill Freeway on flooding characteristics. The study was undertaken using DRAINS and HEC-RAS and was of a reconnaissance nature, based on then existing sources of survey and drainage data.

The Flat Rock Creek Flood Study prepared by Snowy Mountains Engineering

Corporation in 1995 for Willoughby City Council, (SMEC, 1995).

This investigation used the RAFTS rainfall-runoff software to assess flows and the MIKE II dynamic flow model to assess water surface profiles in Flat Rock Creek between Chelmsford Avenue and Willoughby Road. The flood levels derived in this investigation are currently used by Willoughby City Council for planning purposes.

Drainage Investigation on Flooding in the Flat Rock Creek Catchment prepared by

Lyall and Macoun Consulting Engineers in December 1987 for the RTA to assess the potential impacts of the Gore Hill Freeway on flooding. The flood mitigation works proposed to offset the impacts of the freeway included the detention basin, subsequently constructed in Artarmon Reserve, to control flows from the Northern Tributary, which discharges to the main arm of Flat Rock Creek in the vicinity of Chelmsford Avenue. The investigations prepared by LMCE used the RORB runoff-routing software to assess flood flows and the HEC-2 hydraulic modelling software to assess resulting water surface profiles along the open channel section of Flat Rock Creek.

Design Drawings prepared by Lyall and Macoun Consulting Engineers in June 1988,

which show the flood works proposed at that time to mitigate the impacts of the original Gore Hill Freeway construction.



2 COMPARISON OF HYDROLOGIC MODELLING RESULTS

2.1 Peak Flows Peak flows derived from the various modelling approaches are presented in Table 2.1. At Willoughby Road bridge, the differences in peak flows between the investigations is of the order of 10 per cent. A difference of this magnitude, (with some of that difference due to the different detention basin conditions, as identified in the Table), would be acceptable and within the accuracy which could reasonably be achieved by hydrologic modelling. It is only because of the implications of this difference in flows on water levels at Willoughby Road that further consideration of the differences in model results is warranted. The following discussion compares results achieved in the present Flood Study, 2006 with the results of PB, 2004 and WP12, 2001 which were all carried out using the DRAINS software. The remaining investigations were carried out using rainfall runoff models which did not specifically model the piped drainage system upstream of the North Shore Railway.

TABLE 2.1 COMPARISON OF PEAK 100 YEAR ARI FLOWS

VALUES IN m3/s

Investigation

Location Present Investigation

2006 (4)

PB 2004 (3)

WP12 2001(5)

LACE 1987(5)

SMEC 1995(5)

North Shore Railway Culvert

57 65 43 44 (2) 49


73 84 65 68.5 69

Willoughby Road u/s Bridge

82(4) 84 77 Not

Available 76 (1)

Artarmon Reserve Detention Basin

Inflow 32.7 48 35 36 40

Outflow 4.3 23 (3) 8 8 7

Low Flow System 13.4 13 Not

Available 18 18

Notes (1) This flow was derived by the RAFTS hydrologic model and does not allow for any attenuating effects of the

floodplain storage on Flat Rock Creek downstream of Chelmsford Avenue ( which are incorporated in the MIKE 11 dynamic hydraulic model).

(2) This flow was extracted from the 1987 Flood Study by LMCE. (3) PB results are based on as-surveyed detention basin in 2004, and are influenced by the consequent surcharge

of embankment at the 100 ARI level of flooding. (4) These results apply with the Detention Basin in Artarmon raised and the outlet throttled to a 1050 mm diameter

pipe, as constructed in 2006 by Lane Cove Tunnel Design Joint Venture. (5) These results apply assuming that the “as designed’ basin with a 1500 mm diameter outlet had been constructed

in 1991.

(6) All of the above results apply with no blockage of Willoughby Road culvert.



2.2 Comparison PB, 2004 and WP12 The peak flows derived by PB are generally larger than by WP12. At the North Shore Railway culvert, the estimates of 100 year ARI discharge are 65 m3/s and 43 m3/s respectively. At Chelmsford Avenue, which is located downstream of the confluence with the Northern Tributary and its detention basin, the respective flows are 84 m3/s (PB) and 65 m3/s (WP12). Some of this difference in flows is due to the fact that the spillway of the detention basin has been surveyed at a lower level than the design crest of RL 57.8 m. This crest level would result in a surcharge of the as-constructed spillway for 100 year ARI storms of various durations. For the 1988 Design of the basin, runoff from the 100 year ARI storms was intended to be fully controlled by the low level pipe outlet contained in the basin embankment, without surcharging of the spillway. The WP12 analysis was carried out assuming that the basin had been constructed as per the design. Consequently the values shown on Table 2.1 do not permit a direct comparison between PB’s and WP12’s flows downstream of the confluence of Flat Rock Creek and the Northern Tributary. Upstream of Willoughby Road Bridge, the estimates of peak discharge are 84 m3/s (PB), versus 77 m3/s (WP12). Some of the difference between the two estimates would be due to the additional flood flow entering Flat Rock Creek due to the surcharging of the spillway discussed above, and the remainder would be due to the fact that PB’s estimate of the peak discharge through the North Shore Railway culvert is considerably greater than for WP12 (65 m3/s versus 43 m3/s). The model structure of the PB DRAINS model allows a 2 minutes time for the concentration of runoff in individual allotments, followed by a travel time of the floodwave across the sub-areas which is computed on the basis of a water particle velocity of 2 m/s. In the WP12 model, a kinematic wave assumption was used to assess the travel time of the floodwave across each sub-area. This assumption gave a slower travel time than the 2 m/s assumption used by PB and appears to be the main reason for the variation in the results at the North Shore Railway. There are also some differences relating to sub-catchment areas, which appear to be due to the greater level of survey detail available to PB, compared with the previous investigations.

2.3 Comparison Present Flood Study, 2006 and PB, 2004 For major flood events such as a 100 year event it would be expected that the runoff generated by the storm would quickly surcharge the capacity of the piped drainage system in the lateral sub-catchments discharging to the trunk drainage system of the Freeway. Some of the runoff would be temporarily stored on roofs and depressions. That portion of the runoff exceeding the capacity of the lateral drainage system would find its way along the street system to the catchment outlet. As the lateral drainage system is not likely to have a capacity substantially greater than for a 5 year ARI, the overall effect may be for a substantial portion of the catchment runoff to be conveyed above ground as overland flow,



in a manner which more closely replicates a kinematic wave than the uniform 2 m/s velocity adopted by PB. Information contained in ARR, 1998 suggests that for large commercial and industrial buildings, which are typical of the commercial and industrial areas in the catchment particularly on the southern side of the freeway upstream of the North Shore Railway culvert, the response time of the allotments to rainfall would be in the range 5 to 15 minutes. A run of PB’s DRAINS model was carried out with the response time in the commercial and industrial sub-areas increased to 10 minutes. In addition, the path of travel of runoff was adjusted to more closely follow the pattern of the street system. The resulting flow length and slope was then used by DRAINS to assess the travel time of the floodwave. The result of this sensitivity study was a considerable reduction in the modelled peak flow at the North Shore Railway, as shown in Column (2) of Table 2.2. (These results were later adopted in the present Flood Study, 2006.) Column (3) shows PB’s results with the basin in Artarmon Reserve as surveyed in 2004. Column (4) shows the effects of using PB’s DRAINS model and the 2 m/s assumption of travel time, but with the embankment of the basin raised to contain the 100 year ARI inflow. The flood storage in the lower portion of Flat Rock Creek attenuates the flow on the main arm, giving no significent difference in the estimates of 100 year ARI peak flows at Willoughby Road .

TABLE 2.2 SENSITIVITY OF DRAINS MODEL TO

ASSUMED TRAVEL TIME OF FLOODWAVE PEAK 100 YEAR ARI FLOWS

VALUES IN m3/s

Location

Present Flood Study

2006 (1)

PB ,2004 Prior to

Raising the Basin (2)

PB,2004 With Basin Embankment Raised to Contain 100 Year

ARI Flood (2)

(1) (2) (3) (4)

North Shore Railway 57 65 65

Willoughby Road Bridge

82 84 82

Notes (1) These flows were derived assuming 10 minutes delay time for runoff from the large industrial complexes

upstream of North Shore Railway and 5 minutes delay from residential premises. Overland flow times were based on length of travel and slope to outlet.

(2) These flows were derived assuming 2 minutes delay and 2m/s travel time in overland flow paths.



3 FLOOD LEVELS AT WILLOUGHBY ROAD

3.1 Comparison of Results The PB study developed a stage-discharge relationship (rating curve) to describe water surface elevations within the bridge waterway based on the low level culvert/weir scenario of hydraulic control This relationship was used as the downstream boundary condition in their HEC-RAS modelling. Contraction losses as the flow entered the bridge section were assessed within the HEC-RAS model, which gave a 100 year ARI water level of RL 48.1 m AHD upstream of the bridge, for a discharge of 84 m3/s (the peak flow modelled with the detention basin prior to raising the embankment). The WP12 study adopted a similar assumption as to the hydraulic control at the downstream face of the bridge, but assessed the contraction losses at the upstream face manually. These computations gave a rating curve which could be applied at the upstream face of the bridge, thereby allowing the HEC-RAS analysis to commence at the downstream end of the channel. This analysis gave a level of RL 47.6 m AHD for their estimated 100 year ARI discharge of 77 m3/s. The SMEC study gave an estimate for the 100 year ARI flood at the upstream side of the bridge of RL 47.0 m AHD. All of these analyses applied with the culvert operating at full hydraulic capacity, i.e. unrestricted by debris. The PB study simulated the effects of a blockage by reducing the waterway area available to convey flow into the arch. Reduction factors of 25 and 100 per cent were considered. This analysis resulted in the peak water surface upstream of the bridge increasing from RL 48.1 m AHD for the unblocked case to RL 49.5 m AHD for 25 per cent blockage. For 100 per cent blockage, flow would surcharge Willoughby Road. The SMEC 1995 study expressed concern that the bicycle track running along the southern side of the arch would interfere with flow and trap debris. SMEC investigated the effect of the track by increasing the hydraulic roughness of the arch section and reducing the flow area to account for its encroachment into the waterway. However, according to SMEC, the overall effect was quite small amounting to an increase of only 60 mm upstream of the bridge. As their best estimate of conditions which would actually occur at Willoughby Road, PB adopted a 25 per cent reduction in the effective waterway area of the arch bridge. This had the effect of moving the hydraulic control for the 100 year event to the upstream face of the bridge.

With the 25 per cent reduction in waterway area, HEC-RAS considered that pressure flow would occur at the upstream face of the arch bridge. Pressure flow in HEC-RAS is based on the assumption that the arch section would function as a sluice gate, as shown on page 5 – 20 of the HEC-RAS manual attached. Because a sluice gate is hydraulically less efficient than an orifice, HEC-RAS predicts a considerable increase in head required to drive the flow through the arch bridge than would



be the case if free flow conditions were maintained through the arch and the orifice/weir on the eastern face of the Willoughby Road bridge continued as the hydraulic control. PB estimated the peak 100 year ARI flood level with 25 per cent blockage as RL 49.5 m AHD. However, there are considerable differences between the estimates of peak flood level at the upstream side of the bridge as shown on Table 3.1, which applies for the 100 year ARI event and with the waterway operating at full hydraulic capacity, i.e. unrestricted by any blockages. Some of the variation is due to the different estimates of the design flow and the remainder is due to differences in assessed hydraulic capacity.

TABLE 3.1 ESTIMATES OF 100 YEAR ARI PEAK FLOOD LEVELS

AT WILLOUGHBY ROAD BRIDGE BRIDGE OPERATING AT FULL CAPACITY

RL – m AHD

Investigation Present

Flood Study 2006 (4)

PB 2004 (3)

WP12 2001 (4)

SMEC 1995 (4)

47.6 (2)

48.1

47.6

47.0

Note: (1) All results are based on no blockage of bridge opening

(2) This level is based on peak flow of 82 m3/s with detention basin embankment raised.

(3) This level is based on peak flow of 84 m3/s with detention basin embankment raised.

(4) These results apply with the retarding basin raised to its “as designed” level.

3.2 Discussion For the 1988 Design of the basin, runoff from the 100 year ARI storms was intended to be fully controlled by the low level pipe outlet contained in the basin embankment, without surcharging of the spillway. The WP12 analysis was carried out assuming that the basin had been constructed as per the design. Consequently PB’s values shown on Table 3.1 do not permit a direct comparison with the flows derived from the previous investigations downstream of the confluence of Flat Rock Creek and the Northern Tributary. Upstream of Willoughby Road Bridge, the estimates of peak discharge are 84 m3/s (PB), versus 77 m3/s (WP12). Some of the difference between the two estimates would be due to the additional flood flow entering Flat Rock Creek due to the surcharging of the spillway discussed above, and the remainder would be due to the fact that PB’s estimate of the peak discharge through the North Shore Railway culvert is considerably greater than for WP12 (65 m3/s versus 43 m3/s).



4 OPTIONS FOR MITIGATING FLOODING WP12, 2001 suggested that additional throttling of the flows derived from the Northern Tributary Catchment be undertaken as a method of mitigating the impacts of the proposed roadworks. This could be achieved by raising the embankment of the detention basin, in conjunction with reducing the capacity of the existing 1500 mm diameter outlet pipe in the basin embankment and/or throttling the low flow drainage system which runs beneath the Artarmon Reserve to induce greater flow into the storage area. Two options, denoted Option 1b and 1d, were presented in the PB, 2004 report. Each of these schemes would achieve the desired objective of no increase in flow at Willoughby Road. Option 1b, which is PB’s preferred option, involves raising the elevation of the basin embankment by around 0.5 m, together with constricting the outlet to 1050 mm diameter. Option 1d involves constricting the low flow system running beneath Artarmon Reserve, in addition to raising the embankment, but would increase the frequency of inundation of the basin area and was not favoured by Council.


Flat Rock.doc Page i Lyall & Associates 29 March 2006 Rev. 3.0 Consulting Water Engineers

APPENDIX C

FLOOD STUDY RESULTS



APPENDIX C

FLOOD LEVEL, FLOW AND VELOCITY DISTRIBUTION

TABULATIONS - DESIGN FLOODS

HEC-RAS Plan: Plan 01 River: Flat Rock Creek Reach: Flat Rock CreekReach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev Q Left Q Channel Q Right Vel Left Vel Chnl Vel Right Top Width Froude # Chl

(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1899 5year ARI 45.6 46.79 50.1 49.66 50.62 2.86 40.32 2.42 0.55 3.41 0.58 17.89 0.62Flat Rock Creek 1899 10year ARI 53 46.79 49.02 49.93 51.42 0.18 52.68 0.14 0.69 6.88 0.58 6.42 1.56Flat Rock Creek 1899 20year ARI 61.6 46.79 49.2 50.19 51.86 0.44 60.72 0.44 0.84 7.28 0.78 7.74 1.58Flat Rock Creek 1899 50year ARI 67.2 46.79 49.31 50.33 52.12 0.68 65.77 0.75 0.93 7.52 0.94 8.94 1.59Flat Rock Creek 1899 100 year ARI 73.4 46.79 49.42 50.48 52.41 0.94 71.38 1.08 0.85 7.77 0.97 10.86 1.61Flat Rock Creek 1899 200year ARI 83.2 46.79 49.58 50.64 52.78 1.83 79.52 1.85 1.01 8.11 1.11 12.55 1.63Flat Rock Creek 1899 PMF 374 46.79 55.86 53.38 56.02 141.62 105.67 126.72 0.92 3.1 0.88 66.55 0.33

Flat Rock Creek 1890 5year ARI 45.6 46.67 50.19 50.57 4.41 37.86 3.34 0.52 2.99 0.52 24.02 0.53Flat Rock Creek 1890 10year ARI 53 46.67 50.36 49.82 50.77 6.01 42.37 4.62 0.57 3.18 0.58 26.42 0.55Flat Rock Creek 1890 20year ARI 61.6 46.67 50.57 50.07 51 8.23 46.96 6.4 0.61 3.32 0.63 29.67 0.56Flat Rock Creek 1890 50year ARI 67.2 46.67 50.68 50.21 51.13 9.73 49.9 7.57 0.64 3.42 0.67 31.44 0.57Flat Rock Creek 1890 100 year ARI 73.4 46.67 50.87 50.35 51.29 12.31 51.89 9.2 0.67 3.39 0.68 33.14 0.55Flat Rock Creek 1890 200year ARI 83.2 46.67 51.14 50.55 51.54 15.86 55.33 12.01 0.68 3.38 0.71 37.6 0.53Flat Rock Creek 1890 PMF 374 46.67 55.68 56.01 139.27 133.03 101.7 1.21 3.9 1.16 39.64 0.42

Flat Rock Creek 1880 5year ARI 45.6 46.61 50.24 50.53 7.48 31.91 6.21 0.62 2.84 0.64 28.61 0.54Flat Rock Creek 1880 10year ARI 53 46.61 50.43 50.73 9.74 35.21 8.04 0.66 2.94 0.68 31.37 0.54Flat Rock Creek 1880 20year ARI 61.6 46.61 50.66 50.95 13.03 38.18 10.39 0.7 2.96 0.7 33.81 0.52Flat Rock Creek 1880 50year ARI 67.2 46.61 50.78 51.07 14.77 40.48 11.95 0.72 3.03 0.73 35.63 0.52Flat Rock Creek 1880 100 year ARI 73.4 46.61 50.95 51.24 16.94 42.48 13.97 0.71 3.03 0.74 38.73 0.51Flat Rock Creek 1880 200year ARI 83.2 46.61 51.23 51.48 21.65 44.47 17.08 0.73 2.94 0.73 41.31 0.48Flat Rock Creek 1880 PMF 374 46.61 55.75 55.97 145.41 112.78 115.82 1.16 3.44 1.13 42.6 0.38

Flat Rock Creek 1870 5year ARI 45.6 47.25 49.86 49.86 50.49 8.45 33.59 3.56 0.94 4.06 0.89 19.79 0.89Flat Rock Creek 1870 10year ARI 53 47.25 50.02 50.02 50.68 10.76 37.64 4.59 1 4.22 0.94 21.42 0.89Flat Rock Creek 1870 20year ARI 61.6 47.25 50.14 50.14 50.89 13.26 42.64 5.7 1.08 4.55 1.02 22.55 0.94Flat Rock Creek 1870 50year ARI 67.2 47.25 50.23 50.21 51.01 15.36 45.62 6.22 1.14 4.68 1.01 23.54 0.95Flat Rock Creek 1870 100 year ARI 73.4 47.25 50.27 50.27 51.16 17.22 49.68 6.5 1.24 5.03 1.01 24.64 1.01Flat Rock Creek 1870 200year ARI 83.2 47.25 50.45 50.45 51.4 20.12 55.34 7.74 1.23 5.23 0.94 31.15 1.01Flat Rock Creek 1870 PMF 374 47.25 55.71 55.97 146.2 114.76 113.04 1.16 3.69 1 48.67 0.42

Flat Rock Creek 1860 5year ARI 45.6 46.38 49.34 49.61 50.42 3.69 39.59 2.32 1.04 4.92 1.03 14.27 1.09Flat Rock Creek 1860 10year ARI 53 46.38 49.52 49.71 50.61 5.55 44.12 3.33 1.16 5.04 1.1 15.32 1.07Flat Rock Creek 1860 20year ARI 61.6 46.38 49.71 49.8 50.82 7.73 49.26 4.61 1.28 5.2 1.17 16.43 1.07Flat Rock Creek 1860 50year ARI 67.2 46.38 50.36 50.36 50.93 12.65 47.42 7.13 0.89 3.94 0.67 36.32 0.72Flat Rock Creek 1860 100 year ARI 73.4 46.38 50.1 50.44 51.12 11.25 56.02 6.13 1.07 5.08 0.91 28.84 0.96


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1860 200year ARI 83.2 46.38 50.61 50.55 51.18 17.55 53.46 12.19 0.96 4.11 0.83 38.29 0.72Flat Rock Creek 1860 PMF 374 46.38 55.75 55.95 144.92 106.87 122.21 1.13 3.23 1 49.7 0.35

Flat Rock Creek 1852 5year ARI 45.6 46.29 49.52 49.12 49.98 3.34 38.08 4.18 0.69 3.27 0.6 18.24 0.61Flat Rock Creek 1852 10year ARI 53 46.29 49.77 49.32 50.23 4.51 42.28 6.21 0.75 3.35 0.65 19.84 0.6Flat Rock Creek 1852 20year ARI 61.6 46.29 50.01 49.52 50.49 5.8 47.25 8.55 0.81 3.48 0.68 22.08 0.6Flat Rock Creek 1852 50year ARI 67.2 46.29 50.12 49.65 50.65 6.63 51.41 9.16 0.87 3.67 0.65 25.75 0.62Flat Rock Creek 1852 100 year ARI 73.4 46.29 50.35 49.8 50.83 7.51 53.13 12.76 0.86 3.56 0.7 25.75 0.58Flat Rock Creek 1852 200year ARI 83.2 46.29 50.7 49.88 51.12 8.77 56 18.43 0.85 3.44 0.76 25.75 0.54Flat Rock Creek 1852 PMF 374 46.29 55.17 55.89 43.01 178.47 152.52 0.89 5.27 1.35 58.3 0.57

Flat Rock Creek 1850 5year ARI 45.6 46.27 49.11 49.11 49.93 2.41 41.94 1.25 0.76 4.19 0.33 16.47 0.84Flat Rock Creek 1850 10year ARI 53 46.27 49.32 49.32 50.18 3.72 47.22 2.06 0.87 4.37 0.38 18.02 0.84Flat Rock Creek 1850 20year ARI 61.6 46.27 49.54 49.54 50.45 5.41 52.99 3.2 0.98 4.53 0.42 19.62 0.84Flat Rock Creek 1850 50year ARI 67.2 46.27 49.67 50.6 6.52 56.7 3.98 1.04 4.65 0.45 20.36 0.84Flat Rock Creek 1850 100 year ARI 73.4 46.27 49.78 50.77 7.67 60.91 4.82 1.12 4.83 0.48 21.02 0.86Flat Rock Creek 1850 200year ARI 83.2 46.27 49.85 51.03 9.04 68.36 5.79 1.25 5.31 0.53 21.43 0.93Flat Rock Creek 1850 PMF 374 46.27 53.97 55.78 59.92 221.34 92.75 1.95 7.64 1.16 26.5 0.89

Flat Rock Creek 1840 5year ARI 45.6 46.2 48.81 49 49.88 1.99 43.02 0.58 0.82 4.71 0.33 13.07 0.98Flat Rock Creek 1840 10year ARI 53 46.2 49.05 49.24 50.14 3.35 48.5 1.15 0.94 4.82 0.37 15.37 0.96Flat Rock Creek 1840 20year ARI 61.6 46.2 49.31 49.4 50.4 5.13 54.43 2.04 1.04 4.92 0.41 17.84 0.93Flat Rock Creek 1840 50year ARI 67.2 46.2 49.51 49.51 50.56 6.45 57.84 2.91 1.05 4.89 0.43 19.99 0.9Flat Rock Creek 1840 100 year ARI 73.4 46.2 49.77 49.77 50.73 8.06 61.11 4.23 1 4.76 0.44 23.86 0.84Flat Rock Creek 1840 200year ARI 83.2 46.2 49.85 50.99 9.69 68.37 5.14 1.11 5.2 0.49 24.79 0.9Flat Rock Creek 1840 PMF 374 46.2 54.83 55.81 95.15 195.11 83.73 1.29 5.99 0.79 55.08 0.66

Flat Rock Creek 1820 5year ARI 45.6 45.99 48.43 48.86 49.77 1.32 44.1 0.19 0.78 5.21 0.31 10.3 1.13Flat Rock Creek 1820 10year ARI 53 45.99 48.65 49.07 50.04 2.54 49.99 0.47 0.95 5.37 0.31 16.27 1.11Flat Rock Creek 1820 20year ARI 61.6 45.99 48.86 49.3 50.31 4.06 56.35 1.19 1.09 5.56 0.33 20.26 1.1Flat Rock Creek 1820 50year ARI 67.2 45.99 49.36 49.43 50.29 6.91 56.15 4.15 1.03 4.65 0.42 23.96 0.84Flat Rock Creek 1820 100 year ARI 73.4 45.99 49.1 49.55 50.62 6.22 64.35 2.83 1.22 5.81 0.44 22.55 1.1Flat Rock Creek 1820 200year ARI 83.2 45.99 50.22 49.73 50.79 12.59 60.28 10.33 0.96 3.91 0.48 26.03 0.63Flat Rock Creek 1820 PMF 374 45.99 55.07 55.69 142.82 171.44 59.74 1.31 4.99 0.58 60 0.54

Flat Rock Creek 1800 5year ARI 45.6 45.8 48.18 48.61 49.66 0.48 44.74 0.39 0.65 5.45 0.32 11.29 1.2Flat Rock Creek 1800 10year ARI 53 45.8 48.4 48.82 49.93 1.21 50.78 1.01 0.81 5.6 0.44 12.73 1.17


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1800 20year ARI 61.6 45.8 48.63 49.04 50.2 2.38 57.34 1.88 0.99 5.76 0.54 13.19 1.15Flat Rock Creek 1800 50year ARI 67.2 45.8 49.17 49.17 50.23 5.05 58.67 3.48 1.01 4.85 0.55 14.29 0.88Flat Rock Creek 1800 100 year ARI 73.4 45.8 48.92 49.31 50.54 4.31 65.93 3.16 1.15 5.93 0.63 13.79 1.12Flat Rock Creek 1800 200year ARI 83.2 45.8 49.52 49.52 50.7 8.02 70.11 5.08 1.15 5.22 0.63 14.99 0.9Flat Rock Creek 1800 PMF 374 45.8 54.12 55.59 98.87 216.85 58.28 1.94 6.91 0.73 42.85 0.78

Flat Rock Creek 1780 5year ARI 45.6 45.64 47.86 48.37 49.54 1 44.52 0.08 0.83 5.8 0.29 8.95 1.32Flat Rock Creek 1780 10year ARI 53 45.64 48.07 48.59 49.81 2.05 50.71 0.24 1.03 5.97 0.39 9.96 1.3Flat Rock Creek 1780 20year ARI 61.6 45.64 48.3 48.83 50.09 3.56 57.51 0.53 1.18 6.12 0.47 11.03 1.26Flat Rock Creek 1780 50year ARI 67.2 45.64 48.61 48.98 50.12 5.51 60.7 0.98 1.22 5.73 0.5 12.38 1.11Flat Rock Creek 1780 100 year ARI 73.4 45.64 48.59 49.12 50.43 5.94 66.41 1.05 1.33 6.3 0.54 12.32 1.23Flat Rock Creek 1780 200year ARI 83.2 45.64 49.38 49.36 50.48 11.35 69.32 2.53 1.21 5.08 0.5 16.02 0.87Flat Rock Creek 1780 PMF 374 45.64 54.44 55.42 122.38 196.94 54.68 1.69 5.86 0.63 47.39 0.64

Flat Rock Creek 1760 5year ARI 45.6 45.43 47.67 48.27 49.41 0.46 45.05 0.09 0.69 5.87 0.3 8.08 1.34Flat Rock Creek 1760 10year ARI 53 45.43 47.89 48.48 49.7 1.24 51.49 0.26 0.91 6.03 0.39 9.45 1.3Flat Rock Creek 1760 20year ARI 61.6 45.43 48.15 48.7 49.97 2.52 58.42 0.66 1.1 6.14 0.35 14.41 1.25Flat Rock Creek 1760 50year ARI 67.2 45.43 48.96 48.83 49.85 6.13 57.41 3.67 1.01 4.53 0.48 16.44 0.8Flat Rock Creek 1760 100 year ARI 73.4 45.43 49.19 48.98 50.05 7.6 61.08 4.72 1.04 4.5 0.51 16.87 0.77Flat Rock Creek 1760 200year ARI 83.2 45.43 49.54 50.36 10.11 66.72 6.37 1.06 4.46 0.54 17.53 0.73Flat Rock Creek 1760 PMF 374 45.43 54.5 55.37 95.53 193.35 85.12 1.59 5.64 0.72 44.2 0.61

Flat Rock Creek 1745.* 5year ARI 45.6 45.28 47.46 48.08 49.29 0.61 44.92 0.07 0.66 6.05 0.3 9.57 1.4Flat Rock Creek 1745.* 10year ARI 53 45.28 48.64 48.28 49.2 6.92 43.67 2.42 0.84 3.62 0.39 17.12 0.66Flat Rock Creek 1745.* 20year ARI 61.6 45.28 48.99 48.48 49.52 9.69 48.5 3.41 0.88 3.62 0.42 17.94 0.62Flat Rock Creek 1745.* 50year ARI 67.2 45.28 49.19 49.72 11.59 51.58 4.03 0.9 3.63 0.44 18.45 0.61Flat Rock Creek 1745.* 100 year ARI 73.4 45.28 49.41 49.93 13.7 55.01 4.68 0.93 3.66 0.45 18.95 0.6Flat Rock Creek 1745.* 200year ARI 83.2 45.28 49.74 50.25 17.16 60.34 5.7 0.95 3.7 0.46 19.88 0.58Flat Rock Creek 1745.* PMF 374 45.28 54.66 55.29 123.54 175.59 74.87 1.51 4.94 0.62 45.89 0.52

Flat Rock Creek 1740 5year ARI 45.6 45.23 48.36 48.01 48.86 5.99 37.95 1.66 0.77 3.4 0.35 17.01 0.64Flat Rock Creek 1740 10year ARI 53 45.23 48.71 49.16 8.66 41.88 2.45 0.81 3.35 0.38 17.91 0.6Flat Rock Creek 1740 20year ARI 61.6 45.23 49.04 49.49 11.76 46.55 3.28 0.85 3.37 0.4 18.79 0.57Flat Rock Creek 1740 50year ARI 67.2 45.23 49.25 49.69 13.87 49.53 3.8 0.87 3.39 0.41 19.33 0.56Flat Rock Creek 1740 100 year ARI 73.4 45.23 49.45 49.9 16 53.03 4.36 0.88 3.44 0.42 20.19 0.55Flat Rock Creek 1740 200year ARI 83.2 45.23 49.79 50.22 20.21 57.8 5.19 0.92 3.46 0.43 20.81 0.53Flat Rock Creek 1740 PMF 374 45.23 54.7 55.27 132.53 170.23 71.24 1.48 4.75 0.59 46.45 0.5


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1720 5year ARI 45.6 45.04 47.83 47.83 48.78 1.49 43.38 0.73 0.43 4.43 0.34 13.22 0.89Flat Rock Creek 1720 10year ARI 53 45.04 48.04 48.04 49.07 2.27 49.46 1.27 0.49 4.65 0.42 13.54 0.9Flat Rock Creek 1720 20year ARI 61.6 45.04 48.28 48.28 49.39 3.28 56.37 1.95 0.55 4.88 0.49 13.88 0.9Flat Rock Creek 1720 50year ARI 67.2 45.04 48.41 48.41 49.59 3.95 60.86 2.39 0.58 5.03 0.52 14.08 0.91Flat Rock Creek 1720 100 year ARI 73.4 45.04 48.56 48.56 49.8 4.74 65.78 2.89 0.62 5.19 0.56 14.29 0.92Flat Rock Creek 1720 200year ARI 83.2 45.04 48.8 48.8 50.11 6.1 73.37 3.72 0.66 5.4 0.6 14.64 0.92Flat Rock Creek 1720 PMF 374 45.04 53.1 53.1 55.1 55.92 237.7 80.38 1.12 7.82 0.92 43.31 0.89

Flat Rock Creek 1710 5year ARI 45.6 44.94 47.36 47.71 48.7 1.55 43.79 0.25 0.84 5.23 0.33 10.75 1.14Flat Rock Creek 1710 10year ARI 53 44.94 47.54 47.93 48.99 2.67 49.82 0.5 1.01 5.5 0.39 11.79 1.15Flat Rock Creek 1710 20year ARI 61.6 44.94 47.71 48.17 49.31 4.05 56.69 0.86 1.16 5.82 0.45 12.8 1.18Flat Rock Creek 1710 50year ARI 67.2 44.94 47.82 48.3 49.5 5.01 61.05 1.14 1.25 6.02 0.49 13.41 1.19Flat Rock Creek 1710 100 year ARI 73.4 44.94 47.93 48.45 49.7 6.14 65.79 1.48 1.34 6.22 0.52 14.11 1.21Flat Rock Creek 1710 200year ARI 83.2 44.94 48.08 48.65 50.01 7.98 73.13 2.09 1.46 6.53 0.57 15.11 1.23Flat Rock Creek 1710 PMF 374 44.94 52.3 52.4 54.12 95.87 214.28 63.85 2.12 7.76 0.88 45.31 0.93

Flat Rock Creek 1700 5year ARI 45.6 44.82 47.72 47.57 48.42 3.02 40.27 2.31 0.81 3.93 0.43 15.15 0.77Flat Rock Creek 1700 10year ARI 53 44.82 47.98 47.77 48.69 4.45 45.25 3.3 0.89 4.01 0.45 16.54 0.75Flat Rock Creek 1700 20year ARI 61.6 44.82 48.22 47.96 48.96 6.06 51.03 4.5 0.96 4.19 0.49 17.53 0.76Flat Rock Creek 1700 50year ARI 67.2 44.82 47.53 48.14 49.43 3.54 60.89 2.77 1.23 6.4 0.65 14.36 1.31Flat Rock Creek 1700 100 year ARI 73.4 44.82 48.6 48.28 49.33 8.74 58.01 6.64 1.02 4.24 0.54 18.21 0.72Flat Rock Creek 1700 200year ARI 83.2 44.82 48.7 48.47 49.93 5.91 72.79 4.51 - - - - 1.35Flat Rock Creek 1700 PMF 374 44.82 52.56 53.97 89.58 203.87 80.56 1.89 7 0.88 45.21 0.82

Flat Rock Creek 1680 5year ARI 48.3 44.58 47.49 47.49 48.35 2.68 44.09 1.53 0.83 4.29 0.35 16.93 0.84Flat Rock Creek 1680 10year ARI 56.7 44.58 47.75 47.75 48.63 4.16 49.79 2.75 0.93 4.41 0.4 18.9 0.83Flat Rock Creek 1680 20year ARI 66.3 44.58 47.99 47.99 48.9 5.81 56.13 4.36 1.02 4.6 0.47 19.29 0.83Flat Rock Creek 1680 50year ARI 70.9 44.58 47.55 48.07 49.28 4.24 64.17 2.49 1.21 6.1 0.5 17.53 1.19Flat Rock Creek 1680 100 year ARI 79.8 44.58 48.25 48.25 49.26 8.16 65.05 6.6 1.14 4.92 0.56 19.74 0.85Flat Rock Creek 1680 200year ARI 83.2 44.58 48.35 48.32 49.81 5.38 74.56 3.26 - - - - 1.33Flat Rock Creek 1680 PMF 394 44.58 52.12 52.12 53.91 88.03 220.66 85.31 2.07 7.79 0.97 44.4 0.92

Flat Rock Creek 1660 5year ARI 48.3 44.38 47.21 47.2 47.93 3.65 40.81 3.84 0.84 4.08 0.45 19.72 0.81Flat Rock Creek 1660 10year ARI 56.7 44.38 47.42 47.37 48.18 5.23 46.08 5.39 0.94 4.27 0.51 20.1 0.82Flat Rock Creek 1660 20year ARI 66.3 44.38 47.61 47.56 48.43 7.04 52.13 7.12 1.04 4.52 0.57 20.46 0.84Flat Rock Creek 1660 50year ARI 70.9 44.38 47.81 47.65 48.56 8.49 54.03 8.38 1.05 4.39 0.58 20.83 0.79Flat Rock Creek 1660 100 year ARI 79.8 44.38 47.99 47.82 48.78 10.48 59.22 10.11 1.11 4.54 0.62 21.17 0.79


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1660 200year ARI 83.2 44.38 48.06 47.88 48.87 11.26 61.18 10.76 1.14 4.6 0.63 21.3 0.79Flat Rock Creek 1660 PMF 394 44.38 51.5 51.94 53.81 111.28 230.69 52.03 2.42 8.63 0.86 45.5 1.05

Flat Rock Creek 1655.* 5year ARI 48.3 44.32 47.16 47.16 47.92 2.81 41.77 3.72 0.78 4.13 0.45 19.43 0.82Flat Rock Creek 1655.* 10year ARI 56.7 44.32 47.36 47.36 48.17 4.21 47.23 5.26 0.89 4.34 0.51 19.81 0.83Flat Rock Creek 1655.* 20year ARI 66.3 44.32 47.55 47.55 48.43 5.84 53.5 6.96 0.99 4.6 0.57 20.15 0.86Flat Rock Creek 1655.* 50year ARI 70.9 44.32 47.63 47.63 48.54 6.65 56.46 7.79 1.04 4.72 0.6 20.31 0.87Flat Rock Creek 1655.* 100 year ARI 79.8 44.32 47.81 47.81 48.76 8.42 61.88 9.5 1.12 4.9 0.64 20.63 0.87Flat Rock Creek 1655.* 200year ARI 83.2 44.32 47.87 47.87 48.84 9.1 63.96 10.14 1.15 4.97 0.66 20.75 0.88Flat Rock Creek 1655.* PMF 394 44.32 51.93 50.52 53.65 109.18 219.9 64.92 2.06 7.61 0.84 47.49 0.9

Flat Rock Creek 1640 5year ARI 48.3 44.17 46.71 46.99 47.85 1.45 44.98 1.87 0.76 4.89 0.4 18.09 1.04Flat Rock Creek 1640 10year ARI 56.7 44.17 46.92 47.19 48.09 2.53 50.84 3.33 0.9 5.06 0.49 18.46 1.03Flat Rock Creek 1640 20year ARI 66.3 44.17 47.12 47.39 48.36 3.84 57.42 5.03 1.02 5.28 0.57 18.83 1.03Flat Rock Creek 1640 50year ARI 70.9 44.17 47.23 47.47 48.47 4.56 60.41 5.93 1.07 5.35 0.6 19.01 1.02Flat Rock Creek 1640 100 year ARI 79.8 44.17 47.4 47.64 48.69 5.97 66.21 7.62 1.16 5.51 0.66 19.32 1.02Flat Rock Creek 1640 200year ARI 83.2 44.17 47.47 47.7 48.78 6.52 68.4 8.28 1.19 5.58 0.68 19.43 1.02Flat Rock Creek 1640 PMF 394 44.17 51.96 53.59 86.55 225.26 82.19 1.74 7.38 0.89 49 0.86

Flat Rock Creek 1620 5year ARI 48.3 44 46.18 46.66 47.71 0.57 47.64 0.09 0.67 5.51 0.17 13.1 1.27Flat Rock Creek 1620 10year ARI 56.7 44 46.41 46.86 47.97 1.42 54.49 0.79 0.89 5.65 0.33 16.63 1.23Flat Rock Creek 1620 20year ARI 66.3 44 46.62 47.08 48.24 2.49 61.9 1.91 1.05 5.84 0.45 17 1.22Flat Rock Creek 1620 50year ARI 70.9 44 46.72 47.18 48.36 3.06 65.31 2.53 1.11 5.91 0.5 17.17 1.21Flat Rock Creek 1620 100 year ARI 79.8 44 46.89 47.34 48.58 4.23 71.78 3.79 1.2 6.06 0.58 17.47 1.2Flat Rock Creek 1620 200year ARI 83.2 44 47.7 47.41 48.52 7.65 68.32 7.23 1 4.41 0.54 18.87 0.76Flat Rock Creek 1620 PMF 394 44 52.12 53.48 91.08 234.68 68.24 1.63 6.61 0.74 48.21 0.75

Flat Rock Creek 1600 5year ARI 48.3 43.84 46 46.55 47.6 0.15 48.07 0.09 0.47 5.6 0.23 9.06 1.3Flat Rock Creek 1600 10year ARI 56.7 43.84 46.28 46.79 47.87 0.77 55.47 0.45 0.66 5.63 0.33 13.21 1.22Flat Rock Creek 1600 20year ARI 66.3 43.84 46.54 46.99 48.14 1.93 63.16 1.2 0.8 5.74 0.43 16.01 1.18Flat Rock Creek 1600 50year ARI 70.9 43.84 46.64 47.09 48.26 2.61 66.68 1.61 0.87 5.82 0.48 16.53 1.17Flat Rock Creek 1600 100 year ARI 79.8 43.84 47.33 47.25 48.3 7.47 68.55 3.77 1.01 4.7 0.52 16.71 0.84Flat Rock Creek 1600 200year ARI 83.2 43.84 47.72 48.47 9.76 68.74 4.7 0.98 4.2 0.49 16.82 0.71Flat Rock Creek 1600 PMF 394 43.84 51.76 53.43 96.87 247.43 49.7 1.78 7.13 0.7 47.86 0.82

Flat Rock Creek 1580 5year ARI 48.3 43.71 45.87 46.4 47.48 0.11 48.09 0.1 0.47 5.64 0.23 8.86 1.31Flat Rock Creek 1580 10year ARI 56.7 43.71 46.14 46.64 47.77 0.53 55.63 0.54 0.68 5.69 0.33 12.73 1.23


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1580 20year ARI 66.3 43.71 46.41 46.9 48.05 1.28 63.52 1.5 0.82 5.79 0.42 15.61 1.18Flat Rock Creek 1580 50year ARI 70.9 43.71 46.52 47 48.18 1.69 67.12 2.08 0.87 5.85 0.47 16 1.17Flat Rock Creek 1580 100 year ARI 79.8 43.71 47.19 48.26 4.34 70.34 5.12 0.82 4.87 0.54 18.93 0.87Flat Rock Creek 1580 200year ARI 83.2 43.71 47.73 48.43 7.74 68.65 6.81 0.74 4.07 0.49 22.7 0.67Flat Rock Creek 1580 PMF 394 43.71 52.23 53.2 121.04 214.43 58.53 1.51 5.79 0.63 52.63 0.64

Flat Rock Creek 1560 5year ARI 48.3 43.58 45.64 46.23 47.35 0.77 47.47 0.06 0.76 5.84 0.2 11.19 1.39Flat Rock Creek 1560 10year ARI 56.7 43.58 45.84 46.43 47.64 1.73 54.62 0.35 0.98 6.06 0.3 14.51 1.37Flat Rock Creek 1560 20year ARI 66.3 43.58 46.04 46.65 47.93 3.02 62.26 1.02 1.15 6.28 0.38 17.72 1.35Flat Rock Creek 1560 50year ARI 70.9 43.58 47.16 46.72 47.75 7.93 56.86 6.11 0.89 3.8 0.45 21.87 0.67Flat Rock Creek 1560 100 year ARI 79.8 43.58 47.51 48.08 9.36 62.37 8.07 0.74 3.77 0.48 25.95 0.63Flat Rock Creek 1560 200year ARI 83.2 43.58 47.9 48.33 12.49 61.5 9.21 0.72 3.36 0.44 27.04 0.53Flat Rock Creek 1560 PMF 394 43.58 52.5 53.06 147.52 184.85 61.63 1.27 4.73 0.56 60 0.51

Flat Rock Creek 1555.* 5year ARI 48.3 43.52 45.58 46.19 47.32 0.58 47.66 0.06 0.69 5.87 0.19 11.07 1.4Flat Rock Creek 1555.* 10year ARI 56.7 43.52 45.78 46.38 47.61 1.46 54.87 0.37 0.93 6.09 0.3 14.54 1.38Flat Rock Creek 1555.* 20year ARI 66.3 43.52 46.72 46.6 47.5 5.76 56.08 4.46 0.93 4.23 0.45 20.36 0.79Flat Rock Creek 1555.* 50year ARI 70.9 43.52 47.17 47.74 7.48 56.99 6.43 0.82 3.73 0.45 22 0.65Flat Rock Creek 1555.* 100 year ARI 79.8 43.52 47.53 48.07 9.33 62.09 8.38 0.73 3.67 0.47 25.76 0.61Flat Rock Creek 1555.* 200year ARI 83.2 43.52 47.91 48.32 12.18 61.53 9.49 0.7 3.31 0.44 27.03 0.52Flat Rock Creek 1555.* PMF 394 43.52 52.52 53.05 151.57 181.45 60.97 1.25 4.6 0.54 60 0.5

Flat Rock Creek 1540 5year ARI 48.3 43.34 45.39 46.04 47.21 0.16 48.09 0.05 0.46 6 0.18 10.17 1.44Flat Rock Creek 1540 10year ARI 56.7 43.34 45.59 46.24 47.52 0.75 55.56 0.39 0.78 6.21 0.31 14.11 1.41Flat Rock Creek 1540 20year ARI 66.3 43.34 46.82 47.43 5.72 54.94 5.64 0.79 3.79 0.44 21.59 0.68Flat Rock Creek 1540 50year ARI 70.9 43.34 47.22 47.7 7.69 55.91 7.31 0.74 3.42 0.43 22.94 0.57Flat Rock Creek 1540 100 year ARI 79.8 43.34 47.58 48.03 10.16 60.5 9.14 0.75 3.37 0.44 24.13 0.54Flat Rock Creek 1540 200year ARI 83.2 43.34 47.93 48.3 11.55 61.38 10.27 0.68 3.15 0.43 26.4 0.48Flat Rock Creek 1540 PMF 394 43.34 52.58 53.02 161.53 173.16 59.3 1.19 4.27 0.51 60 0.46

Flat Rock Creek 1530.* 5year ARI 48.3 43.31 45.39 45.98 47.13 0.22 47.96 0.12 0.52 5.86 0.2 12.86 1.39Flat Rock Creek 1530.* 10year ARI 56.7 43.31 46.33 46.18 47.04 3.15 49.7 3.86 0.78 3.99 0.42 19.57 0.77Flat Rock Creek 1530.* 20year ARI 66.3 43.31 46.82 47.41 5.38 54.5 6.42 0.8 3.71 0.45 20.7 0.66Flat Rock Creek 1530.* 50year ARI 70.9 43.31 47.22 47.69 6.86 56 8.05 0.73 3.4 0.44 22.16 0.57Flat Rock Creek 1530.* 100 year ARI 79.8 43.31 47.56 48.01 8.8 61.1 9.9 0.72 3.39 0.46 23.85 0.54Flat Rock Creek 1530.* 200year ARI 83.2 43.31 47.91 48.3 10.16 62.02 11.03 0.64 3.17 0.44 26.5 0.49Flat Rock Creek 1530.* PMF 394 43.31 52.56 53.01 158.33 175.28 60.38 1.18 4.32 0.51 60 0.46


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1520 5year ARI 48.3 43.27 45.37 45.92 47.06 0.25 47.77 0.28 0.55 5.79 0.25 14.51 1.37Flat Rock Creek 1520 10year ARI 56.7 43.27 46.36 47.01 3.12 48.88 4.7 0.77 3.84 0.44 19.24 0.73Flat Rock Creek 1520 20year ARI 66.3 43.27 46.83 47.38 5.09 54.02 7.19 0.79 3.63 0.46 20.2 0.64Flat Rock Creek 1520 50year ARI 70.9 43.27 47.23 47.67 6.66 55.51 8.73 0.76 3.33 0.45 20.88 0.55Flat Rock Creek 1520 100 year ARI 79.8 43.27 47.55 48.01 7.72 61.42 10.66 0.69 3.39 0.47 23.25 0.54Flat Rock Creek 1520 200year ARI 83.2 43.27 47.9 48.29 8.83 62.6 11.77 0.6 3.18 0.45 26.65 0.49Flat Rock Creek 1520 PMF 394 43.27 52.54 53 154.81 177.7 61.49 1.18 4.37 0.52 60 0.47

Flat Rock Creek 1500 5year ARI 48.3 43.16 45.26 45.77 46.93 0.2 47.81 0.28 0.58 5.76 0.28 11.87 1.36Flat Rock Creek 1500 10year ARI 56.7 43.16 46.13 46.96 2.29 51.73 2.69 0.76 4.22 0.46 14.4 0.82Flat Rock Creek 1500 20year ARI 66.3 43.16 46.64 47.34 4.41 57.76 4.13 0.78 3.97 0.48 15.89 0.71Flat Rock Creek 1500 50year ARI 70.9 43.16 47.1 47.65 6.39 59.5 5.01 0.74 3.58 0.45 17.29 0.6Flat Rock Creek 1500 100 year ARI 79.8 43.16 47.44 47.98 8.6 65.25 5.96 0.77 3.59 0.45 18.37 0.57Flat Rock Creek 1500 200year ARI 83.2 43.16 47.78 48.27 9.4 67.27 6.53 0.63 3.41 0.43 22.66 0.52Flat Rock Creek 1500 PMF 394 43.16 52.4 52.99 156.94 191.78 45.28 1.25 4.72 0.48 60 0.5

Flat Rock Creek 1480 5year ARI 48.3 43.06 45.92 45.67 46.57 1.84 43.83 2.63 0.65 3.74 0.41 15.22 0.74Flat Rock Creek 1480 10year ARI 56.7 43.06 46.21 46.87 3.04 49.95 3.71 0.72 3.84 0.45 16.11 0.72Flat Rock Creek 1480 20year ARI 66.3 43.06 46.71 47.28 5.38 55.7 5.22 0.74 3.65 0.46 17.65 0.63Flat Rock Creek 1480 50year ARI 70.9 43.06 47.15 47.61 7.49 57.32 6.09 0.71 3.32 0.43 19.01 0.54Flat Rock Creek 1480 100 year ARI 79.8 43.06 47.49 47.94 9.92 62.75 7.14 0.73 3.34 0.44 20.06 0.52Flat Rock Creek 1480 200year ARI 83.2 43.06 47.84 48.22 11.92 63.65 7.63 0.7 3.12 0.41 21.16 0.47Flat Rock Creek 1480 PMF 394 43.06 52.18 52.96 135.22 209.54 49.24 1.3 5.24 0.53 60 0.56

Flat Rock Creek 1460 5year ARI 48.3 42.94 45.96 46.52 1.16 43.61 3.52 0.3 3.51 0.41 17.9 0.68Flat Rock Creek 1460 10year ARI 56.6 42.94 46.23 46.83 1.86 49.89 4.85 0.34 3.65 0.45 18.96 0.67Flat Rock Creek 1460 20year ARI 66.3 42.94 46.71 47.25 3.26 56.27 6.77 0.36 3.56 0.47 20.84 0.61Flat Rock Creek 1460 50year ARI 72.8 42.94 47.09 47.59 4.72 60.01 8.07 0.37 3.41 0.46 21.5 0.55Flat Rock Creek 1460 100 year ARI 79.8 42.94 47.47 47.92 6.3 64.18 9.32 0.39 3.33 0.46 22.31 0.52Flat Rock Creek 1460 200year ARI 85 42.94 47.78 48.21 7.61 67.14 10.25 0.39 3.24 0.45 23.55 0.48Flat Rock Creek 1460 PMF 329 42.94 51.91 52.92 64.53 215.71 48.75 0.61 5.49 0.53 60 0.59

Flat Rock Creek 1440 5year ARI 48.3 42.84 46.11 45.59 46.42 1.91 37.67 8.72 0.35 2.77 0.66 25.18 0.51Flat Rock Creek 1440 10year ARI 56.6 42.84 46.43 45.76 46.71 3.02 41.05 12.53 0.38 2.73 0.72 26.75 0.48Flat Rock Creek 1440 20year ARI 66.3 42.84 46.92 45.93 47.14 5.11 43.63 17.56 0.41 2.53 0.74 27.47 0.41Flat Rock Creek 1440 50year ARI 72.8 42.84 47.3 46.07 47.48 6.66 45.18 20.96 0.4 2.39 0.73 29.06 0.37Flat Rock Creek 1440 100 year ARI 79.8 42.84 47.66 46.18 47.83 8.26 47.25 24.29 0.4 2.3 0.72 30.6 0.34


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1440 200year ARI 85 42.84 47.97 46.28 48.12 9.59 48.64 26.76 0.38 2.21 0.71 31.9 0.32Flat Rock Creek 1440 PMF 329 42.84 52.41 48.46 52.69 63.91 143.21 121.89 0.6 3.4 0.93 51.64 0.36

Flat Rock Creek 1437.5 Bridge

Flat Rock Creek 1430 5year ARI 48.3 42.74 45.77 46.3 0.86 42.77 4.67 0.29 3.41 0.65 19.67 0.65Flat Rock Creek 1430 10year ARI 56.6 42.74 46.23 46.64 1.56 46.02 9.02 0.3 3.16 0.75 21.13 0.56Flat Rock Creek 1430 20year ARI 66.3 42.74 46.75 47.09 2.6 49.72 13.99 0.29 2.93 0.8 22.83 0.48Flat Rock Creek 1430 50year ARI 72.8 42.74 47.15 47.44 3.5 51.94 17.36 0.29 2.77 0.79 24.12 0.43Flat Rock Creek 1430 100 year ARI 79.8 42.74 47.53 47.78 4.5 54.71 20.58 0.29 2.67 0.79 25.35 0.4Flat Rock Creek 1430 200year ARI 85 42.74 47.85 48.08 5.4 56.61 22.99 0.28 2.58 0.78 26.38 0.37Flat Rock Creek 1430 PMF 329 42.74 52.11 52.6 40.35 172.2 116.45 0.57 4.18 1.06 44.49 0.44

Flat Rock Creek 1420 5year ARI 48.3 42.65 45.84 46.25 0.62 40.78 6.9 0.27 3.08 0.68 19.02 0.57Flat Rock Creek 1420 10year ARI 56.6 42.65 46.27 46.61 0.96 44.3 11.34 0.25 2.92 0.75 20.64 0.51Flat Rock Creek 1420 20year ARI 66.3 42.65 46.78 47.07 1.67 48.13 16.5 0.25 2.75 0.79 22 0.45Flat Rock Creek 1420 50year ARI 72.8 42.65 47.17 47.42 2.35 50.42 20.03 0.26 2.62 0.78 22 0.41Flat Rock Creek 1420 100 year ARI 79.8 42.65 47.54 47.77 3.01 53.31 23.48 0.27 2.55 0.78 22 0.38Flat Rock Creek 1420 200year ARI 85 42.65 47.86 48.07 3.51 55.38 26.11 0.27 2.48 0.78 22 0.36Flat Rock Creek 1420 PMF 329 42.65 52.07 52.59 29.62 176.45 122.93 0.41 4.26 1.17 50 0.45

Flat Rock Creek 1400 5year ARI 48.3 42.52 45.87 46.22 5.05 39.91 3.34 0.52 2.86 0.48 26.47 0.52Flat Rock Creek 1400 10year ARI 56.6 42.52 46.31 46.58 7.83 41.94 6.84 0.55 2.63 0.52 30.06 0.45Flat Rock Creek 1400 20year ARI 66.3 42.52 46.83 47.03 10.68 43.54 12.08 0.55 2.38 0.56 31.27 0.38Flat Rock Creek 1400 50year ARI 72.8 42.52 47.23 47.39 12.47 44.38 15.95 0.53 2.21 0.57 32.18 0.33Flat Rock Creek 1400 100 year ARI 79.8 42.52 47.6 47.74 14.17 45.8 19.82 0.52 2.1 0.57 33.05 0.31Flat Rock Creek 1400 200year ARI 85 42.52 47.92 48.04 15.39 46.68 22.93 0.5 2.01 0.56 33.78 0.28Flat Rock Creek 1400 PMF 329 42.52 52.3 52.49 64.84 126.32 137.85 0.62 2.94 1 50 0.3

Flat Rock Creek 1380 5year ARI 48.3 42.34 45.91 46.18 3.32 38.26 6.72 0.28 2.56 0.53 29.44 0.45Flat Rock Creek 1380 10year ARI 56.6 42.34 46.33 46.55 4.65 40.95 10.99 0.28 2.43 0.58 31.84 0.4Flat Rock Creek 1380 20year ARI 66.3 42.34 46.84 47.02 6.32 43.65 16.33 0.28 2.28 0.6 34.87 0.35Flat Rock Creek 1380 50year ARI 72.8 42.34 47.23 47.38 7.44 45.1 20.26 0.26 2.16 0.6 37.71 0.32Flat Rock Creek 1380 100 year ARI 79.8 42.34 47.6 47.73 8.6 46.92 24.27 0.25 2.08 0.6 40.93 0.3Flat Rock Creek 1380 200year ARI 85 42.34 47.91 48.03 10 47.72 27.29 0.25 1.99 0.58 41.85 0.28Flat Rock Creek 1380 PMF 329 42.34 52.28 52.49 53.25 130.08 145.68 0.41 2.97 1.02 50 0.31


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Flat Rock Creek 1360 5year ARI 48.3 42.19 45.98 46.14 7.65 33.13 7.52 0.47 2.08 0.44 36.13 0.35Flat Rock Creek 1360 10year ARI 56.6 42.19 46.39 46.52 10.03 34.88 11.69 0.45 1.96 0.47 39.83 0.32Flat Rock Creek 1360 20year ARI 66.3 42.19 46.89 46.99 13.18 36.65 16.47 0.43 1.83 0.46 45.7 0.28Flat Rock Creek 1360 50year ARI 72.8 42.19 47.27 47.35 15.4 36.85 20.56 0.4 1.69 0.46 47.86 0.25Flat Rock Creek 1360 100 year ARI 79.8 42.19 47.64 47.71 17.82 37.52 24.46 0.38 1.6 0.46 50 0.22Flat Rock Creek 1360 200year ARI 85 42.19 47.96 48.01 20.31 37.53 27.16 0.38 1.51 0.45 50 0.21Flat Rock Creek 1360 PMF 329 42.19 52.35 52.45 104.51 99.72 124.76 0.68 2.23 0.78 50 0.23

Flat Rock Creek 1340 5year ARI 45.3 42.08 45.9 44.95 46.12 1.54 34.81 8.95 0.42 2.36 0.64 19.79 0.4Flat Rock Creek 1340 10year ARI 53.2 42.08 46.3 45.13 46.5 2.82 37.92 12.47 0.47 2.31 0.68 21.08 0.37Flat Rock Creek 1340 20year ARI 63.9 42.08 46.79 45.42 46.97 4.91 42.13 16.86 0.51 2.3 0.72 22.5 0.34Flat Rock Creek 1340 50year ARI 72.6 42.08 47.16 45.59 47.34 7.05 45.34 20.21 0.56 2.28 0.74 22.5 0.33Flat Rock Creek 1340 100 year ARI 81.4 42.08 47.53 45.73 47.7 9.23 48.65 23.53 0.6 2.28 0.76 22.5 0.32Flat Rock Creek 1340 200year ARI 89 42.08 47.83 45.87 48 11.1 51.56 26.34 0.62 2.28 0.77 22.5 0.31Flat Rock Creek 1340 PMF 329 42.08 52.1 48.2 52.42 75.67 144.46 108.87 0.91 3.62 1.03 50 0.37

HEC-RAS Plan: 100year River: Flat Rock Creek Reach: Southern TribReach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev Q Left Q Channel Q Right Vel Left Vel Chnl Vel Right Top Width Froude # Chl

(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 765 5 year ARI 10.9 60.98 62.95 62.95 63.48 0.46 9.92 0.51 0.6 3.37 0.51 6.65 0.83Southern Trib 765 10 year ARI 12.5 60.98 63.09 63.09 63.62 0.74 10.97 0.79 0.65 3.43 0.56 7.53 0.81Southern Trib 765 20 year ARI 14.6 60.98 63.24 63.24 63.79 1.16 12.31 1.14 0.72 3.57 0.62 8.33 0.81Southern Trib 765 50 year ARI 17.2 60.98 63.4 63.4 63.97 1.76 13.83 1.61 0.8 3.69 0.68 9.23 0.81Southern Trib 765 100 year ARI 20.1 60.98 63.56 63.56 64.15 2.49 15.46 2.15 0.87 3.85 0.73 10.08 0.81Southern Trib 765 200yr ARI 22.1 60.98 63.65 63.65 64.26 3.02 16.56 2.53 0.91 3.96 0.77 10.58 0.82Southern Trib 765 PMF 79.2 60.98 65.26 65.26 66.1 25.42 39.18 14.6 1.5 5.58 1.29 18.2 0.89

Southern Trib 750 5 year ARI 10.9 60.53 62.05 62.5 63.32 0.17 10.51 0.22 0.66 5.1 0.43 7.55 1.52Southern Trib 750 10 year ARI 12.5 60.53 62.13 62.58 63.46 0.32 11.71 0.47 0.75 5.28 0.51 9.73 1.52Southern Trib 750 20 year ARI 14.6 60.53 62.22 62.66 63.62 0.55 13.15 0.9 0.82 5.52 0.6 12.24 1.53Southern Trib 750 50 year ARI 17.2 60.53 62.31 62.77 63.8 0.88 14.77 1.55 0.91 5.83 0.71 14.15 1.57Southern Trib 750 100 year ARI 20.1 60.53 62.38 62.87 63.97 1.3 16.44 2.36 1.02 6.16 0.83 15.26 1.61Southern Trib 750 200yr ARI 22.1 60.53 62.43 62.92 64.08 1.62 17.53 2.95 1.09 6.37 0.9 15.91 1.64Southern Trib 750 PMF 79.2 60.53 63.17 63.79 65.83 16.46 40.45 22.29 2.34 9.91 1.89 25.4 2.1

Southern Trib 745 5 year ARI 10.9 60.38 61.85 62.32 63.25 0.16 10.59 0.15 0.67 5.33 0.4 7.34 1.62Southern Trib 745 10 year ARI 12.5 60.38 61.93 62.39 63.39 0.31 11.81 0.38 0.73 5.51 0.48 10.44 1.62Southern Trib 745 20 year ARI 14.6 60.38 62.02 62.48 63.55 0.57 13.23 0.8 0.82 5.76 0.57 13.58 1.63Southern Trib 745 50 year ARI 17.2 60.38 62.1 62.55 63.73 0.94 14.84 1.41 0.93 6.08 0.66 16.52 1.67Southern Trib 745 100 year ARI 20.1 60.38 62.16 62.63 63.89 1.39 16.47 2.24 1.03 6.43 0.79 17.8 1.72Southern Trib 745 200yr ARI 22.1 60.38 62.2 62.68 64 1.72 17.53 2.85 1.1 6.65 0.87 18.53 1.76Southern Trib 745 PMF 79.2 60.38 62.83 63.55 65.72 16.9 39.35 22.95 2.46 10.45 2.01 27.28 2.31

Southern Trib 690 5 year ARI 10.9 58.86 60.1 60.77 62.22 10.9 6.44 1.73 2.08Southern Trib 690 10 year ARI 12.5 58.86 60.22 60.87 62.37 12.5 6.49 2.17 2.2Southern Trib 690 20 year ARI 14.6 58.86 60.35 61 62.53 14.58 0.02 6.56 0.2 4.98 2.24Southern Trib 690 50 year ARI 17.2 58.86 60.47 61.14 62.73 0.01 16.98 0.21 0.38 6.71 0.35 8.44 2.15Southern Trib 690 100 year ARI 20.1 58.86 60.58 61.26 62.92 0.08 19.41 0.6 0.57 6.89 0.5 9.81 2.1Southern Trib 690 200yr ARI 22.1 58.86 60.65 61.33 63.03 0.17 21 0.93 0.66 7.01 0.56 10.67 2.07Southern Trib 690 PMF 79.2 58.86 61.82 62.81 64.79 8.82 54.75 15.63 1.61 9.13 1.18 22.65 1.9

Southern Trib 650 5 year ARI 11.2 57.98 58.78 59.49 61.14 11.2 6.79 2.38 2.61Southern Trib 650 10 year ARI 12.9 57.98 58.88 59.58 61.29 12.9 6.89 2.43 2.51Southern Trib 650 20 year ARI 15 57.98 60.55 59.69 60.68 15 0 1.6 0.05 6.77 0.41Southern Trib 650 50 year ARI 17.6 57.98 60.6 59.81 61.76 17.6 - - - - 2.36Southern Trib 650 100 year ARI 20.6 57.98 60.65 59.95 61.94 20.6 - - - - 2.51


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 650 200yr ARI 22.6 57.98 60.7 60.03 62.03 22.6 - - - - 2.73Southern Trib 650 PMF 80.4 57.98 60.75 61.83 64.16 0 80.37 0.03 - - - - 2.11

Southern Trib 625 5 year ARI 11.2 57.63 59.88 59.14 59.99 11.2 0 1.51 0.02 5.64 0.41Southern Trib 625 10 year ARI 12.9 57.63 60.25 59.23 60.34 0.01 12.86 0.03 0.07 1.36 0.07 9.33 0.34Southern Trib 625 20 year ARI 15 57.63 60.57 60.66 0.09 14.78 0.13 0.11 1.3 0.09 12.44 0.3Southern Trib 625 50 year ARI 17.6 57.63 60.73 59.46 60.83 0.19 17.21 0.21 0.13 1.4 0.1 14.55 0.31Southern Trib 625 100 year ARI 20.6 57.63 60.85 59.6 60.97 0.3 20.01 0.3 0.15 1.54 0.11 17.54 0.33Southern Trib 625 200yr ARI 22.6 57.63 60.92 59.68 61.06 0.39 21.83 0.37 0.16 1.63 0.12 19.76 0.34Southern Trib 625 PMF 80.4 57.63 61.03 61.5 63.79 0.06 80.09 0.24 - - - - 2.06

Southern Trib 615 5 year ARI 11.2 57.63 59.88 59.05 59.98 11.2 0 1.45 0.03 5.96 0.38Southern Trib 615 10 year ARI 12.9 57.63 60.25 59.14 60.33 0.01 12.83 0.05 0.08 1.31 0.07 10.02 0.32Southern Trib 615 20 year ARI 15 57.63 60.57 59.26 60.65 0.11 14.72 0.17 0.12 1.27 0.09 13.53 0.28Southern Trib 615 50 year ARI 17.6 57.63 60.73 59.39 60.83 0.22 17.11 0.27 0.14 1.37 0.1 16.2 0.29Southern Trib 615 100 year ARI 20.6 57.63 60.85 59.52 60.97 0.35 19.85 0.4 0.15 1.51 0.11 19.42 0.31Southern Trib 615 200yr ARI 22.6 57.63 60.93 59.6 61.05 0.42 21.68 0.5 0.15 1.6 0.12 20.92 0.33Southern Trib 615 PMF 80.4 57.63 61.03 61.46 63.6 0.1 79.94 0.35 - - - - 1.96

Southern Trib 609 Culvert

Southern Trib 603 5 year ARI 11.2 57.25 58.19 58.67 59.41 11.2 4.88 2.53 0.38Southern Trib 603 10 year ARI 12.9 57.25 58.21 58.76 59.76 12.9 5.51 2.67 0.32Southern Trib 603 20 year ARI 15 57.25 59.11 59.47 15 2.64 4.65 0.76Southern Trib 603 50 year ARI 17.6 57.25 59.21 59.63 17.6 2.87 4.76 0.81Southern Trib 603 100 year ARI 20.6 57.25 59.35 59.81 20.6 3.02 4.98 0.82Southern Trib 603 200yr ARI 22.6 57.25 59.44 59.24 59.93 22.6 0 3.1 0.05 5.66 0.83Southern Trib 603 PMF 80.4 57.25 61.07 61.07 62.02 3.29 73.04 4.07 0.36 4.53 0.41 29.63 0.84

Southern Trib 593 5 year ARI 11.2 57.18 58.62 58.68 59.13 11.2 3.17 4.2 1.1Southern Trib 593 10 year ARI 12.9 57.18 58.46 58.78 59.48 12.9 4.47 3.53 1.58Southern Trib 593 20 year ARI 15 57.18 59.05 59.44 15 2.77 4.7 0.82Southern Trib 593 50 year ARI 17.6 57.18 59.14 59.01 59.6 17.6 3.01 4.84 0.87Southern Trib 593 100 year ARI 20.6 57.18 59.16 59.16 59.77 20.6 3.46 4.87 1Southern Trib 593 200yr ARI 22.6 57.18 59.24 59.24 59.88 22.6 0 3.54 0.06 5.72 1Southern Trib 593 PMF 80.4 57.18 60.75 60.99 61.97 2.78 73.37 4.25 0.37 5.13 0.44 29.11 0.99


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 509 5 year ARI 14.3 56.01 58.83 57.6 58.89 0.44 11.77 2.08 0.1 1.24 0.14 27.31 0.26Southern Trib 509 10 year ARI 16.7 56.01 59.11 57.76 59.18 0.72 13.2 2.78 0.1 1.24 0.14 31.4 0.24Southern Trib 509 20 year ARI 19 56.01 59.24 59.31 0.94 14.69 3.37 0.11 1.31 0.15 32.3 0.25Southern Trib 509 50 year ARI 22.1 56.01 59.37 59.45 1.22 16.71 4.17 0.13 1.43 0.17 32.87 0.27Southern Trib 509 100 year ARI 25.5 56.01 59.44 58.12 59.54 1.49 19.04 4.96 0.14 1.59 0.19 33.2 0.29Southern Trib 509 200yr ARI 29.4 56.01 59.57 59.68 1.87 21.54 5.99 0.16 1.72 0.21 33.86 0.31Southern Trib 509 PMF 94.2 56.01 60.33 59.72 60.87 9.23 62.35 22.62 0.43 4 0.53 35.17 0.65

Southern Trib 499 5 year ARI 14.3 55.87 58.83 57.43 58.88 0.58 11.15 2.57 0.09 1.11 0.13 30.46 0.22Southern Trib 499 10 year ARI 16.7 55.87 59.12 57.58 59.17 0.89 12.4 3.42 0.1 1.11 0.13 32.93 0.21Southern Trib 499 20 year ARI 19 55.87 59.25 57.69 59.3 1.11 13.8 4.09 0.11 1.18 0.15 33.56 0.22Southern Trib 499 50 year ARI 22.1 55.87 59.38 57.8 59.44 1.41 15.73 4.97 0.12 1.29 0.16 34.26 0.23Southern Trib 499 100 year ARI 25.5 55.87 59.46 57.93 59.53 1.7 17.93 5.86 0.14 1.44 0.18 34.68 0.26Southern Trib 499 200yr ARI 29.4 55.87 59.58 58.03 59.67 2.11 20.3 6.98 0.15 1.57 0.2 35.37 0.27Southern Trib 499 PMF 94.2 55.87 60.4 59.52 60.83 10.12 58.76 25.32 0.42 3.63 0.51 35.42 0.57


Southern Trib 489 5 year ARI 14.3 55.73 56.77 57.31 58.12 14.3 5.14 2.88 0.22Southern Trib 489 10 year ARI 16.7 55.73 57.42 57.42 57.88 0.03 15.64 1.03 0.13 3.11 0.25 16.98 0.86Southern Trib 489 20 year ARI 19 55.73 57.52 57.52 58.01 0.06 17.42 1.52 0.16 3.23 0.29 17.71 0.86Southern Trib 489 50 year ARI 22.1 55.73 57.64 57.64 58.16 0.13 19.74 2.23 0.19 3.38 0.34 18.61 0.87Southern Trib 489 100 year ARI 25.5 55.73 57.74 57.74 58.31 0.22 22.29 2.99 0.22 3.57 0.38 19.41 0.89Southern Trib 489 200yr ARI 29.4 55.73 57.86 57.86 58.48 0.35 25.18 3.87 0.25 3.77 0.41 20.57 0.91Southern Trib 489 PMF 94.2 55.73 59.33 59.33 60.27 6.51 63.6 24.08 0.5 5.2 0.68 35.43 0.92

Southern Trib 479 5 year ARI 14.3 55.57 56.59 57.15 57.99 14.3 5.25 2.87 1.72Southern Trib 479 10 year ARI 16.7 55.57 57.02 57.26 57.81 0 16.42 0.28 0.05 3.97 0.2 13.8 1.21Southern Trib 479 20 year ARI 19 55.57 57.12 57.36 57.94 0.01 18.35 0.65 0.12 4.06 0.26 15.95 1.18Southern Trib 479 50 year ARI 22.1 55.57 57.24 57.48 58.09 0.03 20.81 1.26 0.17 4.21 0.33 16.8 1.17Southern Trib 479 100 year ARI 25.5 55.57 57.35 57.58 58.24 0.08 23.43 2 0.22 4.37 0.39 17.64 1.17Southern Trib 479 200yr ARI 29.4 55.57 57.46 57.7 58.41 0.16 26.36 2.88 0.26 4.56 0.45 18.48 1.18Southern Trib 479 PMF 94.2 55.57 58.84 59.17 60.2 5.3 67.04 21.86 0.56 6.11 0.76 33.6 1.14

Southern Trib 424 5 year ARI 14.3 54.71 55.68 56.29 57.25 14.3 5.55 2.85 1.86Southern Trib 424 10 year ARI 16.7 54.71 55.87 56.4 57.32 16.7 5.33 2.96 1.65Southern Trib 424 20 year ARI 19.2 54.71 56.02 56.51 57.47 19.18 0.02 5.34 0.12 8.81 1.69


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 424 50 year ARI 22.4 54.71 56.14 56.63 57.64 0 22.09 0.31 0.03 5.46 0.26 13.13 1.68Southern Trib 424 100 year ARI 25.4 54.71 56.23 56.72 57.8 0 24.71 0.69 0.14 5.64 0.33 15.68 1.67Southern Trib 424 200yr ARI 29.4 54.71 56.34 56.84 57.98 0.03 27.93 1.45 0.22 5.82 0.43 16.51 1.65Southern Trib 424 PMF 94.2 54.71 57.57 58.31 59.84 3.74 71.9 18.56 0.66 7.63 0.89 30.04 1.54

Southern Trib 369 5 year ARI 14.3 53.85 54.8 55.43 56.43 14.3 5.65 2.84 1.91Southern Trib 369 10 year ARI 16.7 53.85 54.94 55.54 56.61 16.7 5.72 2.89 1.82Southern Trib 369 20 year ARI 19.2 53.85 55.08 55.65 56.77 19.2 5.76 3.16 1.79Southern Trib 369 50 year ARI 22.4 53.85 55.21 55.77 56.96 22.28 0.12 5.86 0.21 11.43 1.84Southern Trib 369 100 year ARI 25.4 53.85 55.3 55.86 57.14 0 24.98 0.42 0.07 6.06 0.31 13.7 1.85Southern Trib 369 200yr ARI 29.4 53.85 55.41 55.98 57.34 0.01 28.38 1.01 0.18 6.28 0.4 15.95 1.83Southern Trib 369 PMF 94.2 53.85 56.54 57.45 59.35 3.11 73.75 17.34 0.7 8.38 0.97 27.9 1.75

Southern Trib 314 5 year ARI 15.6 52.99 54.03 54.63 55.63 15.6 5.6 2.88 1.82Southern Trib 314 10 year ARI 16.7 52.99 54.06 54.68 55.81 16.7 5.86 2.88 1.88Southern Trib 314 20 year ARI 19.2 52.99 54.18 54.78 56 19.2 5.98 3.01 1.85Southern Trib 314 50 year ARI 23.1 52.99 54.36 54.93 56.2 22.96 0.14 6.02 0.22 11.52 1.89Southern Trib 314 100 year ARI 25.4 52.99 54.41 55 56.38 25.08 0.32 6.25 0.29 12.88 1.93Southern Trib 314 200yr ARI 30 52.99 54.53 55.14 56.62 0.01 29.05 0.94 0.18 6.5 0.4 15.85 1.91Southern Trib 314 PMF 108 52.99 55.85 56.8 58.84 4.29 82.45 21.27 0.75 8.75 1.03 30.03 1.77

Southern Trib 300 5 year ARI 15.6 52.76 53.78 54.4 55.43 15.6 5.69 2.87 1.86Southern Trib 300 10 year ARI 16.7 52.76 53.81 54.45 55.6 16.7 5.92 2.88 1.91Southern Trib 300 20 year ARI 20.2 52.76 54.01 54.59 55.79 20.2 0 5.91 0.07 5.1 1.85Southern Trib 300 50 year ARI 23.6 52.76 54.14 54.72 56 23.42 0.18 6.06 0.24 11.86 1.9Southern Trib 300 100 year ARI 29.3 52.76 54.34 54.89 56.16 0.02 28.13 1.15 0.33 6.1 0.41 16.13 1.76Southern Trib 300 200yr ARI 30 52.76 54.29 54.91 56.42 0.01 29.11 0.88 0.28 6.57 0.4 15.77 1.93Southern Trib 300 PMF 108 52.76 55.51 56.52 58.68 6.25 81.79 19.96 1.27 9.05 1.05 28.72 1.87

Southern Trib 275.5 5 year ARI 15.6 52.35 54.39 53.99 54.59 0.24 13.47 1.89 0.23 2.12 0.23 19.64 0.52Southern Trib 275.5 10 year ARI 18 52.35 54.67 54.09 54.84 0.55 14.75 2.69 0.25 1.99 0.22 23.11 0.45Southern Trib 275.5 20 year ARI 21.5 52.35 54.89 54.24 55.06 0.95 16.92 3.62 0.27 2.05 0.24 25.96 0.44Southern Trib 275.5 50 year ARI 25.4 52.35 54.99 54.36 55.19 1.28 19.64 4.48 0.31 2.28 0.26 27.23 0.48Southern Trib 275.5 100 year ARI 29.3 52.35 55.08 54.48 55.32 1.65 22.3 5.36 0.35 2.49 0.29 28.38 0.52Southern Trib 275.5 200yr ARI 35 52.35 55.04 54.66 55.41 1.9 26.78 6.32 0.42 3.04 0.35 27.98 0.63Southern Trib 275.5 PMF 108 52.35 55.06 56.11 58.41 5.98 82.36 19.66 1.29 9.26 1.08 28.23 1.92


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 265.5 5 year ARI 15.6 52.18 54.42 54.56 0.41 12.97 2.22 0.22 1.83 0.2 22.08 0.42Southern Trib 265.5 10 year ARI 18 52.18 54.69 54.82 0.76 14.25 2.99 0.23 1.75 0.2 25.58 0.38Southern Trib 265.5 20 year ARI 21.8 52.18 54.91 55.04 1.23 16.59 3.99 0.26 1.85 0.22 28.39 0.38Southern Trib 265.5 50 year ARI 25.4 52.18 55.01 55.17 1.6 18.96 4.84 0.29 2.03 0.24 29.76 0.41Southern Trib 265.5 100 year ARI 29.3 52.18 55.11 55.29 2.03 21.51 5.76 0.32 2.22 0.26 30.97 0.44Southern Trib 265.5 200yr ARI 35 52.18 55.1 55.37 2.4 25.73 6.87 0.39 2.66 0.31 30.87 0.53Southern Trib 265.5 PMF 108 52.18 55.2 55.95 58.29 5.89 82.56 19.55 - - - - 1.95

Southern Trib 255.5 5 year ARI 15.8 52.02 54.45 53.56 54.55 0.56 12.84 2.4 0.2 1.55 0.18 24.47 0.33Southern Trib 255.5 10 year ARI 18 52.02 54.71 53.68 54.8 0.94 13.92 3.14 0.21 1.5 0.18 27.93 0.31Southern Trib 255.5 20 year ARI 22.2 52.02 54.93 53.85 55.03 1.47 16.51 4.22 0.24 1.64 0.19 30.71 0.32Southern Trib 255.5 50 year ARI 25.6 52.02 55.04 53.98 55.16 1.87 18.67 5.06 0.27 1.78 0.21 32.14 0.34Southern Trib 255.5 100 year ARI 29.5 52.02 55.14 54.08 55.27 2.34 21.06 6.1 0.29 1.94 0.24 32.77 0.36Southern Trib 255.5 200yr ARI 35 52.02 55.15 54.26 55.34 2.8 24.93 7.27 0.35 2.28 0.28 32.83 0.43Southern Trib 255.5 PMF 108 52.02 55.26 55.75 58.17 5.16 84.59 18.25 - - - - 1.93


Southern Trib 242.5 5 year ARI 15.8 51.68 52.64 52.8 53.27 15.8 3.53 6.37 0.33Southern Trib 242.5 10 year ARI 18 51.68 52.89 52.89 53.32 0 18 0 0.02 2.92 0.02 8.6 1.01Southern Trib 242.5 20 year ARI 22.2 51.68 53.04 53.04 53.51 0.01 22.07 0.12 0.09 3.02 0.11 17.38 0.96Southern Trib 242.5 50 year ARI 25.6 51.68 53.15 53.15 53.64 0.02 25.25 0.33 0.13 3.13 0.15 20.12 0.94Southern Trib 242.5 100 year ARI 29.5 51.68 53.26 53.26 53.79 0.06 28.77 0.67 0.16 3.24 0.18 22.91 0.93Southern Trib 242.5 200yr ARI 35 51.68 53.42 53.42 53.97 0.13 33.49 1.38 0.21 3.35 0.23 24.83 0.9Southern Trib 242.5 PMF 108 51.68 54.63 54.63 55.71 1.17 92.89 13.94 0.4 4.98 0.55 25.13 0.99

Southern Trib 232.5 5 year ARI 15.8 51.4 52.5 52.72 53.21 15.8 3.73 6.65 1.49Southern Trib 232.5 10 year ARI 18 51.4 52.62 52.8 53.27 0 18 0 0.02 3.55 0.02 8.25 1.35Southern Trib 232.5 20 year ARI 22.2 51.4 52.74 52.93 53.45 0 22.12 0.08 0.11 3.75 0.12 16.16 1.32Southern Trib 232.5 50 year ARI 25.6 51.4 52.82 53.03 53.59 0.02 25.33 0.25 0.15 3.89 0.18 18.4 1.3Southern Trib 232.5 100 year ARI 29.5 51.4 52.91 53.15 53.73 0.04 28.92 0.54 0.18 4.04 0.22 20.77 1.29Southern Trib 232.5 200yr ARI 35 51.4 53.04 53.27 53.91 0.1 33.82 1.08 0.24 4.2 0.26 23.67 1.27Southern Trib 232.5 PMF 108 51.4 54.12 54.46 55.65 1.17 93.17 13.66 0.48 5.89 0.64 25.07 1.27

Southern Trib 213.25 5 year ARI 15.8 50.9 51.86 52.22 53 15.8 4.73 5.93 2.01Southern Trib 213.25 10 year ARI 18 50.9 51.94 52.3 53.08 18 4.73 6.31 1.95Southern Trib 213.25 20 year ARI 22.2 50.9 52.05 52.43 53.27 22.2 4.91 6.86 1.93


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 213.25 50 year ARI 25.6 50.9 52.13 52.53 53.41 0 25.6 0 0.04 5.03 0.04 8.56 1.91Southern Trib 213.25 100 year ARI 29.5 50.9 52.21 52.67 53.56 0 29.44 0.06 0.13 5.15 0.14 14.89 1.84Southern Trib 213.25 200yr ARI 35 50.9 52.32 52.78 53.75 0.02 34.64 0.34 0.2 5.32 0.24 18.42 1.78Southern Trib 213.25 PMF 108 50.9 53.27 53.94 55.5 1 94.28 12.72 0.57 7.07 0.7 28.33 1.66

Southern Trib 194 5 year ARI 16 50.4 51.31 51.73 52.71 16 5.25 5.68 2.29Southern Trib 194 10 year ARI 18 50.4 51.37 51.8 52.8 18 5.3 5.98 2.25Southern Trib 194 20 year ARI 22.2 50.4 51.47 51.94 53.01 22.2 5.49 6.5 2.22Southern Trib 194 50 year ARI 25.9 50.4 51.56 52.04 53.16 25.9 5.61 6.93 2.19Southern Trib 194 100 year ARI 29.9 50.4 51.64 52.19 53.32 0 29.9 0 0.07 5.75 0.07 9.94 2.16Southern Trib 194 200yr ARI 35 50.4 51.73 52.3 53.53 0.01 34.88 0.12 0.17 5.94 0.19 16.06 2.1Southern Trib 194 PMF 108 50.4 52.62 53.41 55.33 0.91 95.19 11.9 0.62 7.76 0.7 31.64 1.9

Southern Trib 161.3 5 year ARI 16 49.44 50.26 50.72 52.06 16 5.93 5.68 2.75Southern Trib 161.3 10 year ARI 18 49.44 50.31 50.8 52.18 18 6.05 5.7 2.67Southern Trib 161.3 20 year ARI 22.2 49.44 50.41 50.95 52.43 22.2 6.29 5.74 2.56Southern Trib 161.3 50 year ARI 25.9 49.44 50.5 51.08 52.61 25.9 6.44 5.78 2.46Southern Trib 161.3 100 year ARI 29.9 49.44 50.58 51.21 52.8 29.9 6.6 5.81 2.38Southern Trib 161.3 200yr ARI 35 49.44 50.69 51.37 53.04 0 35 0 0.03 6.79 0.03 5.92 2.31Southern Trib 161.3 PMF 108 49.44 51.99 53 55.07 2.76 102.08 3.16 0.52 8 0.52 27.04 1.73

Southern Trib 128.7 5 year ARI 17.1 48.48 49.3 49.83 51.31 17.1 6.29 4.45 2.57Southern Trib 128.7 10 year ARI 19.8 48.48 49.38 49.96 51.49 19.8 6.43 4.46 2.47Southern Trib 128.7 20 year ARI 23.6 48.48 49.48 50.12 51.77 23.6 6.72 4.48 2.42Southern Trib 128.7 50 year ARI 27.5 48.48 49.58 50.28 52 27.5 6.89 4.5 2.33Southern Trib 128.7 100 year ARI 31.9 48.48 49.7 50.45 52.24 31.9 7.05 4.52 2.25Southern Trib 128.7 200yr ARI 37.7 48.48 49.85 50.66 52.52 0 37.69 0 0.16 7.24 0.16 5.26 2.15Southern Trib 128.7 PMF 116 48.48 52.6 52.66 53.78 12.02 94.05 9.93 0.45 5.33 0.43 46.46 0.86

Southern Trib 96 5 year ARI 17.1 47.52 48.4 49.02 50.6 17.1 6.58 3.16 2.32Southern Trib 96 10 year ARI 19.8 47.52 50.17 49.18 50.41 0.62 18.55 0.62 0.19 2.24 0.19 12.9 0.45Southern Trib 96 20 year ARI 23.6 47.52 50.72 49.4 50.92 1.19 21.18 1.24 0.17 2.11 0.18 21.13 0.38Southern Trib 96 50 year ARI 27.5 47.52 50.87 49.6 51.11 1.61 24.25 1.64 0.18 2.3 0.2 23.99 0.41Southern Trib 96 100 year ARI 31.9 47.52 50.99 49.82 51.28 2.08 27.75 2.07 0.21 2.55 0.23 26.01 0.44Southern Trib 96 200yr ARI 37.7 47.52 51.01 50.08 51.4 2.5 32.71 2.49 0.24 2.99 0.26 26.39 0.52Southern Trib 96 PMF 116 47.52 52.97 53.56 24.54 73.3 18.16 0.44 4.25 0.41 61.12 0.58


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 86 5 year ARI 17.2 47.22 49.73 48.7 49.93 0.45 16.31 0.45 0.17 2.06 0.17 11.89 0.42Southern Trib 86 10 year ARI 19.8 47.22 50.21 48.85 50.38 0.83 18.1 0.87 0.16 1.91 0.17 17.45 0.35Southern Trib 86 20 year ARI 23.8 47.22 50.75 49.08 50.9 1.64 20.54 1.62 0.15 1.83 0.16 27.17 0.31Southern Trib 86 50 year ARI 27.7 47.22 50.92 49.29 51.09 2.21 23.37 2.12 0.17 1.99 0.18 30.16 0.33Southern Trib 86 100 year ARI 32.1 47.22 51.04 49.5 51.25 2.83 26.6 2.66 0.19 2.19 0.2 32.39 0.36Southern Trib 86 200yr ARI 37.7 47.22 51.09 49.75 51.36 3.46 31.02 3.23 0.21 2.53 0.23 33.28 0.41Southern Trib 86 PMF 116 47.22 53.1 51.98 53.5 27.65 68.11 20.24 0.4 3.64 0.36 68.3 0.48


Southern Trib 72 5 year ARI 17.2 46.72 47.69 48.15 49.23 17.2 5.5 3.27 0.42Southern Trib 72 10 year ARI 19.8 46.72 47.76 48.29 49.53 19.8 5.89 3.28 0.35Southern Trib 72 20 year ARI 23.8 46.72 48.56 48.56 49.35 0.04 23.7 0.07 0.21 3.96 0.12 10.12 0.94Southern Trib 72 50 year ARI 27.7 46.72 48.79 48.79 49.59 0.09 27.13 0.49 0.24 4.02 0.24 11.3 0.9Southern Trib 72 100 year ARI 32.1 46.72 48.96 48.96 49.84 0.15 30.93 1.02 0.28 4.23 0.3 12.2 0.91Southern Trib 72 200yr ARI 37.7 46.72 49.22 49.22 50.13 0.27 35.53 1.89 0.31 4.34 0.32 16.74 0.88Southern Trib 72 PMF 116 46.72 53.27 53.47 16.85 59.08 40.07 0.26 2.74 0.32 86.71 0.34

Southern Trib 62 5 year ARI 17.2 46.49 47.48 47.96 49.09 17.2 5.63 3.27 1.86Southern Trib 62 10 year ARI 19.8 46.49 47.56 48.1 49.38 19.8 5.98 3.28 1.9Southern Trib 62 20 year ARI 23.8 46.49 48.01 48.38 49.26 0.01 23.79 0 0.16 4.96 0.14 3.77 1.31Southern Trib 62 50 year ARI 27.7 46.49 48.22 48.6 49.51 0.03 27.66 0.02 0.23 5.03 0.21 4.21 1.24Southern Trib 62 100 year ARI 32.1 46.49 48.43 48.75 49.75 0.07 31.75 0.28 0.29 5.13 0.23 10.65 1.19Southern Trib 62 200yr ARI 37.7 46.49 48.64 49.03 50.04 0.15 36.64 0.91 0.33 5.33 0.35 11.71 1.18Southern Trib 62 PMF 116 46.49 53.29 53.46 17.66 57.76 40.58 0.24 2.6 0.29 95 0.32

Southern Trib 34 5 year ARI 17.2 45.84 46.86 47.62 48.89 17.2 6.31 2.76 2.02Southern Trib 34 10 year ARI 19.8 45.84 46.96 47.74 49.17 19.8 6.59 2.76 2.01Southern Trib 34 20 year ARI 23.8 45.84 47.3 47.91 49.12 0 23.59 0.21 0.05 5.99 0.26 9.35 1.6Southern Trib 34 50 year ARI 27.7 45.84 47.45 48.06 49.37 0 26.99 0.71 0.07 6.22 0.35 13.36 1.58Southern Trib 34 100 year ARI 32.1 45.84 47.58 48.2 49.61 0 30.48 1.62 0.08 6.49 0.45 14.8 1.59Southern Trib 34 200yr ARI 37.7 45.84 47.72 48.38 49.9 0 34.66 3.04 0.09 6.82 0.58 14.83 1.6Southern Trib 34 PMF 116 45.84 53.32 53.44 41.17 48.88 25.95 0.24 2.38 0.35 89.99 0.28

Southern Trib 9 5 year ARI 17.5 45.39 46.48 47.18 48.44 17.5 6.21 2.91 2.01Southern Trib 9 10 year ARI 20.3 45.39 46.59 47.34 48.71 20.3 6.46 2.95 2Southern Trib 9 20 year ARI 24.4 45.39 46.85 47.56 48.8 24.4 6.19 3.24 1.79


(m3/s) (m) (m) (m) (m) (m3/s) (m3/s) (m3/s) (m/s) (m/s) (m/s) (m)Southern Trib 9 50 year ARI 28.4 45.39 46.99 47.72 49.06 0.04 28.31 0.05 0.32 6.38 0.32 7.43 1.77Southern Trib 9 100 year ARI 32.9 45.39 47.13 47.87 49.32 0.24 32.35 0.31 0.54 6.6 0.54 9.59 1.74Southern Trib 9 200yr ARI 37.7 45.39 47.25 48 49.6 0.55 36.47 0.68 0.69 6.91 0.68 11.16 1.76Southern Trib 9 PMF 119 45.39 53.33 53.43 30.67 51.49 36.83 0.47 2 0.48 29.17 0.23

Southern Trib 0 5 year ARI 17.5 45.06 46.1 46.74 48.26 17.5 6.51 2.78 2.12Southern Trib 0 10 year ARI 20.3 45.06 46.21 46.9 48.53 20.3 6.75 2.83 2.09Southern Trib 0 20 year ARI 24.4 45.06 46.46 47.32 48.65 24.4 6.55 2.92 1.85Southern Trib 0 50 year ARI 28.4 45.06 46.64 47.62 48.92 28.4 6.7 2.98 1.79Southern Trib 0 100 year ARI 32.9 45.06 46.84 47.83 49.18 32.9 6.79 3.06 1.72Southern Trib 0 200yr ARI 37.7 45.06 47.03 47.98 49.48 0.01 37.68 0.01 0.19 6.94 0.23 5.08 1.67Southern Trib 0 PMF 119 45.06 53.4 49.21 53.4 78.52 19.02 21.46 0.21 0.76 0.16 123.78 0.08

Southern Trib -2 Culvert

Southern Trib -40 5 year ARI 17.5 40.6 45.94 45.95 4.79 8.24 4.47 0.33 0.51 0.39 23.33 0.07Southern Trib -40 10 year ARI 20.3 40.6 46.35 46.36 6.65 8.14 5.51 0.33 0.46 0.37 25.59 2.09Southern Trib -40 20 year ARI 24.4 40.6 46.85 46.86 9.2 8.21 6.98 0.33 0.43 0.36 28.24 0.05Southern Trib -40 50 year ARI 28.4 40.6 47.24 47.25 11.53 8.49 8.38 0.34 0.42 0.37 30.11 0.05Southern Trib -40 100 year ARI 32.9 40.6 47.63 47.63 14.16 8.83 9.91 0.34 0.41 0.37 31.99 0.05Southern Trib -40 200yr ARI 37.7 40.6 47.95 47.96 16.88 9.34 11.48 0.36 0.42 0.38 33.56 0.05Southern Trib -40 PMF 119 40.6 53.4 53.4 81.78 7.66 29.56 0.19 0.2 0.17 127.02 0.02

Southern Trib -41 5 year ARI 17.5 39.62 45.9 41.11 45.94 17.5 0.91 3.05 0.12Southern Trib -41 10 year ARI 20.3 39.62 46.3 41.27 46.35 20.3 1 3.05 0.12Southern Trib -41 20 year ARI 24.4 39.62 46.79 41.48 46.85 24.4 1.12 3.05 0.13Southern Trib -41 50 year ARI 28.4 39.62 47.16 41.69 47.24 28.4 1.23 3.05 0.14Southern Trib -41 100 year ARI 32.9 39.62 47.53 41.9 47.62 32.9 1.36 3.05 0.15Southern Trib -41 200yr ARI 37.7 39.62 47.83 42.12 47.95 37.7 1.51 3.05 0.17Southern Trib -41 PMF 119 39.62 52.92 44.98 53.35 119 2.93 3.05 0.26

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Flat Rock Creek Final Report - edocs.willoughby.nsw.gov.au

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