James Street, Plimmerton - Porirua...1 15/12/2016 Alistair Osborne Steven Cornelius 1.1 19/12/2016...

James Street, Plimmerton NBC Projects Limited

Hydraulic Modelling Analysis of Proposed Development Doc No. J000023.001

16 February 2018

Prepared by Awa Environmental Limited

for NBC Projects Limited

James Street, Plimmerton Hydraulic Modelling Analysis of Proposed Development

iii

James Street, Plimmerton Project No. J000023

Document title Hydraulic Modelling Analysis of Proposed Development

Document number J000023.001

Version number 1.5

Date 16 February 2018

Project manager Steven Cornelius

Author Alistair Osborne

Awa Environmental Limited 115 Tory Street, Te Aro, Wellington 6011, Phone:+64 04 455 0990 www.awa.kiwi

Document History

Version Date Description Author Reviewed

1 15/12/2016 Alistair Osborne Steven Cornelius

1.1 19/12/2016 Alistair Osborne Philip Caughley

1.2 25/01/2017 Alistair Osborne Craig Martell

1.3 16/08/2017 Alistair Osborne Steven Cornelius

1.4 1/12/2017 Alistair Osborne Steven Cornelius

1.5 16/2/2018 Steven Cornelius Steven Cornelius

COPYRIGHT: The concepts and information contained in this document are the property of Awa Environmental Ltd. Use or copying of this document in whole or in part without the written permission of constitutes an infringement of copyright.


iv

Contents Contents ................................................................................................................................................. iv

1 Introduction .................................................................................................................................... 5

2 Study area physical description and known issues ......................................................................... 5

3 Model Development ..................................................................................................................... 10

4 Flood assessment .......................................................................................................................... 12

4.1 Existing site flood assessment ....................................................................................... 12

4.2 Post Development Model Results ................................................................................. 15

5 Discussion ...................................................................................................................................... 23

5.1 Model build ................................................................................................................... 23

5.2 Base Case Flood assessment ......................................................................................... 23

5.3 Proposed Site development flood assessment ............................................................. 24

6 Recommended Building Levels................................................................................................ 25

7 Conclusion ................................................................................................................................... 27

8 References .................................................................................................................................. 28

Appendices .............................................................................................................................................. 1

Appendix A: Model Build Report ............................................................................................................ 2


5

1 Introduction This report details the hydrological and hydraulic modelling analysis undertaken to determine peak flood levels

and to assist in the design of stormwater mitigation for the proposed subdivision development at James Street

Plimmerton.

NBC Projects Limited have proposed a subdivision development on Lot 1 DP489799, a triangular shaped parcel

that fronts the intersection of James Street and State Highway 1 (Figure 1). The site is located in the lower

reaches of the approximately 12km2 Taupo Stream catchment that discharges to the sea at the north end of

South Beach at Plimmerton.

A resource consent is required for the subdivision development and in order to obtain the consent it is necessary

to demonstrate the following:

• The development does not result in increased flood levels off site.

• That the development meets the requirements of the Stormwater Drainage Section of the Porirua City

Council Code and Land Development and Subdivision Engineering.

Furthermore, Porirua City Council consider the property to be at risk of flooding and require a flood model to

be developed.

To this end NBC Projects Limited have engaged Awa Environmental to undertake hydrological and hydraulic

modelling for the site, surrounding environs and upstream catchment.

A coupled 1D-2D hydraulic model of the existing case for the site was developed and simulated for the 10%, 5%,

2%, 1%, and 1% with climate change AEP design events. Following analysis of the results of these simulations,

and earthworks scheme was developed for the proposed subdivision. The hydraulic model was then adapted to

include the proposed earthworks scheme to test the offsite flooding effects of the subdivision.

A full description of the model build process and results presentation is appended in the James Street,

Plimmerton: Hydraulic Modelling Model Build Report.

2 Study area physical description and known issues The proposed subdivision development is Lot1 DP489799, a triangular shaped parcel that fronts the intersection

of James Street and State Highway 1. The site location and the Taupo Stream catchment are shown in Figure 1.

The topography of the Taupo Stream catchment can be seen in Figure 2.

The site is subject to flooding during significant rainfall events, as evident in Figure 3 Figure 5.


6

Figure 1: Site location and Taupo Stream catchment


7

Figure 2: Taupo Stream Catchment Topography


8

Figure 3: Flooding at the site and surrounding environs during the 15th November 2016 event (Source: David Stella; https://www.tvnz.co.nz/one-news/new-zealand/wellington-floods-happened-trains-resume-altered-schedules-flood-clean-up-begins)

Figure 4: Flooding at the site during the 15th November 2016 event, photo taken from the driveway of the show home plot beside the site (Source St Theresa’s School Plimmerton; https://www.facebook.com/StTheresasPlimmerton/photos/a.193775183985900.52708.191696254193793/1411352698894803/?type=3&theater)

Site location

https://www.tvnz.co.nz/one-news/new-zealand/wellington-floods-happened-trains-resume-altered-schedules-flood-clean-up-begins

https://www.tvnz.co.nz/one-news/new-zealand/wellington-floods-happened-trains-resume-altered-schedules-flood-clean-up-begins

https://www.facebook.com/StTheresasPlimmerton/photos/a.193775183985900.52708.191696254193793/1411352698894803/?type=3&theater

https://www.facebook.com/StTheresasPlimmerton/photos/a.193775183985900.52708.191696254193793/1411352698894803/?type=3&theater


9

Figure 5: Flooding in the SH1 roundabout adjacent to the site during the 15th November 2016 event (Source NZTA traffic camera; http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11748286).

http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11748286


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3 Model Development

The catchment consists of a broad valley that runs from North to South, flanked by relatively steep hills that rise

to 260m R.L along the north-western edge of the catchment and 240m R.L along the eastern edge of the

catchment. The main flow paths through the Taupo Stream catchment are also shown in Figure 2. These

flowpaths, the topography, and stormwater pipes/culverts in the vicinity of the development site can be seen

in more detail in Figure 6.

State Highway 1 and the North Island Main Trunk Railway line run north/south through the Taupo Stream

catchment. Midway up the valley the railway line is formed along the western foothills and the Highway along

the eastern foot hills. The area between the railway line and Highway in the centre of the catchment is known

as Taupo Swamp and is densely vegetated with wetland plants.

As shown in Figure 2 and Figure 6 there are flow paths along the western and southern boundaries of the site,

both of these have formed open channels.

The gully to the south west of the site drains in to the open channel that runs along the southern boundary of

the site via two 900mm diameter culverts that cross James Street near the electrical substation. The residential

developments on the hills surrounding this gully have piped stormwater systems that discharge to the gully.

A 450mm diameter culvert under James Street, also near the substation, conveys runoff from the area to the

east of James Street into the open channel along the southern boundary of the site.

The area to the northeast of the site drains via a culvert under James Street, near the State Highway 1 round

about, into the open channel that runs along the western boundary of the site. The gully system to the north of

this area drains through a 1m high by 2m wide box culvert under State Highway 1. If this box culvert is

overwhelmed flows from this area will flood out across the paddocks and may eventually find their way into the

development site via the culvert under James Street.

Several sumps located on James Street also discharge into the proposed development site

The two open channels that run along the western and southern sides of the site convey flows to Taupo Steam

via a single 1.2m diameter culvert that passes under State Highway 1. High water levels in Taupo Stream, due to

either flood flows from upstream, or the downstream tide level could restrict flows through this culvert and

could even result in back flow through the culvert. Flood flows from upstream will be mitigated by the significant

storage that exists in flax swamp.

Full model build details are provided in Appendix A.


11

Figure 6: Site Topography


12

4 Flood assessment A MIKEFLOOD hydrologic and hydraulic model has been developed for the lower Taupo Stream catchment. To

date this model has been used to evaluate the following scenarios:

• Base Case – the existing landform, landuse and drainage network.

• Post Development–proposed earthworks scheme for the site, including a building platform. This

includes the lowering of a significant portion of the site to increase onsite flood storage and cleaning

out the drains to improve conveyance. The site will continue to be drained by the culvert running

beneath SH 1, as the culvert inlet remains well below the lowest portion of the site. A building platform,

approximately 100m long and 30m wide, was raised along the James St boundary. Vehicle access to the

interior of the site will be at the northwest or southeast end of the building platform.

The Base Case scenario has been run for the 10%, 5%, 2%, and 1% AEP events, under existing climate conditions

and the 1% AEP event for future climate conditions. The post development scenario has been run with the 1%

AEP event under existing climate conditions and for future climate conditions.

Results for the modelling is presented in full in the model build reported attached in Appendix A.

4.1 Existing site flood assessment

The Base Case model represents the existing conditions at the site and the flooding expected under these

conditions.

Figure 7 shows the maximum flood depths calculated across the model for the 1% AEP event, without climate

change, under existing conditions. The figure shows there is a significant flooding at the site with water depths

of between 0.5m and 1.0m extending across the parcel. Figure 8 shows the results for 1% AEP event, with climate

change. The figure shows the flooding at the site increased, with the majority of the site flooded to a depth of

over 1.0m. To the south of the site flooding appears to range from 0.25m to 1.0m in depth.

Flood depth maps for the 2% AEP, 5% AEP, and 10% AEP events are provided in Appendix A of the Model Build

report.

Table 1 provides the water levels modelled at each of the result points shown in Error! Reference source not

found.. Figure 6 shows the overland flow paths in the vicinity of the site along with the surrounding stormwater

(SW) network assets. It appears that there is a significant overland flow path originating north of the site

(beginning opposite the entry to the highway weigh station) that flows south along the eastern side of State

Highway 1 (SH1). It is likely this contributes a significant portion of the surface water seen at the site, however,

flow also originates from the southeast near Grays Rd.

It was also noted during a site visit to the proposed development that the open channels along the southern

and western boundaries of the site were not well maintained and were not likely to be able to convey significant

flows in their current condition.


13

Figure 7: Surface flooding during the 1% AEP design event – Base Case


14

Figure 8: Surface flooding during the 1% AEP design event with climate change – Base Case


15

Table 1: Water levels at the selected result points - Base Case

Result Point 10% AEP 5% AEP 2% AEP 1% AEP 1% AEP - CC

P1 2.77 2.86 3.04 3.23 3.51

P2 2.77 2.86 3.04 3.23 3.51

P3 2.77 2.86 3.04 3.23 3.51

P4 3.17 3.54 3.77 3.80 3.84

P5 4.04 4.09 4.15 4.18 4.22

P6 2.76 2.82 3.03 3.23 3.51

P7 2.97 3.00 3.04 3.23 3.51

P8 NA 2.72 3.04 3.26 3.65

P9 2.50 2.74 3.07 3.29 3.66

P10 NA 3.02 3.14 3.32 3.67

4.2 Post Development Model Results The flood depth results for the post development scenario are shown in Figure 9 and Figure 10 for 1% AEP and

1% AEP with climate change events. The results do not show a significant difference between the post

development scenario and base case. At the site there is an increase in water depth, however this is will be due

to the lowering of the site to increase storage.

Table 2 provides the modelled water levels at each of the results points shown in the figures, along with the

difference between the post development and base case water levels for the two events.

Water level difference maps of the post development scenario relative to the base case have been generated

for the 1% AEP and 1% AEP with climate change event. This has been done by subtracting the maximum water

surface elevation of the base case from the maximum water surface elevation for the development scenario.

Error! Reference source not found.Figure 11 provides the water level difference for the 1% AEP event while

Figure 12 shows water level differences for the 1% AEP event with climate change. Positive differences represent

areas where the water level has increased in the post development scenario relative to existing levels, while

negative differences represent a decrease in water levels.

The results show that for the existing climate scenario the development results in a water level decrease off site

of 5-10mm, while in the climate change scenario there is an increase of 5-10mm off site. In both the existing

and climate change scenarios there is an area along the northern edge of the development that has an increase

in peak surface level. This is due to the increase in the ground level and the associated redirection of surface

flows along James Street. This increase in surface level is within the development site and James Street Road

reserve.


16

Table 2: Water levels at the result points and water level differences relative to the Base Case

Result

Point

1% AEP 1% AEP with climate change

Base

Case

Post development

Scenario

Post development

Difference

Base

Case

Post development

Scenario

Post development

Difference

P1 3.23 3.23 -0.01 3.51 3.52 0.01

P2 3.23 3.23 -0.01 3.51 3.52 0.01

P3 3.23 NA NA 3.51 NA NA

P4 3.81 3.80 0 3.84 3.84 0

P5 4.18 4.18 0 4.22 4.22 0

P6 3.23 3.23 -0.01 3.51 3.52 0.01

P7 3.23 3.23 -0.01 3.51 3.52 0.01

P8 3.26 2.26 0 3.65 3.65 0

P9 3.29 2.29 0 3.66 3.66 0

P10 3.32 3.31 0 3.67 3.67 0

Figure 13 shows the areas that were unflooded in the existing case but are now flooded in the post development

scenario. These areas are very small, generally only one model cell in size, and are dominantly located on the

boundaries of the site or in road reserves.

Given the minimal impact of the development under the 1% AEP event, without climate change, it is likely this

minor increase in water level seen during the 1% AEP event, with climate change, is due to a loss of storage

resulting from an increase in sea level of 1.0m.


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Figure 9: Surface flooding during the 1% AEP design event – Post development scenario


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Figure 10: Surface flooding during the 1% AEP design event with climate change – Post development scenario


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Figure 11: Difference in peak surface level for pre and post development 1% AEP event with existing climate


20

Figure 12: Difference in peak surface level for pre and post development 1% AEP event with climate

change


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Figure 13: Areas of new flooding during the post development scenario event – 1% AEP event with climate change


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The modelling results show that flow velocities downstream of the site are not increased by the proposed

development and as such there are no identifiable downstream impacts. Table 3Table 3 provides a comparison

of the base case and post development scenario downstream velocities for the 1% AEP event. Table 4Table 4

shows the equivalent values for the 1% AEP event with climate change.

Table 3: Discharge and velocity downstream of the site – 1% AEP

Base Case Post development scenario

Discharge (m3/s) Velocity (m/s) Discharge (m3/s) Velocity (m/s)

Downstream

Culvert

2.33 2.06 2.33 2.06

Downstream

Stream

19.35 NA 19.33 NA

Table 4: Discharge and velocity downstream of the site – 1% AEP with climate change

Base Case Post development scenario

Discharge (m3/s) Velocity (m/s) Discharge (m3/s) Velocity (m/s)

Downstream

Culvert

2.41 2.13 2.45 2.17

Downstream

Stream

22.41 NA 22.42 NA


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5 Discussion

5.1 Model build The model developed for this study is based on GIS data and LiDAR supplied by PCC. This was supplemented by

a limited site survey and engineering judgement.

The source of all network data used in the MIKE URBAN model has been recorded in a separate geo database.

This will allow the source of data used in the network model to be readily identified.

The MIKE 21 bathymetry has been developed using LiDAR supplied by PCC, and the infiltration and roughness

grids were developed using GIS ground cover data obtained from LINZ. The MIKE 21 model has been developed

to a sufficiently high level of detail for flood hazard management analysis.

Fences and other minor structures are not represented in the model. This may result in some uncertainty in the

modelled ponding and overland flow in the vicinity of buildings and other structures.

Blockages in the network have also not been considered. However, it is unlikely that a blockage of the culvert

draining the site or a blockage of the culvert draining into the site would affect the proposed lots. In the case of

a blockage of the culvert draining the site water will spill into the properties to south of the site – the ground

here at least 0.5m lower than the lowest portion of the proposed building platform and SH1. Maintenance of

the drains on site and the culvert draining beneath SH1 will be carried out by PCC. It is expected that this will be

included in the Council’s programme of works.

In the case of blockages of the culverts draining into the site from the north, overflow is likely to be stored in

the paddocks on the north side of James St or flow across James St and into the proposed storage area of the

site via the vehicle access way or the drain around the proposed building platform. It is recommended that

freeboard is considered when establishing recommended building levels for the site.

It should be noted that the hydraulic model has not been calibrated or verified against any recorded event.

However, an approximate validation of the model hydrology has been undertaken using Flood Frequency

analysis values provided by GWRC for the Taupo Swamp flow gauge.

5.2 Base Case Flood assessment The results show there is significant flooding at the site for during larger events (Error! Reference source not f

ound. and Figure 8).

There are significant storage volumes between SH1 and the hills along the eastern edge of the catchment to the

north of the site. These areas are drained via culverts under the state highway, to Taupo Stream. In larger,

longer duration events these culverts are unable to cope, filling the storage areas. As this storage volume fills

the overland flow path that travels southward along the eastern edge of the state highway activates. This flow

path drains via a culvert under James Street, near the roundabout, into the site. There is also a considerable

volume of storage just upstream of this culvert, which further attenuates the flows through the culvert. As flows

that enter the site from the north are conveyed via a secondary flow path (the primary flow path is through the

culverts under the state highway) and there are significant storage volumes that must be filled prior to these

flows entering the site, it is likely that the site will only be affected by flows from the north during larger longer

duration events.

The modelling also shows that flows are likely to enter the site from the south (Figure 14). The culverts at the

downstream end of the gully between Mo Street and Grays Road discharge to the open channel that runs along

the southern boundary of the site. The model shows that these culverts have limited capacity, which combined


24

with the limited capacity of the stormwater network in the southern part of James Street results in flooding of

the school, church and residential properties here. This then flows overland into the site.

Figure 14: Overland flow originating from gully between Mo Street and Grays Road

Although the site is lower than the surrounding properties, due to the mechanism of flooding it is one of the last

locations to flood and is only likely to be flooded during larger longer duration events.

It should be noted that the flood maps generated do not include an allowance for freeboard.

5.3 Proposed Site development flood assessment The potential flooding effects of the development site have been presented in Section 4.2.

Flow to culvert

Overland flow

to James St

Flow down gully

Flow to culvert

Overland flow

to James St

Flow down gully


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The proposed development results in a loss of storage where the building platforms is elevated along the

northern boundary of the site. This is offset by increasing the storage volumes across the remainder of the site

and cleaning out the two existing onsite drains.

The modelling shows that the development of the site as proposed will result in a negligible decrease in flooding

beyond the site under current climate conditions. There is a negligible increase in offsite flooding under climate

change conditions. The consequences of this increase are less more than minor, these areas are already

significantly affected by flooding under the exiting climate and existing development scenario. The flood levels

here are anticipated for the 1% AEP event are anticipated to increase by 280mm as a result of climate change.

As such it is unlikely that the existing dwellings, school and church will remain

Flow velocities downstream of the site are not increased by the proposed development and as such there are

no identifiable downstream impacts.

6 Recommended Building Levels A map of recommended building levels (RBLs) has been generated across the building platform at the proposed

development, Figure 15. The RBLs have been generated by adding a freeboard of 0.5m to adjacent flood levels

for the 1% AEP event with climate change. Freeboard is adopted to account for uncertainties in the data applied

in the model.

NZS4404:2010 for land development and subdivision infrastructure identifies minimum freeboard requirements

as follows;

4.3.5.2 Freeboard

The minimum freeboard height additional to the computed top water flood level of the 1% AEP design storm

should be as follows or as specified in the district or regional plan:

Freeboard Minimum height

Habitable dwellings (including attached garages) 0.5m

Commercial and industrial buildings 0.3m

Non-habitable residential buildings and detached garages 0.2m

The minimum freeboard shall be measured from the top water level to the building platform level or the

underside of the floor joists or underside of the flood slab, whichever is applicable.

It is recommended that any buildings in the proposed subdivision adopt the RBLS presented in Figure 15 or the

proposed ground surface, whichever is higher.


Figure 15: Recommended Buildings Levels


27

7 Conclusion A coupled 1D-2D stormwater hydraulic model has been developed for the James St site and Taupo Stream

catchment. The model includes 1D pipe network and 1D open channel data based on data supplied by PCC and

Wellington Water Ltd as well as a 2D model generated from LiDAR data. Errors, omissions and anomalies

identified in the GIS data have been resolved using engineering judgment and site inspections undertaken by

Awa Environmental Ltd staff.

The hydraulic model has been developed to a high level of detail and includes all of the sumps and small diameter

pipes that could be identified in the catchment. The Taupo Stream open channel has been modelled as one

dimensional channel, while minor channels and drains have been modelled in the 2D. The ground model has

also been developed to a high level of detail with a 2m by 2m cell size.

The hydrological model has been built to a comparable level, with sub-catchments delineated for each sump,

open channel and building with a direct connection to the stormwater network.

The assessment of offsite flooding effects resulting from the proposed development has shown that during the

1% AEP there is a negligible decrease in flooding beyond the site.

For the 1% AEP event with climate change the model predicts a negligible increase in flooding beyond the site.

The affected areas are already significantly impacted by flooding in the existing case model.

The model results also show there is negligible difference between velocities through the culvert draining the

site for the existing case and post development scenario.

Recommended building levels have been supplied. These have been generated by applying a freeboard of 0.5m

to the flood levels modelled for the 1% AEP event with climate change.


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8 References

DHI, 2013. Guidelines for Stormwater Modelling using Mike Flood

Capacity Infrastructure Services, 2013. Regional Stormwater Hydraulic Modelling Specifications.

Ministry of Business, Innovation & Employment, 2010. NZS4404:2010 Land Development and Subdivision Infrastructure.


A1

Appendices


A2

Appendix A: Model Build Report

16th February 2018

Plimmerton MIKE FLOOD Modelling Report Addendum 2

Peer Review - Response Awa Environmental have developed a hydraulic model to support the consent application for a proposed

subdivision at James Street Plimmerton.

DHI have undertaken a peer review of the model and report and presented their findings in the report

dated January 2017 appended to this letter.

This letter details our response to that peer review. The peer review uses a ranking system from 0 to 3

where 0 is not an issue and 3 is a major issue. The review includes 80 Items Checked, our response below

address Items Checked ranked 1 -3, and uses the Items Checked numbering from the DHI Review.

Items Checked Rank Awa Response

2. Model file Names

and dates

1 Agreed, we have implemented an improved naming convention.

4. Extent of the

model

1 The extent of the model was verbally discussed with PCC when initiating

the project. The extent covers the full catchment upstream and

downstream of the development site. The extent is appropriate.

5.Entire catchment

boundary check.

2 The catchments have generated using the ArcGIS Hydrology Toolkit and

are limited by the LiDAR extent which may truncate portions of some

catchments. We agree this will make little difference to the model and

effort required to rectify this is not warranted in this instance.

6. Sub-catchment

area ranges.

1 The level of detail used in the catchment delineation exceeds the

specification. In our experience the use of a finer, more detailed

catchment delineation results in improved model performance, both in

terms of stability and representation of flooding. The distribution of a

larger number of smaller inflows across the network avoids the issue of

lump catchments creating erroneous flooding at point loading locations.

The catchments were generated using an automated process, we have

since improved this process to merge small catchments. Whilst we

agree that the smallest catchments should be merged the effect is

negligible and the effort not warranted in this instance.

8. Hydrological

modelling

methodology used

2 Erroneous records in msm_HModB have been deleted. This does not

affect the model results.

9. Spot check of

sub-catchment

length.

1 Ref 6.


14. Node diameter

in the model.

2 Additional data has been provided in the report as to how node

diameters were determined.

15. Node cover type

in the model.

1 Cover type has been changed to normal.

16. Node invert

levels in the model

compared to the GIS

asset database and

LiDAR.

2 Additional detail has been added to the report.

17. Node ground

levels in the model

compared to the GIS

asset database and

LiDAR.

1 Additional detail has been added to the report.

19. Basin modelling

method.

1 The three basins that are identified in the peer review are located

upstream of the Taupo Swamp Stream gauge in the part of the model

that is represented as a single hydrological catchment. Given the size of

these basins, the distance of the basins to the catchments outlet, and

the insignificant storage volume in these basins relative to the storage

volume in the swamp, there is no benefit to explicitly modelling these

basins in the hydraulic model. The large hydrological catchment which

contains these basins has been validated against the stream gauge and

as such this will provide a better representation of the behavior of the

catchment than would be achieved by attempting to model these basins

in the hydraulic model.

20. Manhole head

losses (outlet

shapes) in the

model set on the

manholes in MIKE

URBAN.

3 Manhole head losses have been changed to Mean Energy Weighting

with Km = 0.3. Additional detail has been added to the report.

21. Culvert inlet &

outlet head losses

(outlet shapes) in

the model.

3 Manhole head losses have been changed to Mean Energy Weighting

with Km = 0.3. Additional detail has been added to the report.

26. Link frictional

losses (Manning’s n

or CW).

2 Link friction type has been changed to Colebrook White. Materials types

were based on PCC GIS data, RCRRJ has been mapped to concrete

normal, HDPE and PVC to plastic and AC to Asbestos Cement.

Agree this will make little difference to the model.

27. 1D-2D linkage

method (e.g. orifice

vs. weir etc.).

1 The coupling for manhole ID 510301 has been changed to use the orifice

equation.

33. Network extent. 3 Additional detail has been added to the report including the

construction drawing and a description of network survey that was

undertaken.

34. Modelling of

culverts.

2 The culverts appear to be well represented in MIKE URBAN and there

are no stability issues.


36. dhiapp.ini

parameters.

2 Reservoir height has been updated to 100.

37. Difference

between the 4 MIKE

URBAN models

2 The ground levels for outlets SurveyedOutlet16 and SurveyedOutlet07

appear to have been changed in error. These have been set back to the

original values. This parameter is not used by the model engine.

SurveyedOutlet12 has been moved as part of the proposed

development, hence the changes to the ground and invert levels.

Additional detail has been added to the report.

38. Design

rainstorm shape and

total rainfall in the

model.

3 This has been corrected

41. Channel

roughness.

1 Typo in report fixed.

42. Spot check of

model cross-

sections compared

to the LiDAR

contours.

1 The banks were adjusted in places to reflect the surveyed cross section

levels. Detail added to report.

43. Spot check of

model cross-

sections whether it

included the low

flow channel.

2 As per 42.

44. Boundary

conditions.

1 MIKE11 initial conditions updated to match MIKE21 initial conditions.

45. Structures

(culverts and

bridges).

2 Three additional bridges have been included along the modelled reach

of Taupo Stream.

46. Cross-section

parameters.

1 Detail added to the report.

49. Blocking out

cells when open

channels modelled

in 1D model.

1 Lateral links have been rebuilt and blocked cells corrected.

60. Linear features

representation

(roads, railways,

etc.)

2 Linear features such as kerb lines, roads, and railway were not

accentuated during the LiDAR processing and development of the

model bathymetry as it was considered that they adequately

represented model bathymetry.

A key addition to the model bathymetry was the inclusion of a ground

model provided by Orogen covering the site and a small area upstream

of the site. Detail has been added to the report.

63. Earthworks 2 No onsite drains were removed, this has been clarified and confirmed

with the peer reviewer. The 650m3 of fill in the development scenario is

not necessarily a removal of this volume from available flood storage as

the building platform (filled area) rises above the flood storage.


64. Surface friction 1 Built-up Area (settlement) Commercial is within guide line specs for

roads and car parks, the spec does not include a value for the remainder

of the site.

Transport is within the range for roads and car parks.

The spec does not contain values for sand/gravel or ponds/lakes.

The development site is above the peak flood level and as such changes

to roughness here will have no impact on the model results.

67. Standard links 2 Have change M11 boundary to water level and directly linked the MU to

M11. The cross sections in the M11 model have been lowered to align

with the pipe outlet invert.

68. Lateral links 2 Lateral links updated.

70. River/Urban

links

1 Agreed catchment discharges would be better directed directly to the

channel, however the effort to accomplish this is not warranted for the

current model given the negligible benefit. This has however been

noted for future projects.

71. Difference

between the 4 MIKE

FLOOD .couple

models

3 Survey Outlet12 has been moved as part of the development, this is

now detailed in the report. The positions of Survey Outlet14 and Survey

Outlet03 were moved in error and have been returned to their original

position.

72. Overall mass

balance (should be <

5%).

1 Mass Balances are less than 5%.

73. Spot check of

any instability in

model results.

2 The instabilities have been reduced slightly by the various updates to

the model. Agree with reviewer that these do not affect the results.

75. Channel results. 1 Minor instabilities at the tide boundary remain despite improvements

made to the model. Agree with the reviewer that these do not affect

the results.

77. Hydraulic

neutrality.

2 Report has been updated.

78. Effects of model

errors /

assumptions on

predicted pipe

performance.

2 The model has been run for a range of events (10, 20, 50 and 100 year)

and is suitable for use in system performance assessment, however this

is not the intended purpose of the model. The model will only be used

to assess the impact of the proposed development.

79. Effects of model

errors /

assumptions on

predicted flood

levels.

1 The majority of the recommended changes have been made. The

model will not be used as the basis of flood investigations at other

locations.

80.

Recommendations.

1 All key recommendations have been addressed.

26th February 2018


Peer Review - Response Awa Environmental have developed a hydraulic model to support a consent application for a

proposed subdivision at James Street Plimmerton.

NZTA has engaged AECOM to review the proposed development with respect to flooding impacts at

the State Highway 1 James Street Roundabout.

The response below relates to the four bullet points included in the recommendations section of the

AECOM report.

• That the scenario where storage in the Taupo swamp and Plimmerton Domain reaches capacity and has no further beneficial effect is included in the hydraulic model (if it is not already included) and in the assessment of flood effects associated with the proposed development.

The storage volume for the Plimmerton Domain is explicitly described in the model. The model has been run for 24 hours and much of the available storage in the Plimmerton Domain is utilized in the 1 in 100 year ARI. The model shows significant flooding in the domain even in the smaller design event simulated. Taupo Swamp, and its upstream catchment have been included in the model as a hydrology catchment. The parameters for the hydrology catchment have been adjusted to better reflect the flood frequency analysis for the Flax stream gauge provided by Greater Wellington Regional Council. Even so, the model is over estimating the peak flow in the 1 in 100 year ARI event by 2m3/s (14m3/s model vs 12m3/s flood frequency analysis) and as such the model is conservative in terms of peak flows from the swamp. The edits made to the hydrology parameters, to better reflect the flood frequency analysis, reduce the peak flow by slowing the run off, rather than by reducing the total volume of runoff. As such we feel that the model is representing the scenario where the storage capacity of the domain and swamp are fully utilized as all of the rainfall falling on the catchment is converted to run off, with the exception of initial wetting losses and infiltration losses. In the advent that a longer duration model run was undertaken, or a greater flow from the swamp were modelled it is likely that the predicted peak water levels in both the existing and post development scenario models would increase equally and the development would show a negligible affect on peak water levels outside the development site.

The 15 November 2016 event discussed in the AECOM report resulted in extensive flooding of the domain, as seen in Figure 2. This event was approximately a 60-80 Year return period when assessed against the HIRDS design storms. Figure 1 provides a comparison between the recorded rainfall and the 100 year ARI (1% AEP) rainfall event used in the model.

Figure 1 Wheua Tapu rain gauge data Vs HIRDS 100 Year ARI event

0

20

40

60

80

100

120

14-11-16 0:00 14-11-16 12:00 15-11-16 0:00 15-11-16 12:00 16-11-16 0:00

mm

/ho

ur

Whenua Tapu Rain Gauge 1% AEP Design Event

Figure 2 Aerial view of flooding in Plimmerton 15th Novermber 2016. Source: https://www.tvnz.co.nz/one-news/new-zealand/wellington-floods-happened-trains-resume-altered-schedules-flood-clean-up-begins

Comparing Figure 2 with Figure 17 (100 year ARI) and Figure A.3 (50 year ARI) of the model build report suggests that the model would accurately represent the flooding that occurred on 15 November 2016 if this event were run through the model.

• That any increase in stormwater levels in this high flood risk area is unacceptable.

The model has been updated subsequent to a peer review conducted by DHI (see attached copy of updated flood model reports, dated 16 February 2018). Although updates were minor they have had an impact on the results. The model now shows a reduction in peak surface water level off site due to the development in the existing climate scenario of almost 10mm. For the climate change scenario the model shows an increase in peak water level due to the development of less than 10mm. It is our understanding that the effected properties are susceptible to flooding well above the floor level in the existing development existing climate scenario during the 1 in 100 year ARI event. The model shows that the peak surface level will increase at these properties by about 280mm due to climate change for the 1% AEP event. The peak flood depths on the southern part of James Street are predicted to be more than 1.1m in the existing development climate change scenario. Our professional opinion is that the less than 10mm increase in flood level resulting from the development is less than minor in the context of this flooding and is likely to have a negligible affect on the cost of the required works to resolve flooding here.

Our expectation is that by 2090 NZTA and Porirua City Council will have undertaken works to address the inadequacy of the existing levels of service.

• That provisions to protect the flood mitigation measures contained within Lot 13 (primarily flood retention) over the long term are formally defined.

Agreed.

• That maintenance responsibilities for all stormwater mitigation measures are formally defined and agreed to by parties with maintenance responsibilities.

Agreed.

26th February 2018


Greater Wellington Feedback - Response Awa Environmental have developed a hydraulic model to support a consent application for a


Having viewed the hydraulic modelling report prepared by Awa, Greater Wellington have raised

concerns relating to safe access to and from the development site and the impacts of culvert

blockage on the flood risk. These concerns are addressed below.

Safe Site Access

It is essential that safe access to and from the properties is maintained during an extreme weather

event. The risks and consequences of attempting to navigate deep or fast flowing flood waters

either on foot or by car are significant.

The Capacity Regional Stormwater Modelling Guidelines that were in use at the time the modelling

was undertaken require Flood Hazard classification using the methodology described in the

Floodplain Development Manual (New South Wales Government, April 2005). This methodology

widely accepted and applied.

The NSW manual provides a depth and velocity based methodology for categorising hydraulic flood

hazard as low or high as shown in Figure 1. The manual also provides guidance on depths and

velocities at which vehicles become unstable and wading becomes unsafe for an able-bodied adult.

We have used this guidance to assess the safety of access to and from the site.

The analysis undertaken uses the peak depth and peak velocity at each location. As the peak depth

and peak velocity at a given location may not be coincident our analysis overstates the risk. Each

calculation point in the MIKE 21 surface model has been classified into the following categories

based on depth and velocity:

• No Safe Access,

• Safe pedestrian access,

• Safe pedestrian and vehicle access.

The results are presented as a map Figure 3 are for the 100-year ARI rainfall event under climate

change conditions with the development in place.

The peak depths and peak velocities at each location shown in Figure 5 are plotted in Figure 4 on top of

the NSW Velocity and Depth Relationships.

Figure 1 Provisional Hydraulic Hazard Categories taken from the NSW FloodPlain Management Guidelines.

Figure 2: Velocity and Depth Relationships taken from the NSW FloodPlain Management Guidelines.

Figure 3: Depth and Velocity based safe access classification.

Figure 4: Depth and Velocities along site access path overlaid on NSW Velocity and Depth Relationship.

Figure 5 Model Computation Points along safe access path

There are a few of computation points on the James Street berm/foot path that fall into the

category of safe pedestrian access (but not vehicle access). The location of the point that falls

closest to the “wading unsafe from here line” line in Figure 4 is marked in Figure 5 as the “Highest

Depth and Velocity Point”. The time series for depth and velocity at this location are presented in

Figure 6. The maximum depth of 0.38m coincides with a velocity of 0.17m/s, while the peak velocity

of 1.23m/s coincides with a depth of 0.28m. These combinations of depth and velocity are closer to

the “vehicles unstable from here” line than the “wading unsafe from here line”. Although it would

be possible to walk through the location for the duration of the rainfall event, it would not be safe to

drive a vehicle through this particular location for a period of around 4 hours.

The few computation points that are located along the safe access route that fall into the safe

pedestrian access category are located on the berm and foot path.

Figure 6: Depth and Velocity time series at most hazardous point on James Street.

In our opinion residents in the proposed development could safely evacuate by travelling along

James Street and southward along the state highway, either on foot or by car.

Safe access to the site from the South would require cutting across the southern side of the

roundabout. However, as the State Highway to the North would be closed, and there would most

likely be traffic control in place, it is likely that residents and emergency vehicles would be able to

safely access the proposed development site if required.

The State Highway Roundabout does not allow for safe vehicle access, other than making a left turn

out of James Street. The highway north of the roundabout does not allow for safe vehicle access

and in places pedestrian access is unsafe. There is no safe vehicle or pedestrian access to the school,

church and properties along the southern side of James Street. There is also no safe vehicle access to

the Ulric Street industrial area.

In our opinion safe access to and from the site will not be impacted during the 1% AEP Rainfall event

with climate change.

Culvert Blockage

Greater Wellington have raised concerns around the blockage of culverts near the site. Key culverts

near the site are shown in Figure 7. The downstream culvert is a 1.2m diameter circular pipe that

discharges to Taupo Stream behind the garden center. The upstream culvert entering the site from

the north, near the round about is 0.6m diameter circular pipe with 0.3m and 0.45m diameter inlet

pipes. The upstream culvert that crosses James Street from the north at the eastern corner of the

site is 0.45m in diameter. There are two upstream culverts that enter the eastern corner of the site

from the south, these are both 0.9m diameter circular pipes.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1/23/2016 12:00 1/23/2016 14:00 1/23/2016 16:00 1/23/2016 18:00 1/23/2016 20:00

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45V

elo

city

(m

/s)

Dep

th (

m)

Depth Velocity

Figure 7 Upstream and down stream culvert locations

The blockage of culverts has been represented in the model by removing the coupling from the MIKE

FLOOD .couple file. Three culvert blockage scenarios have been run:

• Downstream culvert blocked,

• Upstream culverts blocked,

• Downstream and upstream culverts blocked.

The blockage scenarios have been tested using the 1% AEP rainfall event with climate change. The peak

level differences between the base case and post development for each blockage scenario are shown in

Figure 8 toFigure 10.

The maximum flood depth across the three post development culvert blocked scenarios is mapped in

Figure 11. The peak depth at the various result points shown in Figure 11 are tabulated in Table 1 for

various culvert blockage scenarios and without culvert blockage.

The development has a negligible impact on peak flood levels under the various culvert blockage

scenarios tested.

The peak flood levels adjacent to the development increase by as much as 300mm due to blockage of the

down stream culvert. This is within the 500mm allowed for free board.

Figure 8 Peak water level difference between existing case and post development with down stream culverts blocked for the 1% AEP event with climate change.

Figure 9 Peak water level difference between existing case and post development with upstream and downstream culverts blocked for the 1% AEP event with climate change.

Figure 10 Peak water level difference between existing case and post development with upstream culverts blocked for the 1% AEP event with climate change.

Figure 11: Maximum peak flood depth from the three post development culvert blockage scenarios.

Table 1: Water levels at the result points

Result

Point No Culvert

Blockage

Culvert Blockage Tests

Downstream culvert Upstream culverts Downstream and

upstream culverts

Base

Case

Post

development

Scenario

Base

Case

Post

development

Scenario

Base

Case

Post

development

Scenario

Base

Case

Post

development

Scenario

P1 3.51 3.52 3.81 3.82 3.51 3.52 3.76 3.76

P2 3.51 3.52 3.81 3.82 3.51 3.52 3.76 3.76

P3 3.51 NA 3.81 NA 3.51 NA 3.76 NA

P4 3.84 3.84 3.84 3.85 3.85 3.86 3.85 3.86

P5 4.22 4.22 4.22 4.22 4.23 4.23 4.23 4.23

P6 3.51 3.52 3.81 3.82 3.51 3.52 3.76 3.76

P7 3.51 3.52 3.81 3.82 3.51 3.52 3.76 3.76

P8 3.65 3.65 3.61 3.62 3.65 3.65 3.61 3.61

P9 3.66 3.66 3.64 3.65 3.67 3.67 3.62 3.62

P10 3.67 3.67 3.64 3.65 3.68 3.68 3.63 3.64

6th July 2018


Modification to Proposed Earthworks Design Awa Environmental has developed a hydraulic model to support a consent application for a


The proposed earthworks include for filling part of the site to create raised building platforms, which

will reduce the available flood storage volumes within the site. The loss in flood storage volumes has

been offset by lowering the remainder of the site. This approach has been tested using the hydraulic

model.

After the modelling of the proposed development was completed the developer advised that some

filling had occurred at Lot 1 within the proposed development that was not shown on the proposed

earthworks plans for the site. This filling will reduce the available flood storage volumes. To offset

the impacts of the filling at Lot 1, the earthworks design and subdivision layout has been revised.

The revisions include widening the access ramp to lot 12, reducing the number of lots, and adjusting

the batter around the proposed lots.

The post development bathymetry used in the modelling and the revised post development

bathymetry are shown in Figure 1.

Stage storage curves have been developed based on the modelled bathymetry and the revised

earthworks bathymetry, these are presented in Figure 2.

The storage curve for the revised earthworks bathymetry has greater volume at all elevations than the

storage curve for the modelled bathymetry. As such the modelled water levels within the development,

and at adjacent properties will be lower if the model is run with the revised earth works bathymetry.

Re-running the models with the revised earthworks bathymetry is unnecessary as the modelling has

previously shown the development does not have an impact of site. The revised earthworks design will

not adversely affect the off-site impacts and in all likely hood will result in a reduction in offsite flood

levels.

Figure 1 Modelled bathymetry (left) and bathymetry derived from revised earthworks (right).

Figure 2 Stage storage curves for the modelled bathymetry and revised earthworks bathymetry

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

Elev

atio

n (

m)

Storage Volume (m3)

Stage Storage Curves for Modelled and Revised Earthworks

Modelled Bathymetry Revised Earthworks Bathymetry

Implications of Earthworks Design Change on Other Conclusions

We provide an assessment of the ability to achieve safe access to and from the proposed allotments

during an extreme weather event in our Addendum Report 4. It is concluded in the report that

residents in the proposed subdivision will be able to safely evacuate the proposed lots via James

Street and southward along SH1. This conclusion was based on the previously proposed subdivision

and earthworks design (see Figures 3 and 5 in Addendum 4).

We have considered the conclusion reached in Addendum 4, having regard to the changes to the

earthworks design and the revised storage curves. We consider that the new lot layout and amended

earthworks design will not affect flows across James Street, other than to reduce velocities into the

accessway of proposed Lot 12 (previously Lot 13), which is now wider. The result of the change will

therefore be positive in terms of the ability to safely exit the proposed lots in a high rainfall event.

The other matter covered in Addendum Report 4 was the effect blocked culverts would have on

predicted flood levels. We reached the conclusion that the development has a negligible impact on

peak flood levels under the various culvert blockage scenarios that were tested.

The change to the design of the earthworks and subdivision will not alter our conclusions in this

regard. The proposal to reduce the proposed earthworks may in fact reduce peak flood levels in both

culvert-blocked and culvert-unblocked scenarios.

Our Addendum Report 5 responds to advice prepared by AECOM to NZTA (letter dated 29 March

2018). The advice raises a concern that there will be a point between a moderate and severe rainfall

event where even a small increase in stormwater levels will be the difference between water flowing

from the highway into private properties or not. Our response to that advice was that our modelling

shows that the proposed development is better than hydraulically neutral for all events, up to a 1%

AEP event.

Our conclusion in this regard does not change as a result of the proposal to change the design of the

proposed earthworks. The change to the design of the earthworks is likely to result in a reduction in

off-site flood levels.

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