FINAL REPORT FOR 100% DESIGN HIGHWAY 14 EMERGENCY WORKS ... · FINAL REPORT FOR 100% DESIGN HIGHWAY...

FINAL REPORT FOR 100% DESIGN

HIGHWAY 14 EMERGENCY WORKS, PORT RENFREW BC DRAINAGE REPORT

McElhanney 200 – 858 Beatty Street Vancouver, BC V6B 1C1

Contact: Michael Thiessen, P.Eng. Drainage Lead 604-674-7471 | [email protected]

REVISION RECORD

REVISION DESCRIPTION DATE (YYYY-MMM-DD) ISSUED BY 1 Issued for 100% Design 2020-APR-10 McElhanney

CONTENTSCONTENTS ...................................................................................................................... III TABLES ............................................................................................................................................... III FIGURES .............................................................................................................................................. IV EXECUTIVE SUMMARY ........................................................................................................................ I 1. INTRODUCTION ............................................................................................................................ 2

1.1 Background Information .............................................................................................................................. 2 1.2 Existing Conditions ...................................................................................................................................... 7

2. DESIGN CRITERIA AND ANALYSIS METHODOLOGY .............................................................. 10 2.1 Design Criteria .......................................................................................................................................... 10 2.2 Climate Change Analysis .......................................................................................................................... 11 2.3 Hydrologic / Hydraulic Modelling ............................................................................................................... 15

3. MODEL RESULTS ....................................................................................................................... 18 3.1 Existing Conditions .................................................................................................................................... 18 3.2 Future Conditions ...................................................................................................................................... 19

4. CONCLUSION ............................................................................................................................. 22

TABLES Table 1: IDF Station Details ................................................................................................................................................ 4 Table 2: Existing Culvert Summary ..................................................................................................................................... 9 Table 3: MoTI Design Guidelines ...................................................................................................................................... 10 Table 4: Runoff Coefficient Values Expected for Coastal Type Basins. Forested Cover with Moderate

Slope Best Represents The Catchments in This Study ...................................................................................... 11 Table 5: Mean Percent Change in Precipitation for the Projection Percentiles ................................................................ 14 Table 6: Percent Change of Precipitation Comparison ..................................................................................................... 14 Table 7: Catchment Parameters and Conduit Roughness Coefficients ............................................................................ 15 Table 8: Assumed Soil Parameters for Green-Ampt Infiltration Model ............................................................................. 16 Table 9: Design Storm Characteristics ............................................................................................................................. 16 Table 10: Culvert Capacity by Diameter and Type assuming HW/D = 1 .......................................................................... 18 Table 11: Existing Culvert Conditions ............................................................................................................................... 18 Table 12: Future Conditions Modelled Results for the 100-Year, 24-hour Storm with Climate Change ........................... 19

FIGURES Figure 1: Location Map ....................................................................................................................................................... 3 Figure 2: IDF Station Locations .......................................................................................................................................... 5 Figure 3: Existing Drainage Infrastructure .......................................................................................................................... 8 Figure 4: Canadian Climate Normals for Port Renfrew Showing Peak Precipitation Occurs in Winter ............................ 11 Figure 5: 2018 Hydrometric Daily Discharge for San Juan River Showing Peak Flows Occur in Winter ......................... 12 Figure 6: Summary of climate change for the Capital Region in the 2080s showing a 9% increase for

winter precipitation .............................................................................................................................................. 13 Figure 7: Projected Change in Winter Precipitation for the Capital Region with 10, 25, 50, 75, and 90

Percentiles ........................................................................................................................................................... 13 Figure 8: Percent Change in Precipitation for the 2080’s Projection Percentiles .............................................................. 14 Figure 9: Comparison of Plan2Adapt and IDF_CC Mean Percent Change Projection Percentiles .................................. 15 Figure 10: Proposed Upgrades ......................................................................................................................................... 20

REPORT SIGNATURE

This report is prepared for the sole use of the BC Ministry of Transportation and Infrastructure. No representation of any kind is made by McElhanney or its employees to any party not affiliated with the BC Ministry of Transportation and Infrastructure. The information provided in this report represents McElhanney’s best professional judgment in light of the knowledge available to McElhanney during the time of preparation.

Please direct any questions or clarification regarding the contents of this report to the following team members who prepared this report.

Prepared by:

Michael Thiessen, P.Eng. Drainage Lead [email protected]

Reviewed by:

Nav Sandhu, P.Eng. Senior Drainage Engineer [email protected]

Highway 14 Upgrades, Parkinson Hill Drainage Report: Draft Final Report for 100% Design | April 10, 2020 Prepared for Ministry of Transportation and Infrastructure

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EXECUTIVE SUMMARY

McElhanney has been retained by the Ministry of Transportation and Infrastructure (MoTI) to complete the detailed design for a 1-kilometer segment of Highway 14 that has experienced recent roadway failure in the Capital Regional District, 6-kilometers east of Port Renfrew.

Runoff from very steep, forested slopes flow from the western mountainsides, though culvert crossings under Highway 14, and continue east down steep slopes to Falls Creek. Ditches are only located on the western side of Highway 14 which are overgrown with vegetation and constricted in areas where bedrock cliffs are adjacent to the highway. Culverts along the project extents range from being in good condition to being completely infilled with debris. The scope of the project is the construction of a replacement retaining wall where an existing wall failed, as well three additional walls to address embankment stability. An existing drainage assessment and drainage mitigation design has been completed in conjunction with the retaining wall.

A computational modelling exercise was undertaken to determine peak runoff flows generated from the catchment areas within the project area. Model inputs included increased precipitation due to climate change. Modeling was carried out in alignment with chapter 1000 of the MoTI Supplement to TAC Geometric Design Guide (Apr 2019).

Flows were compared to culvert capacity assuming inlet conditions to create a summary of the drainage infrastructure in this area. Most culverts and ditch sections are undersized and design efforts have been made to improve the capacity of the existing drainage along the entire length of the project area.


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1. INTRODUCTION

McElhanney Ltd. (McElhanney) was retained by the BC Ministry of Transportation and Infrastructure (MoTI) in response to a structural wall and roadway failure on a 1-kilometer portion of Highway 14 approximately 6-kilometers east of Port Renfrew in the Capital Regional District.

The current highway has been reduced to single lane traffic as a result of a retaining wall failure on the eastern side of the highway. Three additional locations will require the construction of retaining walls to address embankment stability. There are steep slopes to the east of the highway that meet with Falls Creek at the bottom of a valley. Efforts to re-establish both lanes and the improve roadway and drainage conditions are being undertaken.

There are nine (9) identified culvert crossing under Highway 14 within the project limits. The catchment areas of the crossing vary from 0.12 to 13.8 hectares. All catchments are located on the slopes to the west of the highway which currently drain to an undersized drainage network of ditches and culverts. A map of the site showing the catchment areas are presented in Figure 1.

The purpose of this report is to:

• Highlight the existing drainage conditions and infrastructure in the area • Present an assessment of the capacity of the existing culverts including climate change • Present proposed culvert improvements which include climate change

1.1 BACKGROUND INFORMATION Information relevant to the drainage design was gathered from several sources:

• MoTI Supplement to TAC Geometric Design Guide (2019); • iMapBC hydrologic data; • Government of Canada historical rainfall and flow data; • BC Ministry of Environment soil maps; • LiDAR data and supporting topographic maps; • Pacific Climate Impacts Consortium (PCIC) Plan2Adapt climate change tool; • Western University IDF_CC climate change tool; and, • Site visits and anecdotal information.

Hwy 14 Parkinson Emergency Works0 125 250 375 50062.5

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FOR DISCUSSION AND REGULATORYPURPOSES ONLY

INFORMATION SHOWN ON THIS DRAWING REGARDING EXISTINGUTILITIES IS COMPILED FROM SOME RECORD DRAWINGS AND SOMESURVEYS, AND MAY NOT BE COMPLETE. CONTRACTOR SHALL EXPOSEAND CONFIRM THE LOCATIONS AND ELEVATIONS OF ALL EXISTINGUTILITIES AND ADVISE THE ENGINEER OF ANY POTENTIAL CONFLICT.

THIS DRAWING AND DESIGN SHALL NOT BE USED, REUSED ORREPRODUCED WITHOUT THE CONSENT OF McELHANNEY CONSULTINGSERVICES LTD. McELHANNEY WILL NOT BE HELD RESPONSIBLE FORTHE IMPROPER OR UNAUTHORIZED USE OF THIS DRAWING AND DESIGN.

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1.1.1. Historical Rainfall Data The project area is situated in the Western Vancouver Island hydrologic zone (according to the “Hydrologic Zones” layer provided by iMapBC), in a region that receives an average of 3,500 mm of precipitation every year.

Historical Rainfall Data Intensity-Duration-Frequency (IDF) data from the Government of Canada was used to determine the existing condition design storms for this project. There was one IDF station situated near the project area which is located northeast of the project (Port Renfrew). Details on the IDF station are provided in Table 1.

Table 1: IDF Station Details

Name ID

Approx. Distance from Project

(km) Years

(Range) Years

(Number)

100-Year, 24-hour Rainfall

(mm)

Port Renfrew 1016335 4.5 1973-1982 10 277.9

As previously mentioned, the project area is located in a region with significant annual rainfall. The Port Renfrew station only has 10 years of rainfall data. This is considered a minimal amount of data for the creation of accurate design storms. In order to verify the accuracy of the Port Renfrew IDF data, a comparison of Environment Canada (EC) climate normals and other IDF stations along the west coast of Vancouver Island was completed. Based on the hydrological conditions along the west coast of the island, rainfall amounts generally increase from south to north and the gradient is especially steep between Victoria and Port Renfrew.

Annual precipitation from EC climate normals were collected from the Tofino A, Amphitrite Point (near Ucluelet), Bamfield East (near Bamfield), Port Renfrew, and Victoria Marine (near Victoria) stations. From the annual precipitation between these stations, the rainfall gradient was confirmed, however, Port Renfrew received nearly the most precipitation. This is likely due to orographic precipitation at the Port Renfrew weather station as it is located in a valley, versus Tofino, Ucluelet, and Bamfield are further way from mountains. IDF data from these stations were compared and found to be consistent with the climate normal findings. Jordan River, located south of the project site and north of Victoria, has no climate normal data, but two IDF stations, Jordan River Diversion and Jordan River Generating. The Jordan River Diversion station is also located in a valley southeast of the project and was found to have relatively large precipitation depths as seen in the Port Renfrew IDF data.

Based on these findings, the Port Renfrew IDF station was selected for this project. Station locations are presented in Figure 2.

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1.1.2. Soil Maps South Vancouver Island soil maps from the Soil Survey Report No. 44 (BC Ministry of Environment, 1986) characterises the soil groups throughout the drainage catchment areas as gravelly loam soils less than 1-meter thick overlying bedrock. Although described as well draining near the surface, these soils are poorly developed due to the steep slopes resulting in unstable terrain conditions for soils. Upstream soils in the catchments are expected to have higher initial abstraction rates, while downstream soils will have much lower rates as a result of infiltrated runoff flowing along the subsurface bedrock layer and discharging into downstream soils.

Soil information was used for determining infiltration parameters within the hydraulic model (see Section 2.3.1).

1.1.3. LiDAR and Topographic Maps LiDAR data was received on July 7th, 2019 and covers the full length of highway through the project site. A topographic survey was also completed for the culverts located along Highway 14 and data was received on December 5th, 2019. Information gathered at the time of this survey was used to discern flow patterns and delineate minor catchments in the project area. Relevant information from this survey has been incorporated into Sections 1.2 and 3.1 of this report.

Contour maps from the Government of Canada’s Atlas of Canada – Toporama website for the project area and surrounding lands were used to determine approximate drainage catchment boundaries discharging through the project area. Further details on the drainage catchment boundaries are provided in Section 1.2.1.

1.1.4. Site Visit Information A site visit to assess existing drainage conditions was conducted on November 7th, 2019. The full project length was visited, and stops were made at known culvert locations so that condition assessments and measurements could be taken. Observations from the site visit were as follows:

• Additional culvert crossings were identified from the preliminary drawing package by McElhanney, dated June 30th, 2019

• Nine culverts were identified, one had an infilled inlet, one completely infilled, and one had its outlet buried in rip rap;

• Several culvert inlets are infilled with debris, damaged, or overgrown with vegetation; • Ditches are located on the west-side of the road and east-side of the road drops off at steep slopes to Falls Creek

and; • Ditches are overgrown with vegetation and constrained in locations by bedrock cliffs.


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1.2 EXISTING CONDITIONS

1.2.1. General Drainage Pattern Catchments for the project area are all located on the west side of Highway 14. The total drainage area is approximately 33.1 hectares (ha) in size and is primarily made up of steep, dense forest with bedrock cliffs. Runoff interacting with the highway flows from west to east over the steep terrain with slopes between 28% and 78%. Catchment areas and locations of surveyed culverts are presented in Figure 3. A minimum percent impervious of 5% was applied to all catchments with a maximum of 7% to accounted for the highway.

Six (6) catchments were identified in the project area with a total of seven (7) culverts. Flows from catchments are conveyed though culverts which cross under Highway 14. Once on the east side of the highway, steep slopes and natural channelization convey flows to Falls Creek. Of the 7 culverts identified, 1 of these culverts are plugged and not functioning. Currently, the 6 highway catchments drain to a corresponding culvert crossing with a single culvert.

Catchment C1 is the largest at 14.8ha. It consists of a very steep bedrock cliff and a moderately steep bank. There is a narrow ditch along the west side of the highway allowing runoff to drain North to Culvert E1. Catchment C2 is 8.8ha with moderately steep slopes (32%) that has channelization at the culvert crossing. A narrow, overgrown ditch separates C2 from the highway. Catchment C3 and C4 are 4.8ha and 2.7ha respectively. They are relatively narrow catchments that have slopes of approximately 40%. Channelization is not evident in surveyed surfaces for C4, but C3 has a major channel down the center of the catchment. Flows from C3 and C4 enter culvert E3 and E4 respectively. The ditch along this portion of Highway 14 is overgrown with vegetation and is constricted by bedrock along C3. The ditch widens and remains a consistent size from C3 to the culvert crossing for C5. Catchment C5 is the smallest catchment at 0.67ha. It consists of steep slopes of 48% which transition into the highway ditch. Catchment C6 is 1.4ha and has a moderate slope of 28%. Most of the catchment that runs adjacent to the highway is bedrock cliffs that constricts the ditch. The cliff face ends at the northern most portion of the C6 before the culvert crossing.

1.2.2. Drainage Infrastructure As described in Section 1.1.4, ditches are overgrown with vegetation and become constricted next to bedrock cliffs that are adjacent to the highway. This results in inconsistent cross-sections of varies sizes between culvert crossings.

The sizes and locations of surveyed culverts within the project area are presented in Figure 3. Measured diameters in the field ranged from 600mm to 700mm, with one 1400mm x 800mm arch culvert. The size, material type, inlet and outlet elevations, and condition of culverts were assessed at the time of the field visit and follow up survey, as discussed in Section 1.1.3 and Section 1.1.4. Culvert conditions that cross the highway in the project area are summarized in Table 2.

Several of the deficiencies summarized in Table 2 are addressed by the proposed maintenance and replacements discussed in Section 3.2.1 of this report.


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Table 2: Existing Culvert Summary

Culvert ID

Diameter and Material Type

Inlet Invert Elevation (m)

Outlet Inlet Elevation (m) Condition

- 400mm CSP** 209.7* 208.9* Inlet not located, outlet of culvert rusted and in poor conditions; Assumed not functional

E1 700mm CSP 201.1 200.5 Good inlet condition, poor outlet condition

E2 700mm CSP 187.2 186.2 Inlet inaccessible, good outlet condition

E3 600mm CSP 164.0 163.4 Good inlet condition, outlet buried under riprap armouring

E4 600mm CSP 161.4 160.9 Fair inlet and outlet conditions

E5 600mm CSP 159.7 158.2 Fair inlet and outlet conditions

E6 1400mm x 800mm Arch CSP 141.7 140.7 Fair Inlet and outlet conditions

*Survey was not available; Invert elevations based on surveyed topographical surface **Actual diameter unknown; approximately 400mm in diameter


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2. DESIGN CRITERIA AND ANALYSIS METHODOLOGY

2.1 DESIGN CRITERIA

2.1.1. Guidelines The MoTI Supplement to TAC Geometric Design Guide (2019) was the primary source for this project’s design criteria. Key guidelines are presented in Table 3 below.

Table 3: MoTI Design Guidelines

MoT Section Criteria Value

1010 – General Design Guidelines

Design Return Period (Collector Road) • Highway Ditches • Culverts with < 3m Span

25 years

100 years

1030 – Open Channel Design

Longitudinal Slope • Minimum • Recommended

Channel Depth • Minimum (unless exceeded by 25-

year flow) Minimum Freeboard Minimum Channel Bottom Width Channel Side Slopes (H:V)

• Minimum • Recommended Minimum • Maximum

0.3% 0.5%

0.30m from SGSB layer to ditch bottom

0.3m 1m

1.5:1 2:1 4:1

1040 – Culvert Design

Minimum Culvert Size • Under highway or main road • Driveway culvert

Longitudinal Slope • Minimum

• Maximum Roughness Coefficients

• CSP • Concrete

Hydraulics • Inlet Control HW/D • Outlet Control Headloss

600mm 400mm

0.5% (except for fish bearing culverts, which

can be less) 20% (CSP) / 10% (Concrete)

0.021 – 0.027

0.012

Shall not exceed 1.0 Shall not exceed 0.3m


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Section 1020 from the MoTI Supplement to TAC Geometric Design Guide highlights maximum runoff coefficients to be used for coastal type basins and these values are presented in Table 4.

Table 4: Runoff Coefficient Values Expected for Coastal Type Basins. Forested Cover with Moderate Slope Best Represents The Catchments in This Study

Surface Cover Physiography Impermeable Forested Rural

Mountain (>30%) 1.00 0.90 -

Steep Slope (20–30%) 0.95 0.80 -

Moderate Slope (10-20%) 0.90 0.65 0.75

Rolling Terrain (5-10%) 0.85 0.50 0.65

Flat (<5%) 0.80 0.40 0.55

Return Period 10-25 Years +0.05 +0.02 +0.05

Return Period >25 Years +0.10 +0.05 +0.10

Catchment area slopes throughout the project area generally fall under the steep slope to mountain categories shown above. As such, modelled runoff coefficient results were expected to be around 0.80 for the 100-year storm. Modelled runoff coefficient results are discussed in Section 2.3.3.

2.2 CLIMATE CHANGE ANALYSIS To account for future climate change, Technical Circular T-04/19 for adaptation to climate change has been followed and adopted for this project by incorporating adjusted rainfall volumes into the drainage analysis. The adjusted rainfall volumes were used in the drainage model precipitation input. Climate change projections vary depending on the season; therefore, it is critical to understand the climatological factors that influence peak flows.

2.2.1. What Drives Peak Flows? Climate data for the Port Renfrew was collected from Environment Canada to determine the annual rainfall distribution. Figure 4 shows that precipitation is highest during the winter months.

Figure 4: Canadian Climate Normals for Port Renfrew Showing Peak Precipitation Occurs in Winter


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Hydrometric data was also collected from Environment Canada for the San Juan River near Port Renfrew (Station 08HA010) to determine when peak flows occur throughout the year. The San Juan River station provides real-time and historical data with records ranged from the years 1959 to 2018. Figure 5 contains flow data from 2018 for the San Juan River which shows that peak runoff rates generally occur during winter.

Figure 5: 2018 Hydrometric Daily Discharge for San Juan River Showing Peak Flows Occur in Winter

Based on the rainfall distribution and discharge hydrograph shown in Figure 5 and Figure 6, respectively, peak flows in the project area appear to be driven by precipitation during the winter months. The below climate change analysis will, therefore, focus on precipitation changes specific to the winter season.

2.2.2. Precipitation Changes Two projection tools, PCIC Plan2Adapt and Western University’s IDF_CC Tool were used to estimate climate change impacts on precipitation in the project area. In this section, the methodology used for these projection tools are discussed and the results are compared.

Projection 1 – PCIC Plan2Adapt Tool The Pacific Climate Impact Consortium’s (PCIC) Plan2Adapt tool was used to generate the percent change in precipitation based on climate change models. Winter climate change projections for the Capital Region were obtained for the horizon year of 2080 and are summarized in Figure 6. PCIC reports that the selected horizon yar is representative for the years 2070-2099, which includes the end of the anticipated 75-year design life of the proposed infrastructure (approx. 2095). The median change in precipitation shows an increase of 9% with 25th and 75th percentiles of approximately 3.3% and 18%, respectively. The range of projected changes in precipitation for the 2080 horizon year is shown in Figure 7.


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Figure 6: Summary of climate change for the Capital Region in the 2080s showing a 9% increase for winter precipitation

Figure 7: Projected Change in Winter Precipitation for the Capital Region with 10, 25, 50, 75, and 90 Percentiles

Projection 2 – Western University IDF_CC Tool Western University’s IDF_CC Tool 4.0 (IDF_CC) was used to develop future IDF curves accounting for climate change. Climate change projections were generated for the time period between 2006 and 2080, which represents the 2080 horizon year. IDF_CC produces three Representative Concentration Pathways (RCP) for different radiative forcing peaks by the year 2100. The median RCP 4.5 scenario, representing a radiative forcing peak of 4.5 W/m2, was chosen to compare with the median PCIC results. Future total precipitation depths for different storm durations were generated by IDF_CC for the 100-year return period event. Historical and future precipitation depths were then used to calculate the percent change in precipitation. The median and quarterly percent change percentiles for different storm durations can be seen in Figure 8.


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Figure 8: Percent Change in Precipitation for the 2080’s Projection Percentiles

To compare results with the PCIC tool, the percent change in precipitation from all durations were averaged and are presented in Table 5.

Table 5: Mean Percent Change in Precipitation for the Projection Percentiles

25th Percentile Median 75th Percentile

-2.4% 3.8% 10.8%

Comparison of Projection Tool Results A comparison of projected precipitation changes from PCIC and IDF_CC can be seen in Table 6.

Table 6: Percent Change of Precipitation Comparison

Percentile Percent Change of Precipitation

PCIC Plan2Adapt IDF_CC Tool (Averaged % Change)

25th 3.3% -2.4%

50th 9.0% 3.8%

75th 18.0% 10.8%

As seen in Table 6, the projections generated by PCIC predict larger increases in precipitation than IDF_CC. The 25th and median percentiles for PCIC are mostly consistent with the median and 75th percentiles from IDF_CC, respectively. These results are shown graphically in Figure 9.

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Figure 9: Comparison of Plan2Adapt and IDF_CC Mean Percent Change Projection Percentiles

Based on these results shown in Table 7 and Figure 9, the median projection by PCIC was selected to account for climate change impacts on precipitation. The 9% increase represents the most probable increase in precipitation from PCIC projections and provides a more conservative increase when compared to IDF_CC projections. The 9% increase was applied to the historical IDF total precipitation depths of all storm durations for the design storm.

2.3 HYDROLOGIC / HYDRAULIC MODELLING MoTI design standards allows for the designer to use several methods to estimate peak discharge rates for sizing of drainage infrastructure. For this project, the method of hydrologic / hydraulic computer modelling has been utilized to estimate the runoff rates from the study area catchments in order to assess the capacity of the existing culvert crossings.

2.3.1. Model Build Single event computational modeling was undertaken to evaluate the existing and proposed drainage systems under peak design discharge conditions. The hydrologic / hydraulic model was developed in PCSWMM version 7.2. PCSWMM is an adaptation and enhancement of the well-known and widely used United States Environmental Protection Agency’s (USEPA) Stormwater Management Model (SWMM) version 5.1. PCSWMM was developed by Computational Hydraulics International (CHI) as a combination hydrology-hydraulic model.

Hydrologic / hydraulic modelling requires several inputs to simulate the rainfall-to-runoff and routing processes. The global input parameters required for the hydrologic / hydraulic simulation of the catchment areas and conduits are presented in Table 7.

Table 7: Catchment Parameters and Conduit Roughness Coefficients

Parameter Value

Overland Impervious Manning’s n 0.012

Overland Pervious Manning’s n 0.35

Catchment Slope Based on Topographic Maps

Impervious Depression Storage (mm) 1.6

Pervious Depression Storage (mm) 12

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Infiltration losses for this analysis were calculated using the Green-Ampt infiltration model. Infiltration parameters were assumed universal for the catchment areas. A conservative infiltration rate was used in the model to account for the thin layer of soil and presence of bedrock close to the surface as per the soil maps described in Section 1.1.2. Soil parameters are presented in Table 8.

Table 8: Assumed Soil Parameters for Green-Ampt Infiltration Model

Soil Characteristics Saturated Hydraulic Conductivity

(mm/hr) Suction Head

(mm) Initial Deficit

• Stony soils on steep slopes • Less than 1-meter thick soils • Bedrock located near the

surface

1.2 239 0.091

Other modelling assumptions are as follows:

• No further development will occur in the modelled catchments under future conditions (no increase in development-based drainage flows); and

• Overland flooding / flow paths were not modelled.

The scenario assessed in the current modeling effort is the existing drainage system’s (with proposed modifications) ability to convey the 100-year flow (with climate change).

2.3.2. Design Storms The Port Renfrew Intensity-Duration-Frequency (IDF) curve was selected for this analysis as described in Section 1.1.1.

Various duration synthetic design storms were created using the IDF curve with the 9% increase as described in Section 2.2.2. The Atmospheric Environmental Service (AES) distribution for coastal British Columbia was used to generate the synthetic design storms for the short duration rainfall event (1 hour) and the Soil Conservation Service (SCS) Type IA distribution was used to generate the synthetic design storms for longer duration events (6, 12 and 24 hours). The runoff at culvert crossing locations for the 1-, 6-, 12-, and 24-hour rainfall events were evaluated to determine which duration created peak flow conditions.

Based on the analysis completed, the 6-, 12-, and 24-hour duration design storms generated the largest peak flows at the outlet locations. Thus, the larger of the 6-, 12-, and 24-hour duration storm events was used for analysis purposes for each catchment. Details of the storm are presented in Table 9.

Table 9: Design Storm Characteristics

Event Total Rainfall Adjusted for Climate

Change (mm)

Peak Rainfall Intensity Adjusted for Climate Change

(mm/hr)

SCS Type IA 100-Year, 6-Hour 152.4 78.4




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2121-00389-01

2.3.3. Model Validation Calculated runoff coefficient results were also assessed in the existing conditions model. Values ranged from 0.76 to 0.79. This is further validation that the model is generating reasonable results based when compared to values expected in Table 4.


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3. MODEL RESULTS

3.1 EXISTING CONDITIONS The focus of the current project is the construction of a replacement retaining wall where an existing wall failed and three additional walls to address embankment stability. This project will also improve the capacity of the existing drainage along a 1-kilometer segment of Highway 14 approximately 6-kilometers east of Port Renfrew, in the Capital Regional District.

A preliminary assessment of the existing capacity of the circular and arch CSP culverts was assessed using inlet-control nomographs provided by the Handbook of Steel Drainage & Highway Construction Products (2007). The capacity of CSP culverts of various standard diameters is provided in Table 10.

Table 10: Culvert Capacity by Diameter and Type assuming HW/D = 1

Culvert Type Inlet-Controlled Capacity (Lps)

400mm CSP 150

500mm CSP 200

600mm CSP 310

700mm CSP 460

1400mm x 800mm Arch CSP 1000

The existing culvert crossing capacities results are summarized in Table 11. There was a culvert found at Station 65+880 that was plugged and not passing any flow. Therefore, it will not be included in Table 11 as it is not contributing to the drainage system.

Table 11: Existing Culvert Conditions

Catchment ID Culvert ID

Diameter and Material Type

Catchment Area (ha)

Culvert Capacity (QCap) (Lps)

(HW/D=1)

C1 E1 700mm CSP 14.77 460

C2 E2 700mm CSP 8.76 460

C3 E3 600mm CSP 4.80 310

C4 E4 600mm CSP 2.75 310

C5 E5 600mm CSP 0.67 310

C6 E6 1400mm x 800mm Arch CSP 1.40 1000


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2121-00389-01

3.2 FUTURE CONDITIONS

3.2.1. Culverts Although drainage improvements are not a focus of the current project, some culvert replacements are recommended. A summary of the proposed drainage improvements, the 100-yr flood flow with climate change, and the ability of the culverts in each catchment to handle this flow are summarized in Table 12.

Table 12: Future Conditions Modelled Results for the 100-Year, 24-hour Storm with Climate Change

Culvert ID

Station (see design drawing

package) Proposed Design /

Improvements 100-Yr Flow (Q100) (Lps)

Q100/QCap (Inlet

controlled) Flood Routing

E1 66+073 Replace with 1200mm HDPE 1808 0.82

Bypass inlet and continue north in north

ditch



ditch



ditch



ditch



ditch



ditch

The Q100/QCap ratios indicate that the proposed culvert upgrades should be able to convey the runoff of 100-year, 24-hr storm with climate change. This analysis is conservative and does not include any storage provided by the ditch network in the region.

Protecting the area immediately around the inlet and outlet of the culvert is necessary to prevent channel degradation and erosion. Therefore, riprap will be put in place outside each culvert’s inlet and outlet.

Due to the large difference in elevation between the inlet and outlet of culvert E1, a manhole will be installed to allow for a milder slope. This will decrease the velocity of water travelling through the culvert minimizing downstream erosion and prevent disturbances to the road foundation, preserving the road stability.

Upgrades to the existing system are shown in Figure 10.


MetersFigure 10 - Proposed Drainage Infrastructure


April 2020




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2121-00389-01

3.2.2. Ditches

The current system is insufficient to handle future storm events and upgrades are recommended. Ditch regrading is recommended to be completed along the extents of the highway to create a formalized flow path for runoff and minimize highway overtopping during large events. As noted previously, any regrading presents a grading challenge to property outside of the highway right-of-way. Where feasible, steep ditches will be armoured to minimize the risk of erosion.

Ditch regrading will be completed to the extent possible given the property and budget constraints of the project.


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2121-00389-01

4. CONCLUSION

McElhanney has been retained by the Ministry of Transportation and Infrastructure to complete the detailed design for the construction of a replacement retaining wall and three additional walls to address embankment stability along a 1-kilometer segment of Highway 14 approximately 6-kilometers east of Port Renfrew, in the Capital Regional District. The project will improve the capacity of the existing drainage along the entire length of the project area. The scope of this report is to assess the highway drainage system consisting of ditches and culverts.

Existing highway drainage flow patterns consists of runoff from the forested, mountain slopes draining east towards the highway where ditches along the western portion of the highway collect and convey runoff to culvert crossings. Once through crossings, runoff continues eastward down steep slopes to Falls Creek. The cross-section geometry of the ditches and culverts vary across the project area.

A computational model was undertaken to determine peak runoff flows generated from catchment areas including the effects of climate change. The results of this model were validated using Section 1020 from the MoTI Supplement to TAC Geometric Design Guide.

The existing functioning culverts were assessed for their capacity to convey the future (including climate change) flows. It was found that four out of six culverts are undersized and do not have capacity to convey the 100-year flow. Additionally, all 6 existing culverts have flood routes that overtop and cross the highway presenting a safety hazard for road users. These culverts should be seen as high priority.

While the existing system is undersized, this project will not exacerbate any existing drainage problems. The culvert replacements will improve existing capacity.

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