ATTACHMENT 8: Storm Water Management Plan
Technical report by McElhanney Ltd. outlining the strategy for managing stormwater at the proposed 27-unit residential development.
STORM WATER MANAGEMENT PLAN
for
Camden Avenue and Helmcken Road Residential Development, View Royal, BC (#14 & 14A Helmcken Road and #6 Camden Avenue, View Royal)
Prepared for: HFT Ventures Ltd. 4038 Prospect Lake Road Saanich, BC
Prepared by: McElhanney Ltd. #500 – 3960 Quadra Street Victoria, BC V8X 4A3
Date: September 27, 2023
Table of Contents
- Introduction......................................................................................................................................................3
- Existing Conditions ...........................................................................................................................................4
- General Storm Water Management Approach ...................................................................................................5
- Storm Water Management Strategy ..................................................................................................................5 4.1. Natural, Revegetated and Landscaped Areas............................................................................................5 4.2. Building and Hardscaped (Non-vehicular) Areas ........................................................................................5 4.3. Hardscaped (Vehicular Accessible) Areas..................................................................................................6
- Stormwater Quantity Control.............................................................................................................................6 5.1. Pre-development Runoff...........................................................................................................................6 5.2. Post Development Runoff.........................................................................................................................7 5.3. Stormwater Detention ..............................................................................................................................8 5.4. Climate Change........................................................................................................................................8
- Stormwater Quality (Treatment) ........................................................................................................................9
- Erosion and Sediment Control During Construction ...........................................................................................9 7.1. Silt Fences-Perimeter Ditching ............................................................................................................... 10 7.2. Sediment Control Ponds ........................................................................................................................ 10 7.3. Slope and Surface Protection................................................................................................................. 11
- Operations and Maintenance......................................................................................................................... 11 8.1. Detention / Infiltration Tanks ................................................................................................................. 11 8.2. Drainage Mains and Manholes .............................................................................................................. 11 8.3. Catchbasins .......................................................................................................................................... 12 8.4. Oil Interceptors...................................................................................................................................... 12
1. Introduction
McElhanney Ltd. (McElhanney) has been retained by HFT Ventures Ltd. (Client) to provide a Storm Water Management Plan for the properties located at 14 and 14A Helmcken Road and 6 Camden Avenue in the Town of View Royal.
The purpose of this report is to support an application for Building Permit on the subject property.
The development combines the three existing lots into one 0.16-hectare property. The development will include:
- 27 residential units in one building
- 27 underground and 2 surface parking stalls
The site is bounded on the east and north with streets followed by multi-family dwellings. The areas to the south and west are single-family dwellings.
This Storm Water Management Plan will:
- Review the existing surface drainage conditions.
- Consider the approach to be used for storm water management.
- Describe a proposed storm water management strategy.
- Provide guidelines for the long-term operation and maintenance of the stormwater management systems.
2. Existing Conditions
The existing site includes single family dwellings, see Figure 1.
The terrain generally slopes to the southwest, at approximately 4%, away from the intersection of Helmcken Road and Camden Avenue.
Surface drainage from adjacent properties do not appear to discharge onto the property. Road drainage along Helmcken Road generally flows to the southwest along an asphalt and water control curbs to a catch basin for connection to the municipal system. Drainage along Camden Avenue travels south along a roadside swale with no evident connection to a municipal drain system.
3. General Storm Water Management Approach
The proposed developments’ Storm Water Management Plan will be a collaboration between the design professionals (Civil Engineer, Arborist, Landscape Architect, Geotechnical Engineer) and the Town of View Royals staff (Engineering, Parks, Planning). The approach taken will address and promote the following:
- Detain post-development runoff to pre-development runoff rates
- Provide storm water systems that promote ground water recharge
- Minimize impacts to storm water quality
- Minimize sedimentation and erosion, stabilize, and re-vegetate impacted areas
- Consider impacts of extreme rainfall (1 in 200 year) events
- Consider impacts of climate change
4. Storm Water Management Strategy
The proposed storm water management facilities will separate rainfall on the development into three general categories:
- Natural, Revegetated and Landscaped areas
- Building and Hardscaped (non-vehicular) areas
- Hardscaped (vehicular accessible) areas
4.1. Natural, Revegetated and Landscaped Areas
Rainfall runoff in these areas will be encouraged to follow the natural terrain of the area and infiltrate into the ground. No additional runoff from pre-development conditions anticipated for these areas.
4.2. Building and Hardscaped (Non-vehicular) Areas
Rainfall runoff in these areas will be directed to area drains and landscaped area. The runoff will be directed to underground detention/infiltration tanks, prior to discharge to the municipal drainage system on Camden Avenue, with an orifice-controlled outlet to limit flows to the 2 and 10-year existing (pre-development) rates. The intent is to utilize the detention/infiltration tanks to promote infiltration (to the extent possible given the soil conditions) and provide detention. Designed overflow facilities will be provided for events greater than 10-year return. Major storm events (200-year event) will be directed overland.
4.3. Hardscaped (Vehicular Accessible) Areas
Rainfall runoff in these areas will be directed to area drains and routed through oil / water separators or directed to landscaped areas. The runoff will ultimately be directed to the underground detention / infiltration tanks for detention, prior to discharge to the municipal drainage system on Camden Avenue with an orifice-controlled outlet to limit flows to the 2 and 10-year existing (pre-development) rates. The intent is to utilize detention / infiltration tanks to promote infiltration (to the extent possible given the soil conditions) and provide detention. Designed overflow facilities will be provided for events greater than 10-year return.
Major storm events (200-year event) will be directed overland. Given the site topography, overland flow will discharge at the southwest corner of the site. This matches the current overland flow route.
Concrete curb and gutter will be provided along Helmcken Road and Camden Avenue complete with catch basins to intercept and direct storm water from the municipal right-of-way to the municipal storm system.
5. Stormwater Quantity Control
5.1. Pre-development Runoff
Calculation of the pre-development runoff is contingent on the site’s runoff coefficient value. As shown on Figure 1, the pre-development condition of the site was single-family residential with a mix of buildings, gravel / asphalt driving surfaces and landscaped areas.
Typical C Coefficients*
| Description of Area | Runoff Coefficients Range | Runoff Coefficients Selected | Contributing Area (m²) | C x A |
|---|---|---|---|---|
| *single family residential area | 0.30 – 0.50 | 0.40 | 1612 | 645 |
| Contributing Area (ha) = | (1612 m²) 0.16 ha | |||
| Pre-development C = | 0.40 |
Based on the Rational Method, the resultant pre-development flows from the site are calculated based on the following:
Q=ciA/360
Where
- “i” 2-year 15 min = 13.2 mm/hr
- “i” 10-year 15 min = 20.4 mm/hr
- “c” = 0.40
- “A” = 0.16 ha
Pre-development Q₂ = 0.0024 m³/s (2.4 L/s) Pre-development Q₁₀ = 0.0036 m³/s (3.6 L/s)
5.2. Post Development Runoff
Following development, substantial portions of the site will have been excavated and regraded to provide level areas for the buildings (footprints), driveway, sidewalks, and landscape areas. Similar to the pre-development calculations above, the following table calculates the post-developed site’s runoff coefficient.
Typical C Coefficients*
| Description of Area | Runoff Coefficients Range | Runoff Coefficients Selected | Contributing Area (m²) | C x A |
|---|---|---|---|---|
| *Asphalt & Concrete (d/w, s/w) | 0.80 - 0.95 | 0.95 | 336 | 319 |
| *Buildings | 0.75 - 0.95 | 0.95 | 756 | 718 |
| *Lawns, Average 2-7% | 0.18 – 0.22 | 0.20 | 520 | 104 |
| Contributing Area (ha) = | (1612 m²) 0.16 ha | |||
| Post Development C (rounded) = | 0.71 |
Post-development Q₂ = 0.0047 m³/s (4.9 L/s) vs Pre-development Q₂ = 0.0024 m³/s (2.4 L/s) Post-development Q₁₀ = 0.0075 m³/s (7.6 L/s) vs Pre-development Q₁₀ = 0.0037 m³/s (3.7 L/s)
As expected, with the post-development runoff coefficient greater than that of the pre-development, the resultant post-development flows are higher. As such, storm water detention with flow-controlled release is proposed to reduce the 2- and 10-year rainfall event peak runoff rates to match pre-development levels.
5.3. Stormwater Detention
The development is storm water detention / infiltration tanks with an orifice-controlled outlet to restrict the post-development runoff to the peak pre-development runoff rates. The size of the orifice will be determined during detailed design as it is directly related to the onsite grading and contributing area.
The required total detention volume, based on the controlled release rates and calculations above, are:
- 4.7 cu.m. for 1 in 2-year storm events
- 7.9 cu.m. for 1 in 10-year storm events
Refer to Appendix A for detailed calculations on the detention volume.
5.4. Climate Change
“On a provincial scale, climate change projections generally indicate warmer, shorter winters and longer summer periods. Total precipitation is projected to increase, with drier summers and a greater proportion concentrated in the shorter winter periods. However, the magnitude of these changes is not uniform over the province, and can vary significantly due to topographic or orographic effects and local influences.”
The development’s storm water runoff has been reviewed in consultation with the Engineers and Geoscientists of British Columbia’s (EGBC) “Professional Practice Guidelines, Developing Climate Change – Resilient Designs for Highway Infrastructure in British Columbia”. These guidelines provide projected percent change in precipitation by 2050s by region and links to models for assistance in determining these projected changes. The projected percent change for the south coast region, per EGBC, for summer is -2% to +11% and for winter -4% to +14%.
The IDF_CC (Version 6.5) developed at the University of Western Ontario (UWO), in the Faculty for Intelligent Decision Support utilizes the most recent Environment Canada IDF datasets. This tool projects that for the Victoria Gonzales (#1018611) weather station that the 24 hr rainfall events change as indicated below:
| Event | Existing | Projected | Percent Change |
|---|---|---|---|
| 2 year / 24 hr | 47 mm | 54 mm | +15% |
| 10 year / 24 hr | 72 mm | 85 mm | +18% |
| 100 year / 24 hr | 107 mm | 125 mm | +17% |
The IDF curves generated by the UWO model have been used in our Appendix A calculations.
6. Stormwater Quality (Treatment)
Storm water from the developments hardscaped (vehicular access) areas will be treated through oil / water separators or landscaped areas prior to entering the storm water infiltration / detention system.
7. Erosion and Sediment Control During Construction
A key factor in erosion and sediment control is the interception and management of site runoff. Careful planning of construction site activities and phasing, to prevent the generation of erosion, is more effective and less expensive than downstream sediment control with sediment ponds only. Sediment ponds should be considered the last line of defense.
As outlined in the "Land Development Guidelines for Protection of Aquatic Habitat", the following general principles will be required at this site:
- Where possible, conduct earthworks activities during dry months of the year
- Stage activities to allow "green-up" or re-establishment of vegetation and minimize bare areas
- Halt construction during periods of significant precipitation and runoff
- Restrict vehicular/equipment access or provide working surfaces/roads. At each access point to the development site, a sediment pond or trap should be constructed to retain wash down water and sediments that may flow down the adjacent catchment to the access point.
- Retain existing vegetation until excavation commences, i.e. do not strip vegetation long before the following activities
- Minimize clearing and stripping areas
- Physically mark clearing boundaries on the construction site
- Seed or re-vegetate cut and fill slopes, and disturbed natural areas
- Cover temporary fills or stockpiles with sheeting or tarps and/or use silt fence surround
- Use mulches and other organic stabilizers to minimize erosion until
- Vegetation is established
- Plan seeding and planting to allow establishment before the end of the growing season
Although ultimately the responsibility of the site contractor, the following general site-specific control measures should be followed:
7.1. Silt Fences-Perimeter Ditching
Silt fences provide an effective filter for sediment laden runoff from bare soil slopes and surfaces. Silt fences are effective boundary control devices, trapping the sediment close to the erosion source and reducing the mobilization into runoff.
Silt fences should be installed along the downstream edges of activity, lower property lines and at the top of the slope to the foreshore. In temporary runoff control ditches and in proposed bio-swales, silt fences may be overwhelmed by the quantity of runoff and should be reconsidered, in favour of traps or rock berms, particularly where ditch slopes are steeper than approximately 3%.
Adequacy of the silt fence locations should be confirmed in the field, inspected, and maintained on a regular basis as construction proceeds.
7.2. Sediment Control Ponds
Sediment control ponds are the last line of defense before runoff is discharged from the development site.
Sediment ponds should ideally be a minimum of 1% of the contributing catchment area and be as large as practical within the topographical constraints of the site. Catchment boundaries and proposed locations of the sediment ponds will be shown on the detailed design drawings. Adequacy of the proposed sediment ponds should be reviewed in the field.
7.3. Slope and Surface Protection
The required protection will be determined by the type of material, the grade of slope, and the expected exposure time, and shall be in accordance with the "Land Development Guidelines for Protection of Aquatic Habitat".
In dry conditions, all cut/fill and cleared natural slopes and surfaces should have erosion controls implemented within 14 days.
In wet conditions, erosion control should be implemented immediately on completion of the grading operations of the area.
Slopes exceeding 3.0 meters in height and steeper than 2H:1V should be reviewed by a Professional Engineer, to assess slope stability, erosion, and drainage control requirements.
8. Operations and Maintenance
The on-site storm water system should be inspected and maintained regularly. We have summarized key system components and inspection/maintenance requirements below:
8.1. Detention / Infiltration Tanks
The storm water detention/ infiltration tank system should be inspected at regular intervals and maintained when necessary to ensure optimum performance. Please refer to the detailed Operations and Maintenance Guidelines included in Appendix B.
- As a minimum, the detention / infiltration tanks should be inspected twice annually, preferably in the fall and the spring.
- Inspect inlet and outlets facilities annually in the fall prior to major rainfall events. Remove floatables and any accumulated garbage
- Remove accumulated sediments (if required).
8.2. Drainage Mains and Manholes
The drainage mains are constructed at gradients that are sufficiently steep to ensure self-cleansing. Access to the pipe is available at either a manhole or the beginning of line dean out. Under normal conditions of operation, the pipe system should require no other maintenance. A visual inspection at the manhole
should be conducted yearly to confirm that pipe flow in the manhole channel is uninhibited. Remove visual obstructions.
8.3. Catchbasins
The best way to determine when catch basins should be cleaned is to perform regular visual inspections. At a minimum, facility managers should dispatch a trained worker to inspect all catch basins twice annually, preferably in the fall and the spring.
There are several trees/landscape areas that will generate debris on-site. Cleaning of catch basins can be done using a hydrovac system when debris is accumulated. Cleaning can also occur in the spring and/or fall (or as needed) depending on the outcome of visual inspections and site conditions.
8.4. Oil Interceptors
The oil interceptors should be inspected at regular intervals and maintained when necessary to ensure optimum performance. Please refer to the detailed Operations and Maintenance Guidelines included in Appendix B.
This report has been prepared by:
McELHANNEY LTD.
Ian McCall, P.Eng. Senior Civil Engineer
Reviewed by:
Nathan Dunlop, P.Eng. Project Manager
2023-09-27
Document Images
(5)
