Westbrook Consulting Ltd. - Re: 231 Heddle Avenue – Storm Water Management Concept
Technical report on the proposed storm water management system, including runoff coefficients and detention strategies.
January 17, 2018
3214-02
Sterling Plaza Inc. | K.Star Home Improvements c/o Praxis Architects Inc. 401-1245 Esquimalt Road. Victoria, BC V9C 3J2
Attn: Gerry Conrod | John Koren
Re: 231 Heddle Avenue – Storm Water Management Concept
Dear Sirs:
Westbrook Consulting Ltd. has prepared a preliminary storm water management concept plan for the development of 231 Heddle Avenue, as outlined in the following letter.
Background
The existing site at 231 Heddle Avenue slopes steeply from the center down towards the property boundaries. One single family home with a gravel driveway and parking area has been constructed on the site. The center of the site is cleared and rock has been exposed. Towards the outer edges of the site boundaries, there are forest covered slopes that will be retained.
Pre-Post Development Comparison
Due to the rocky nature of the site, a pre-development runoff coefficient of 0.4 has been used.
The post-development runoff coefficient was calculated using a combination of a runoff coefficient of 0.9 for the 2740 m² of proposed impervious surface area and a runoff-coefficient of 0.4 for the remaining 1760 m² of vegetated green space.
The post-development runoff coefficient has been calculated by taking a weighted average of the various post-development runoff coefficients as follows:
CPost = (Cimpervious X Aimpervious + Cvegetated X Avegetated) / (Aimpervious + Avegetated) = (0.9 X 2740 + 0.4 X 1760) / (2740 + 1760) = 0.70
The post-development runoff coefficient will be used to determine the required storm water detention volume.
Conceptual Storm Water Management System
The storm water management system is proposed to consist of a rain garden and a detention gallery located under the western driveway.
The building rain water leaders will release rain water to rock swales leading into a rain garden. The runoff produced by the driveway will be collected in catch basins and stored in a detention gallery. The eastern portion of the driveway, fronting Building 1 will be constructed out of permeable pavers to aid in reducing the amount of runoff created by the development.
The system will detain the storm water created by the development and release the storm water at the pre-development rate to the vegetated area fronting the site. The storm water will then be infiltrated into the ground, as presently occurs. In the event that a storm occurs that exceeds the system capacity; an overflow will be installed to release the excess water directly to the drain on Heddle Avenue, as shown in the enclosed figure.
Summary
A storm water management system will be required to limit the runoff from the site to the pre-development flow rate. The system will be composed of a combination of a rain garden, detention gallery, permeable pavers, swales, and a piped system.
If you have any questions please do not hesitate to contact the undersigned.
Yours truly,
WESTBROOK CONSULTING LTD.
Tracey Klatt, EIT Project Engineer
Reviewed by,
Mike Wignall, P.Eng, LEED AP Project Manager
ENCL: SERVICING CONCEPT PLAN
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