Stormwater Management Plan Letter - Associated Engineering

Re: THETIS VALE PHASE 7 - STORMWATER MANAGEMENT

This letter is to address the stormwater management issues related to the above noted subdivision. Phase 7 is one of the highest phases, in elevation, of all the past and future phases of Thetis Vale development. Phase 7 is a predominately sloping, rocky site that will require blasting for site grading. There are inherent problems in building stormwater facilities on this type of site. However, stormwater facilities built in previous phases of Thetis Vale were specifically designed to provide stormwater management for runoff from Phase 7. This letter will outline the downstream stormwater management measures that were previously implemented.

Stormwater Management consists of two main elements; water Quality treatment and water Quantity attenuation. There are various Low Impact Development (LID) or "Green" methods to manage stormwater runoff and address water Quality & Quantity issues. They include the use of bio-swales, rain gardens, detention ponds and underground storage techniques as well as implementing designs that minimize hard surfaces. These methods aim to improve water quality through filtration and settlement of pollutants as well as to mimic pre-development conditions by encouraging groundwater recharge.

In the natural or pre-development condition, stormwater infiltrates the ground and seeps into the water table through a process known as groundwater recharge. This process works best where native soils are conducive to percolation, i.e. sandy or gravelly soils. Gravity and capillary mechanisms cause the runoff to migrate through the ground to the water table. This migration process acts like a filter removing contaminants from the runoff before it reaches the water table. LID systems such as bio-swales, rain gardens and detention ponds can be used to hold stormwater runoff and allow it to percolate into the ground from the surface. Alternatively, underground systems such as "soak away" pits or perforated pipes can be used to introduce stormwater runoff directly into the ground.

The problem with attempting to induce groundwater recharge with underground systems in a fractured rock site or a site that has been filled with blasted rock is that you cannot control the path of the water through the rock. It will always follow the path of least resistance. While water introduced into uniform gravels will generally flow down toward the water table, capillary forces can cause water introduced into fractured rock to move laterally or even upward. Stormwater from the surface can concentrate along fracture lines and break out of the rock fill downstream causing flooding or erosion. Furthermore, limiting the impervious area on this type of site is meaningless as the pre-developed, rocky condition generally has a similar level of imperviousness.

Bio-swales and rain gardens are other methods being used extensively to attenuate and treat stormwater runoff. The process for both is similar. They use biology, in the form of plants and engineered soils to remove pollutants. Storing stormwater and slowing down the rate at which it leaves the site encourages groundwater recharge and reduces the flow rate. The challenge with using these methods on a site like Phase 7 is that much of it is steeply sloping and the small lot nature of the subdivision does not allow room for the large flat areas that would be required. Once the subdivision is established the runoff that will generate the most pollutants will come from the roadways. This means that bio-swales would have to be placed near the road within the Right-of-Way (R.O.W.). The frequency of driveways, streetlights, fire hydrants and other above ground utilities within the R.O.W. in a small lot subdivision makes this very difficult to achieve.

Stormwater ponds are also used for attenuation and treatment of stormwater runoff. Ponds rely on storing stormwater and releasing it at a controlled rate. This allows solids to settle out of the stormwater and can prevent downstream flooding. Ponds are best suited for fairly flat sites with open spaces available. As has been previously stated, this is not the case in Phase 7.

The planning for the Thetis Vale development started in earnest back in 2002. That is when View Royal had the foresight to commission a Transportation Assessment and Master Drainage Plan for the Atkins Local Planning Area. The report was prepared by Stantec and was completed in 2003. It formed the basis for design of roadways and drainage systems in the Thetis Vale area. The conditions that exist in Phase 7 and indeed all phases were known and considered when planning and design was undertaken for Phase 1. Following recommendations by the project Biologist, Ray Hallady R.P.Bio., a detention and treatment pond was built by Limona Construction at the northeast corner of Phase 1 near the Highway off ramp. Bio-swales were also constructed along Six Mile Road downstream of the pond and along the railway next to Chilco Road. These facilities were designed to accommodate upstream flows from all phases of Thetis Vale, including Phase 7. The wetland was there because the area is flat and the native soils are better suited for infiltration and groundwater recharge than elsewhere in the development. In addition, the pond and bio-swale along Six Mile Road are hydrologically connected to the rock fill that exists under much of the Phase 1 area through the porous, gravelly soils. This means that a significant amount of runoff can be stored underground while it slowly percolates into the native soils.

The pond and bio-swales constructed in Phase 1 of the Thetis Vale development function as stormwater management facilities for all the upstream properties, including Phase 7. So while stormwater management facilities may not be constructed within Phase 7, stormwater treatment and attenuation is being provided for all of the Thetis Vale development in facilities constructed in Phase 1. The ravine in Phase 7 also offers some treatment opportunities. The drainage system within this phase will be tight piped and we will utilize this natural area as much as possible.

Erosion and sedimentation control measures will be outlined by the project biologist and adhered to by the contractor.

Yours truly,

Dennis Desjardins, ASct Project Manager

DD