Stormwater Management in the Town of Oakville
The Town of Oakville have provided detailed requirements for stormwater management, as published in the Development Engineering Procedures and Guidelines document, dated September 2023. As a developer, you may receive a comment from the development engineering department saying:
“2.10. Proposed increase in impervious surface on the property. Please, demonstrate as per site’s drainage design, how runoff will be managed and mitigated on-site and will not cause adverse impact to subject site and adjacent properties including Town Right of Way. As per development engineering guidelines, increased runoff shall be retained (infiltrated) on-site. SWM system to be designed by a Professional Engineer and sealed design document to be provided. Supporting soils analysis data from site to be provided with the design document. Please refer development engineering procedures and guidelines manual section 7.3.1 storm drainage criteria.”
Stormwater management system for a new single family home in the Town of Oakville.
What does this mean?
The first requirement is that the first 25mm of rain on site must be captured and controlled. Specifically, Section 7.3.1 titled Storm Drainage Criteria states that:
“the runoff from a 25 mm event shall be retained (infiltrated) on site for all site areas (rooftop, hardscape, and softscape yard areas), as per the recommendation of the Town SWMP. This can be completed through various LID methods such as cisterns, soak-away pits, dry wells, grey water systems, rooftop storage, green roofs, infiltration trenches, modular underground infiltration systems, and bioretention areas. Rain barrels and pools will NOT be considered for SWM storage requirements.” Note that rooftop storage is not permitted in residential applications. The requirements in this section detail that every dry well must have an inlet (catch basin), and if over 8m2, must have two inlets. A cleanout typically functions as a secondary inlet.”
The Town requirements state that the runoff must be infiltrated, however only about one third of the Town has Sandy soils. Where soils do not allow infiltration, such as clay or silty soils, the volume must still be controlled but may be released to either a storm sewer or roadside ditch. In this case the Towns storm sewer capacity mapping should be reviewed for surcharging sewers. Where a storm sewer is surcharging anywhere downstream, the runoff must be controlled to a higher degree.
A stormwater management system hidden under the lawn of this stunning single family home in the Town of Oakville.
Additional elements are required to ensure the design will function and for long term maintenance, including soils testing, inlets, and cleanouts. In the design phase a soils test is required to confirm the following:
The soil permeability, measured in mm/hr
That the high groundwater level is 1.0m or great below the system.
That bedrock or impermeable soils are 1.0m or greater below the system.
The Town requirements state:
“Infiltration systems (e.g., soak-away pits) shall provide data from a percolation test indicating that the soils are suitable to provide a drawdown time of 48 hours and seasonal groundwater and bedrock elevations (if applicable) to confirm elevations are sufficiently low to allow stormwater percolation. A 1 m separation between the bottom of the infiltration system and the seasonal high groundwater elevation is required.”
Section 7.3.1. states that an inlet is required for all dry wells. This means that a catch basin is needed to allow the runoff to get into the dry well. One or more cleanouts are typically needed to ensure the system can be easily maintained for the long term viability of the system.
Mini stormwater storage system in the Town of Oakville. This site had very little space available to meet the Town’s requirements. Everwest Engineering designed a mini system in the front yard to achieve the Towns objectives.
When working on the design of stormwater management systems, many clients often ask if the rainwater leaders (downspouts) can be piped underground directly into the system. The Town standards specifically state that:
“All downspouts shall discharge onto approved sodded areas using splash pads”.
This provides a long term passive solution, typically with little to no maintenance.
Our designers at Everwest Engineering are experts with the Town of Oakville design guidelines. After working on so many designs, we understand the requirements thoroughly and how they are enforced by the Town. We complete our designs looking for the lowest build cost, while getting your design plans approved quickly. If you have any questions or if your looking for support with a design please feel free to reach out.
