Performance Assessment of a Stormwater Retrofit Pond – Harding Park, Richmond Hill, Ontario

Retrofit pond
From the mid 1970s to the late 1980s, stormwater management was narrowly focused on preventing downstream flooding and channel erosion.  Dry and wet ponds were designed to provide temporary storage of runoff to reduce peak flows.  When pollutant removal was added as a central objective of stormwater management in the mid to late 1980s, many of these older quantity ponds needed to be retrofitted to address issues of water quality.  Since there is often limited space available for expanding the surface area of older ponds, the necessary control must be provided by other innovative measures.

The retrofit pond in Richmond Hill is a good example of what can be done within a limited space to provide the desired level of water quality enhancement.  Originally constructed as a dry flood control pond, the facility was retrofitted to include a permanent pool, a large volume of active storage (i.e. temporary storage above the permanent pool) and three cells: a forebay, wet pond and a small wetland.   To minimize short circuiting the forebay and wetland were separated from the main pond by aggregate berms with control structures (Hickenbottom risers) for hydraulic control.  French drains consisting of emplaced sand lenses were installed in the berm separating the wet pond and wetland to provide for continuously wet conditions during dry periods.

A two year monitoring program was initiated at the Richmond Hill site in 1996 and 1997.  Through continuous monitoring of water quality, water temperature and flow, the program aimed to assess the overall performance of the pond against design objectives.  Algae sampling was used to indicate spatial variations in stormwater quality within the facility.  Water quality and quantity monitoring data were subsequently used to calibrate a continuous hydrologic model to estimate long term performance over the historical rainfall period (1986-1997).

For monitoring results on this project, see the report entitled Performance Assessment of a Stormwater Retrofit Pond – Harding Park, Richmond Hill, Ontario