March2011_ES&E D3_2010 01/04/11 10:04 PM Page 53
Stormwater Management
Ottawa area flood control project uses innovative stacked storage system By Lynne Chwyl, Adrien Comeau and Jeff Deloyde
T
he Sandy Hill community in Ottawa has long suffered the results of a topography and antiquated sewer/drainage system that subjected the area to persistent flooding during major storms. Parts of this area are typically lower in elevation than water levels of the nearby Rideau River during storms. Also, because the sewer and drainage systems in the area were old and inefficient, surface flooding and sewer backup led to surcharge or overflow in the drainage pipes, that often resulted in residential basement flooding, when precipitation of any significant amount occurred. Although the City of Ottawa tried to resolve the problem by separating the combined sewer lines in problematic areas, this was minimally effective and flooding was still a recurring issue in Sandy Hill. Recognizing that the topography could be used as an advantage, the design team from Stantec, contracted by the City, created an innovative stacked water storage system that included both a surface stor-
There were a number of design constraints that made construction of the flood control tank challenging.
age area in Sandy Hill Park and an underground storage tank. Project engineers recommended re-grading the existing
The new design incorporates an improved park layout, modernized facilities and comfortable outdoor places. www.esemag.com
roadways to direct stormwater runoff to the centre of the park where it could be collected in the surface storage area. To alleviate surcharge in the limitedcapacity combined trunk sewer, a diversion chamber was designed to direct excess combined sewage during extreme rainfall events to the storage tank buried under the park. After the storm event passes, combined sewage stored in the underground tank is pumped back to the combined trunk sewer for treatment at the wastewater treatment plant. The surface storage area within the park is located above this underground tank and provides approximately 4,000 m3 of runoff storage. Placement of inlet control devices in catch basins where the roadway dips near the park ensure that the first flush flows are captured and force “cleaner� stormwater runoff that may surcharge the sewer pipes into the park and the surface storage area. Sub-drains and engineered soil assist with faster field drainage, following the spring melt period and after each rainfall event. This is expected to allow the park continued overleaf... March 2011 | 53