Flood Control in Sandy Hill

Many Canadian municipalities face chronic flooding issues. The problem is particularly prevalent in older communities, where there is aging infrastructure, changing land uses and the increasing use of basements as living spaces.

Modern design practice and standards for managing stormwater in urban settings are based on the dual drainage principle, which provides a much higher level of protection than in the past.

The dual drainage system is based on providing a cost-effective balance between an underground drainage network of pipes that convey all runoff up to a prescribed limit (typically based on a 2 to 10 year storm recurrence), and the provision of adequate surface or overland drainage routes for conveying excess flows from rarer events that exceed this limit. By controlling excess flows at the source and safely managing them overland along natural or engineered drainage routes, municipalities can mitigate the impacts of extreme storm events and climate change.

Stantec’s experience over a decade has shown that the dual-drainage approach is an effective solution for established urban areas with a history and an elevated risk of flooding. One of Stantec’s recent applications was at Sandy Hill, a residential area with a community park near the Rideau River, in the eastern portion of downtown Ottawa.

Historically, plans and efforts to reduce flooding in the Sandy Hill area had focused on separating the combined sewers and finding a suitable outlet for the storm water. But this solution was unreliable. Constructing and operating a large-scale remediation tank in the community presented challenges because of the impacts on existing community features. These constraints had stalled previous attempts to solve the flooding situation.

How could the massive storage facility be incorporated within the heart of the community and, in particular, in its highly valued community park? It was no small task.

Stantec demonstrated that an innovative “stacked-storage” facility was the most viable and technically effective solution.

The “stacked storage” facility consists of a 12,500-m³ underground combined sewage storage tank topped with a 4,000-m³ dry pond for managing excess stormwater runoff.

The underground sewage storage tank measures 55 m x 55 m, and is 4.5-8 m high. It is located at a depth of 6-10 metres, and was excavated primarily in bedrock within the heart of Sandy Hill Community Park.

The surface water detention area lies over the tank, separated from it vertically by a sub-drain system of perforated pipes and a 1.3-metre layer of engineered soil that retains enough moisture to sustain grass.

The original grade level of the park had to be lowered by 1 to 1.5 metres to create the surface storm water storage area, which affected a significant portion of the park’s usable area. The existing roadways were modified to direct storm water runoff towards the storage area. Also, inlet control devices in the catch basins in the roadway sags near the park ensure that the runoff from smaller and more frequent storms, as well as the initial drainage volume from larger storms, is directed to the underground sewers. In this way only the excess, “cleaner,” storm water runoff that may surcharge the sewer pipes enters the park and goes into the surface storage area.

The surface storage area will only be used for very rare storm events and most of the time is a dry grassed area. The underground tank will be used more often. During extreme storm events, a diversion chamber on the trunk sewer directs excess combined sewage to the underground storage tank, where it is stored until it can be pumped back to the combined trunk sewer for treatment at the wastewater treatment plant approximately 20 kilometres away.

The underground tank has two cells. Cell No. 1 occupies one third of the tank’s total volume and is intended for more frequent use (once every two years on average). Cell No. 2, occupying the balance of the tank, is intended for storm frequencies greater than one in five years.

Flushing gates use some of the stored sewage to clean accumulated debris from the tank floor after each use.

Since 2009 when the project was completed, the underground tank has been used twice. During these high rainfall events it successfully stored combined sewage, preventing basement flooding.

Working closely with the community, Stantec realized an opportunity to enhance the park without compromising the flood protection goals.

The new design incorporates a superior park layout, modernized facilities and comfortable outdoor places. Grade changes for the surface storage area were used to shape a new landscape with natural slopes and visually appealing stone walls. While the redesigned sports field was smaller in area than the original, its design and functionality were greatly enhanced.

The storage tank’s remotely operated control equipment, as well as the tank’s carbon filter odour control system, is stored in a new park support building. This building is combined with new change facilities and rest rooms for the public.

The landscaping design conserves mature trees as much as possible. It uses native species and species chosen for their year-round beauty as well as their ability to survive submergence in deep water. Once fully grown, the vegetation will buffer the park from the surrounding roadways.

An amphitheatre was integrated between the storage facility and community centre to accommodate a variety of community needs. Built out of natural stone and precast flagstone, the amphitheatre was designed with organic lines that are consistent with the existing grading. To create visual interest and foster a sense of play, a large chess board area was also installed, using two colours of concrete pavers.

Construction of the facilities and Stantec’s landscape architectural design of the park were completed in less than two years. The finished project has provided the community with an enhanced public space and protection from basement flooding up to a design target level of service of 1 in 100 years.

The park is now an established destination for community gatherings and the project has won several awards, including the American Public Works Association’s 2010 Project of the Year Award in the environment category ($5M to $25M). cce

IPEX Action Sandy HillDoran IBI Group (surface pond layout and underdrain system design); Golder Associates (geotechnical and hydrogeology); Civitas Architecture (control and park support building) Stantec Consulting (Adrien Comeau, P.Eng., Brett Byce, P.Eng., Stéphane D’Aoust, P.Eng., James Ricker, P.Eng., Chantal Gaudet)City of Ottawa