Stormwater Basins

A common method to manage stormwater runoff is through stormwater basins or ponds, which collect and release runoff at a controlled rate to avoid flooding or erosion of downstream areas. Stormwater basins can either be detention basins, which are dry between storms, or retention basins, which have a permanent pool of water. Detention basins only contain water after a rain event, while retention basins always hold water.

The Canada Games Centre on Lacewood Drive in Halifax has an underground detention basin on-site that collects stormwater runoff from the building’s roof, parking lot and lawn area. Underneath a field beside the Centre lies a 10m deep hole that is filled with rock. Large, boulder size rocks were used to increase infiltration. Two metres beneath the surface lies a perforated pipe that runs across the length of the field and catches any overflow. During a heavy precipitation event, water enters the dry detention pond and infiltrates the rock layer. Water only enters the perforated pipe if the water level rises to 2 metres below the surface, and Jeff Pinhey, the site engineer has never witnessed an overflow event. The detention pond was built during the construction of the building. 

Underground detention pond at the Canada Games Centre

Underground detention ponds are safe, do not need to be fenced and provide additional space for on-site recreation. Detention ponds can also be above ground, such as this example in Dartmouth Crossing. We’ll explore wet retention ponds in more detail in future posts.

Above-ground dry detention pond in Dartmouth Crossing

Bioswales

The Prospect Road Community Centre was built in 2010 with several stormwater features  incorporated into the building's design. Runoff from the roof is collected in a large underground cistern and reused in the building for non-potable uses such as flushing toilets.

Runoff from the site and parking lot are collected in bioswales, which are vegetated depressions where water collects and soaks into the ground. This filtration improves water quality by removing pollutants and allows for water to re-enter the ground, recharging groundwater supply.

Water enters the bioswale through curb openings

The bioswales are composed of an underground perforated pipe surrounded by 5 feet of rock and gravel. During an extreme precipitation event, high volumes of water flow overland into the bioswales, flooding the rock area and overflowing into the perforated pipe. The water is then collected and piped off-site, ultimately draining into a natural wetland adjacent to the building. 

 Bioswales reduce strain on built stormwater systems and reduce likelihood of flooding and infrastructure damage by passively allowing water to move off-site. This engineered system mimics how water would naturally flow in an unaltered environment. Bioswales are not only useful, but also can be an attractive landscape feature.

Water drains off-site into a natural wetland

Stormwater Infiltration and Inflow

In HRM, there are two types of wastewater collection systems. Separated wastewater systems are designed to carry wastewater only. In older areas of Halifax and Dartmouth, combined systems exist that accept both wastewater and stormwater. When stormwater unnecessarily enters the wastewater system, the excess water load can cause a range of problems.

Stormwater can enter the wastewater system in two ways:

Infiltration (groundwater entering through cracks and leaks in the wastewater pipes) or Inflow (stormwater entering the system through a direct connection like sump pumps, drains or catchbasins).  Stormwater that enters the wastewater system ends up being collected and treated unnecessarily, which causes extra strain on  treatment plants and can lead to  combine sewer overflows (CSO) in heavy rains. CSOs cause untreated wastewater to overflow directly into natural water bodies leading to environmental and health risks. Infiltration and inflow can also cause sewer backups in people’s homes and can add to operating costs of treatment facilities which ultimately leads to higher rates for customer.  To address this serious issue, stormwater needs to be better managed at the source, by retaining runoff (allowing it to soak into the ground, or be collected and reused) on-site instead of allowing it to enter the wastewater system.

To reduce the frequency and magnitude of infiltration and inflow events,  Halifax Water has designed a Stormwater Inflow Reduction (SIR) Program to help customers indentify ways to better manage stormwater on their properties. The number one tip for property owners? Disconnect your downspout if it is directly connected to the wastewater system. This water can be redirected onto your lawn and collected in a rain garden or rain barrel, or allowed to directly soak into the ground.


If you have concerns about where your water is going, you can call 490-RAIN (7246) to have a free inspection of your property. Click here to find out more.