June 1, 2014
By By Andrew Sobchak, P.Eng.
Elevated temperatures have long been cast in the role of villain in the world of stormwater management. In fact, Canada’s Department of Fisheries and Oceans (DFO) considers warm water a deleterious substance, a potential killer, keeping...
Elevated temperatures have long been cast in the role of villain in the world of stormwater management. In fact, Canada’s Department of Fisheries and Oceans (DFO) considers warm water a deleterious substance, a potential killer, keeping company with pesticides, petroleum products and detergents.
Warm water is particularly troublesome in our cities, where sprawling impervious surfaces like parking lots, roads and roofs bake in the summer sun. Rainfall contacts these surfaces, heats up and runs off into urban waterways, increasing their temperature in the process. Adding to the problem, stormwater management ponds — the very features designed to improve stormwater quality — can also increase water temperatures by as much as 5 °C. By the time stormwater is discharged to a river or stream it can often top 25 °C, far exceeding the upper bearable threshold for desirable cold water fish species like brook trout, at 19 °C.
Regulatory agencies like the DFO have long wanted to control run-off temperatures. But there is very little regulatory guidance on how, exactly, to do it, and even fewer case studies of proven concepts. Stepping into the void, consultants are returning to engineering first principles to pioneer candidates for best management practices.
From theory to practice
In Ontario, 125 kilometres west of Toronto and buried three metres underground near the crossroads of the village of Baden, lies one of those candidates: a 32-metre-long, clear stone-filled cooling trench. Connected to the outlet of a stormwater management wetland that services a 64-ha residential subdivision, the 64-m3 trench is designed as an end-of-pipe cooling solution for run-off from small-to-moderate storms with less than 15 mm of rainfall.
“The science is quite simple,” says Steve Brown, P.Eng., surface water lead for Stantec Consulting in eastern Canada, whose team based in Kitchener, Ontario designed the feature. “Run-off that passes through the trench transfers heat to the cooler stone, exiting with a reduced temperature. It’s all sized using heat transfer theory.” Discharge from the trench is directed via a secondary up-welling trench that allows the water to passively percolate up to a shaded swale, before ultimately reaching a sensitive cold water stream tributary.
Following a three-year monitoring study, Brown presented the performance record of the trench at the 2013 Canadian Water Resources Association Annual Congress in Saskatoon last May. “The stone temperature remains remarkably consistent,” he says. “The greatest capacities for cooling appear to be in the spring when the difference between water and stone temperatures is greatest. In ideal conditions, the trench was able to reduce wetland discharge temperatures by as much as 10 °C, and on average about 4 °C.” Most importantly, the data showed trench discharge temperatures were almost always cooler than those occurring naturally in the tributary. “Overall, we are pleased,” Brown says.
Tailoring to specific sites
In the 7,000-sq. km. Grand River watershed — southern Ontario’s largest — the Baden cooling trench is one of approximately 15 end-of-pipe solutions that have been or are being constructed. Brown’s team is responsible for designing roughly one third of them.
The first option is to infiltrate clean or treated stormwater whenever possible, but when soils are unsuitable for infiltration, or when a more rigorous cooling regimen is required due to the sensitivity of a receiving watercourse, cooling trenches are gaining popularity. “The science behind [trench] sizing is simple,” says Brown, “but there are opportunities to adjust stone size and location. For example, submerging the trench in the groundwater table can sometimes provide benefit, but the results of these variations are less proven.”
In fact, it is this variability that makes Baden an important case study. Until now, some regulatory agents weren’t convinced that the cooling theory on paper was translating into field performance. With three years of monitoring data, the Baden trench proves this approach can consistently work. The key now for consultants is to identify the nuances of integrating the trenches into their site-specific geography to promote universal efficacy. “We in the industry need to develop a proven toolkit so engineers can pick from a few options based on site conditions and project goals,” says Brown, “but we aren’t quite there yet.”
Trenches are starting to appear in other regions of the country where additions like artificial shade screens and floating wetland vegetation are being used in stormwater management facilities to minimize the absorption of solar energy. “People are talking about piped geothermal cooling too,” notes Brown. This technology routes stormwater underground through small diameter tubes. “It operates on the same principles as [the Baden] trench, but I haven’t yet seen it applied in the field.”
Brown’s team is also considering breaking an unwritten rule of stormwater management: escaping passive-only systems and employing active means to improve the discharge quality. “We are investigating using solar-powered valves to time the discharge of run-off from the wetland,” he says. “This way, we can capitalize on the diurnal fluctuations of run-off temperatures, and release at night when water can be several degrees cooler.”
A cascading approach
The knowledge base behind cooling trenches is advancing, but end-of-pipe solutions are just one type of weapon in a developing arsenal for the thermal mitigation of stormwater. Others, such as improved planning techniques and the selection of building materials, fall outside the direct jurisdiction of a stormwater engineer but firmly in the realm of land development professionals.
The Toronto and Region Conservation Authority (TRCA) refers to this approach as “cascading,” with mitigation measures employed at various scales to “address all aspects of thermal enrichment.” The TRCA recently convened a working group with other public agencies to compile the expertise accumulating in regional pockets like Brown’s to inform the development of policy. Although the release of the group’s white paper has been delayed and is now scheduled for December 2014, regulatory guidance for engineers is on the way.
In the meantime, Brown and his team continue in the cycle of engineering first principles and are busy applying the lessons learned from previous trench installations to new designs: “We are starting a monitoring project this summer for a much larger trench in Waterloo which was commissioned in 2013.” At 442 metres long, bigger may not necessarily mean better, but Brown is hoping to improve upon the success in Baden.
Test. Revise. Repeat.
Andrew Sobchak, P.Eng., is principal of Toronto-based Tributary Consulting and has worked for 15 years in the water resources industry. He was a consultant on the Baden cooling trench monitoring study.