Thermal regulation of stormwater ponds using HDPE balls

By Rohit Sati and Amy Woods

A common challenge with municipal stormwater management ponds relates to the effect of solar heating of the receiving watercourse. Some species of aquatic life are very sensitive to temperature changes, and even a shift in water temperature by a few degrees can impact their health and survival.

Conventional approaches to thermal mitigation that include deeper ponds or bottom draw outlets have been used with some success. However, the City of Brampton and the Toronto and Region Conservation Authority (TRCA) wanted to develop alternatives where traditional approaches are not desirable or feasible.

The alternative needed to achieve the desired thermal mitigation benefit, while also being reasonably inexpensive to install and maintain over the life of the pond. In recent years, parts of eastern Canada have experienced prolonged and intense heat waves. One meteorologist commented that “the summer of 2018 had the hottest stretch of weather in more than a decade, and record-high temperatures have been shattered.”

The Ontario Ministry of Natural Resources and Forestry currently requests that temperatures from stormwater management facilities not exceed 24oC for the protection of endangered and sensitive aquatic species. With increasingly warmer summers, water within stormwater facilities will see a further rise in temperature. Measures will be required to mitigate this.

Brampton and the TRCA, the project partners, were interested in a low impact economical solution for thermal mitigation. They were also interested in a system that was fast and easy to install, and would not require frequent and costly maintenance. After some research and discussions with Layfield Group Ltd. and others, they opted to use shade balls (otherwise known as bird balls) as cover.

Shade balls are fairly easy to install, can be left in over the winter and require no maintenance. During a storm event, the balls automatically adjust to fluctuating water levels and allow maintenance equipment through the water during pond cleanup.

Shade balls have been used to reduce evaporation and slow light-driven chemical reactions in reservoirs, but are more commonly used in applications where a pond is suspected to contain toxic water not ideal for bird habitat. These balls also act as a deterrent to birds landing on the pond surface.

A total of approximately 200,000 200 mm white HDPE shade balls were deployed on the surface of a stormwater pond chosen for a pilot study. In order to keep them away from the shore where residents could access them, the balls were installed away from the shore and outlet location, which can be accessed from the shore during dry weather.

The containment section outline has a floating turbidity curtain anchored to the bottom of the pond. A boat was used to deploy the turbidity curtains and anchors to hold the curtain in place. After the curtain was deployed and anchored, balls were poured into the containment section where they kept spreading until they covered approximately 75% of the pond surface area.

The white balls reduce solar heating of the pond by reflecting most of the light back into the environment, while allowing the pond to "breathe" through the spaces between the balls. Results of this pilot study will be monitored for the next couple of years in relation to a control pond of similar size and configuration without balls.

Rohit Sati and Amy Woods are with Layfield Group. Email: amy.woods@layfieldgroup.com