Stormwater Basin Algae Blooms and Ways to Minimize Them

Shallow basin showing early stages of an algae bloom.

"April showers bring May flowers". Those same rain showers can contribute to other types of blooms; algal blooms. Plants need sunlight, nutrients, water and a favorable environment in which to grow. Algae have similar requirements. A shallow basin allows intense sunlight to shine to the bottom and allows the water to heat up quickly. Longer spring days mean more sunlight available to the algae. Warming temperatures also contribute to the growth of the algae. Nutrients that are bound to sediments can wash into the basin from the surrounding drainage area and help drive algal blooms.
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Why does algae in a basin matter? Algae is a part of any heathy aquatic ecosystem so at normal levels they are not a problem. When one species of algae blooms, it can lead to maintenance issues and other problems. Let’s look at algae blooms and their impact on the function of the stormwater basin. Dense algal scums interfere and obstruct parts of the basin’s outlet structure, causing it to lose storage capacity or overfill. Algae that die off will sink to the bottom, decay and will eventually fill the basin with muck. This can lead to costly maintenance. 

Beyond the maintenance implications of an algae bloom, there are health and aesthetic implications. As an algal bloom dies off and decays dissolved oxygen in the water is depleted. This can lead to a fish kill. While not all algae blooms would cause a health impact, there is one class of “algae”, the cyanobacteria, that can release harmful toxins when they bloom. The toxins they release can harm pets that wade or drink the water. There can also be respiratory impacts from severe blooms.  Water quality also includes aesthetics. A homeowner who just paid $500K to move into a nice community is less likely to appreciate a basin with a thick algal scum. This can hurt real estate values in the area.

The best way to reduce the impact of a harmful algae bloom is to work to prevent blooms. This starts in the design phase. Designing a wet extended retention basin to a depth of 6-8 feet and with sufficient bank grading will reduce plant growth and algae blooms in the basin. A dry basin should be constructed so it doesn’t hold water for more than 72 hours. During the construction phase, limit the amount of sediment that flows into the basin. This sediment will have bound up nutrients and will provide the food leading to future algae blooms. Remove built up sediment in the basin, the presence of a forebay should make this maintenance easier. Stabilize the basin walls and surrounding ground with seed and straw to help lock soil and nutrients in place, keeping them out of the basin.
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Post-construction measures like keeping a vegetated buffer around the basin will help keep the sides of the basin from caving in. The taller vegetation will also discourage geese from hanging out around the pond and adding more nutrients. The vegetation buffer will act as a filter strip. Native sedges, blue flag iris and rushes are a few plants recommended as native buffers. Woody vegetation and nuisance vegetation should be discouraged. Keep grass clippings out of the basin. Bottom-up aeration can help support a healthy ecosystem and help regulate temperature. Also encourage residents to be responsible if they are applying fertilizer to their lawns. Adopting the four Rs of nutrient management for your lawn can be useful.

​Considering that fertilizer will lead to more costs, mowing, and maintenance, some may decide to forego it.  With proper design and maintenance, the stormwater basin will remain functional and be more enjoyable to have in the community.

​For more information, contact our office at 513.695.1337