Slowing the flow of stormwater

Following a rainstorm, any water that doesn’t soak into the ground will flow downhill carrying particles and pollutants with it into local rivers and lakes. When runoff from paved surfaces hits a little stream, it can be too much all at once for the waterway to safely handle, leading to erosion and damage of delicate habitat. Stormwater ponds are man-made pools designed to capture and hold that water – dropping out pollutants and slowing the flow to the stream. Ponds are really good at reducing flow and large suspended particles, but they aren’t so good at removing tiny particles or dissolved pollutants – including nutrients like phosphorus.

People next to a stormwater pond

Stormwater ponds have been a popular method for managing urban runoff for decades. Now that we understand more about their limitations, regulatory agencies are moving away from their use.  But there are thousands of stormwater ponds in New England alone. Replacing all of them with a different practice would be time consuming and costly and may not be possible with existing available land.

Studying stormwater pond improvements

As part of her Ph.D. research at the University of Vermont, Water Quality Program Manager, Dr. Becky Tharp investigated methods to improve stormwater pond performance using natural systems and green infrastructure.  Using floating treatment wetlands – buoyant rafts with wetland plants growing on top – Dr. Tharp studied the rafts ability to improve pond performance and also the most appropriate selection of plants to survive long cold winters and short growing seasons. Details on the plant-specific portion of the research was recently published in the scientific journal Ecological Engineering in an article entitled, Macrophyte performance in floating treatment wetlands on a suburban stormwater pond: implications for cold climate conditions.

Three photos - person holding raft with plant roots; plant roots; person next to floating rafts.
Measuring root growth of the plants in the deployed rafts.

Of the four wetland plant species studied, Carex comosa (Longhair Sedge) did the best overall – performing among the best for winter survival, biomass development, phosphorus uptake, and root size and shape (important for trapping particles).

This research is relevant to the work of Watershed as we design stormwater systems to reduce pollution and improve water quality in local streams. We will be using the lessons from this Vermont-based study to improve our work and are always seeking to push the boundaries of our current understanding of best stormwater treatment design.