What Happens to Stormwater in a Retrofitted Stormwater Detention Basin?

What Happens to Stormwater in a Retrofitted Detention Basin?

Water from the neighborhood enters the basin through the inlet,
carrying trash, pollutants, and sediments from the ground surface.

Water slows when it hits the forebay, allowing some sediment to settle out.

Water hits the berm, forcing it to either side, slowing and spreading the flow.

Water meanders along the curved edges, slowing and spreading the flow.

As water passes through the plantings,
it soaks into the ground, slowing and spreading.

Water enters a rain garden pocket, slows, collects
and soaks into the ground. Water can also evaporate.

Water flows through the outlet after a certain volume is contained by the basin.

Outflow discharges a smaller volume of water than enters the basin,
and during some small storms, no water flows out of the basin.

Reduce Water Quantity
- Action: Remove low flow channel, add berms and flatten basin edges.
  Creates a longer and slower path for water flowing through the basin, allowing time for the water to soak into the ground, evaporate and be absorbed by plants. See Photos
- Action: Modify outlet structure.
  Raise the height and reduce the size of the outlet opening, allowing water to be retained in the basin for a short period of time and soak into the ground. See Photos
- Action: Install native plantings.
  Meadows, Shrubs, and/or Trees have deep roots that can filter, absorb and infiltrate more water. See Photos
- Action: Install rain garden pockets.
  Excavate basin floor to collect and hold water for short periods of time, increasing infiltration and evaporation volumes. See Photos
- Action: Increase overall volume of basin.
  Excavate a deeper basin and use the material to build up basin walls and flatten basin edges to increase the total volume of water a basin can manage. See Photos
Improve Water Quality
- Action: Retrofit forebay.
  Install erosion controls on the forebay to disperse the flow of water into the basin, and to allow for easy clean out of collected sediment and trash. See Photos
- Action: Remove low flow channel and install berms.
  Berms or other attenuation features slow the flow of water, increase infiltration and allow sediment to drop out of the water. See Photos
- Action: Install naturalized plantings such as meadows, shrubs, and trees.
  Meadows, shrubs, and trees have a deep root structures that slows down water and allows pollutants to be absorbed by plants and soil. See Photos
- Action: Install rain garden pockets.
  Excavate basin floor to collect and hold water for short periods of time. Soil and plants will filter sediment, pesticides, fertilizers and other pollutants from the water. See Photos
Lower Maintenance Costs
Action: Install low maintenance plantings. Meadows can contain grasses and flowers that are intended to grow tall, eliminating the need for weekly mowing. Excavate basin floor to collect and hold water for short periods of time. Soil and plants will filter sediment, pesticides, fertilizers and other pollutants from the water. See Photos
Help with Regulatory Compliance
Action: Install forebays, berms, edges, rain garden pockets and plantings to achieve volume reductions and water quality improvements. Stormwater regulations are becoming stricter to address the cumulative impacts of runoff on stream health. Retrofitted basins can help municipalities comply with new regulatory mandates.
Improve Recreation
Action: Install native plantings and rain garden pockets. Diverse native plant communities can add bird and butterfly viewing opportunities, improve the natural beauty of the landscape, and provide passive recreation such as mowed walking paths around basins. Retrofitted basins can provide aesthetic and recreational benefits to landowners and the larger community.
Reduce Safety Risks
Action: modify basin edges.Where space allows, flattening out side slopes to maximize volume and infiltration area can also reduce steep slope mowing risks and improve worker safety.