Sediment Basin Function and Maintenance

​Sediment basins are designed to trap sediment from a construction site. If not maintained properly the basin can become filled with sediment and algae. While keeping sediment and algae out of the basin may seem like an impossible task, there are steps that can lessen the buildup and costly maintenance associated with dipping out or treating the basin. The following article covers sediment basin function, maintenance, and considerations to reduce algae.
Sediment Basin Basics
A sediment basin is a settling pond. By slowing down the flow of sediment-laden runoff, solids can settle out and sink to the bottom of the basin. The sediment basin is constructed as a first step, prior to land disturbing activities. The settling basin is usually converted to a wet retention basin or a dry detention basin at the end of the construction project. During construction, the basin functions for sediment control while after construction, the basin functions for stormwater flood control and water quality improvement practice.
How it Works 
As construction site runoff flows into the basin its flow rate is slowed on reaching the basin. Heavier sediment particles sink to the bottom while clean water remains in the top dewatering zone. As the water makes its way to the outlet a skimmer pulls water from near the surface and directs it through the principal spillway. During extreme rain events water would build up in the basin then flow through the riser and the principal spillway.

Figure 1 Components of a settling basin

The depth of the Dewatering Zone and the Sediment Zone are spelled out in the design plans. The water quality function of the basin depends on its ability to collect sediment and keep it out of rivers and lakes.

Figure 2 Water and sediment storage in a basin

Maintenance for Proper Function and Control of Algae
For a basin to function, accumulated sediment must be removed from the sediment storage zone once it exceeds 50 percent of the minimum required sediment storage design capacity and prior to the conversion to the post-construction practice. Excess sediment in the basin decreases the depth of the dewatering zone, brings in other potential pollutants, such as nutrients. Sediment buildup also increases the maintenance required to keep the basin functional.  
The shallow water also allows additional sunlight to make its way to the bottom of the basin further warming the water. Shallow water promotes the growth of vegetation, algae, and mosquitos. Algae can increase maintenance needs by creating additional solids that settle to the basin bottom and fill the sediment storage zone with muck. Some blue green algae can also produce toxins making the water unsafe.
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Prevent Early Fill-up of the Sediment Storage Zone
The following steps can help minimize maintenance needs while keeping the basin functional.

1. Stabilize the inner bank of the basin as early as possible utilizing grass seed and straw matting.
2. Remove sediment from inflow into the basin by utilizing stabilized grass swales, silt fence, or filter sock and other best management practices on the construction site.
3. Excavate the construction site in phases so that less of the dirt is exposed and subject to erosion.
4. Keep the disturbed area of the site as small as possible.
5. Stabilize the construction site as soon as possible.
6. Have a pond management company treat algae blooms before they become severe.
7. Dip sediment once it exceeds (50%) of the minimum required sediment storage and prior to conversion to post-construction.

Prevention of excess sediment in the basin is often more cost effective than bringing in heavy equipment to dip out the basin and then hauling off the sediment. These steps can also reduce resident and local homeowner association complaints down the road.
 
Additional information can be found in the Rainwater and Land Development Manual, Chapter 6

​Questions? Contact our office at 513.695.1337