REPORT: Evaluating Inlet Sediment Traps

In 2010, the Georgia Soil and Water Conservation Commission (GSWCC) received funding to revise the Manual for Erosion and Sediment Control in Georgia. One of the parameters was to incorporate new BMP’s into the Manual. This was done by characterizing full-scale, installed performance of commonly used best management practices (BMPs) for sediment control at storm water manhole and curb inlets. Some of the specific BMPs tested included what the GSWCC refers to as inlet sediment traps, or inlet protection. Inlet sediment traps are commonly site-built and comprised of one or more generic components, such as stone, open-cell concrete blocks, fence posts, and/or wire fabric, or they may be pre-manufactured products such as geotextiles (silt fence), sediment retention fiber rolls (SRFRs) or proprietary 3-D structures. Critical elements of inlet sediment traps are their ability to: (a.) slow and/or pond concentrated runoff to encourage sedimentation, thereby reducing soil particle transport into the inlet, and (b.) trap soil particles upstream of an inlet.

Recognizing that the actual performance of BMPs is system or installation dependent, the GSWCC determined that a large-scale test that could incorporate full-scale “as installed” conditions would be the best evaluation procedure. To this end, the GSWCC selected a large-scale test procedure that generally conformed to ASTM D 7351, with a modification to present a concentrated flow, rather than sheet flow, to an area inlet. The procedure includes an inlet area comprised of an approximate 24-inch x 24-inch opening simulating a manhole inlet positioned at the center of an approximate 96-inch x 96-inch containment area. The BMPs tested were installed adjacent to the opening and exposed to simulated (concentrated) runoff. The measurement of sediment that passes through, over, and/or under the BMP compared to the amount in the upstream flow is used to quantify the effectiveness of the BMP in retaining sediments while allowing continued seepage. The measurement of water that passes through, over, and/or under the BMP compared to the amount of the upstream flow is used to quantify the effectiveness of the BMP in allowing continued seepage.

The test results appear to establish appropriate baseline performance characteristics for standard BMPs used in either unpaved or paved applications. The results of the testing reported herein strongly suggest that in both paved and unpaved applications, it is possible to differentiate between BMPs that provide maximum sediment retention and those providing maximum seepage. This may facilitate separate application-specific specifications for BMP systems.

Keywords: sediment trap, inlet protection, BMP, inlet testing, GSWCC, ASTM D 7351