REPORT: Evaluating Channel Check Dams

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. Some of the specific BMPs tested included what the GSWCC refers to as check structures or check dams. Check dams have traditionally been constructed of straw bales, riprap mounds, and occasionally silt fence structures. More recently “wattles”, “socks” and other alternatives have been used. These check dams slow, or “check”, concentrated flows to make them less erosive until the associated channel can vegetate sufficiently to resist flow erosion. Critical elements of this protection are the ability of the temporary check structure to: (a.) slow and/or pond runoff to encourage sedimentation, thereby reducing soil particle transport downstream, (b.) trap soil particles upstream of a structure, and (c.) decrease soil erosion.

Since there is relatively little performance data available for most BMPs, including check dams, and the limited data that is available has generally been developed using widely differing protocols, the testing protocol chosen should, as much as possible, conform to an existing standardized procedure so that future check structure BMPs could be subjected to the same protocols and easily and reliably be compared to the results of this program.

Recognizing that the actual performance of many check structure 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 standard test method – ASTM D 7208 – for the evaluations. ASTM D 7208, “Determination of Temporary Ditch Check Performance in Protecting Earthen Channels from Stormwater-Induced Erosion” has been developed to simulate this condition. It uses full-scale channel flow in a trapezoidal channel with check structure(s) installed. The test protocol included one replicate each at increasing flow levels of 0.5, 1.0, and 2.0 cubic feet per second. The testing evaluated compost socks, straw bales, and 2”-10” rock checks, as well as, a silt fence check. The test soil was classified as a Sandy Clay as shown on the USDA soil triangle.

Soil loss and the associated flow depth and velocity measurements were made at numerous locations along the channel during the testing. This data was used to calculate soil accretion and loss volumes using cut/fill calculations based on the Simpson Rule. From this data a Soil Accretion Index (SAI) and a Clopper Soil Loss Index (CSLI) were determined.

In general, as a check dam gets taller it may be able to increasingly reduce channel soil loss by creating greater ponding and, thus, greater slowing of water. Yet, in the process, the check dam must provide greater structural integrity and adjacent scour resistance. The original single row straw bale system and the silt fence system both offered taller damming, but even at the lowest flow level they provide insufficient structural integrity and scour resistance to function effectively. Conversely, the compost sock, rock check, and the enhanced, i.e. double row + deep trench, (NRCS) straw bale systems provided the necessary balance between damming and scour resistance to perform effectively under all flow levels.

Keywords: check dam, BMP, channel testing, GSWCC, ASTM D 7208