REPORT: Evaluating Sediment Barriers

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 sediment barriers. Sediment barriers have traditionally been constructed of two rows of silt fence or one row of silt fence backed by straw bales for sensitive areas and one row of silt fence for non-sensitive areas, with the silt fence being categorized into three different types. More recently “wattles”, “socks” and other alternative BMP’s have been used. These sediment barriers are used as so-called “perimeter control devices” around construction and building sites to intercept sheet flows when no obvious low point or ponding capacity exists on-site.

Since there is relatively little performance data available for most BMPs, including sediment barriers, 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 sediment barriers 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 sediment barriers 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 that is being developed within ASTM for the evaluations. This proposed test method uses test plots having a slope of 3:1 and a 27 ft slope length. The test soil was classified as a Sandy Clay as shown on the USDA soil triangle. The sediment laden flow is generated by simulated rainfall falling on the slope, eroding the bare soil plots, and collecting against the sediment barrier at the toe of the slope. The rainfall sequence was run according to ASTM D 6459 – 2 in/hr, 4 in/hr, and 6 in/hr each for 20 minutes. All runoff seepage and associated sediment passing the sediment barrier was collected, dried, and measured. The measured soil loss value is used to calculate the P-Factor.

The Practice Management Factor, or P-Factor, from the Revised Universal Soil Loss Equation (RUSLE) of the USDA-ARS Agricultural handbook 703 is the reported performance measure from this testing. Total sediment loss and the associated rainfall depth measured during the testing are the principle data used to determine the P-Factor. The P-Factor thus calculated is the reported performance value. This facilitates product-to-product comparison of test results at a common point of the storm event. Additionally, using the regression equations for the protected and the control (or unprotected) conditions, the users of the test report can evaluate performance at other points in the model storm by selecting the R factor (and the corresponding A-Factor) that may fit local conditions and calculating the ratio.

In general, lower system seepage rates correlate with lower system sediment loss rates. Related to this, lower fabric permittivity rates parallel lower system seepage rates and thus lower sediment loss, and higher fabric percent open area (for woven fabrics) correlates with maintaining higher system seepage rates along with associated higher sediment loss rates.

It was not possible to make similar comparisons for non-fabric (i.e. non-silt fence) systems, since there are no standardized index tests for these 3-dimensional (3-D) materials. Still, it would be likely that these 3-D systems have lower open area and size (i.e. straight-thru open spaces) but as high or higher flow (similar to permittivity). This suggests that 3-D structures may be able to provide superior balance of properties (greater filtration and greater flow) as long as there is no piping, undermining, or overtopping. The data suggests that products fall into one of two categories: “High Retention” or “High Flow”. A lower P-Factor is generally associated with the High Retention systems, while High Flow systems typically have higher seepage rates. Straw bales are not recommended as sediment barriers for slopes greater than or equal to 3:1 and, perhaps, not even for lower slopes.

Keywords: sediment barriers, perimeter sediment control, BMP, slope testing, GSWCC, Method 11340