|Filter Fence Design Aid for Sediment Control at Construction Sites (70 pp, 5.11 MB) (EPA/600/R-04/185) September 2004
The focus of environmental policy and regulation is increasingly on water quality issues. Particularly, there is a more widespread awareness that sediment is one of the most prevalent pollutants and that the effects of excess sediment released into lakes and rivers can be as damaging as those caused by agricultural or industrial chemicals.
Construction sites are the typical source of undesirable sediment releases. To make construction activity easier, sites are generally cleared of all vegetation. The exposed soil is susceptible to erosion by grading and vehicle traffic. Frequently, the only action taken to control sediment releases is the installation of a filter/silt fence. This approach is not generally successful, for several reasons:
The first two items can best be addressed through public education along with adoption and enforcement of regulations. The third and fourth items can be addressed through development of a design aid, which was the objective of this research.
Development of the design aid required the ability to mathematically model the delivery of runoff and sediment to a silt fence from the drainage area, the erosion along the toe, and the behavior of water impounded behind the fence.
Many of the functions in the design aid were adopted from well-known, established modeling practices. However, existing relationships describing sediment delivery and concentrated flow erosion are not applicable to a highly disturbed construction site, particularly because the soil at these sites is usually excavated and replaced. Accordingly, this document provides adjustments to these relationships or options for further development. In addition, the hydraulics of flow through and along the silt fences had to be modeled because there were still gaps in understanding these mechanics.
To develop the additional information needed to complete a silt fence model, a limited series of flume experiments was completed and a comprehensive series of field-scale tests was conducted. Sufficient information was obtained for a first-generation model. The purpose of the field tests was to study erosion along the toe and quantify the amounts of water and sediment delivered to the fence, flowing along the toe, and flowing through the fence. Primarily, the field data were used in the model development. In some cases, the existing relationships required adjustment.
Observations made during a series of visits to construction sites and during the field experiments were summarized in a series of recommendations for silt fence siting, installation, and maintenance. The design algorithms were incorporated into a spreadsheet model wherein the user can enter site, rainfall, and fabric information, run a hydrologic/hydraulic computation, and assess the likelihood of failure and the performance of the silt fence. The user can vary parameters to see the impact on performance in order to make the best possible use of the silt fence on a particular construction site. Finally, the model can assist the user in determining when maintenance may be required.
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