Submitted to: Environmental Protection Agency
Publication Type: Government Publication
Publication Acceptance Date: 1/11/2005
Publication Date: 1/11/2005
Citation: Stevens, E., Barfield, B.J., Hunt, S., Hays, J.S. 2005. Filter fence design aid for sediment control at construction sites. Environmental Protection Agency. EPA/600/R-04/185. 2005 CDROM.
Interpretive Summary: Sediment is the number one pollutant in our nation’s streams, lakes, rivers, and other water bodies. Sediment in runoff waters is damaging to the environment and even more so because of the constituents (i.e. agricultural and industrial chemicals) that attach themselves to the sediment. This ultimately makes environmental policy makers and regulators more aware of the increasing water quality issues related to sediment release. Construction sites are typically primary sources of undesirable sediment releases due to the vulnerability of the exposed soil that results from the clearing of vegetation when preparing the site for construction. The installation of filter/silt fence is a common approach for controlling sediment; however, it is not generally a successful practice. The objective of this research was to develop a design aid for locating areas where silt fence can be an effective sediment control. The development of this design aid required the ability to mathematically model the delivery of runoff and sediment to a silt fence from the drainage area, erosion along the toe, and the behavior of water impounded behind the fence. To assist with the development of the design aid, a limited series of flume experiments were completed, and a comprehensive series of field-scale tests were conducted. Observations made during a series of construction site visits and during the field experiments were summarized into a series of recommendations for silt fence siting, installation, and maintenance. The model was incorporated into a spreadsheet wherein the user can enter site, rainfall, and silt fence fabric information, run a hydrologic/hydraulic computation, and then assess the likelihood of failure and the performance of the silt fence. The user can vary parameters and see the impact on performance and thereby make the best possible use of the silt fence on a particular construction site, and finally, the model can assist the user in determining when maintenance may be required for the silt fence.
Technical Abstract: The focus of environmental policy and regulation is increasing on water quality issues. Particularly, there is a more widespread awareness that sediment is one of the most prevalent pollutants and that the impacts of excess sediment released into lakes and rivers can be as damaging as those caused by agricultural or industrial chemicals. Due to their nature, construction sites are typically principal sources of undesirable sediment releases. To make construction activity easier, sites are generally cleared of all vegetation. The exposed soil is then made further susceptible to erosion by being disturbed by grading and vehicle traffic. Frequently, the only action taken to attempt to control sediment releases is the installation of a filter/silt fence. This approach is not generally successful, for several reasons: • The fence is not installed as recommended by existing guidelines. • The fence is not adequately maintained over time. • The fence is not located for effective control of sediment. • The site is not suitable for a silt fence. 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, 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 the highly disturbed construction site environment, particularly since it is likely that the soil at these sites is typically excavated and replaced. Accordingly, these relationships either required adjustment or new relationships had to be developed. In addition, the hydraulics of flow through and along the silt fences had to be modeled as 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 merely required an adjustment. Observations made during a series of construction site visits and during the field experiments were summarized into 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 then assess the likelihood of failure and the performance (in terms of sediment trapped) of the silt fence. The user can vary parameters and see the impact on performance and thereby make the best possible use of the silt fence on a particular construction site. Finally, the model can also assist the user in determining when maintenance may be required.