Hydraulics of flow through and along silt fences

Silt fence is a widely used and mis-used construction site BMP. Limitations imposed by the sites themselves make following recommended installation practices difficult to follow, particularly keeping the fence on contour. The results of off-contour installation are the development of concentrated flow with erosion and failure along the toe and the development of overtopping at low points. The objectives of this project are to (1) observe the failure mechanisms, particularly erosion and undercutting along the toe, through field testing; (2) improve algorithms for predicting sediment trapping through incorporation of field testing data; (3) develop a mathematical simulator for silt fence performance; and (4) use the simulator to develop user-friendly design aids to help site managers assess if silt fence will be structurally sound under anticipated rainfall and sediment loading and to determine if water quality objectives will be met. Integral to producing (3) and (4) is the ability to model simultaneous flow through and along the fence. Concentrated flow along the toe of silt fence was studied in a rectangular flume with a solid wall on one side and silt fence on the other. The slope and flow rate were varied, and for each slope and flow rate the discharge through the fence and at the downstream end of the flume were measured. The data revealed that for a given slope, discharge through the fence was a function of head. The distribution of flow through and along the fence was primarily controlled by the slope of the flume. A model was developed that gave the flow through the fence as a function of slope, velocity head, and static flow under the same head. The model gave accurate predictions, but was only applicable to the specific fabric used in the development.