The energy of rain water droplets impacting the soil particles, or the weight and velocity of water moving against soil particles, causes them to detach and become mobile — suspended in the water. Erosion control products and practices attempt to minimize that energy by covering the soil or conditioning it in some manner. Sediment control is containing the sediment on the construction site, after it has become mobile. And once it becomes mobile, it can be very difficult to control. A major factor is the tremendous volume of runoff that can be generated from a single storm event. A one inch storm on a single acre can generate more than 27,000 gallons of storm water!  If the drainage area happened to be flowing to a single low point, what could possibly control the volume, either from a structural or an effectiveness viewpoint?

For a sediment control structure to be effective, generally the water must be still to allow sedimentation to occur. There are three sizes of soil particles that make up sediment — sand, a relatively large particle; silt, much smaller particle; and clay, a very small particle. Sand falls out of water quickly once stilled, silt particles somewhat later, and clay particles can take days to clear from the water. Probably the best sediment control device is a properly constructed and vented sediment pond because it requires less maintenance and should be able to handle a larger storm event. Available space on a construction site for a properly sized sediment pond is often a challenge because developers want to develop the entire site at once. Achieving proper discharge for the sediment pond is also a challenge — gaining enough time for sedimentation to occur versus waiting enough time to release the water before the next storm event.

Other sediment devices would include silt fence of several varieties, foam-structure silt fence, compost berms and logs, and socks filled with different types of supposed filtering media, and daily new products. These all have varying degrees of effectiveness.

Please click here for a list of key contacts for the County's Erosion and Sediment Control program.