Have you ever wondered what role environmental agencies such as the Chester County Conservation District or Pennsylvania Department of Environmental Protection play in construction projects? One of their major roles is to review and approve the Erosion & Sediment Control Plan.
When you decide that you want to build something, it is easy to forget about the stormwater management and erosion control that will be required to get the project approved through the municipality, conservation district, and PADEP, if necessary.
So, what is erosion control, and what purpose does it serve in the civil engineering industry? Well, for starters, during the engineering portion of a project, we will determine an area called the limit of disturbance, which is exactly what it sounds like, the edge of where the grass will be dug up and the earth disturbed. In this area, the soil is exposed, and has a much higher CN, or curve number value, creating more runoff than if there was still grass there.
This runoff, which not only now has a greater volume, will typically also carry more solids suspended in solution from the disturbed soil (sediment). For smaller projects, erosion controls such as silt fence and compost filter sock will be placed downslope of the disturbed area to decrease the rate at which the runoff flows and capture the sediment. For larger projects, erosion controls such as sediment basins with baffles are used to hold the runoff and encourage the suspended solids to come out of solution before exiting the basin through the skimmer.
Prior to the start of construction, a survey crew will mark the limits of disturbance for a project and the contractor will place the Erosion & Sediment Control BMPs (Best Management Practices). This includes things such as the silt fence, compost filter sock, or a sediment basin, as discussed in the above paragraph. When a sediment basin is constructed, it will often be used as an infiltration basin once the site is stabilized. In this case, the sediment that has accumulated at the bottom of the basin will be removed and amended soils will replace them to help encourage infiltration.
Just like stormwater management, erosion control measures can be expensive, so why is it such a big deal if a little bit of sediment washes into the creek downslope of a construction project? Well, if everyone just let their sediment flow into the creek, it would, and already does, have serious impacts. In terms of environmental concerns, increased sediment deposition has been shown to cause algae blooms and reduce water depth, causing increases in temperature in the aquatic environment. In terms of economic impacts, there is a constant battle in our navigable waters to keep them safe for ships to pass, requiring extensive and costly dredging.
If any of this information sounds interesting to you, below is a link to the PADEP Erosion and Sediment Pollution Control Manual which goes into much further detail: