Specific Measures for Erosion and Sediment Control

Minimizing Impacts at the Local or Site Level

Erosion Control

This is probably the most effective way of reducing or preventing soil erosion. The amount and the type of vegetative cover determine the amount of soil erosion taking place at a particular site. The vegetation absorbs the energy of the falling rain and slows down the velocity of the runoff thereby reducing its capacity to carry sediments. The grass or vegetative cover also acts a filter to catch sediments and helps maintain the absorptive capacity of the soil. The roots of the plants hold the soil in place and prevent it from being carried off as sediments with the runoff.

Temporary seeding also makes the site green and more attractive for neighbors and potential buyers. A market survey conducted by Purdue University students (Herzog, 1997) to investigate factors which influence homebuyers, realtors and developers when buying/selling a lot in a residential housing development found that vegetated lots are perceived to be more important. Realtors perceived vegetated lots to be worth more than barren lots and thought that homebuyers would be willing to pay more for grassed lots. The added lot value is particularly significant because the price differential of seeding the plots to the added value is about 200% more than the cost involved in vegetating the plot.

The study (Herzog, 1997) also found that effective vegetated cover for erosion control has not been widely implemented even though most developers are aware of the regulations and environmental benefits of erosion control. This is often because to do is seen more as complying with rules and regulations than the added economic benefits derived from it. In fact, in some cases, developers using such erosion control practices see it as an apparent cost disadvantage. Furthermore it is not seen as a major economic advantage because the developer does not have to pay to mitigate the environmental impact caused by sediment leaving the site. That cost is borne by the taxpayers.

Construction phasing

This involves selective clearing of land that is to undergo development. Only land that has to undergo immediate construction is cleared and developed. This is repeated till the project is over. This prevents large areas from being bare and undergoing erosion. This process might be perceived as more expensive but it is more economical in the long run because soil does not have to be replaced after construction, it does not have to be removed from sewers, storm water basins and street.

Detailed information about construction site erosion and sediment control management measure can be found at the Environmental Protection Agency (EPA) http://www.epa.gov/OWOW/NPS/MMGI/Chapter4/ch4-3a.html Information about measures that can be taken to reduce soil erosion at plot level can also be obtained from the following web page: http://www.abag.ca.gov/bayarea/enviro/erosion/eyoudo.html

Sediment Control

Since it is not possible to prevent erosion altogether from construction sites some kind of sediment control method must be used to remove soil that is carried in runoff. Some of the measures that can be taken are:

Sediment Ponds

Ponding the water is the most effective way of sediment control. Ponding works by slowing down the water so that it can no longer carry the sediments within it. The sediments settle down and the water on top can be drained off. This process may take just a few minutes for heavier sediments such as sand, but hours or even days for finer sediments such as silt or clay. It is important to design a sufficiently large sediment pond because the water that the basin receives might include areas outside the actual plot being developed.

Gravel Entrance

Gravel entrances in construction sites prevent soil from the site being carried off onto the streets and thereby later clogging the drainage for the streets. Care must be taken while making gravel entrances that the entrance is wide enough for construction vehicles. It is also essential that the depth be maintained at a minimum of 6inches. Adding stones when required can do this.

Silt Fences

These are one of the most common forms of sediment control. It is not very effective in ditches or channels though. The stakes must be placed at the bottom of the slope. If the slope is steep or very long the fence must be placed further from the base of the slope. It is important to monitor the sediment build up at the fence. The sediment deposits need to be removed when they reach the fence height. Care must be taken that the filter fabric is buried deeply enough that no water can run underneath it.

Other sediment control techniques include downspout extenders, curb inlet protection, drop inlet protection, straw bales etc. Information on related things can also be found at the following web sites: http://www.epa.gov/owow/nps/education/runoff.html

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