Sheetflow Construction Erosion and Sediment Control

October 20, 2017

The More Grass the Cleaner the Water

The Clean Water Act requires construction sites to meet water quality standards. One of the standards is turbidity, the measure of the cloudiness of water. Turbidity is measured in NTUs, nephelometric turbidity units, using a turbidity meter. The turbidity meter shines light through a water sample and measures the light intensity difference between the light emitter and collector; the cloudier the water, the higher the NTUs. The chief cause of turbidity in construction stormwater runoff is fine soil particles; fine silts and smaller. Persistent turbidity is caused by colloidal soil, particles which are so small the soil particle’s negative electrical charge is stronger than the force of gravity. This causes the particles to remain in the water column for long periods of time. Particles of this size do not settle out. The best way to keep the colloidal particles from entering stormwater is to protect the soil surface, both from the force of raindrop impact, as well as the shear stress of flowing water. Vegetation is the single best way to protect bare soil from these forces; the more vegetative cover the better. In this video, the affect of grass cover is apparent. Turbidity samples, taken from runoff from a grassy area with 70 to 80 percent soil cover is under 25 NTUs, the allowable discharge number.

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