Check dams are used in ditches to slow the velocity of flowing water, reducing erosive potential. An additional benefit of check dams is to allow some sedimentation in the ponded water behind the dam.
Sand and some larger silt can be removed using check dams; colloidal particles cannot. These microscopic particles, with a negative electrical charge, resist sedimentation and are the chief cause of turbidity, or cloudiness of the water.
This video clearly shows that the turbidity of water flowing in a ditch is the same after traveling through 8 check dams.
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