What is the maximum flow length of sheet flow from adjacent impervious areas?

What is the maximum flow length of sheet flow from adjacent impervious areas?

• A. 150 ft
• B. 125 ft
• C. 75 ft
• D. 175 ft

Answer: (C)

The idea is to keep runoff in sheet form long enough to spread over a filter or treatment area. Limiting sheet flow to 75 feet ensures the water stays as a thin, uniform layer, which promotes contact with vegetation and other treatment surfaces and reduces erosive energy. If the flow length were longer, the water is more likely to start concentrating into small channels or rivulets before reaching the BMP, which both lowers treatment efficiency and increases the risk of erosion. So the maximum sensible length for sheet flow from adjacent impervious areas is 75 feet.

The idea is to keep runoff in sheet form long enough to spread over a filter or treatment area. Limiting sheet flow to 75 feet ensures the water stays as a thin, uniform layer, which promotes contact with vegetation and other treatment surfaces and reduces erosive energy. If the flow length were longer, the water is more likely to start concentrating into small channels or rivulets before reaching the BMP, which both lowers treatment efficiency and increases the risk of erosion. So the maximum sensible length for sheet flow from adjacent impervious areas is 75 feet.