Sediment Basin and Trap Design Flashcards

What is the hydraulic residence time of a sediment basin?

Answer: The average time water spends in the basin, which determines how much sediment can settle out

Hydraulic residence time is the average time water spends in the basin, calculated as basin volume divided by flow rate; longer residence time allows more and finer particles to settle.

Which particle size is the most difficult to remove in a sediment basin?

Answer: Clay and fine silt particles

Clay and fine silt particles have very low settling velocities due to their small mass, requiring very long residence times or coagulation and flocculation treatment to be effectively removed.

Polyacrylamide (PAM) is sometimes applied to sediment basin inlets to:

Answer: Cause fine clay particles to flocculate and settle more rapidly

PAM causes fine clay and silt particles to aggregate into larger flocs that settle much faster than individual particles, significantly improving sediment removal efficiency in basins.

The principal spillway of a sediment basin should be sized to pass the:

Answer: Design storm flow while maintaining water quality treatment in the basin

The principal spillway is sized to safely pass the design storm flow while maintaining the water surface elevation within the basin at a level that provides adequate treatment before discharge.

Why should the embankment of a temporary sediment basin be inspected after every significant storm?

Answer: To identify signs of seepage, settlement, or erosion that could lead to embankment failure

Post-storm inspections identify early signs of embankment distress including piping seepage, settlement, cracking, or erosion that must be corrected before they progress to catastrophic failure.

What practice should be followed when decommissioning a temporary sediment basin at the end of construction?

Answer: Stabilize or remove the embankment, redistribute accumulated sediment to upland areas, and revegetate the basin area

Proper decommissioning requires safely removing or breaching the embankment, redistributing accumulated sediment to stable upland areas, and revegetating the disturbed area to achieve final stabilization.