Detention Basin and Retention Pond Grading

Detention basin grading requires: verifying the outlet invert elevation within ±0.05 ft before any other grading (it controls all water volume calculations); shaping the basin bottom with ≥1% continuous slope toward the outlet; checking side slopes at 3:1 or as designed; and verifying the emergency spillway invert is at the correct elevation. A wrong outlet invert — even by 0.10 ft — changes the storage volume enough to fail stormwater management design criteria.

Outlet Invert Elevation Is Everything

Every volume calculation for a detention basin is based on the outlet invert elevation — the elevation at which water starts leaving the basin. If the outlet is 0.10 ft too high, the basin stores less volume than designed and may overflow during a design storm. If 0.10 ft too low, water releases too early and the basin does not provide the intended detention time. Always verify the outlet invert with a level shot before staking the rest of the basin. If the outlet is already installed and is off by more than ±0.05 ft, notify the engineer.

Basin Bottom Grading

The basin bottom must drain fully and continuously to the outlet, with no flat spots or reverse-grade areas that would leave standing water after a storm. Stake the basin bottom on a 25–50 ft grid, targeting a minimum 1–2% slope toward the outlet. After grading, verify the flowline at each stake station and confirm no reverse grade exists anywhere between the far end and the outlet.

Side Slope Grading

Most basins are designed with 2:1 to 4:1 side slopes. Set slope stakes at the top of bank and at the toe using the slope staking procedure (see Slope Staking). Verify as-built slopes after grading with a slope board or digital inclinometer. Slopes steeper than design may slough during heavy rainfall, particularly in wet or clay soils.

Verification Checklist

See also: Setting Drainage Swales and Channels to Grade and Cut and Fill Calculator.