Dewatering Discharge Through a Sediment Basin to an Impaired Creek
A 4.2-acre disturbed area drains to a sediment basin with a permanent pool 60 ft × 90 ft and 4 ft of usable storage above the forebay. Design criteria require 3,600 cf of storage per acre disturbed. A 250 gpm trench pump will run continuously for three weeks and discharge into the basin; the basin outlet is a riprap channel to a creek listed as impaired for sediment. The 10-year design storm produces an intensity of 5.0 in/hr for the 15-minute time of concentration, and the bare, compacted ground has a runoff coefficient of 0.45. The contractor plans to start pumping as soon as dewatering begins and to keep pumping until the trench is backfilled.
1. Compute the design peak inflow and check the required versus provided basin storage.
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Rational method: Q = C·i·A = 0.45 × 5.0 × 4.2 = 9.45 cfs for the 10-year storm.
Storage:
- Required: 3,600 cf/acre × 4.2 acres = 15,120 cf
- Provided: 60 × 90 × 4 = 21,600 cf (about 43% above the minimum)
Volume sanity check: 9.45 cfs × 900 s (15-min storm) ≈ 8,500 cf of runoff, well inside 21,600 cf. The basin is volumetrically adequate.
What the volume check does not prove:
- The outlet must pass the peak after storage, and the riprap channel must carry the peak outflow. If the outlet is undersized, the basin fills and the embankment overflows, which is worse than no basin.
- Freeboard above the 21,600 cf must be adequate for wave and debris.
- The forebay must be sized to drop the coarse load before it reaches the main cell, or the 4 ft of usable storage becomes sediment storage within one season.
2. Convert the pump discharge and determine whether the basin provides effective treatment for it.
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Convert: 250 gpm × 0.1337 cf/gal = 33.4 cfm = 0.557 cfs
If the outlet were blocked: 21,600 cf ÷ 0.557 cfs = 38,800 s ≈ 10.8 hours to fill the basin. The basin cannot serve as a holding tank; it must be a flow-through system with a functioning outlet from day one.
Detention time: 21,600 cf ÷ 0.557 cfs ≈ 10.8 hours at steady state. That is enough to settle sand and coarse silt but not clay-sized fines, which is exactly what a continuously pumped trench produces.
So the basin alone will not control turbidity. Options, in order:
- Discharge the pump through a weir tank or portable settling tank with a flocculant injection and metering, then to the basin.
- Route the pump to a vegetated buffered area rather than the creek outfall.
- Reuse the water for dust control or compaction, which removes the discharge entirely.
Run the pump only after the basin, forebay, and outlet are installed and stabilized, and stagger pumping to keep from slugging the basin.
3. What permit obligations and inspection requirements drive this decision?
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Coverage: 4.2 acres disturbed is well over 1 acre, so NPDES Construction General Permit coverage applies. A SWPPP must exist, be implemented, and identify the basin, the outlet, and the dewatering operation as BMPs.
Dewatering discharge: construction dewatering is a point-source discharge requiring permit authorization. It must not cause or contribute to a water quality standard violation. Because the receiving creek is already listed as impaired for sediment, the permit may carry a numeric turbidity limit (commonly 280 NTU daily maximum, with a reporting threshold far lower) and may require treatment, not just settling.
Inspection: a qualified person inspects the BMPs at least every 7 days and within 24 hours after a storm of 0.5 in or more; the basin must be cleaned when sediment reaches the design capacity, and records retained.
Monitoring and judgment: log pump rate, pH, and turbidity daily at the discharge point; sample as the permit requires. Starting the pump before the outlet works, or discharging visible plumes to an impaired creek, is both a permit violation and a practical enforcement target - a stop-work order costs far more than the settling tank.