Reusing a Farm Pond as the Sediment Basin: Storage and Drawdown Judgment
Your firm is designing erosion and sediment control for a 22-acre residential subdivision. Grading will disturb 14.5 acres in one phase over 9 months on Type C soils with 3–8% slopes. The state construction general permit requires a sediment basin with 3,600 ft³ of storage per disturbed acre and drawdown in no more than 48 hours. The owner wants to reuse an existing farm pond on the property with 38,000 ft³ of usable storage below the emergency spillway and an average water surface area of 9,500 ft² at operating stage. The pond also receives runoff from 9 acres of undisturbed pasture that will remain in grass. The contractor proposes a 6-inch orifice dewatering from a 4-ft operating depth and asks you to sign off so grading can start Monday.
1. Does the existing pond satisfy the storage requirement? Show the calculation and state the decision you would issue.
Read the worked answer
Required storage: 14.5 disturbed ac × 3,600 ft³/ac = 52,200 ft³ Available: 38,000 ft³ below the emergency spillway Deficit: 52,200 − 38,000 = 14,200 ft³, about 27% short
In area terms the pond covers 38,000 ÷ 3,600 = 10.6 disturbed acres, not 14.5.
Two refinements make the number worse, not better:
- If the permit reserves dead/sediment storage (many states require roughly half the total volume be held for sediment), only ~19,000 ft³ is live settling volume, and the area the pond can actually serve drops to about 5.3 ac.
- The 9 ac of pasture does not count toward the disturbed-area requirement, but its inflow does consume storage and does pass through the embankment. The emergency spillway must pass the full 23.5 ac contributing area, not 14.5 ac.
Decision: Do not sign off. Defensible options are (a) add at least 14,200 ft³ of live storage by excavation, embankment raising, or a berm, on top of any reserved sediment storage; (b) build a supplementary basin for the balance of the drainage; or (c) constrain phasing so no more than 10.6 ac - fewer if sediment storage is deducted - is exposed at any one time, with each area stabilized before the next is opened.
2. Is the proposed 6-inch orifice acceptable for the 48-hour drawdown criterion? Show the check and identify what you still need to confirm.
Read the worked answer
Orifice discharge: Q = C·A·√(2gh). With A = π(0.25 ft)² = 0.196 ft², C ≈ 0.6, and h = 4 ft, √(2×32.2×4) = 16.05 ft/s, so Q = 0.6 × 0.196 × 16.05 ≈ 1.9 cfs at full head.
Drawdown for a pond of constant plan area: t = [2·A_s / (C·a·√(2g))] × (√h₁ − √h₂) t = [2 × 9,500 / (0.6 × 0.196 × 8.02)] × (√4 − 0) t = (19,000 / 0.945) × 2.0 ≈ 40,200 s ≈ 11.2 hours
Against a maximum 48-hour criterion, 11.2 h passes. Before signing, confirm:
- Direction of the criterion. Some permits set a minimum detention (e.g., 24–48 h) to let fines settle. If that is the case here, 11.2 h fails and the orifice must be reduced - a smaller orifice gives a slower drain but raises clogging risk.
- Reference elevation. The time must be computed from operating stage down to the sediment-storage elevation, not to the bare bottom, and excluding the volume reserved for sediment.
- Clogging. A 6-inch orifice receiving 23.5 ac of runoff in a farm pond will trash up. Require a trash guard/gravel jacket and a cleanout method, or the basin sits full and the next storm passes untreated over the spillway.
3. The contractor argues the pasture drainage has always crossed the pond and 'nothing changes.' How do you handle the run-on in the design, and what do you tell the owner about risk?
Read the worked answer
Quantify before arguing. Under the Rational method the pasture in good grass has C ≈ 0.2 versus roughly 0.6 for bare graded ground, so 9 ac of pasture behaves like about 3 ac of bare ground in peak-flow terms. That is real but secondary - the governing problem remains the 14,200 ft³ storage deficit, and diverting the pasture does not fix it.
Options and their implications:
- Include the pasture in the design. Honest but expensive: sizing for the combined inflow increases required storage further, and the pond will be permanently loaded, so a cleanout schedule and a sediment-storage reserve become mandatory.
- Divert the pasture around the basin. This removes a chronic sediment source and reduces required storage, but the diversion needs a stabilized channel and a protected outlet. Without that you have simply relocated the erosion, and the new discharge point may require a permit modification and downstream-owner notification.
- Do neither. Unacceptable: the spillway is then sized for the wrong area.
Tell the owner plainly: the pond is an existing embankment on their property that will now be subject to much higher and more frequent inflow. Overtopping or embankment distress is their liability, and the permit makes them the operator. The design decision belongs in writing before grading starts, not in a phone call after the first storm.