The Detention Basin That Isn't the Size of the Plan

Orifice equation: Q = C A √(2 g h)

Solve for the head that passes 8 cfs: 8.0 = 0.6 × 0.785 × √(64.4 h) 8.0 / 0.471 = 16.99 = √(64.4 h) 64.4 h = 288.7 h = 4.48 ft

So the outlet does not reach the allowable release until the water surface is about 4.5 ft above the orifice invert. Sanity check at 4.0 ft of head: Q = 0.6 × 0.785 × √(257.6) = 7.6 cfs - consistent.

Why this matters: the outlet is the capacity limit, not the performance limit. The controlling question is whether the basin can hold the flood long enough that the inflow peak is attenuated to 8 cfs before the stage reaches 4.5 ft. If the stage rises past 4.5 ft, the release exceeds the allowable and the downstream creek sees a higher peak than pre-development - the exact thing the permit forbids.

The basin is short by 0.6 ac-ft, a 37.5% reduction. For the same inflow hydrograph, less storage means a higher stage and a higher discharge for the same 55 cfs inflow. The outlet will pass more than 8 cfs at peak and the 100-year release criterion is not met.

Do not assume the shortfall scales linearly with peak flow - storage routing is nonlinear and the peak is insensitive to the last increment of storage. Run the routing with the actual stage-storage and stage-discharge from the as-built survey rather than arguing from the 0.6 ac-ft figure.

Corrective action, in order:

1. Remove the access ramp and the sediment and re-survey. Part of the 0.6 ac-ft may be recoverable at essentially no cost; sediment accumulation is a maintenance item that will recur, so a sediment forebay and a maintenance schedule should be added.
2. If storage is still short, restrict the outlet with a smaller orifice or a weir, which lengthens the detention time and lowers the peak. Verify that the smaller outlet does not create a nuisance ponding/drainage time or mosquito problem, and re-check the emergency spillway.
3. Enlarge the basin - the most reliable and most expensive option; also check that the embankment and spillway are re-evaluated for the new stage-storage.

Recurring risk to flag: a 12 in. orifice is easily blocked by debris. Confirm that the design does not rely on a blocked orifice, and confirm the emergency spillway can pass the full 100-year inflow with the orifice plugged.

It compares the wrong quantity. Peak flow is only one of three things a detention requirement protects.

• Peak discharge: judged against the pre-development 8 cfs, at the correct stage - which is what the 4.48 ft calculation tests, and which the reduced storage fails.
• Runoff volume: the post-development site generates a far larger volume than the pre-development site (14 acres, essentially 100% impervious, versus the original cover). Detention attenuates the peak but cannot reduce the volume; the added volume still reaches the creek, and the basin's performance must be evaluated on the full hydrograph, not just the peak.
• Duration and timing of the peak: a basin with a restricted outlet shifts the peak later and stretches it. If the creek's pre-development peak arrives at a different time, the detention discharge can coincide with the downstream peak and increase flooding downstream even while the on-site peak is lower.

Second problem: 'the post-development peak leaving the site is small' is an assertion about performance that was never demonstrated - it is precisely the quantity the routing analysis is supposed to compute, and it cannot be assumed from the basin's existence.

Finally, the 1.0 ac-ft is measured below the emergency spillway crest; the permit condition usually ties performance to a specific design event, so the as-built condition must be re-routed against that event, documented, and reported to the reviewing agency - self-certifying compliance on an unverified basis is not a defensible position.