Detention Basin Storage Estimator

Preliminary detention volume two ways: a Modified Rational critical-duration sweep against an allowable release rate, or a simple post-minus-pre runoff volume difference. Runs entirely offline; inputs persist in this browser via localStorage.

Use limits: This is screening-level sizing — enough to know whether the corner of the site set aside for the basin is remotely big enough, and to sanity-check a drainage report. Final design is reservoir routing: a real inflow hydrograph through a stage-storage-discharge relationship, per the local criteria (which also set the design storms, release rules, drawdown times, and safety factors). The Modified Rational sweep here uses a rectangular inflow approximation and full allowable release for the storm duration — a common preliminary form; jurisdictions each have their own flavor and theirs governs.

Method Sizing approach

Watershed (post-development)

Runoff coefficient C

Area (ac)

Time of concentration (min)

IDF Equation — i = a / (D + b)ⁿ (D in minutes)

Coefficients from the local hydrology manual for the detention design storm's return period. The sweep needs the full IDF relationship — a single intensity can't find the critical duration.

Release

Allowable release rate (cfs)

Max duration to sweep (min)

Runoff Volumes (e.g. from the TR-55 tool)

Post-development runoff volume

Pre-development runoff volume

Units

Same storm, same method, both sides — run the TR-55 tool twice (existing vs. proposed CN) and bring the volumes here.

Results

Critical duration	—
Intensity at critical duration	—
Peak inflow at critical duration	—
Required storage	—

How the Numbers Are Derived

The Modified Rational sweep

For each trial storm duration D (from Tc up to the sweep limit): intensity i = a/(D+b)ⁿ, inflow Q_in = C·i·A, and required storage V(D) = (Q_in − Q_allow) · D · 60 cubic feet — inflow volume for the duration minus what the outlet releases over the same window, using a rectangular inflow approximation. Short storms are intense but brief; long storms are big but weak — somewhere between, V(D) peaks. That maximum is the critical duration and the required storage. It is always longer than Tc, which is why sizing detention with the peak-flow storm alone undersizes it — the counterintuitive result this tool exists to make visible.

Volume-difference mode

V = V_post − V_pre for the same design storm. This is the retention/water-quality framing (capture the new runoff the development created) rather than the peak-shaving framing, and some criteria require one, the other, or the larger of both. It's also a quick reasonableness check on a Modified Rational answer.

Why routing governs final design

A real outlet doesn't release the allowable rate at all stages — orifice and weir flow vary with head, so early in the storm the basin releases less and fills faster than the sweep assumes.
Stage-storage is set by the actual grading; the last foot of a shallow basin holds far more than the first.
Criteria add requirements the sweep can't see: multi-storm performance (2-, 10-, 100-yr simultaneously), drawdown time limits (vector control is serious business in Southern California), freeboard, and emergency spillway sizing.
Rule of thumb: if the preliminary volume barely fits the available footprint, it won't survive routing. Leave margin.

References

The local county hydrology/drainage manual — governing method, storms, release rates, and drawdown requirements.

NRCS TR-55 / TR-20 and reservoir routing texts — hydrograph methods for final design.

ASCE MOP 77 / WEF MOP FD-20 — urban stormwater detention design practice.