TR-55 Detention Storage Volume

Given the peak inflow qi (from a graphical peak-discharge calculation), the allowable peak outflow qo the downstream channel or ordinance permits, and the runoff Q and area, it estimates the storage Vs a basin must hold to throttle qi down to qo. Vs/Vr = C0 + C1 (qo/qi) + C2 (qo/qi)^2 + C3 (qo/qi)^3 from figure 6-1 (Table F-2 coefficients by rainfall type), Vr = 53.33 Q Am (acre-ft), Vs = (Vs/Vr) Vr. TR-55 example 6-1 -- a 0.117 mi^2 type-II watershed whose developed 360 cfs peak must be held to the channel's 180 cfs (qo/qi 0.5), Q 3.4 in -- gives Vs/Vr 0.28, Vr 21.2 acre-ft, and Vs 5.9 acre-ft. An approximate preliminary single-stage size; the final basin comes from a stage-storage routing. A design aid; the local drainage manual and the engineer of record govern.

Vs/Vr = C0 + C1 r + C2 r^2 + C3 r^3 (r = qo/qi), C0-C3 from TR-55 Table F-2 by rainfall type (I/IA vs II/III). Vr = 53.33 Q Am (acre-ft). Vs = (Vs/Vr) Vr.

The NRCS TR-55 (Urban Hydrology for Small Watersheds, 1986) Chapter 6 approximate detention-storage sizing with the figure 6-1 / Appendix F Table F-2 regression, by name; verified against TR-55 example 6-1 (0.117 mi^2 type-II, 360 cfs held to 180 cfs -> Vs 5.9 acre-ft).

TR-55 is a free USDA NRCS publication; the storage-ratio equation, the figure 6-1 coefficients, and the Vr conversion are published in it.

Estimate. AHJ and licensed professional govern.

Field names used by the API: qi_cfs, qo_cfs, runoff_in, area_mi2, rainfall_type, vs_acreft, vs_vr, qo_qi, vr_acreft

• Storage ratio Vs/Vr = C0 + C1 r + C2 r^2 + C3 r^3, r = qo/qiTR-55 figure 6-1 / Table F-2
• Runoff volume Vr = 53.33 Q Am (acre-ft), Q the curve-number runoff, Am in mi^2TR-55 eq. 6-1
• Storage volume Vs = (Vs/Vr) VrTR-55 eq. 6-2
• Range/use qo/qi about 0.1 to 0.8, single-stage preliminary size, final by stage-storage routingTR-55 Ch. 6 limitations