TR-55 Graphical Peak Discharge

qp = qu Am Q Fp, where the unit peak discharge qu (csm/in) comes from the time of concentration Tc and the ratio Ia/P through the Appendix F regression, Am is the drainage area (mi^2), Q the runoff depth (in) from the curve number, and Fp the pond/swamp factor. TR-55 example 4-1 -- 0.39 mi^2, CN 75, a 6 in type-II storm, Tc 1.53 hr -- gives Ia/P 0.11, qu 269 csm/in, Q 3.28 in, and qp 344 cfs. Ties the time of concentration and the runoff depth into a peak flow. One homogeneous watershed with a single CN and main channel; no reservoir routing and no hydrograph. A design aid; the local drainage manual and the engineer of record govern.

qp = qu Am Q Fp (cfs). qu = 10^(C0 + C1 log10(Tc) + C2 (log10 Tc)^2) csm/in, with C0/C1/C2 from TR-55 Table F-1 by rainfall type (I, IA, II, III) and Ia/P. S = 1000/CN - 10; Ia = 0.2 S; Q = (P - Ia)^2/(P - Ia + S); Fp from Table 4-2.

The NRCS TR-55 (Urban Hydrology for Small Watersheds, 1986) Chapter 4 Graphical Peak Discharge method with the Appendix F Table F-1 regression, by name; verified against TR-55 example 4-1 (0.39 mi^2, CN 75, a 6 in type-II storm -> qp 345 cfs).

TR-55 is a free USDA NRCS publication; the peak-discharge equation qp = qu Am Q Fp, the Appendix F unit-peak-discharge coefficients, and the Table 4-2 pond/swamp factors are published in it.

Estimate. AHJ and licensed professional govern.

Field names used by the API: tc_hr, curve_number, rainfall_in, area_mi2, rainfall_type, qp_cfs, qu_csm_in, runoff_in, ia_over_p, fp

• Peak discharge qp = qu Am Q Fp (cfs)TR-55 Ch. 4
• Unit peak discharge log10(qu) = C0 + C1 log10(Tc) + C2 (log10 Tc)^2, coefficients by type and Ia/PTR-55 Appendix F Table F-1
• Pond/swamp factor Fp from percent pond/swamp area (1.00 at 0%, 0.75 at 3%)TR-55 Table 4-2
• Range Tc 0.1 to 10 hr, Ia/P 0.1 to 0.5, CN above 40, one homogeneous watershedTR-55 Ch. 4 limitations