Tools › Civil and Geotechnical › Hydraulics › Stormwater Runoff and Detention › Example
50-acre watershed before and after development, 25-year storm: Rational, TR-55 and unit-hydrograph peaks, detention storage and level-pool routing
| Tool | Stormwater Runoff and Detention |
| Source | U.S. Department of Agriculture, Soil Conservation Service (1986). Urban Hydrology for Small Watersheds, 2nd ed. Technical Release 55 (eqs. 2-1 to 2-4, Table 2-2, eqs. 3-1 to 3-4, eq. 4-1, Tables F-1 and F-2); NRCS National Engineering Handbook Part 630, Chapter 16 (2007), Table 16-1 and Appendix 16A; FHWA HEC-22, 3rd ed. (2009), eqs. 3-1, 8-17 to 8-22, 8-29, 8-30; NRCS WinTR-20 version 3.30 rainfall tables (types I, IA, II, III at 0.1-hour steps). Illustrative watershed with the 6.0 in / 3.6 in rainfall of the TR-55 examples. |
| Level | Professional |
Problem
A 50-acre pasture on group C soil (CN 74, C 0.30, time of concentration 45 min) is developed into 30 acres of quarter-acre lots, 10 acres of commercial area and 10 acres of open space. For the 25-year, 24-hour storm of 6.0 in (type II), find the peak runoff before and after development by the Rational method, the TR-55 graphical method and a unit-hydrograph computation, estimate the detention storage, and route the developed hydrograph through a basin with a 24-in orifice, a 4-ft weir and an emergency spillway.
Inputs to Enter
- Storm: 25-year, P 6.0 in, P_2 3.6 in, type II, I_a/S 0.2, no ponds, peak rate factor 484; IDF i = 83/(t + 12)^0.80 in/h, C_f 1.1
- Subareas: residential 1/4 ac 30 ac (C 0.50, CN 83), commercial 10 ac (C 0.85, CN 94), open space good 10 ac (C 0.25, CN 74), all soil group C; before: C 0.30, CN 74, t_c 45 min
- Time of concentration after: sheet 100 ft, n 0.24, s 0.02; shallow paved 600 ft, s 0.02; channel 1,500 ft, n 0.013, r 0.75 ft, s 0.005
- Basin: storage 0, 28,000, 60,000, 96,000, 136,000, 180,000, 228,000, 280,000, 336,000 ft³ at elevations 100 to 108 ft; orifice D 2 ft at 100 ft (C_o 0.6), weir L 4 ft at 103.5 ft (C_d 0.62), spillway L 20 ft at 107 ft (C_d 0.5)
Answers
- Composite C = 0.52, CN = 83.4 → 83; S = 2.048 in, I_a = 0.410 in, runoff 4.091 in after (3.185 in before, CN 74)
- t_c after = 0.224 + 0.058 + 0.062 = 0.344 h (20.7 min)
- Rational: i = 3.269 and 5.102 in/h, Q = 53.9 ft³/s before and 145.9 ft³/s after
- TR-55 graphical: q_u = 417.6 and 635.0 csm/in, q_p = 103.9 ft³/s before and 203.0 ft³/s after
- Unit hydrograph (Δt 2 min, T_p 0.223 h after): peaks 108.9 ft³/s at 12.33 h before and 216.3 ft³/s at 12.10 h after
- Storage: TR-55 curve V_s/V_r = 0.272 at q_o/q_i = 0.512, 201,886 ft³; modified rational 148,737 ft³ at a 57.4-min storm
- Routing: peak outflow 100.9 ft³/s at 12.33 h (allowable 108.9), peak stage 106.41 ft, peak storage 249,166 ft³, drawdown 25.7 h, mass balance residual below 1e-9 ft³
Notes
The tool's default inputs are this case; press Submit. The watershed and the IDF curve are illustrative (stated on the page); the built-in type II distribution is the NRCS table at 0.1-hour steps (WinTR-20 and WinTR-55 rainfall file; before 2026-11-12 the half-hour tabulation of HDS-2 Table 6.5, which gave hydrograph peaks of 99.5 and 173.3 ft³/s), and the hydrograph peaks are now within 7 percent of the graphical peaks. Checked by test_stormwater_runoff_detention.php (2,958 checks): TR-55 Table 2-1 and Examples 2-1 to 2-4, 3-1, 4-1 and 6-1 to 6-4, NEH Part 630 Example 16-1, HEC-22 Examples 3-1, 3-3, 3-5, 8-1 to 8-6 and the routing of Example 8-9 to every printed digit, HDS-2 Examples 8.2 and 8.3, and an independent numpy / scipy implementation on this case and 20 seeded watersheds (References/Stormwater/).