Stormwater Runoff & Detention

Pre- and post-development runoff by the NRCS TR-55 method: curve numbers, time of concentration, runoff and peak flow for the 1- to 100-year storms, the Pennsylvania CG-1 volume increase, a release-rate check and a detention storage estimate, with every equation on the calc sheet.

TR-55 (NRCS, 1986)
PA Stormwater BMP Manual, CG-1

Example loaded: a 12-acre, 12-lot subdivision in Chester County, PA. Change any input.

Project and rainfall Project

Rainfall distribution Ponds and swamps % of area

24-hour rainfall depths inches, by return period (years)

Land cover

Pre-developmentPost-development

Cover type (TR-55 Table 2-2)SoilAcres

Model pre-development cover per PA DEP CG-1: non-forested pervious areas as meadow (good), and 20% of existing impervious area as meadow.

Time of concentration

Pre-developmentPost-development

Set a length to 0 to skip a segment. Sheet flow uses the 2-year depth above.

Peak rate control Allowable release rate % of the pre-development peak, same storm

CG-1 asks for no increase in peak rate for the 1- through 100-year storms. Many municipal ordinances and Act 167 plans set lower release rates.

Results

Project—

Drainage area—

Curve number, pre → post—

Method—

Checked against the TR-55 worked examples

The values in the last column are computed live by the same engine that runs the tool. TR-55 reads the unit peak discharge from its chart (270 csm/in for Example 4-1); this tool uses the Appendix F equation for the same curves.

Case	Quantity	TR-55	This tool
Example 2-1 · pasture (good), CN 70, P = 6.0 in	Q (in)	2.81	—
Example 2-2 · ½-acre lots on 75 ac HSG B and 100 ac HSG C, open space (good) on 75 ac HSG C	CN / Q (in)	75 / 3.28	—
Example 2-3 · CN 77, P = 6.0 in	Q (in)	3.48	—
Example 3-1 · sheet, shallow concentrated and channel flow	T_t (hr) and T_c	0.30 + 0.24 + 0.99 = 1.53	—
Example 4-1 · 250 ac, CN 75, T_c 1.53 hr, P = 6.0 in, Type II	q_u (csm/in) / q_p (cfs)	270 / 345	—