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Start from a topic below. For anything else, write to [email protected].
Free calculators
- Curve number and runoffComposite CN from cover type and hydrologic soil group, then TR-55 runoff depth for a 24-hr rainfall.
- Time of concentrationTR-55 Tc from a sheet flow, a shallow concentrated flow and a channel flow segment, in hours and minutes.
- Rational method peak flowPeak runoff Q = Cf·C·i·A from the runoff coefficient, rainfall intensity and drainage area, with the frequency factor for the storm.
- Detention volume estimateStorage needed to hold post-development runoff to an allowable release rate, by the modified rational critical-duration method.
- NY water quality volumeWQv = P·Rv·A/12 from the 90th percentile rainfall, impervious cover and contributing area, per the NYSDEC design manual.
PondSheet
- How results are checkedFixtures, worked examples, and what PRELIMINARY means.
- Pricing and the trialOne price per firm, the 14-day trial, and what read-only means.
- About PondSheetWhat it does, what it does not, and who makes it.
- Terms of serviceIncluding who is responsible for a sealed report.
- Privacy policyWhat is collected, and how to have it deleted.
Where the methods come from
- TR-55, Urban Hydrology for Small Watersheds (USDA SCS, 2nd ed., June 1986)Runoff equation, curve number tables, sheet and shallow flow, minimum Tc.
- NRCS National Engineering Handbook Part 630, HydrologyWeighted runoff, the 100 ft sheet-flow limit, the unit hydrograph and routing.
- FHWA HEC-22, Urban Drainage Design Manual (4th ed., 2024)The rational method and its limits; the 3rd edition (2009) for the frequency factor.
- NYC DEP, Criteria for Detention Facility Design (2012)The critical-duration detention volume and its outflow ratios.
- NYS Stormwater Management Design Manual (July 31, 2024)WQv, channel protection, overbank and extreme flood criteria, freeboard.