Stormwater Runoff Hydrology Calculator 
Determine Rainfall Runoff Peak Discharge for Watersheds using TR55 Method 

Register to enable "Calculate" button. Units in stormwater runoff peak discharge calculator: cfs=ft^{3}/s, cm=centimeter, ft=foot, gal=U.S. gallon, gpm=gal/min, hr=hour, km=kilometer, m=meter, mi=mile, min=minute, s=second. Introduction
Top of Page Though the TR55 document mentions specific units (all English) for its equations, our hydrology calculation allows a variety of input and output units (English and metric). We have tried to make the calculation useful for the international community. Unfortunately, TR55 only presents rainfall distribution maps for the USA. Therefore, nonUSA users need to determine whether a typical 24hr rainfall resembles a Type I, IA, II, or III distribution and determine 24hr rainfalls from local sources. Equations (SCS, 1986)
Top of Page where: A = total watershed area (mile^{2}). CN = overall curve number for the watershed. F_{p} = pond and swamp adjustment factor from Table 42 in SCS (1986) to which we fit a 3rd order polynomial; input a number in the calculation for the % of watershed area (0 to 5%) occupied by ponds and swamps unless you accounted for ponds and swamps in your curve numbers. I_{a} = initial abstraction (inch); losses before runoff begins (surface depressions, interception by leaves, evaporation, infiltration)  SCS determined the above equation for I_{a} after numerous studies. P = precipitation (inch) for 24hr duration storm of return period for which you are interested. Q = depth of runoff over entire watershed (inch). Q_{p} = peak stormwater discharge (cfs). Q_{u} = unit peak discharge (cfs/mile^{2}inch); computed from equations given in SCS (1986) Appendix F representing its Figures 4I thru 4III for various rainfall distribution types; if you are outside the USA, use the rainfall distribution type that best represents your typical storm. s = potential maximum watershed water retention after runoff begins (inch). T_{c} = time of concentration for the watershed (hr); time for stormwater runoff to travel from the furthest distance (by time) in the watershed to the location where you wish to determine Q_{p}. Our stormwater hydrology calculator allows the user to divide a watershed into a maximum of five subregions represented by different curve numbers. Then, the overall curve number and total area are computed. Alternatively, if there are more than five hydrologic subregions, you may compute the overall curve number by hand and enter that value into our calculation. Table of curve numbers as a function of land use. Overall curve number is computed from: After decades of research, SCS (1986) indicates that there are typically three distinct stormwater runoff patterns in a watershed  sheet flow, shallow concentrated flow, and channel flow. Sheet flow occurs in the upper reaches of a watershed and persists for a maximum of 300 ft. After flowing in sheets, water then typically becomes less sheetlike and more concentrated. Following shallow concentrated flow, stormwater typically collects in natural or manmade channels. Each of the flow patterns requires a unique mathematical hydrology expression: where: L = length of flow pattern (ft); include all wiggles in channels. n = Manning's n value; for sheet flow, n represents the ground cover to a depth of about 1.2 inches (3 cm); for channel flow, n represents bank full conditions for an open channel or full conditions for a culvert (Manning n's for channel flow were assembled from Manning's n values). P_{2} = 2yr return period, 24hr duration precipitation for the geographic region where your watershed is located (inch); click for USA rainfall maps. R = hydraulic radius (ft) of bank full open channel or culvert flowing full; computed automatically if channel crosssection dimensions are input. S = average ground slope of each flow pattern (ft vertical/ft horizontal). T_{c} = time of concentration for the watershed (hr); time for runoff to travel from the furthest distance (by time) in the watershed to the location where you wish to determine Q_{p}. T_{t} = travel time for flow regime of interest (hr)  sheet, shallow concentrated, or channel flow. V = average velocity of water in each flow regime (ft/s). For channel flow, our stormwater calculator allows you to input the type of channel and the crosssection dimensions. Channel flow information is used for computing channel travel time. SCS (1986) states that bank full dimensions for open channels (or full flow conditions for culverts) should be used for this calculation. The diagrams below indicate the types of channels that are coded into our hydrology calculation. The hydraulic radius (R) is calculated by our program, but is provided below for your information. (R is used in the Manning equation to determine flow velocity and then travel time.) If your channel does not match one of the four types shown below, our program can still be used to compute travel time: You should compute R by hand for your channel, then select "Circular Culvert" and enter 4R for the culvert diameter. When our program computes R, it will compute R=D/4=(4R)/4=R, so the R used in the Manning equation will be what you computed. Runoff Curve Numbers
Top of Page Table of Stormwater Runoff Curve Numbers (SCS, 1986)
Error Messages
Top of Page "Overall CN must be 40 to 100." The runoff and peak discharge hydrology calculations are only valid for CN between 40 and 100. If overall CN is input, runoff will be computed. If CN for subregions is input, runoff will not be computed. "Most accurate if Q>0.5 inch." The runoff and peak discharge stormwater calculations using the TR55 method have been found to lose significant accuracy if the runoff (Q) is <0.5 inch (1.3 cm). This is just a warning message; outputs will be computed. "Sheet length must be 0 to 300 ft." T_{t} for sheet flow not computed. Relying on decades of research, the SCS has found that sheet flow occurs only for flow lengths up to 300 ft. Enter 0.0 if the sheet flow regime does not exist. "Ponds, Swamps must be 0 to 5%." Pond and swamp factor (thus Q_{p}) not computed. The percent of the total watershed area occupied by ponds and swamps can only be between 0 and 5%. If you have accounted for ponds and swamps in your CN's, then enter 0% for ponds and swamps. "Total Area must be >0." Q_{p} not computed. The total area of the watershed must be greater than zero. If you are entering subregion information, at least one of the subareas (A_{1} through A_{5}) must be >0. "P_{2} must be >0." T_{t} for sheet flow (thus Q_{p}) not computed. The 2yr, 24hr duration rainfall is used for computation of travel time (T_{t}) for sheet flow. If you don't want to compute this T_{t} or sheet flow does not exist, just enter any positive value for P_{2} and enter L (for sheet flow) as 0.0. "Manning n must be >0." T_{t} 's (thus Q_{p}) not computed. Manning n describes the roughness of the sheet flow terrain and the channel flow material. If either (or both) flow regimes do not exist, just enter any positive value for the n's and enter L=0 for the flow regime(s) that does not exist. "S must be >0." T_{t} 's (thus Q_{p}) not computed. Average ground slope of each flow regime must be positive. If a certain flow regime (sheet, shallow concentrated, or channel flow) does not exist, just enter any positive value for S and enter L=0 for the flow regime. "Precip must be >0." Runoff (Q) and Peak
Discharge (Q_{p}) not computed. U.S. Soil Conservation Service (now called Natural Resources Conservation Service). Department of Agriculture. Technical Release 55: Urban Hydrology for Small Watersheds. June 1986. Available on the web at http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf.
© 19992024 LMNO Engineering, Research, and Software, Ltd. (All Rights Reserved) Please contact us for consulting or questions about hydrology, peak discharge, and runoff. LMNO Engineering, Research, and Software, Ltd.  To: LMNO Engineering home page (more calculations) Precipitation maps for 24hr duration storms in USA Detention Basin Volume Calculation Other Time of Concentration Equations Rational Equation for Peak Discharge
