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###### Rational Formula Methods

Tc - Overland Flow (kinematic wave formula)
• Calculate the time of concentration (Tc) for overland flow across small areas without defined channels using the kinematic wave formula.  Good for paved areas such as parking lots and streets.
Tc - Overland Flow
• Calculate the time of concentration (Tc) for overland flow in shallow concentrated flow across an area.
Tc - FAA Formula
• Calculate the time of concentration (Tc) for Stormwater flow using the FAA Formula.
Tc - Kirpich's Equation
• Calculate the time of concentration (Tc) for Stormwater flow using Kirpich's Equation.
Tc - Kerby's Equation
• Calculate the time of concentration (Tc) for Stormwater flow using Kerby's Equation.
Rainfall Intensity
• Calculate the rainfall intensity (I) for different storm frequencies using the Steel formula.
Rational Formula Discharge
• Calculate the stormwater peak discharge (qp) using the Rational Method.
Rational Formula Runoff
• Calculate the stormwater runoff (Q) using the Rational formula peak discharge.
Rational Formula Detention Storage
• Calculate the stormwater detention storage volume (Vs) using the Rational Formula Hydrograph Method.
Q - Modified Rational Method
• Calculate the stormwater runoff (Q) using the Modified Rational Method.

###### USDA NRCS/SCS - TR-55

NRCS Runoff
• Calculate the USDA, NRCS, TR-55 runoff rate by curve number method.
Composite CN - Connected Impervious Area
• Calculate USDA, NRCS, TR-55 composite Curve Number for area with a connected impervious area or for an area with an unconnected impervious area and a total impervious area greater than 30%.
Composite CN - Unconnected Impervious Area
• Calculate USDA, NRCS, TR-55 composite Curve Number for area with an unconnected impervious area and a total impervious area less than 30%.
Weighted CN
• Calculate USDA, NRCS, TR-55 weighted Curve Number for a basin from the curve numbers and areas of its subbasins.
Tt - Channel Flow
• Calculate the travel time (Tt) for stormwater runoff in channelized flow or shallow concentrated flow using USDA, NRCS, TR-55.

Tt - Sheet Flow
• Calculate the NRCS travel time (Tt) for sheet flow of less than 300 feet using the NRCS version of Manning's kinematic wave formula.

Travel Time Average Velocity
• Calculate the average velocity for shallow concentrated flows to calculate the USDA, NRCS TR-55 travel time or time of concentration.
Graphical Peak Discharge Method
• Calculate peak discharge using USDA, NRCS, TR-55 Graphical Peak Discharge Method.
Tabular Hydrograph Method
• Calculate peak discharge using USDA, NRCS, TR-55 Tabular Hydrograph Method.
NRCS Runoff Volume
• Calculate USDA, NRCS TR-55 stormwater discharge runoff volume to use in detention basin storage volume calculation.

###### USDA NRCS/SCS - Advanced Hydrology Calculations

SCS Cumulative Rainfall, Runoff and Volume
• Calculate USDA, NRCS TR-55 design storm cumulative rainfall, cumulative runoff and cumulative runoff volume for a drainage area. Includes all SCS 24 hour, 18 hour, 12 hour and 6 hour, Florida Type II modified 24 hour, SJRWMD 96 hour, SFWMD 72 hour and Florida Department of Transportation 10 day, 7 day, 3 day, 24 hour, 8 hour, 4 hour, 2 hour and 1 hour storms.
SCS Unit Hydrograph
• Calculate USDA, NRCS, SCS Unit Hydrograph time to peak, tp, unit peak discharge, qp, and the SCS unit hydrograph ordinates for a watershed area.
SCS Gamma UH
• Calculate USDA, NRCS, SCS Unit Hydrograph time to peak, tp, unit peak discharge, qp, and the SCS unit hydrograph ordinates for a watershed area using the Gamma equation. Referencing Part 630, Hydrology, Chapter 16, Hydrographs, National Engineering Handbook, USDA, NRCS this calculation is to be used to calculate the unit hydrograph for a watershed using the gamma equation.
SCS Storm Hydrograph
• Calculate USDA, NRCS, SCS Storm Hydrograph, unit hydrograph time to peak, tp, unit peak discharge, qp, volume under the hydrograph and runoff volume for an SCS design storm and drainage basin. Referencing Part 630, Hydrology, Chapter 16, Hydrographs, National Engineering Handbook, USDA, NRCS this calculation is to be used to calculate the flood hydrograph for a watershed using an SCS design storm and the gamma equation. Includes all SCS Type I, IA, II, and III storms and local storms up to 24 hrs.
Storm Hydrograph - 24hr to 96hr
• Calculate USDA, NRCS, SCS Storm Hydrograph, unit hydrograph time to peak, tp, unit peak discharge, qp, volume under the hydrograph and runoff volume for an SCS design storm and drainage basin. Referencing Part 630, Hydrology, Chapter 16, Hydrographs, National Engineering Handbook, USDA, NRCS this calculation is to be used to calculate the flood hydrograph for a watershed using an SCS design storm and the gamma equation. Includes all SCS local storms greater than 24 hrs and up to 4 days.
Storm Hydrograph - greater than 96hr
• Calculate USDA, NRCS, SCS Storm Hydrograph, unit hydrograph time to peak, tp, unit peak discharge, qp, volume under the hydrograph and runoff volume for an SCS design storm and drainage basin. Referencing Part 630, Hydrology, Chapter 16, Hydrographs, National Engineering Handbook, USDA, NRCS this calculation is to be used to calculate the flood hydrograph for a watershed using an SCS design storm and the gamma equation. Includes all SCS local storms greater than 96 hrs and up to 10 days.

###### Example Design Problems

Design Problem 1
• Given a 600'x600' rural parcel of pasture land located in Region 1, determine the 10 year stormwater discharge to size a culvert that will be located on the SE corner of the property. Use the Steel formula and rational method to determine Q.
Design Problem 2
• Using the NRCS method, determine the volume of stormwater runoff that must be stored in an onsite retention pond to meet local land development regulations for a 5 acre commercial development site.

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