Runoff Hydrograph - SCS Method

This function applies the SCS hydrograph procedure and the same methodology as used in TR-20 to generate runoff hydrograph by specifying either a triangular hydrograph shape or a curvilinear one. There are five unit hydrograph options: Delmarva, SCS Curvilinear, SCS Triangular, Clark and Snyder unit hydrograph method. The SCS method is best suited for large watersheds.

SCS Curvilinear Unit Hydrograph uses the dimensionless unit hydrograph developed by Victor Mockus. This dimensionless hydrograph has its ordinate values expressed in a dimensionless ratio Q/Qp (discharge at time t to total discharge) or Qa/Q (accumulated volume at time t to total volume) and its abscissa values as T/Tp (time t to peak time). When calculating the unit hydrograph for certain watershed, Delta, the internal calculation increment, is 0.1333Tc for each given subarea. Slightly adjust Delta so that Tp on the unit hydrograph will fall exactly on one of the ordinates spaced at Delta. This method creates the unit hydrograph for one inch of direct runoff using the 51 evenly spaced Q/Qp ratios used internally by TR-20. 

SCS Triangular Unit Hydrograph uses an equivalent triangular hydrograph having the same units of time and discharge to represent the dimensionless curvilinear unit hydrograph. Please refer to National Engineering Handbook Section 4 for the equations used in generating both SCS Curvilinear and SCS Triangular unit hydrograph.

Delmarva Unit Hydrograph is an alternative to the 484 dimensionless unit hydrograph developed for the Delmarva region (Delaware, Maryland, Virginia peninsula) based on data from four gaged watersheds. Peak rate factor of 284 is used in this method.

Clark Unit Hydrograph is a synthetic unit hydrograph method. There's no need to develop a unit hydrograph through the analysis of past observed hydrograph. Instead, a time vs. area curve built into the program is used to develop the translation hydrograph resulting from a burst of precipitation. The resulting traslation hydrograph is routed through a liner reservoir to account for storage attenuation effects across the subbasin. The time of concentration defines the maximum travel time in the subbasin; the storage coefficient is used in the linear reservoir that accounts for the storage effects.

Snyder Unit Hydrograph is based on a study of watersheds in the Appalachian Highlands. In basins ranging from 10 to 10,000 square miles.

Snyder relations are: t_p = C_t(LL_c)^0.3

Where:
      t_p = basin lag (hr)
      L  = length of the main stream from the outlet to the divide (mi)
      L_c = length along the main stream to a point nearest the watershed centroid (mi)
      C_t = Coefficient usually ranging from 1.8 to 2.2

            Q_p = 640C_pA/t_p

Where:
      Q_p = peak discharge of the unit hyrograph (cfs)
      A   = drainage area (mi2)
      C_p = storage coefficient ranging from 0.4 to 0.8, where larger values of C_p are associated with smaller values of C_t

Select SCS Method from Watershed > Hydrograph Routing > Watershed Hydrograph menu. This command opens a dialog for hydrograph calculation. First choose which method to use, SCS Curvilinear or SCS Triangular. Then enter the sub-basin and rainfall data. When creating an SCS Triangular unit hydrograph, you need to enter the unit hydrograph peak attenuation factor, which controls the area under the unit hydrograph before the time to peak. Usually a factor of 484 is used. This factor varies from about 600 in steep terrain to 300 in very flat swampy areas. When creating an SCS Curvilinear unit hydrograph, the peak attenuation factor is default to 484.

The program can get Rainfall Depth automatically based on user specified Rational rainfall ID, return period and time of concentration.

After entering all information, click Calculation button to generate the unit and runoff hydrograph. Unit Hydrograph button and Runoff Hydrograph button open a dialog with the tabular and graphic data of the unit and runoff hydrograph correspondingly, from there you can draw the hydrograph on screen, generate the hydrograph data in the report format and save the hydrograph to a file.

The calculation result can be reported in PDF format with design data and hydrograph curve.

Unit Hydrograph Dialog

Runoff Hydrograph Dialog