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. The SCS method is best suited for large watersheds.
SCS Curvilinear method 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, ∆D, the internal calculation increment, is 0.1333Tc for each given subarea. Slightly adjust ∆D so that Tp on the unit hydrograph will fall exactly on one of the ordinates spaced at ∆D. 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 method 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.
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.
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.
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| Runoff Hydrograph - SCS
Method |
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| Unit Hydrograph Dialog |
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| Runoff Hydrograph Dialog |