|
The dimensionless unit hydrograph is one of several watershed related parameters incorporated into NRCS (SCS) hydrologic modeling procedures. The unit hydrograph influences the shape of the runoff hydrograph generated by the model, particularly the peak rate of discharge. It does not affect the volume of runoff (which is determined by curve number).
A unit hydrograph’s shape varies by watershed based on many factors, including:
+ Watershed size, slope, and length
+ Geomorphic and geologic characteristics
+ Amount of storage
+ Degree of urbanization
The default SCS unit hydrograph represents an average condition for much of the country. It is generally sufficiently accurate for the hydrologic design of SCS conservation practices. Detailed studies, however, have been conducted in some watersheds and regions to develop more representative dimensionless unit hydrographs. StormNET allows the user to define these unique unit hydrographs, as well as provides several standard unit hydrographs used in different parts of the country.
The Dimensionless Unit Hydrographs dialog is used to specify the peak rate factor that defines the shape of the dimensionless unit hydrographs. These dimensionless unit hydrographs are used in both the SCS TR-20 and SCS TR-55 hydrology computations. The selected peak rate factor and corresponding unit hydrograph is applied to all subbasins.
Select INPUT | DIMENSIONLESS UNIT HYDROGRAPH to display the Dimensionless Unit Hydrographs dialog. Note that the Dimensionless Unit Hydrographs dialog is only available if the SCS TR-20 or SCS TR-55 hydrology method is selected from the SUBBASIN HYDROLOGY METHOD drop down list in the Project Options dialog, General tab.
|