Subcatchment FAQ
This document discusses general issues about the use of a "subcatchment" within
HydroCAD.
What is a subcatchment?
A subcatchment is used to model the runoff from a given area of land. Each subcatchment generates a runoff hydrograph, that is typically routed into a downstream reach or pond. A subcatchment can also be used to account for the rain falling directly on the surface of a pond. A subcatchment cannot be used to route an inflow hydrograph. Instead, use a subcatchment to calculate the runoff and a separate reach to perform the routing.
How is the runoff calculated?
HydroCAD provides a choice of runoff techniques, including the SCS/NRCS Unit Hydrograph procedure (aka TR-20), the Santa Barbara Urban Hydrograph (SBUH).
How is the
rainfall defined?
For the SCS and SBUH runoff procedures, a predefined rainfall distribution is selected for the project area. When using the Rational method, a custom IDF file can be created. For details, read about rainfall options.
How is the subcatchment characterized?
Each subcatchment must have a curve number (or c-value) that characterizes the soil type and ground cover. HydroCAD also provides for special handling of impervious areas.
Each subcatchment also requires a Time-of-Concentration. HydroCAD provides a range of techniques for calculating the Tc. Details here.
Why can't I route an inflow through my subcatchment?
A subcatchment is used only to generate a runoff hydrograph. To route a separate inflow hydrograph you may want to use a reach. Also read about overland flow.
For further information please read about runoff calculations.