With the exception of purely sanitary flow systems with no wet weather effects, Bentley StormCAD starts its hydraulic calculations from a hydrograph for each catchment. There are numerous ways of generating those hydrographs. Most involve starting with Storm Data then calculating a runoff hydrograph using one of the following methods:

• SCS
• Unit Hydrograph
• EPA SWMM
• Modified Rational Method
• RTK Unit Hydrograph Method

Virtually any hydrograph and loss method can be used with any numerical engine with the following exceptions:

1. Modified rational method only works with Implicit engine and only with peak intensity (IDF) rainfall (Local rainfall is not supported).
2. If you choose the SWMM engine and specify EPA-SWMM runoff method, then all catchments must use that runoff method and the loss method specified on the calculation options manager (only Green -Amt, Horton, or SCS).

However, it is also possible to directly enter a hydrograph by specifying a user-defined hydrograph for any catchment.