Technical information

146 | Chapter 10 Hydrology and Hydraulics
Before calculating runoff you should check with your local city or county for
their applicable requirements. For a general guide refer to the following table.
Key Concepts
You can use the commands from the Hydrology menu to determine
■ Applicable rainfall distribution type
■ Size of the drainage area (A)
■ Runoff curve number (RCN)
■ Runoff coefficient (C)
■ Adjustment factor, the time of concentration (T
c
)
■ Time of travel (T
t
)
■ Size of the pond and swamp area
■ Rainfall frequency
■ Rainfall intensity for each subarea
■ Slopes and elevations across a site can be extracted from a surface model.
You can also build a surface and model the watershed before calculating
runoff by using Terrain Model Explorer in AutoCAD Land Desktop.
■ Establish an intensity duration frequency (IDF) curve file (.idf extension)
for the project location.
If you want to …Then use …
size a storm pipe or culvert the Rational Method or TR-55 Methods.
calculate runoff from multiple subareas the Rational Method or TR-55 Tabular
Method.
create a hydrograph for a storm event
with a 24-hour duration
the TR-55 Tabular Method or the TR-20
Method.
create a hydrograph for a storm event
of different length than 24 hours
the TR-20 Method.
calculate runoff volume for designing
storage facilities using the storage
indication method (reservoir routing)
a hydrograph method, like the Tabular
method or the TR-20 method.
calculate runoff volume for designing
storage facilities
TR-55 methods.