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Chapter 5 Working with Hydrology Commands
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first published by SCS (now NRCS) in 1975, and updated to its current form in
1986. TR-55 presents methods to calculate storm runoff volumes, peak discharge
rates, and pre- and post-developed hydrographs. The methods apply to small
watersheds (around 2000 acres or less) in the United States. TR-55 was originally
developed to be a by-hand method, but with the advent of personal computers,
it has been computerized in many forms.
The TR-20 method uses the procedures described in the SCS National
Engineering Handbook, Section 4, Hydrology (NEH-4).
Autodesk implements these easy-to-use methods in the drafting environment
and lets you analyze the before and after site hydrology, as well as develop
graphs of the output information.
NOTE
It is important you have some familiarity with the methods and
terminology described above. If you need more information about Urban
Hydrology for Small Watersheds, review the SCS Technical Release 55
document, which you can obtain from your local SCS (now NRCS) or
county Soil & Water Conservation District office, most college libraries, or
the National Technical Information Service in Washington, D.C. It is also
available in downloadable PDF format from
www.ncg.nrcs.usda.gov/tech_tools.html.
Selecting a Runoff Calculation Method to Use
Autodesk Civil Design provides four different runoff calculation methods you
can use to calculate pre- and post-development runoff for a site. Often local
ordinances will dictate which methods should be used for given conditions.
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:
If you want to} Then use}
size a storm pipe or culvert the Rational Method or TR-55 Methods
for larger drainage areas.
calculate runoff from multiple subareas the 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 for volume got designing
storage facilities using the detention
storage method for detention pond design
TR-55 methods.