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Table Of Contents
Calculating Runoff With the TR-20 Method
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being investigated, then it might be more accurate to use the observed data
rather than using a unit hydrograph based on such widely varying data.
Compared with the TR-55 methods, which are used for smaller, simpler
watersheds, the TR-20 runoff calculation method has a broader range and can
be used for larger and more complex watersheds. The TR-20 Method is best used
for watersheds ranging from 2 to 400 square miles with subareas from 0.1 to 20
square miles.
The TR-20 Method provided in Autodesk Civil Design is a limited version
intended to be used to calculate a single subarea. TR-20 is published by the Soil
Conservation Service (now NRCS). The TR-20 Method uses TR-20 distribution
curve tables (tr20t1 through tr20t7). These files are stored in the c:\Land
Desktop R2\Data\hd folder and have a .dst extension.
NOTE
To see which calculations are used for the TR-20 method, see the "National
Engineering Handbook: Section 4-Hydrology," in Chapter 16.
Distribution curves show the relationship between the fraction of rainfall that
occurs and a given time period. The TR-20 distribution curve tables use both 24-
and 48-hour time periods, and also include a table for emergency spillway
conditions (table tr20t6). The different TR-20 distribution curve tables are based
on different rainfall distribution types (I, IA, II, and III) that are described on
page B-1 of the TR-55 manual.
NOTE
When you perform calculations using the TR-20 Method, it is
recommended that the units be Imperial, the rainfall in inches and the area
in square miles. If the units for rainfall are set to something other than
inches, then the TR-20 distribution curve table (.dst file) must match the
rainfall units.