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Chapter 5 Working with Hydrology Commands
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by-hand method, but with the advent of personal computers, it has been
computerized in many forms.
The TR-55 presents methods to calculate storm runoff volumes, peak discharge
rates, and pre- and post-development hydrographs. The methods apply to small
watersheds (around 2000 acres or less) in the United States. You can use the TR-
55 methods for larger drainage areas and storm water detention calculations.
Both of the graphical peak discharge and tabular hydrograph methods use a 24-
hour rainfall event to calculate runoff.
The Graphical and Tabular methods are roughly equal in their accuracy.
Because they use a time of concentration value, they can be used for watersheds
of any size. The Graphical Method is the simpler of the two TR-55 methods. Use
the Graphical Method if you require a peak discharge value, but not a complete
hydrograph. The Graphical Method is limited to calculations for one watershed
subarea at a time. The Tabular Method can accommodate multiple watershed
subareas and produces a peak discharge value as well as a complete hydrograph.
The following table can help you decide which TR-55 method to use:
If} Then use}
the watershed cannot be delineated
as a single sub-area
the Tabular Method.
the watershed has multiple subareas the Tabular Method.
you only require a peak discharge
value
the Graphical Method.
you require a hydrograph the Tabular Method.
NOTE
It is important that 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.
To calculate runoff using the TR-55 tabular and graphical methods, you must
assemble the following data:
n
Rainfall distribution for the selected region
n
Storm frequency
n
Watershed area(s)
n
CN number, or the composite (weighted) CN number
n
Time of Concentration
Some of this information, such as CN number, is provided in look-up tables
from within Autodesk Civil Design. You can use a Autodesk Civil Design
calculator to calculate the Time of Concentration, based on the sheet, shallow,
and channel flow lengths. You can calculate watershed areas from a TIN surface
by using the Terrain Model Explorer, or you can delineate the watershed data by
hand and draw the areas in your drawing as closed polylines.