Technical data

Using the TR-55 Graphical Peak Discharge Method to Calculate Runoff

Using the TR-55 Graphical Peak Discharge

Method to Calculate Runoff

Technical Release 55 (TR-55), prepared by the Soil Conservation

Service (SCS), presents two simplified methods for estimating storm

water runoff from urbanizing watersheds. Although the procedures

found in TR-55 are particularly well suited to urban and urbanizing

watersheds, the methods can be applied, in general, to any small

watershed when the governing limitations of either method have

been adequately addressed.

The simpler of the two methods is the Graphical Peak Discharge

Method (GPDM). The Graphical Peak Discharge Method is intended

for use on hydrologically homogeneous watersheds for which land

use, soils, and cover type are uniformly distributed throughout the

watershed. The TR-55 Graphical Peak Discharge Method, as the name

of the method implies, determines the peak discharge only. If the

watershed in question is heterogeneous, or if hydrographs are

required, the TR-55 Tabular Hydrograph Method should be used.

NOTE

The Soil Conservation Service is now called Natural Resources

Conservation Service.

Key Concepts

■

Determine the applicable rainfall distribution type, the size of the

drainage area, the runoff curve number (RCN), the time of

concentration (Tc), the size of the pond and swamp area, and the

amount of rainfall. This can all be calculated or selected using

commands from the Hydrology menu.

■

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 the AutoCAD Land Development

Desktop Terrain Model Explorer.

■

Refer to the SCS (Soil Conservation Service) TR-55 manual for more

information regarding the Graphical Peak Discharge Method,

particularly the implied limitations of the method.