Technical data

Chapter 3 Performing Hydrologic Studies

After you enter all the information, the peak discharge is calculated

automatically and is displayed in the Peak Discharge line.

To determine the runoff peak discharge for other storm events, select

the new storm frequency using the Select button next to the rainfall

edit field in the Graphical Peak Discharge Method dialog box,. The

software will automatically re-calculate the appropriate rainfall

intensity and the runoff peak discharge.

For example, if you select 100 from the Define Rainfall Frequency

dialog box, the runoff peak discharge for the 100-year storm event will

be calculated.

Using the TR-55 Tabular Hydrograph

Method to Calculate Runoff

The second runoff procedure outlined in Technical Release 55 (TR-55)

is the Tabular Hydrograph Method. The Tabular Hydrograph Method

can be used on heterogeneous watersheds that can be subdivided into

homogeneous subareas. By dividing the heterogeneous watershed into

homogeneous subareas, estimated peak discharges and hydrographs

for the heterogeneous watershed can be obtained.

Key Concepts

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Determine the applicable rainfall distribution type for the entire

watershed. Additionally, you must know the hydrologic parameters

for each subarea, including the size, the time of concentration (Tc),

the time of travel (Tt) and, if applicable, the amount of rainfall, and

the runoff curve number (RCN). Note that these values can all be

calculated or selected from within 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 Tabular Hydrograph Method,

particularly the implied limitations of the method.