Runoff Hydrograph - TR-55 Tabular Method

The TR-55 Tabular Method approximates TR-20, which is a more detailed hydrograph procedure. This method can develop partial composite runoff hydrographs at any point in a watershed by dividing the watershed into homogeneous subareas. The tabular discharge values for the various rainfall distribution, presented in TR-55, Urban Hydrology for Small Watersheds (U.S. Soil Conservation Service), are used in this method. Tabular discharges expressed in csm/in are given for a range of subarea Tc's from 0.1 to 2 hours and reach Tt's from 0 to 3 hours.

Select TR-55 Tabular Method from Watershed > Hydrograph Routing > Watershed Hydrograph menu. This command opens a dialog for hydrograph calculation. Choose the area unit and select the appropriate storm type for the project location. See the storms maps in the TR-55 manual to determine the storm type. Enter the rainfall depth for the watershed. Select the Ia/P Interpolation option to interpolate the Ia/P ratio, otherwise the calculated Ia/P value would be rounded to the constants 0.10, 0.30 or 0.50. Next, enter the watershed data in the spreadsheet. In the Subarea Name column, enter a short name for each subarea, using numbers or letters. In the Area column, type the area value. You can also click the Select Area button on the bottom of the spreadsheet to calculate the drainage area by selecting a closed polyline.

In the Tc (time of concentration) and Tt (Travel Time) columns, enter the corresponding values. Tt specifically refers to the total time for water to flow from a subarea's outfall to composite outfall point. When the Tc value for a particular subarea doesn't exist in the tabular hydrograph for the given distribution, it will be rounded off in order to determine which hydrograph table to read from. Please refer to Tr-55 for the method used for rounding off Tc and Tt values.

In the Downstream Subarea Name column, type the list of the names of the downstream subareas. Separate each name with a comma. In the CN (Runoff Curve Number) column, enter the CN value of the subarea. Insert Row and Delete Row buttons allow you to do the spreadsheet editing. You can also load a runoff tabular file that you previously saved by clicking on Load button, and save current calculation by Save and SaveAs buttons. The data is stored in a .tab file.

After entering all information, click on Calculation button to generate the runoff hydrograph. Runoff Hydrograph button opens a dialog with the tabular and graphic hydrograph data, from there you can draw the hydrograph on screen and save it to a file.

Pulldown Menu Location: Watershed > Hydrograph Routing > Watershed Hydrograph > TR-55 Tabular Method
Keyboard Command: runoffhyd2
Prerequisite: None