WMS:TR-20 Time of Concentration

Time of concentration should be in hours for the unit hydrograph. Several different equations exist for determining the time of concentration. Two different methods exist for computing the time of concentration using computed basin data or using map data. The list of basin geometric attributes computed automatically when basins have been delineated from a TIN can be useful in many of these equations. These attributes can be viewed and edited from within the TR20 Basin data dialog by choosing the Basin Geometrical Attributes button.

Basin Geometric Attributes

When drainage data are computed, select this button to view and edit the geometric attributes of a basin, including the basin area, the flow length, and the basin slope. The different basin attributes can be viewed and edited only after computing the basin data. The basin data may be computed by selecting the Compute Basin Data command in the TIN or DEM Drainage module menus, or in the Feature Objects map module menu.

Compute TC – Basin Data

Pressing this button will bring up the Unit hydrograph parameter computation dialog. In this dialog, define the type of computation method to use in computing the time of concentration for TR20. After exiting this dialog, the time of concentration, lag time, time to peak, and Snyder coefficient will be calculated. Many of these coefficients and times can be used in HEC-1 models, and the time of concentration can be used in TR20 watershed models. By de-selecting the Do not auto-recompute parameters option, unit hydrograph parameters will be automatically re-calculated when the basin area is re-computed or other basin parameters are changed.

Compute TC – Map Data

Times of concentration may also be computed using a time computation coverage. The basin time of concentration dialog is accessed by selecting the Compute TC – Map Data button.

Related Topics

• TR20 Overview
• Editing TR20 Parameters
• Entering Basin Data