Compute TOC Widths Command

The EPA SWMM platform requires users to define the TOC equivalent width for subbasins in order to calculate the rectangular runoff plane dimensions. The calculated values are used in the Kinematic Wave Time of Concentration (TOC) computations for sheet flow and shallow concentrated flow.

In GeoSTORM software, the Compute TOC Widths command allows the user to compute the EPA SWMM TOC equivalent width (W) for subbasins by computing the contributing drainage areas on both sides of the longest flow path bisecting the subbasin.

The software will use the computed longest flow path to determine the length of the subbasin channel (L) along with the two separate contributing drainage areas–A1 and A2, as shown below.

The Subbasin TOC Width (W) is calculated using the following equation:

Where:

S_k = Skew factor (0≤Sk≤1) = (A₂-A₁)/(A₁+A₂)

A₁ = Area on one side of the subbasin channel

A₂ = Area on the opposite side of the subbasin channel

L = Length of the subbasin channel

Follow these steps to use the Compute TOC Widths command:

1. From the Watershed ribbon menu, select the Compute TOC Widths command.
   
2. The Compute TOC Widths dialog box will be displayed.
   
3. From the Select Subbasins section, check the checkboxes corresponding to the subbasins. Alternatively, click the [Pick] button to manually select subbasins from the Map View.
   
   Note: To select/deselect all subbasins at once, check/uncheck the Subbasin checkbox.
4. The number of selected subbasins will be displayed in the Total selected read-only field.
   
5. After selecting the subbasins, click the [Compute] button, and the software will then compute the TOC equivalent widths for the selected subbasins.
   
6. The TOC equivalent widths computed for the subbasins will be displayed in the TOC Equivalent Width editable column.
   
   Note that if the longest flow paths have not been computed for selected subbasins, then the following informational dialog box will be displayed.
   

The computed TOC widths for the corresponding subbasins will also be populated in the TOC equivalent width entry of the Subbasin Data dialog box. To learn more about the Subbasin Data dialog box, refer to this article in our knowledge base.