Subbasin TOC Data – Selecting FAA TOC Method

The Subbasin TOC Data command allows the user to compute the time of concentration (TOC) for one or more subbasins using the Federal Aviation Administration (FAA) method. The user can select the TOC/Lag time method from the Subbasin – TOC/Lag Time Method dropdown combo box in the General Preferences section of the Options backstage page. Refer to this article in our knowledge base to learn more about TOC methods.

The FAA TOC method computes TOC using the following equation:

Where:

• L = Longest flow path in the subbasin
• S = Flow path average slope
• C = Rational Method runoff coefficient

Follow the steps below on how to compute time of concentration using the FAA TOC method:

1. From the Watershed ribbon menu, select the Subbasin TOC Data command.
   
   Alternatively, from the Input ribbon menu, click the Drainage Subbasins dropdown menu and select the Subbasin TOC Data command.
   
2. The Subbasin TOC Data dialog box will be displayed, as shown below.
   

The Subbasin TOC Data dialog box contains three tabs as described below:

• Compute TOC
• Convert CN to Runoff Coefficient
• Rainfall Intensity Lookup

Compute TOC

In the Compute TOC panel, the Select Subbasins section allows the user to select single or multiple subbasins defined in the current scenario for computing the time of concentration using the FAA method.

The user can select the subbasin(s) using any of the following methods:

• By checking the checkboxes corresponding to each subbasin in the Select Subbasins section.
• By clicking the [Pick] button. In this selection method, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the subbasin(s) from the Map View. After selecting the subbasin(s), press the [Enter] key or right-click and choose Done from the displayed context menu. The Subbasin TOC Data dialog box will be redisplayed.

After selecting the subbasin(s), the total number of selected subbasins will be displayed in the Total selected read-only field, as shown below.

The table in the Select Subbasins section contains the following columns:

• Subbasin
  This read-only column lists all subbasin IDs contained in the current scenario.

• Flow Path Slope (ft/ft)
  This read-only column lists the computed flow path slopes using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Flow Path Length (ft)
  This read-only column lists the computed flow path lengths using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Runoff Coefficient
  This editable column lists the computed runoff coefficients. Clicking the […] lookup button displays a Runoff Coefficients dialog box, which allows the user to select the runoff coefficient.
  

• Computed TOC (minutes)
  This editable column lists the computed TOC value. By default, this column lists the TOC value defined in the Subbasin Data dialog box. To learn more about the Subbasin Data command, refer to this article in our knowledge base.

Convert CN to Runoff Coefficient

This panel is used to convert computed curve number (CN) values into equivalent runoff coefficient values for the selected subbasin(s).

The table in the Select Subbasins section contains the following columns:

• Subbasin
  This read-only column lists all subbasin IDs contained in the current scenario.

• Flow Path Slope (ft/ft)
  This read-only column lists the computed flow path slopes using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Curve Number
  This editable column lists the computed curve numbers using the Compute CN command. To learn more about the Compute CN command, refer to this article in our knowledge base.

• Impervious Area (%)
  This editable column lists the computed impervious areas using the Compute % Impervious command. To learn more about the Compute % Impervious command, refer to this article in our knowledge base.

• Runoff Coefficient
  This editable column lists the runoff coefficient that is computed when the user clicks the [Compute] button.

Clicking on the [Compute] button allows the software to compute the runoff coefficient for the selected subbasin(s) and display the result in the Runoff Coefficient column.

Rainfall Intensity Lookup

This panel is used to retrieve the rainfall intensity value required to convert a previously computed curve number (CN) into an equivalent runoff coefficient.

Location Selection

This section allows the user to select the location from which the rainfall intensity value is to be retrieved.

The following options are available in this section:

• Centered on current view extents
  If this radio button option is selected, the software automatically selects the central location of the region displayed in the current Map View extents for retrieval of rainfall intensity value.

• Centered on subbasins
  If this radio button option is selected, the software automatically selects the central location of the subbasin model defined in the current scenario. If no subbasins are defined, this option will be disabled (i.e., grayed out).

• Select location
  This radio button option allows the user to pick the required location from the Map View interactively by using the [Pick] button. The address of the selected location will be displayed in the read-only field next to this option. The latitude and longitude values for the selected location will be displayed in the Latitude & longitude fields. The user can click the [Clear] button to clear the selected location and redo the entire process.
  

Perform Rainfall Intensity Lookup

This section allows the user to retrieve the rainfall intensity value for the selected location.

The following entries are available in this section:

• Rainfall data source
  This dropdown combo box entry lists the rainfall data sources that are available for the selected location. The following rainfall data sources are available:

• • Austria – Bundesministerium Wasserwirtschaft eHYD Data Service
  • Canada – Environment Canada & Ontario Ministry of Transportation
  • Canada – Ontario Ministry of Transportation
  • Germany – Deutscher Wetterdienst – Klima und Umwelt
  • USA – ISWS (Illinois) Bulletin 75 (PFDS) Precipitation Frequency Data Server
  • USA – NOAA Atlas 14 (PFDS) Precipitation Frequency Data Server
  • USA – NOAA Atlas 2 (PFDS) Precipitation Frequency Data Server
  • USA – NRCC (Northeast Regional Climate Center) PFDS
  • USA – SUDAS (lowa) Bulletin 71 (PFDS) Precipitation Frequency Data Server

Note: The specific rainfall data source entry will be displayed based on the selected location.

• Return interval
  This dropdown combo box allows the user to select the return intervals that are provided by the selected rainfall data source. By default, the 2 Year return interval is selected.

• Rainfall intensity
  This entry field contains the rainfall intensity value that is returned for the selected location, rainfall data source, and return interval. Clicking the [Retrieve] button allows the software to retrieve the rainfall intensity value and display the results in the entry field.

Computing TOC

Once all the data have been defined in the Subbasin TOC Data dialog box, click the [Compute] button. The software will then compute the time of concentration for the selected subbasin(s) and display the results in the Computed TOC column.