Subbasin TOC Data Command

The Subbasin TOC Data command allows the user to subdivide the subbasin flow path into individual segments in order to assign the appropriate SCS TR-55 runoff flow types to compute the SCS TR-55 TOC/Lag Time. The defined flow path and corresponding computed TOC/Lag Time is then stored for each subbasin and used in the HEC-HMS runoff computations in the following runoff methods:

To use the Subbasin TOC Data command, follow the steps below:

1. From the Watershed ribbon menu, select the Subbasin TOC Data command.
   
2. The Subbasin TOC Data dialog box will be displayed.
   

The following sections describe how to compute subbasin flow path TOC/Lag Time and interact with the above dialog box.

Selecting Subbasin Flow Path

Using the dropdown combo boxes and corresponding [Pick] buttons provided under the Select Subbasin Flow Path section, the user can select the desired subbasin and associated flow path for purposes of computing the SCS TR-55 TOC/Lag Time.

Flow Path Profile Plot

After the subbasin and associated flow path have been selected, the software will display a profile view of the terrain surface along the flow path line, starting at the highest endpoint of the flow path line at the left side end of the profile. This profile view allows the user to subdivide the flow path into flow segments, where each segment corresponds to a specific flow type (i.e., sheet flow, shallow concentrated flow, channel flow).
The user can extract the flow path profile for different terrain surfaces by selecting the terrain surface from the Terrain dropdown combo box and clicking the [Extract] button. The new flow path profile for the selected terrain surface will be displayed in the profile plot.

Editing Flow Segments on Profile Plot

The user can grab the joint between any two segments and drag the joint to the left or right. User adjustments will be reflected in the flow path profile line. As the joint is moved, the length and slope will automatically update in the data table shown in the Define SCS TR-55 TOC Data section. In addition, the length of each segment is shown above the segment so that the user can see how the edits are impacting segment length.

Adding/Merging Flow Segments on Profile Plot

The user can also add new segments or merge existing segments of the flow path using the Add Segment or Merge Segments option from the profile plot context menu.
To add a flow segment, the user can right-click at a location on the profile line and select the Add Segment option from the displayed context menu. The software will insert a new segment breakpoint at the cursor location. After adding segments, the user can define the runoff flow type for each segment using the data table provided under the Define SCS TR-55 TOC Data section.
To merge two flow path segments, the user can select the segment breakpoint between the two segments on the profile plot. The selected segment breakpoint will be highlighted. The user can then right-click and select the Merge Segments option from the displayed context menu.

The software will merge the segments adjacent to the segment break point. However, the flow line (and profile line) will not shorten. The two bracketing flow segments surrounding the segment breakpoint will fill-in the gap.

Editing Flow Path Segments in Data Table

The flow path segments can also be edited using the data table entries provided under the Define SCS TR-55 TOC Data section.
If the Maintain total flow length (adjacent segments will automatically adjust their lengths) checkbox entry is checked, then:

If the Maintain total flow length (adjacent segments will automatically adjust their lengths) checkbox entry is unchecked, the user can freely edit the segmented flow path lengths and slopes. The profile plot will adjust accordingly, ignoring the profile that was extracted from the selected terrain surface.

If the user rechecks the checkbox entry after making manual edits in the table, the software will recompute the segment lengths and slopes. It will restore the leftmost segment to 20 ft (6 m) long. It will then evenly distribute the other segments based on actual flow path length.

Data Table Entries

Editing Flow Path Segments on Map View

The user can click the [Edit TOC Segments] button to add, edit or delete flow path segments on the Map View.

On clicking the [Edit TOC Segments] button, the Subbasin TOC Data dialog box will temporarily disappear. The user can then select the flow segment nodes and drag them along the flow path line on the Map View.

Adding/Deleting Flow Path Segments on Map View

If necessary, the user can add or delete a flow segment while editing the flow path on the Map View. To add or delete a flow segment from a TOC flow path, the user can right-click at a location where a new flow segment node is to be inserted or select an existing TOC flow segment that is to be deleted. Then select the Add Segment or Delete Segment option from the displayed context menu.

On selecting the Add Segment option, the software will insert a node at the selected location and subdivide the previous flow segment into two separate segments. The two flow segments will have the same runoff flow type as the previous flow segment. The inserted node will be selected. The user can click and drag the node along the TOC flow path to adjust the length of the two adjacent flow segments.

On selecting the Delete Segment option, the software will remove the selected TOC flow segment. The remaining flow segments will fill-in (backfill) any removed flow segments. When removing a flow segment, the next downstream flow segment will fill-in the removed flow segment (by convention) where possible. If the most downstream flow segment is removed, then the next upstream flow segment will fill-in the removed flow segment.

Note that if there is only one flow segment on the flow path, the user cannot remove the flow segment, and the Delete Segment option is grayed out (unavailable).

After editing the flow paths, press the [Enter] key or right-click and select Done from the displayed context menu. The Subbasin TOC Data dialog box will be redisplayed.

SCS TR-55 Time of Concentration Computation

The software requires a 2-year-24-hour rainfall depth value to calculate TOC for sheet flow. For the regions falling within the United States, the software automatically retrieves and displays the 2-year-24-hour rainfall depth value in the 2yr-24hr rainfall depth entry field.

The Precipitation data source dropdown entry is used to select the precipitation data source for regions such as Austria, Germany, Ontario (Canada), and the USA. The precipitation data source is used for computing the 2yr-24hr rainfall depth data for the selected location.

The available precipitation data source options are:

Clicking the [Retrieve] button causes the software to display the Rainfall Depth dialog box. This dialog box displays the rainfall depth values for various durations of 1, 2, 5, 10, 25, 50, 100-, 200-, 500- and 1000-years storm events.

For the regions falling outside the United States, the user must the precipitation data coverage area, the user needs to manually enter the rainfall depth value in the 2yr-24hr rainfall depth entry field.

Furthermore, the user can click the [Compute All] button to compute the TOC/Lag Time for all subbasins at once. On clicking the [Compute All] button, the software will go through each subbasin and compute the TOC/Lag Time. If a subbasin has multiple flow paths, the software will compute the TOC/Lag Time for each flow path. However, the flow path with the largest TOC/Lag Time will be assigned to the subbasin. Click the [Restore] button to restore the TOC flow segments back to their original segments and computed values.