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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 Time of Concentration (TOC) / Lag Time. The defined flow path and corresponding computed Time of Concentration / Lag Time is then stored for each subbasin and used in the HEC-HMS runoff computations in the following runoff methods:

Runoff MethodParameter Utilized
Clark Unit HydrographTime of Concentration
SCS Unit HydrographLag Time
Unit HydrographLag Time
User Specified S-GraphLag Time
ModClarkTime of Concentration
Synder Unit HydrographLag Time

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

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

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 Time of Concentration (TOC) / Lag Time.

The following entries are provided in the Select Subbasin Flow Path section:
  • Subbasin ID
    This dropdown combo box allows the user to select the subbasin. If only one subbasin is present in the project, then that subbasin will be automatically selected.
    Subbasin ID dropdown combo boxNote that if no subbasin is already selected, the dropdown combo box status will appear as “Not Selected”.

    Alternatively, the user can click the [Pick] button to interactively select the subbasin from the Map View. On clicking the [Pick] button, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can then click on the desired subbasin on the Map View to select it. The Subbasin TOC Data dialog box will be redisplayed, and the Subbasin ID dropdown combo box will automatically update to show the ID of the selected subbasin.
  • Flow Path ID
    This dropdown combo box allows the user to select the subbasin flow path. If only one flow path is defined for the selected subbasin, then that flow path will be automatically selected.
    Flow path ID dropdown combo boxNote that if no flow path is already selected, the dropdown combo box status will appear as “Not Selected”.

    Alternatively, the user can click the [Pick] button to interactively select the flow path from the Map View. On clicking the [Pick] button, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can click on the desired flow path to select it. The Subbasin TOC Data dialog box will be redisplayed, and the Flow path ID dropdown combo box will automatically update to show the ID of the selected flow path.
  • Terrain
    The Terrain dropdown combo box allows the user to select the terrain surface associated with the selected subbasin. If only one terrain surface is present in the project, then that terrain surface will be automatically selected. Note that if there is no terrain surface corresponding to the selected subbasin, the dropdown combo box status will appear as “Not Selected”.

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).
Flow path profile plotThe 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.
Extracted flow path profile

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.
Editing flow segments

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.
Add Segment and Merge Segments commandsTo 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.
Segment breakpointTo 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.
Merged Segments

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.
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:
  • The total length of the flow path will not change. Note that the total length of the flow path is displayed in the Total flow path length read-only field.
  • The slope values are read-only and change with changes to the flow segments.
  • If the user manually edits the segment length values in the table, the segment length below the current entry will change accordingly to reflect the manual edit.
  • If the user enters a segment length value larger than feasible relative to the segment length below it, the following informational dialog box will be displayed, and the entry is discarded.
    Invalid Segment Length informational dialog box

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

The following entries are provided in the data table:
  • Runoff Flow Type
    This dropdown combo box is used to define the runoff flow type for each segment. It lists four options to define segment flow type: None, Channel Flow, Shallow Concentrated Flow (default), and Sheet Flow.
    Runoff Flow Type dropdown combo box
  • Flow Length (ft)
    This entry is used to alter the length of flow segments. This entry accepts only positive, non-zero integers.
  • Slope (ft/ft)
    This entry represents the slope of the flow segment line on the selected terrain surface. This entry accepts only positive, non-zero floating numbers with four-digit decimal precision. Note that this entry is read-only when the Maintain total flow length (adjacent segments will automatically adjust their lengths) checkbox is checked.
  • Surface Roughness/Type
    This entry automatically changes depending upon the runoff flow type selected.
    Runoff Flow TypeEntry Type
    Channel FlowSurface Roughness
    Sheet FlowSurface Roughness
    Shallow Concentrated FlowSurface Type

    When Shallow Concentrated Flow is selected as the Runoff Flow Type, a dropdown combo box entry is provided to select the surface type, as shown below.
    Surface Types for Shallow Concentrated FlowWhen Channel Flow is selected as the Runoff Flow Type, an entry field is provided to enter the roughness value. The user can click the […] lookup button to display the Manning’s Roughness lookup table dialog box, as shown below.
    Manning's Roughness lookup dialog boxWhen Sheet Flow is selected as the Runoff Flow Type, an entry field is provided to enter the roughness value. The user can click the […] lookup button to display the Overland Flow Roughness lookup table dialog box, as shown below.
    Overland Flow Roughness lookup dialog box
  • TOC (minutes)
    This read-only entry displays the computed time of concentration for that row.
  • Total TOC (minutes)
    This read-only entry displays the summation of all the individual flow segment TOC values.
  • Lag time (minutes)
    This read-only entry displays the corresponding lag time based upon the total TOC.

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.
[Edit TOC Segments] button

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.
Flow segment node

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.
Add and Delete Segment commands

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.
2yr-24hr rainfall depth entry field

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, 50, 100-, 200-, 500- and 1000-years storm events.
Rainfall Depth diaglog boxFor the regions falling outside the United States, the user must manually enter the rainfall depth value in the 2yr-24hr rainfall depth entry field.

To compute the Time of Concentration (TOC) / Lag Time for the selected flow path, the user can click the [Compute] button. On clicking the [Compute] button:
  • The TOC for individual segments will be displayed in the corresponding TOC (minutes) read-only field.
  • The TOC and Lag Time of the complete flow path will be displayed in the Total TOC (minutes) and Lag time (minutes) read-only field, respectively.
  • All HEC-HMS stormwater runoff methods that require the SCS TR-55 TOC Time of Concentration (TOC) / Lag Time will have the data populated into the appropriate fields.

Computed TOC and Lag TimeFurthermore, 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.

About the Author Chris Maeder

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