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 Method | Parameter Utilized |
|---|---|
| Clark Unit Hydrograph | Time of Concentration |
| SCS Unit Hydrograph | Lag Time |
| User-Specified Unit Hydrograph | Lag Time |
| User-Specified S-Graph | Lag Time |
| Kinematic Wave | Time of Concentration |
| Snyder Unit Hydrograph | Lag Time |
To use the Subbasin TOC Data command, follow the steps below:
- From the Watershed ribbon menu, select the Subbasin TOC Data command.
- The Subbasin TOC Data dialog box will be displayed.
The following sections describe how to compute the 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 required 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
The Subbasin ID dropdown combo box allows the user to select the subbasin. If only one subbasin is present in the current scenario, then that subbasin will be automatically selected.
Note 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 required 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
The Flow path ID 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.
Note 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 required 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, pipe flow, and zero travel time (ponds)].
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:
- 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 editable and can be changed for 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 will be discarded.
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.
Data Table Entries
The following entries are provided in the data table:
Runoff Flow Type
The Runoff Flow Type dropdown combo box is used to define the runoff flow type for each segment. It lists five options to define segment flow type: Channel Flow, Pipe Flow, Shallow Concentrated Flow, Sheet Flow, and Zero Travel Time (Ponds).
- Channel Flow: It is assumed to begin where surveyed cross section information has been obtained (where channels are visible on aerial photographs or blue lines that indicate streams appear on USGS quadrangle sheets).
- Pipe Flow: It is a flow through a pipe network.
- Shallow Concentrated Flow: Sheet flow usually becomes shallow concentrated flow after a distance of 300 feet.
- Sheet Flow: It is a flow over plane surfaces and usually occurs in the headwater of streams.
- Zero Travel Time (Ponds): It accounts for flow segments through a reservoir, lake, pond, or other structure that has a zero travel time.
Flow Length (ft)
The Flow Length (ft) entry is used to alter the length of flow segments. This entry accepts only positive, non-zero integers.
Slope (ft/ft)
The Slope (ft/ft) editable entry represents the slope of the flow segment line on the selected terrain surface. This entry accepts only positive, non-zero floating numbers. The user can click the adjacent [Recalc] button to recompute the segment slope for the defined flow length and elevation terrain surface. Similarly, the [Recalc All] button allows the user to recompute the slope value for the flow path segments available in the scenario.
Hydraulic Radius (ft)
The Hydraulic Radius (ft) entry is used in the Channel Flow and Pipe Flow equations while computing the Time of Concentration (TOC) values. This entry is only enabled for the Channel Flow and Pipe Flow rows, and its default value is 0.45 ft.
Surface Roughness/Type
The Surface Roughness/Type entry automatically changes depending upon the runoff flow type selected.
| Runoff Flow Type | Entry Type |
|---|---|
| Channel Flow | Surface Roughness |
| Pipe Flow | Surface Roughness |
| Sheet Flow | Surface Roughness |
| Shallow Concentrated Flow | Surface Type |
Shallow Concentrated Flow
When Shallow Concentrated Flow is selected as the Runoff Flow Type, a dropdown combo box will be displayed with the following entries:
- Agricultural Crops
- Bare Ground
- Forest With Underbush
- Forest Without Underbush
- Long Grass
- Paved
- Short Grass
- Unpaved
The TOC for the Shallow Concentrated Flow is computed using the following equation.
The Shallow Concentrated Flow computational procedure uses the same mathematical formula as used in the NRCS Upland Method (USDA NRCS NEH National Engineering Handbook, Chapter 15, Travel Time, Time of Concentration and Lag). The basic difference between the two methods is that the Shallow Concentrated Flow utilizes two surface types (i.e., paved and unpaved). Due to the similarity between these two methods, the software includes the capabilities in the NRCS Upland Method to provide additional flexibility in computing the TOC for the Shallow Concentrated Flow.
Note that the surface roughness values used to calculate the TOC for the Shallow Concentrated Flow are shown in the below table.
| Surface Type | Manning’s n value | Kv (ft/sec) | Kv (m/sec) |
|---|---|---|---|
| Agricultural Crops | 0.058 | 9.00 | 2.74 |
| Bare Ground | 0.051 | 10.00 | 3.05 |
| Forest With Underbush | 0.202 | 2.50 | 0.76 |
| Forest Without Underbush | 0.101 | 5.00 | 1.52 |
| Long Grass | 0.050 | 16.13 | 4.91 |
| Paved | 0.025 | 20.33 | 6.20 |
| Short Grass | 0.073 | 7.00 | 2.13 |
| Unpaved | 0.050 | 16.13 | 4.92 |
Channel Flow
When 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.
Pipe Flow
When Pipe 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 Channel and Pipe Manning’s Roughness lookup table dialog box, as shown below.
A default value of 0.29 ft is used for Hydraulic Radius. This value is for a 12 in pipe with maximum discharge. For maximum discharge, the depth must be 0.939 times the diameter, or the hydraulic radius must be 0.286 times the diameter.
Sheet Flow
When 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.
Zero Travel Time (Ponds)
When Zero Travel Time (Ponds) is selected as the Runoff Flow Type, the software does not require any roughness value or surface type. It accounts for flow segments that pass through a reservoir, lake, pond, or other structure. This travel time is normally very small and can be assumed as zero.
Velocity (ft/s)
The Velocity (ft/s) is a read-only entry that displays the computed flow velocity for that row.
TOC (minutes)
The TOC (minutes) is a read-only entry that displays the computed time of concentration for that row.
Total TOC (minutes)
The Total TOC (minutes) is a read-only entry that displays the summation of all the individual flow segment TOC values.
Lag time (minutes)
The Lag time (minutes) is a read-only entry that 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.
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:
- 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 – NOAA Atlas 2 (PFDS) Precipitation Frequency Data Server
- USA – NOAA Atlas 14 (PFDS) Precipitation Frequency Data Server (default)
- USA – ISWS (Illinois) Bulletin 75 (PFDS) Precipitation Frequency Data Server
- USA – SUDAS (lowa) Bulletin 71 (PFDS) Precipitation Frequency Data Server
- USA – NRCC (Northeast Regional Climate Center) PFDS
Note: The specific precipitation data source entry will be displayed based on the selected location.
The 2yr-24hr rainfall depth entry field is used to define the 2-year-24-hour rainfall depth value. Clicking the [Retrieve] button causes the software to automatically retrieve the 2-year-24-hour rainfall depth value for the regions falling within the United States.
For regions falling outside the precipitation data coverage area, the user needs to manually enter the rainfall depth value in the 2yr-24hr rainfall depth entry field.
- 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.
![[Compute] button](https://knowledge.civilgeo.com/wp-content/uploads/2024/12/Subbasin-TOC-Data-Command-Img-22.png)
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.