The Assign Flow Paths command allows the user to select an individual polyline for each subbasin to function as the overland flow path for computing the SCS TR-55 Time of Concentration / Lag Time.
Lag Time and Time of Concentration (TOC) are two different but related terms that are used in hydrology computations. They represent the time required for runoff to travel from the hydraulically most distant point in the watershed to the outlet. The hydraulically most distant point is the point with the longest travel time to the watershed outlet, but it is not necessarily the point with the longest flow distance to the outlet. It is a function of the topography, hydrologic soil type, and land use within the subbasin. There are many different methods for computing Lag Time and Time of Concentration, but generally, the computations are dependent upon the slope and character of the subbasin and the flow path.
Follow these steps to use the Assign Flow Paths command:
- From the Watershed ribbon menu, select Assign Flow Paths.
- The Assign Flow Paths dialog box will be displayed.
The following sections describe how to interact with the above dialog box.
Select Subbasin Flow Path Polyline
The Flow path polyline entry specifies whether a flow path line has been selected. Clicking the [Pick] button causes the dialog box to disappear, at which point the user is then prompted to select a corresponding subbasin flow path line for computing the TOC.
After selecting the flow path line, the user is immediately returned to the dialog box. The software then determines which subbasin the flow path line corresponds to and displays the subbasin ID in the dialog box.
Note that the Assign Flow Path command will check whether the drawn polyline is at least 90% contained within a subbasin polygon. If more than 10% of the line resides outside of the subbasin, the following informational dialog box is displayed, and the selected polyline is not assigned as a flow path polyline.
Extract Elevation Data
This optional section is used to extract elevation from the defined terrain surface.
This option is enabled by default but recalls the user selection at the project level (when the file is saved).
The Terrain surface dropdown combo box only lists elevation grids (i.e., DEMs). By default, it will show the following:
- Elevation grid if there is only one elevation grid available in the project.
- Elevation grid associated with the delineation of the subbasins.
Create SCS TOC Flow Segments
This optional section is used to subdivide the assigned flow path into flow segments for assigning SCS TOC. It is enabled by default but recalls the user selection at the project level (when the file is saved).
The flow segments are as follows:
- Create sheet flow segment: If a terrain surface is selected, then this option will be enabled. Otherwise, this entry will be disabled (i.e., grayed out). This entry will specify that there is a sheet flow segment at the upstream end of the selected TOC flow path. The default segment length is 20 ft (or 6 meters). This option is enabled by default but recalls the user selection during the application session.
- Create channel flow segment: If a terrain surface is selected, then this option will be enabled. Otherwise, this entry will be disabled (i.e., grayed out). This entry will specify that there is a channel flow segment at the downstream end of the selected TOC flow. The default segment length is 100 ft (or 30 meters). This option is disabled by default but recalls the user selection during the application session.
Note that the flow path default flow type is shallow concentrated flow.