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Longest Flow Path Command

The Longest Flow Path command allows the user to select subbasins and have the software compute the longest flow path for one or more subbasins. The computations rely on the previously computed flow direction grid and flow accumulation grid using the Delineate Subbasins command. Therefore, the user should only select subbasins that were computed using the Delineate Subbasins command.

The longest flow path is widely used in hydrology for computing time of concentration and lag time.

An example is shown below.

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 and 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.

Follow these steps to use the Longest Flow Path command:

  1. From the Watershed ribbon menu, select the Longest Flow Path command.

  2. The Longest Flow Path dialog box will be displayed.

The following sections describe how to interact with the above dialog box.

Terrain Elevation Source

This section specifies which terrain grid layer should be used. This should default to the same terrain surface used in the Delineate Subbasins dialog box or, if not defined there, in the Scenario Manager dialog box. The dropdown combo box will only list elevation grids.

Select Subbasins to Compute Flow Paths

This section controls which subbasins should have the longest flow paths computed.

All Subbasins

This entry causes the software to compute longest flow paths for all subbasins on the HEC-HMS layer.

Select Subbasins

This entry allows the user to interactively select subbasin polygons from the HEC-HMS layer on the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear and allow the user to select HEC-HMS subbasin polygons from the Map View. The total number of selected subbasin polygons will be displayed in the entry upon return to the dialog box.

Note that the Longest Flow Path command confirms that the selected subbasins overlay the selected elevation grid terrain surface. If not, the software will display the following message.

If one subbasin was selected:

If more than one subbasin was selected:

Create Flow Path Drawing Layer

This section controls if the computed flow path will be drawn on a “normal” drawing layer.

By default, the Flow path layer entry will present the current drawing layer.

Clicking on the pencil icon will allow the user to rename the selected drawing layer.

The Flow path layer dropdown combo box lists all drawing layers that are not locked in the project in alphabetical order. Locked drawing layers will not be listed. In addition, at the top of the listing is an Add New Drawing Layer entry which will create a new layer.

Create SCS TOC Flow Path

Note that the SCS TOC flow path will be drawn on a “custom” drawing layer that only supports SCS TOC flow path polylines that are part of the HEC-HMS layer. Once the flow paths have been created, they are associated with the HEC-HMS subbasin elements. The user can toggle ON and OFF the display of the SCS TOC flow path in the HEC-HMS Properties dialog box.

The data entries that appear below allow the user to specify the default lengths for the sheet flow and the channel flow segments.

  • Create sheet flow segment: 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).
  • Create channel flow segment: 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).

Note that the software will confirm that there is a flow direction grid and a flow accumulation grid corresponding to the selected terrain surface elevation grid. If not, the software will inform the user that these grids need to be computed and the following message will be displayed.

If one subbasin was selected:

If more than one subbasin was selected:

About the Author Chris Maeder

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