The Automated Flow Paths command allows the user to select subbasins and direct the software compute the longest flow path for one or more subbasins. The computations rely on the previously computed flow direction grid generated with 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.
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 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.
Follow the steps below to use the Automated Flow Paths command:
- From the Watershed ribbon menu, select the Automated Flow Paths command.
- The Automated Flow Paths dialog box will be displayed.
The following sections describe how to use the Automated Flow Paths command and interact with the above dialog box.
Selecting HEC-HMS Subbasins to Compute Flow Paths
The Select HEC-HMS Subbasins to Compute Flow Paths section controls which subbasins should have the longest flow paths computed.
The All Subbasins entry causes the software to compute the longest flow paths for all subbasins on the HEC-HMS layer.
The Select subbasins 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 will 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 Automated Flow Paths 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:
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.
Terrain Surface Thinning Options
This section allows the user to define the cell size that will be used for thinning the terrain surface. The smaller the cell size value, the longer the processing time.
The user can select one of the following options to define the cell size:
- Selecting the Compute optimal cell size radio button option causes the software to use the default cell size value computed by the software. By default, the software selects this option.
- Selecting the Define cell size radio button option allows the user to manually define the cell size value. The user can use the spin control button to define the cell size value.
- Selecting the Use terrain surface cell size radio button option causes the software to use the default cell size value of the terrain surface.
Creating Flow Path Drawing Layer
This section controls whether the computed flow path will be drawn on a “normal” drawing layer.
By default, the Flow path layer entry will be present on the current drawing layer.
Clicking on the [New] button will allow the user to create a new 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.
Creating 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.
The Delete previously computed longest flow paths checkbox option allows the user to delete previously computed longest flow paths from the model.
Create Sheet Flow Segment
The Create sheet flow segment checkbox option will specify that there is a sheet flow segment at the upstream end of the selected TOC flow path. This option provides different ways of computing the sheet flow segment, as detailed below.
The Fixed distance entry allows the user to specify the length to use for the sheet flow segment. By default, the software uses a segment length of 20 ft (or 6 meters). The user can enter a different value or click the […] button to measure sheet flow length from the Map View.
The Use McCuen-Spiess equation, Manning’s n entry allows the user to specify the Manning’s n roughness value and the software will automatically compute the sheet flow length. The default value of this entry is 0.1500. Alternatively, click the adjacent […] button to measure Manning’s n roughness value from Manning’s Roughness table. This equation is used for post-construction conditions. The sheet flow length is computed using the following equation.
Create Channel Flow Segment
The Create channel flow segment checkbox option will specify that there is a channel flow segment at the downstream end of the selected TOC flow. This option provides different ways of computing the channel flow segment, as detailed below.
The Fixed distance entry allows the user to specify the length to use for the channel flow segment. By default, the software uses a segment length of 100 ft (or 30 meters). The user can enter a different value or click the […] button to measure channel flow length from the Map View.
The Maximum shallow concentrated flow distance entry allows the user to enter the maximum shallow concentrated flow distance. The default value of this entry is 1000 ft (or 300 meters). The user can also manually enter a value or click the adjacent […] button to measure the maximum shallow concentrated flow distance from the Map View.
The Percentage of total length spin control is used to compute the length of the channel flow segment, based upon a percentage of the total flow path length. This spin control ranges from 1 to 100%, with a default value of 50%.
The Intersection with routing reach option causes the software to compute the channel flow length when the flow path polyline intersects the routing reach polyline.
Note that the software will confirm that there is a flow direction 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.
Computing Longest Flow Paths
Once the data has been defined in the Automated Flow Paths dialog box, click the [Compute] button. The software will then compute the longest flow path for the selected subbasin(s), as shown below.