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Draw and Assign Flow Paths Command

GeoHECHMS allows the user to draw or assign a 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 slope and character of the subbasin and flow path.

The flow paths can be defined by either drawing or assigning the polyline using the following commands:
  • Draw Flow Paths
  • Assign Flow Paths

Drawing Flow Paths

The Draw Flow Paths command allows the user to draw a polyline on the Map View to be used as a subbasin flow path line for computing the TOC / Lag Time.

Follow the steps below to use the Draw Flow Paths command:

  1. From the Watershed ribbon menu, select the Draw Flow Paths command.
    Draw Flow Paths Command
  2. The Draw Flow Paths dialog box will be displayed.
    Draw Flow Paths Dialog Box

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

Drawing a Subbasin Flow Path Polyline

The Flow path polyline entry specifies whether a flow path line has been drawn. Clicking the [Draw] button causes the dialog box to disappear, at which point the user is then prompted to draw a corresponding subbasin flow path line for computing the TOC.

After drawing 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.

The user can turn on the Draw curvilinear polyline check box to add the curvilinear segments or can press the [Ctrl] key to enable the straight segments while drawing flow paths.

Note that the Draw Flow Paths command will check whether the drawn polyline is at least 90% contained within a subbasin polygon. If not, then the following informational message is displayed.

Extracting Elevation Data

The Extract Elevation Data 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.

Creating SCS TOC Flow Segments

The Create SCS TOC Flow Segments 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).

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.
  • Fixed distance: This 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 […] Measure button to measure sheet flow length from the Map View.
  • Use McCuen-Spiess equation: This option will automatically compute the sheet flow length. This equation is used for post-construction conditions. The sheet flow length is computed using the following equation.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.
  • Fixed distance: This 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 […] Measure button to measure channel flow length from the Map View.
  • Percentage of total length: This 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%.
  • Intersection with routing reach: This option causes the software to compute the channel flow length when the flow path polyline intersects the routing reach polyline.

Note that the flow path default flow type is shallow concentrated flow.

Once all the data has been defined in the Draw Flow Paths dialog box, click the [Apply] button. The software will then create a corresponding subbasin flow path line for computing the TOC / Lag Time.

Assigning Flow Paths

The Assign Flow Paths command allows the user to assign a polyline as subbasin flow path line for computing the TOC / Lag Time.

Follow the steps below to use the Assign Flow Paths command:

  1. From the Watershed ribbon menu, select the Assign Flow Paths command.
    Assign Flow Paths Command
  2. The Assign Flow Paths dialog box will be displayed.
    Assign Flow Paths Dialog Box

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

Selecting a 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.
Invalid Flow Path Error BoxNote that the Extract Elevation Data and Create SCS TOC Flow Segments sections of the Assign Flow Paths command are similar to the Draw Flow Paths command, both of which have been explained in the previous sections of this article.

Once all the data has been defined in the Assign Flow Paths dialog box, click the [Apply] button. The software will then assign the selected polyline as a corresponding subbasin flow path line for computing the TOC / Lag Time.

About the Author Chris Maeder

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