Using Catchment Delineation

Product(s): SewerGEMS, CivilStorm, StormCAD, SewerCAD
Version(s): 10.XX.XX.XX
Area: Layout and Data Input


This article contains common workflows for using the Catchment Delineation tool. Catchment delineation is available in the stand-alone and MicroStation platforms only and enables you to create and assign catchment drainage areas to catchbasins based on a draped terrain model.

For earlier versions of OpenFlows products, this functionality is not built-in. However, Bentley "InRoads Hydraulics and Hydrology add-in", which works in MicroStation, has the ability to do watershed delineation. You could then import the delineated catchments using ModelBuilder. For more information on how to use ModelBuilder please refer to this link .

Table of Contents

  • Video Demonstration
  • Delineation for newly placed catch basins
  • Delineation for existing catch basins
  • Moving catch basins
  • Adding catch basins within existing catchments
  • Deleting catch basins
  • Advanced Settings

Video Demonstration

Placing new catch basins to delineate contributing catchments (ie. Catch basin positions are not yet known)

1. Attach the Digital Terrain Model (DTM) which will be used to determine the contributing catchment areas.

a. Open the Terrain Models Manager (View > Surface Views > Terrain Models).

b. Click the New icon to open the DTM (Digital Terrain Model) settings dialog.
See: Help "Terrain Model Settings Dialog"

c. Specify the Source Type, Terrain Model Source file, and the Source Unit to be used.

d. Check the box "Show Contour Lines". See this short video about more functionality on this: A tip on displaying terrain model contours in your hydraulic model

e. Adjust the interval values and colors as needed. Click OK


  • Change the minor interval contour line color to something other than red, such as green. The downstream trace color is red, which may make it difficult to differentiate between the two.
  • By making the Minor Interval smaller it will be easier to see where things drain to with the Downstream Trace tool.

2. Click the Zoom Extents button to show the terrain model (View > Zoom and View > Zoom Extents).

3. In the Terrain Models Manager, select the Downstream Trace button. Use the downstream trace tool to locate the upstream end and to locate where the trace lines drain to.

4. In the Terrain Model Manager, press the "Run Catchment Delineation" button. The button will be placed in the depressed position.


  • With this enabled, placing or moving a catch basin within the Digital Terrain Model will identify and create a corresponding catchment.
  • The catchment delineation tool works by first performing an upstream trace from a point at the center of the catch basin until the trace reaches a high point. Additional traces are then performed until sufficient traces are available to complete a catchment boundary.

5. Place a catch basin in the candidate spot based on the downstream trace.

Example of misplaced catchment (catch basin is not on downstream trace line)
See the section below "Moving the catch basin and updating the contributing catchment area".

Example showing good placement (all upstream area turns into catchment)

6. Repeat step 5 for the other catch basins. Start at the upstream end and work your way downstream. When working from upstream to downstream, the downstream catchment is aware of the upstream catchment and they won't overlap.

7. Uncheck the box in the Terrain Model Manager to turn off contours and better see catchment separation.

8. Repeat this process until the area in question is completed. Then a new section can be started.

Delineating catchments draining to a pond

If your catchments will drain to a pond instead of a catchbasin, you can first lay out the pond polygon (or an approximation), then lay out a few catchbasins within the pond's polygon perimeter. After the contributing catchments are created, delete the catchbasins, then re-direct the catchments' "outflow node" to the pond.

Delineating catchments for existing catch basins (ie. Catch basins are already in the model, either from manual layout or ModelBuilder)

If a model already contains catch basins just select all of them, right-click in the drawing pane and select "Update Catchments for Inlets". Catchments will be automatically created for all of the catch basins that are selected without any overlapping.

Moving the catch basin and updating the contributing catchment area

When the "Run Catchment Delineation" button is enabled, moving a catch basin will automatically update the contributing catchment area as well as adjacent downstream catchment areas. For example, when the middle catch basin below is moved to the right, the downstream catchment boundary is automatically updated.

When the "Run Catchment Delineation" button is disabled and a catch basin is moved, then it's catchment and the downstream catchment will not be updated automatically. Both will need to be updated by right-clicking on the catch basin and selecting "Update Catchments for Inlets" (starting at the upstream catch basin).

For example, when the middle catch basin below is moved to the right, with the "Run Catchment Delineation" button disabled, then the catchments remain the same.

Adding a catch basin in an already defined catchment

Example: If a catchment area is too large and an additional catch basin is required to divide the catchment area.

1. Use the downstream trace tool to better understand the slope of the catchment.

2. Place an additional catch basin based on the downstream trace line.

If the "Run Catchment Delineation" button is enabled, adding a catch basin will automatically update the contributing catchment area, but not the adjacent downstream catchment area.

If the "Run Catchment Delineation" tool was disabled, then a catchment will not be created automatically. You will need to Right-click on the new catch basin and select "Update Catchments for Inlets".

3. Right-click on the downstream inlet (associated with the catchment being divided) and select "Update Catchments for Inlets".

Deleting catch basins or catchments

Example: You might have placed too many catchments and want to remove one of them. Deleting the catch basin and the associated catchment will leave a gap.

The downstream catch basin can be updated to include the deleted upstream catchment area by right clicking on the catch basin and selecting "Update Catchments for Inlets".

Advanced Settings

There are several parameters that can be adjusted to improve the performance of the automated catchment delineation. The default values for these parameters are usually adequate so you should generally avoid changing them without good reason.  The parameters are described in the help topic "Catchment Delineation Settings".

  • Step Size: Step size is distance from the last found point to the next found point. A smaller search step will generate more accurate results, but it will increase the total time to search all points in catchment boundary. Range of the value is from 1 to 3 ft.
  • Step Angle: Step Angle is the angle to rotate the line from the last found point to next point when trying to find the next point within the catchment or a point outside of the catchment boundary. Value can be 0.15 radiant / step.
  • Step reduction angle tolerance: When a point is found in catchment boundary, the bend angle of the last found 3 points is calculated. If the bend angle is larger than the value, the search step will be decreased by half to find a more accurate point to replace the last found point. The step reduction angle tolerance value can be PI / 16 or 0.196 radiant.
  • Catchment boundary closure tolerance: When a point within catchment boundary and a point outside of the catchment boundary are found, bisection method is used to find a point closer to the real catchment boundary. If the distance between the last found 2 points is smaller than the value, the search for the point will be stopped.
  • Adjacent catchment gap tolerance: 3 lines with length of this value are drawn to the outside of the catchment boundary from 3 points in the line between the last found 2 points in catchment boundary. By checking the ending points in the downstream trace paths from those 3 extended points, the program can tell if the gap is larger than gap tolerance. If the gap is larger than the gap tolerance, the search step will be decreased by half to find a more accurate new point to replace the found last point.
  • Ignore pit/depression depth less than: We use terrain model downstream trace to check if a point is within the boundary of the catchment for the inlet. If the depth of the local pit is smaller than the value, the downstream trace will pass the local pit and continue to the downstream low point. For terrain models on road, the value can be between 0.01 ft and 0.1 ft. For natural surface, a relatively large value can be used.
  • Simplify boundary vertices: If the box is checked, maximum perpendicular algorithm is used to reduce the number of points in catchment boundary.
  • Vertex removal tolerance: When the distance from a point to the line between other 2 points is smaller than the value, the point can be removed from the catchment boundary point list.

Display Triangles

When adding a terrain model, you now have the ability to show the Civil product triangles or TIN (Triangulated Irregular Network) when loading the file. Triangles can be useful when reviewing catchments that are generated from using the Terrain Model. In addition, Triangles can be useful when doing the downstream water-drop trace.

What if I don't have a Digital Terrain Model (DTM)?

Catchment Delineation needs to know the slope of the surface to be able to know where the water flows. so it needs elevation information to do that.  If you don't have a Digital Terrain Model (DTM) to use with Catchment Delineation, Catchments can be placed in your model from the Place Catchment command (Layout - Catchment).

Simply place your Catchments in your model and link them to Catch Basins (as outflow elements).  More information on how to accomplish this can be found in this Wiki article - Entering catchment data directly to a catch basin.

See Also

Blog Video: Save time with automatic catchment delineation

SIG Video: Catchment Delineation

Help: Catchment Delineation

Help: Catchment Delineation Settings

Help: Terrain Models Manager