Troubleshooting Urban Surface Watershed Delineation

[JennaB2]

I am trying to run a city-wide analysis to define the stormwater catchments using the DEM and catchbasins only, as we are looking at just surface runoff watersheds and also don't have access to the stormwater infrastructure pipe network.

I am seeing this anomaly where there are long straight lines at certain points throughout the city (see left side in the screenshot), but in other areas, the watersheds output looks as expected.  I am using the following workflow with Spatial Analyst tools:

1. Fill
2. Flow Direction
3. Flow Accumulation
4. Snap Pour Points (Catch Basin locations)
5. Watershed

Any insights on how best to troubleshoot this is much appreciated!

[Robert_LeClair]

Those long, straight “cuts” are almost always coming from the DEM/flow-direction surface and raster alignment, not from Watershed itself. In an urban DEM with D8 flow routing, small artifacts (flats, embankments, resampling seams) can produce very straight drainage divides.

Here are some steps you can try (in order):

1) Verify raster alignment environments for every tool

In Geoprocessing Environments for Fill, Flow Direction, Flow Accumulation, Snap Pour Point, and Watershed, set consistently:

• Snap Raster = your DEM
• Cell Size = Same as DEM
• Processing Extent = Same for all steps (often “Same as DEM”)
• (Optional) Mask = your city boundary (to prevent edge effects)

Misalignment/resampling between steps is a common reason for straight-line artifacts.

2) Diagnose whether the lines are in Flow Direction (they usually are)

Symbolize/inspect the Flow Direction raster. If you see directional “seams” or large uniform patches, the watershed boundaries will reflect them.

Background: Watershed delineation is based on Flow Direction + pour points: How Watershed works—ArcGIS Pro

3) Tune Snap Pour Point (distance matters a lot)

Snap Pour Point moves your catch basins to the highest flow accumulation cell within the snap distance; too small = not snapping onto a real drainage path, too large = snapping across a divide into a different drainage path.

• Confirm snapped points land on the intended “drainage lines” in the Flow Accumulation raster.
• Reference: Watershed tool, Snap Pour Point function

4) Consider DEM conditioning for urban barriers/flats

Road crowns/berms, bridges/culverts, and lidar artifacts create unrealistic divides (often straight). Fill helps with sinks, but it won’t solve barriers/culverts.

• Esri’s hydrology workflow starts with Fill to address sinks: Hydrologic analysis sample applications

5) Check DEM resolution and vertical quality

If your DEM is coarse relative to curb-to-curb drainage, D8 routing can create grid-aligned divides that look like straight lines.