In HEC-HMS, when you click on GIS in the menu bar and then select Preprocessing Sinks, you perform a GIS operation to identify and handle sinks in the watershed model.
Once performed this operation, it is not possible do undo it. Therefore, before doing it, be careful to have your DEM file correctly saved in your folder or create a copy of it.
What do you get?
- The software analyzes the Digital Elevation Model (DEM) to detect depressions in the terrain (sinks), which are areas where water could accumulate and not flow out based on the digital model.
- If the DEM has errors or irregularities, these sinks can interfere with the correct flow path simulation.
- The preprocessing step fixes these artificial depressions by either filling or removing them, creating a hydrologically coherent surface.
Why is it done?
- It ensures that water flow is properly modeled without artificial interruptions.
- It is an essential step in preparing the DEM before generating watersheds, drainage networks, and sub-basins in hydrologic analysis.
- If skipped, the model might produce inaccurate results or unrealistic flow paths.
The Preprocessing Sinks function in HEC-HMS is used to fill in "holes" or "puddles" (sinks) in the DEM, which are depressions where water could stagnate instead of flowing downstream.
These depressions can result from errors in the digital elevation model data or real terrain features. However, for accurate hydrological modeling, it is often necessary to remove or fill them so that water follows a continuous path toward the main watercourses instead of getting trapped in isolated spots.
This step ensures that water is properly modeled within the watershed and that there are no artificial obstacles to runoff.
In summary, Preprocessing Sinks in HEC-HMS helps correct the DEM to ensure a more accurate hydrological simulation.
In the following pictures the Terrain before and after this operation is shown.
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