When performing a watershed delineation in GIS, the DEM usually needs to be pre-processed by “filling” any depressions or sinks (an area lower than its surroundings) to create a hydraulically connected and filled DEM. However, sometimes the operation of filling depression will significantly change the original DEM by filling too large or too deep an area, which may cause an issue to the following watershed delineation.
Breaching a depression is a low impact alternative to fill depression (Figure 1). Preferably, a DEM can first be breached to get rid of majority of the depressions, if not all, and then filled to eliminate any remaining depressions.
WhiteboxTools has an very effective depression breaching tool – BreachDepressionsLeastCost. As illustrated in Figure 2, the maximum search distance, which may be better named as maximum breach channel length, can be set as 10 or 20 cells, or twice of the expected roadway embankment width to be breached. The fill unbreached depressions option should be turned off since it will be conducted separately next.
The breached DEM should be further “filled” to eliminate any remaining depressions (Figure 3). Since it is believed that the breach tool has gotten rid of the majority of the depressions, the following fill depression operation will have a much less impact on the original DEM than if it is performed without the DEM being breached first. WhiteboxTools has several fill depression tools and the FillDepressionsWangAndLiu is recommended here.
After the above pre-processing steps, a watershed delineation can be conducted using WhiteboxTools as explained in this post.
To further minimize the impact to the original DEM, some discussions on the internet suggest using WhiteboxTools‘ FillSingleCellPits (Figure 4) or BreachSingleCellPits (Figure 5) to remove small depression areas by filling or breaching just one single cell before applying the breaching or filling depressions tools which usually are meant for large depressions features. Note these two tools are optional, and therefore use them at your own discretion.
It always prudent to investigate different DEM pre-processing methods and their resulted DEM – fill depressions directly, breach depressions and then fill them, applying breach or fill single cell pit before breach/fill depressions, and use the one that generates the most reasonable watershed delineation results.