The Manual Cutter and Dam Builder tool is an iterative step in this process. This step is also the most important tool in accurately developing a stream network.

This tool works by the user identifying errors within the DEM flow network and creating cut and dam lines to correct these. Cut lines are used when an obstruction of flows is present within the DEM and it needs to be burned through to allow the flow to move. These lines will be used in cases where you want to show changes to the minimum elevations, such as where there are culverts or where the flow will be going under a bridge.

Another option is to develop dam lines, which are used when an obstruction to flow is desired, but is absent within the DEM. In this case, when a dam line is drawn, the maximum elevation values that fall along that line will be extracted and then applied to all the cells in the DEM that the line crosses. If no cut or dam lines are made, there’s a good chance your flow paths will be inaccurate.

In the images below, you can see the original flowpaths in purple. We can tell from the imagery and depth grid that there should be cuts across the east/west road where there are probably culverts. For the north/south road on the left side, cuts were added across driveways to simulate culverts. After the cuts were added in red, the Visualize Flowpaths tool was run and the new flowpaths are in yellow.

In the images below, you can see the original flowpaths in pink. In the left image, there are two main flows out of our watershed (boundary in green) going north. We want only one flow out of a watershed so a dam (blue line) was added along the boundary to reroute one of the flowpaths (center). The Visualize Flowpaths tool was run, showing the new flowpaths in yellow (right image). Now the flow leaves only in one path as expected.

It’s very important to go through the DEM one area at a time and evaluate very precisely where dams and cut lines should be placed. This process will take you about half a day to complete but can vary widely based on watershed characteristics.

It is important to note that, when making changes to the DEM, the derived terrain products must be regenerated as well. The output data: flow direction, flow accumulation, filled DEM, and the hillshade raster, must use the edited DEM in order to faithfully represent the flow network. Only one version of these rasters must be present in the file geodatabase.

The image below shows a DEM with areas of potential cuts outlined in blue ovals. These situations are road culverts or other infield blockages to flow.

Tool Summary

Inputs:

Cut lines (Optional)
Dam lines (Optional)
Unfilled DEM raster (Required)

Outputs:

New Unfilled DEM raster (NewDEM + inHUC)
Filled DEM raster (DEMFill + inHUC)
Flow Direction raster (D8FlowDir + inHUC)
Flow Accumulation raster (D8FlowAcc + inHUC)
Hillshade raster (Hshd + inHUC)