This tutorial shows how to create some catchment areas for a terrain grid using the tools in the Surface water menu.

When we're creating catchments we need to work with gridded surfaces. The process Muk3D goes through is as follows:

1. Define the location of any ponds/dams/sinks that allow water to collect.
2. Fill depressions in the grid. If a point can't flow anywhere and its not regarded as a sink, then the point elevation is modified until all points can flow somewhere (except sinks) unless they are on the edge of the grid.
3. Determine flow direction from each point.
4. Count the number of predecessor points that contribute to flow through each point.
5. Create streams for points that have more than a threshold area that contributes to flow.

The command Surface water/Watershed/Watershed wizard runs through all these steps using reasonable default values and identifies the streams.

To generate catchments, we select a point along one of the streams, and the contributing area will be identified and extracted from the base grid.

Tips

Once the flow_grid has been created, its no longer suitable for doing geometric modelling.  The Fill depressions part of the algorithm may modify surface elevations by an unspecified amount.

Depending on the size/extents of the base grid, a coarser grid spacing can be used for the watershed calculations than we'd normally use for geometric modelling.  

Pre-requisites

• Have a base grid to work with

Commands used

• Surface water/Watershed/Watershed wizard
• Surface water/Watershed/Generate catchment