Defining Basins

Each time a feature outlet point is created, a basin for each upstream feature arc is created for the hydrologic modeling tree. This means that the stream arcs themselves are associated with a basin even before the Define Basins command is issued. When the command is used the DEM points intersected by the stream arcs are assigned the basin ID already given to the arcs. The Define Basins procedure then continues by tracing the flow paths of the remaining DEM points until a point which has already been assigned a basin ID is intersected. The result is that each DEM point gets assigned the ID of the sub-basin it belongs to.

Knowing that DEM flow paths only run until a stream is intersected can help you modify the results of a delineation by creating your own stream arcs (independent of those created using the DEM->Stream Arcs command) that "intercept" flow along a line that might not be represented by the underlying elevations. An example of this is along a highway embankment. The scale of most DEMs does not capture a feature such as a highway embankment, yet it is an important feature in basin delineation. You can create a stream arc along the highway alignment and leading to a culvert or bridge crossing. The resulting basin delineation will then "capture" all flow that intersects the line and account for the raised highway embankment. The following figures illustrate the use of this concept.

Delineation from the TOPAZ results only (not including the roads as streams).

Delineation after including the roads as streams. It is important to note that streams have been made in both directions from the outlets in order to "capture" the area that will flow to each outlet.

If you leave the outlet of the basin at the bottom node it will create separate basins for the main branch and each branch along the road. In order to combine them into a single basin a small (about one cell in length or less) stub stream is created so that only a single drainage will be used. Remember that WMS will create a separate basin for each upstream branch from an outlet point.

Close up of an outlet so that a single basin is defined.

Additional outlet points can be created by changing the attribute of existing arc nodes to outlets or by converting arc vertices to nodes and then changing the attribute to outlet. The Node<->Outlet command can be used to accomplish this. Any selected node or vertex will automatically be converted to an outlet node when using this command. Any selected outlet node will automatically be converted to a generic node type when using this command.

The Define Basins command can be used any number of times to redefine basins after the addition/deletion of outlet nodes.