WMS:TIN Basins

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Defining Basins

The Define Basins command assigns each triangle in the TIN to a drainage basin. This is accomplished by initiating a flow path from the centroid of each triangle and "flowing" down until an outlet point is encountered. The triangle is then assigned the appropriate basin ID. The boundaries may appear rough or jagged because each triangle is assigned according to the flow from its centroid, when in fact the triangle may actually straddle basin boundaries. Boundaries may be corrected by issuing the Refine Boundaries command. The drainage basin boundaries option in the Drainage Display Options dialog is automatically set when defining basins.

Refining Boundaries

After the initial definition of drainage basins, many of the boundaries are rough or irregular. Triangles straddling true basin boundaries can be split using the Refine Boundaries command. This process is accomplished by tracing paths of maximum upward gradient along boundaries, splitting triangles when the path crosses over them, and then reassigning all affected triangles to their new basins. The process is displayed graphically.

Merging Basins

For each stream branch upstream from an outlet point, a drainage basin is automatically created. The recommended method of merging basins is to establish the outlet at a node just downstream of the junction.

However, if an alternative method to merge basins is needed, there is an option to merge selected basins together using the Merge Basins command. The Merge Basins command allows combining the basins for a given outlet. In order to merge basins, they must be adjacent to each other and belong to the same outlet. In order to select drainage basins, the Select Drainage Basins tool must be active.

Editing Basin Variables

The Basin Variables button will let viewing/editing any of the basin variables that are computed by WMS. While it is unnecessary to edit these variables, some may find some cases where overriding what WMS has computed is desired so as to use a value derived through a separate analysis.

Assigning Triangles to Basins

Occasionally it is desirable to manually assign triangles to a basin. This is particularly important if there is a large flat area (such as a lake) within a basin and there is no need to edit the elevations such that they would flow to the outlet.Select the triangles to assign and then choose the Assign Triangles to Basin command in order to assign the triangle to a basin. A prompt will appear to select the outlet of the basin to assign the selected triangles to.

Deleting NULL Basin Triangles

The Delete Null Basin Triangles command can be used to delete all triangles whose flow path does not encounter an outlet. Before defining drainage basins all triangles are classified as belonging to the null drainage basin. After defining drainage basins some triangles still belong to this null basin since they do not contribute flow through any of the given outlets. They should not be deleted if further editing is to be done. However, once all sub-basins have been properly defined, they can be deleted in order to reduce the size of the model to the region of interest.

Computing Basin Data

After defining basin boundaries, attributes such as basin areas and slope and stream lengths and slopes can be computed using the Compute Basin Data command. These are all geometric parameters used in defining basins and routing networks in HEC-1 and TR-20. If the TIN is edited or sub basin configuration changed, the drainage data must be recomputed using this command.

Once computed, they can be displayed along with the basins in the Graphics Window or written to a file using the Export File command found in the File menu. By default, basin IDs and areas are displayed after computing drainage data. The other attributes can be toggled on for display in the Basin Attributes dialog accessed from within the Drainage Display Options dialog.


When the Compute Basin Data command is given the Model and Parameters Units dialog appears which allows specifying the current Units of the model (TIN vertices) and the units to use for computed parameters. TIN vertices must either be in feet or meters, but the computed areas and lengths can be given separate units. If wanting to change the units of computed parameters at a later point, recompute the basin parameters with different selections. It may be desirable to convert the coordinate system prior to computing the basin data.

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