The Delineate Catchments tool allows the user to delineate catchments for existing catch basins if the catchment wasn’t delineated when the catch basin was initially created. This is useful for creating catchments for catch basins that were created by importing from ModelBuilder for example.

Delineate Catchments

The purpose of Catchment Delineation is to analyse a terrain model, and calculate the area of it that will drain to a catch basin or a point. It is intended to be used with the final terrain model of the area to be served, which is normally the proposed design surface. Terrain models should be sufficiently precise that features such as gutters and swales are included, so that (for example) the valley formed by a gutter and curb is present, and contours clearly show this break line.

Terrain model which includes gutter and curb

Detail of contours and triangles which include the gutter and curb

It is not intended to be used to identify the area that will drain to a location on a plane, because in this case only a very thin area will be found.

No clear valley line to collect runoff

Example of thin catchment area, created where there is no clear valley line

Note that gutter elements in the drainage model are not used in catchment delineation.

The catchment delineation tool works by first performing an upstream trace from the catch basin or point, until the trace reaches a high point. Additional traces are then performed until sufficient traces are available to complete a catchment boundary.

The catchment delineation tool cannot identify portions of the catchment that lie outside of the terrain model. The boundaries of this portion of the catchment must be identified with some other approach as it is considered a separate catchment. The outflow from this external catchment can be assigned to the same catch basin as that within the terrain model. The catch basin can have two catchments associated with it - one from within the terrain model and one from outside the terrain model.

There are several parameters that can be adjusted to improve the performance of the automated catchment delineation. These are described in the Catchment Delineation Settings topic