Backwater

Backwater

Along flowing open channels the depth of water will vary due to (a) the boundary conditions at the beginning and end of the channel, (b) changes to the channel geometry (longitudinal- and cross-sections) and (c) the presence of structures such as weirs, gates, bridges and culverts.

Some changes can be abrupt, as at a weir, but most variations occur under conditions of gradually-varied flow, with the channel depths gradually increasing or decreasing relative to the channel bed.  A water surface profile controlled by a feature downstream, such as a weir or a tailwater level, is called a backwater curve.

Profiles can be calculated by dividing a channel into a number of reaches and (a) performing calculations that project water surface profiles, or (b) solve the equations of conservation of mass and momentum. The pipe and open channel flow calculations in the DRAINS Lite hydraulic model use the latter method. In the Full unsteady hydraulic model, this procedure also applies to overflow routes.

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