Normal depth

If a steady flow occurs in a long, straight channel, the velocity and water level will become constant, as the loss of energy through friction will equal the energy lost through the fall along the channel. The water surface slope will be the same as the bed slope of the channel. The depth under these conditions is known as the normal depth. 

It depends on the flowrate, the channel shape, slope and roughness. It must be calculated iteratively, but this is a fairly easy process. Pipe and channel friction equations such as the Colebrook-White equation, Manning’s equation and the Chezy equation can be used to determine normal depths.

In many situations, channels are too short to establish normal depth, or flowrates are not constant along the channel, due to lateral inflows or outflows (spatially-varied flow). In such cases, the normal depth should be seen as an indicator, rather than an exact depth. This is important when considering results in DRAINS for overflow routes when using the Lite hydraulic model, which displays results of normal depth calculations using the peak flowrate along the route, with allowance for part of the flows from the sub-catchment through which the overflow route runs. 

The unsteady flow calculations applied to overflow routes in the Full Unsteady hydraulic model provide full water surface profiles and accurate depths.

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