Question

The hydraulic mean depth or the hydraulic radius is the ratio of

Answer 1

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Hydraulic radius is defined as the area of the flow section divided by the wetted perimeter, whereas,

Hydraulic mean depth is defined as the area of the flow section divided by the top water surface width.

In rectangular channels, hydraulic radius does not equal depth, but approaches depth as the channel becomes very wide. However, the hydraulic mean depth is the same as the depth of the rectangular flow section. It is more important to understand which of the two is used where. Eg. While calculating frictional head losses, Hydraulic Radius is used and while in Froude number and energy relationships in open channel flow hydraulic mean depth gives desired results