Depth of Flow

Uniform depth (d
u
) of flow (sometimes referred to as normal depth of flow) occurs when there is uniform flow in a channel or conduit. Uniform depth occurs when the discharge, slope, cross-sectional geometry, and roughness characteristics are constant through a reach of stream. See
Slope Conveyance Method
for how to determine uniform depth of flow in an open channel (Chapter 7).
By plotting specific energy against depth of flow for constant discharge, a specific energy diagram is obtained (see Figure 6‑2). When specific energy is a minimum, the corresponding depth is critical depth (d
c
). Critical depth of flow is a function of discharge and channel geometry. For a given discharge and simple cross-sectional shapes, only one critical depth exists. However, in a compound channel such as a natural floodplain, more than one critical depth may exist.
Typical Specific Energy Diagram (click in image to see full-size image)
Figure 6-2. Typical Specific Energy Diagram
You can calculate critical depth in rectangular channels with the following Equation 6‑12:
EquationObject206227
Equation 6-12.
where:
You can determine the critical depth for a given discharge and cross section iteratively with Equation 6‑13:
  • q
     = discharge per ft. (m) of width (cfs/ft. or m
    3
    /s/m).
You can determine the critical depth for a given discharge and cross section iteratively with Equation 6‑13:
EquationObject207228
Equation 6-13.
where:
  • T
    c
    = water surface width for critical flow (ft. or m)
  • A
    c
    = area for critical flow (sq. ft. or m
    2
    ).