Coastal Engineering Technical Note IV-18
March 1999
Equilibrium Scour Depth
at Tidal Inlets
by Steven A. Hughes
PURPOSE: The Coastal Engineering Technical Note (CETN) herein introduces a simple
expression relating maximum discharge per unit width at a location in a tidal inlet to the depth of
scour at that location. Application of this provisional guidance is illustrated by three examples.
BACKGROUND: One scour problem of concern at improved navigation inlets occurs where
the maximum depths of the equilibrium inlet throat cross section are adjacent to a stabilizing
jetty structure. These deep portions along a jetty have the potential to undercut the structure toe
and cause subsequent damage to the structure armor layer.
Within the inlet channel, tidal currents play a major role in erosion and deposition of sediment.
If scour occurs close to the seaward end of the inlet, wave action and longshore currents also
contribute to the scour action and perhaps dominate the process. However, where scour occurs
well inside the entrance channel, wave action is reduced; and it is reasonable to assume sediment
movement at that location is driven primarily by the tidal flow.
Over many tidal cycles, scour in regions with minimum wave action will eventually reach a
"live-bed" equilibrium depth where the maximum shear stress acting on the bottom is no longer
sufficient to initiate scour of the bed. Additional scour can occur only if the maximum flow
discharge is increased at that particular location. Flow increases might occur because of an
overall increase in tidal prism or because of flow redirection resulting from structure alterations,
dredging activities, or channel realignment.
In this technical note, a new relationship for use at tidal inlets is developed for the maximum
tidal flow discharge per unit width as a function of the water depth and sediment characteristics.
Measurements of maximum discharge from Ponce de Leon Inlet and Shinnecock Inlet are used to
establish an upper-bound empirical coefficient. This equilibrium discharge relationship implies
that there is an equilibrium depth that can tolerate a given discharge per unit width. Increases in
discharge will result in scour and a corresponding increase in water depth. Practical applications
of this simplified engineering approximation are suggested.
FORMULATION: Assume the vertical velocity profile at times near the maximum discharge
through a tidal inlet can be represented as a steady, fully developed, rough, turbulent boundary
layer extending from the bottom to the free surface. Any contribution by waves is neglected.
The boundary layer velocity profile can be adequately approximated by a 1/8 power curve (Yalin
1971) with the shear stress at the bed given as
1
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