ERDC/CHL CETN-IV-26
June 2000
Simulation of Current-Induced
Scour in Movable-Bed Inlet Models
by Steven A. Hughes
Purpose: This Coastal Engineering Technical Note provides guidance on using movable-bed
physical models to predict erosion and deposition caused by currents in tidal inlet channels. A
scaling relationship based on equilibrium scour depth allows observed model scour depths to be
quantitatively scaled to full-scale dimensions. Appropriate situations are listed for which this
modeling guidance can be applied.
Background: Historically movable-bed physical models of jettied and jettied inlet systems
have seen limited use due to uncertainty about similitude relationships for scaling model results
to full size (prototype scale). Consequently, most movable-bed inlet model results were viewed
as qualitative indicators of general inlet behavior and evolution, and little attempt has been made
to use movable-bed models to quantify depth of scour or amount of deposition under given flow
conditions.
Area constraints in laboratory facilities generally force the prototype-to-model geometric length
scale (NL) of a typical inlet to be on the order of 75-to-1 or greater (NL ≥ 75). For a model in
which the flow kinematics conform to the Froude similitude relationship, it is not practical to
reduce the model sediment size by the geometric length scale and still have noncohesive
sediment in the model. Instead, noncohesive sediment is used in the model that has a grain-size
diameter relatively larger than the diameter required by Froude scaling. The consequence is that
model currents need to be relatively faster than currents specified by Froude scaling to move the
sediment and scour the inlet channel to a depth similar to what would occur in nature. Therefore,
it is necessary to "distort" the prototype-to-model Froude velocity scale to achieve similarity in
scour patterns between the model and prototype. Unfortunately, distorting the velocity scale
limits the movable-bed model to situations where inlet channel scour is caused by either currents
or waves, but not by both.
This note focuses on movable-bed modeling of inlet channel scour problems stemming from
bedload sediment transport caused by tidal currents. Such scour is most common in the throat of
inlet systems where waves have only a secondary effect because of reduced wave height or fairly
deep channel depths. Situations where waves are thought to be a dominant scour mechanism are
not correctly simulated by the relationship given in this note.
Scaling Relationship: An appropriate scaling relationship for movable-bed modeling of
channel evolution produced by the tidal current is derived in Appendix A of this note. The
scaling relation was derived by assuring that an expression for equilibrium scour depth at tidal
inlets is the same in the model as in the prototype. The resulting scaling relationship is given by
the expression:
1
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