Coastal Engineering Technical Note IV-19
Physical Model for Coastal Inlet
by William C. Seabergh
PURPOSE: The Coastal Engineering Technical Note herein provides information about the
potential use of a physical model facility dedicated to the study of inlets and equipped to
represent the most significant physical processes at and around inlets.
BACKGROUND: An inlet is a region connecting two or more large bodies of water by a
relatively short and narrow channel. The water bodies may be an ocean and lagoon, a large lake
and a bay, or a river entering a sea or lake. Many processes at inlets can be examined in a
thorough and efficient manner in a dedicated inlet physical model.
FACILITIES: As part of the Coastal Inlets Research Program, an idealized inlet was constructed
in a 46-m- (150-ft-) wide by 99-m- (325-ft-) long concrete basin with 0.6-m- (2-ft-) high walls.
The approach was to design an inlet with simplified bathymetry and fairly steep beach slopes so
that additional features (such as an ebb shoal) can easily be added. Also, it was anticipated that a
fine sand serve as both a tracer and as a fully mobile bed that can be placed over the concrete
bottom in a thick veneer. A 1:50 undistorted scale was assumed to determine reasonable inlet
dimensions to model. However, other scales can easily be assumed to accommodate the study of
specific processes because of the simplified bathymetry in the model. Also, the bathymetry can be
remolded in the inlet entrance area to the more complex bathymetry of an actual inlet, either in
fixed bed (concrete) or movable bed (sand). Ebb- and flood-shoal areas can also be modified to
represent more complex bathymetries.
Figure 1 shows the basin area of the Idealized Inlet Model Facility. The ocean-side parallel
contours were an equilibrium profile according to the equation from Dean (1977), h = Ax0.67, where
h is depth, x is distance offshore, and A is determined by sediment characteristics. A value of A
= 0.24 ft1/3 was taken, as it represented a relatively steep beach. The contoured ocean beach
slope extends to the 18.3-cm (0.6-ft) mean low water (mlw) depth (or 9.1-m (30-ft) depth if
scaled by 1:50) and is linearly transitioned to the basin floor at a depth of 30.4 cm (1.0 ft) (or
15.2-m (50-ft) depth when scaled by 1:50). The inlet throat region converges to a depth of
15.2 cm (or when scaled to 1:50, 7.6 m (25 ft)) relative to a mlw datum.