qualitative validity of Eq. 5, the cross-sectional area depends weakly, but inversely on
the gross longshore transport rate, meaning that for all other factors being equal, the
inlet channel cross-sectional area will be larger for areas with smaller transport rate.
Inlet channel stability is promoted by a smaller ratio of its width to hydraulic radius at
mean sea level, W/R. As a first approximation, R can be replaced by average depth in
the entrance channel. Jarrett (1976) found that most dual-jettied inlets had W/R < 100.
Such channels tend to be deep and, therefore, more hydraulically efficient.
A classical approach to inlet stability is that of Escoffier (1940), in which a "stability
curve" is developed relating channel cross-sectional area to the velocity through the
inlet. Empirically, a mean-maximum velocity (mean of maxima of spring tides, for
example) of 1.1 m/sec is necessary to maintain a minimal channel cross-sectional area,
and the Escoffier analysis is compatible with that result. A PC program is available to
perform this analysis (Seabergh and Kraus 1997).
Jetties are typically extended seaward to at least the depth of the navigation channel to
protect the channel against intrusion of longshore sediment transport and to shelter
vessels from breaking waves and the longshore current in the surf zone under non-storm
conditions. Channels may be dredged deeper over the entrance bar or ebb shoal because
of the presence of breaking waves there. Jetties are sometimes oriented and configured
with doglegs to provide protection against higher waves from their incident direction.
Therefore, jetty length and orientation, wave height and direction, and channel depth
and orientation are three sets of interconnected parameters entering functional design of
navigable inlets. As the ebb shoal grows at a new or modified inlet, it will reach a
limiting depth that may be a concern to navigation channel design. Guidance is
available to predict this minimum depth (Buonaiuto and Kraus 2003).
A design conflict may arise in that small W/R, preferable for scouring the channel and
maintaining cross-sectional area, also promotes a strong ebb and flood current. A strong
ebb current increases wave steepness (wave height divided by wavelength) in the inlet
entrance, degrading navigation reliability. A strong tidal current reduces rudder control
of larger vessels, which approach inlets at moderate speed.
If feasible, channel orientation is into the predominant waves, typically "straight out."
However, if there is a saddle in the ebb shoal, vessel captains will tend to maneuver
through it toward deeper water, which gives greater under-keel clearance and is
typically an area of reduced wave height because of the deeper water. Price (1952)
advocated taking advantage of the natural orientation of the main (ebb) inlet channel for
navigation, but such an orientation may put vessels abeam to incident waves. Ship
simulations based on computed waves and currents can assist in designing channel
6. Response of Bay
This functional design consideration covers such as aspects as (a) change in magnitude,
phasing, and duration of storm water levels; (b) change in bay flushing, (c) salinity