12 December 2001
Where the results of this approximation appear to be critical to sustained
operation or to project costs, additional investigations using the appropriate hydraulic
model studies should be performed.
Bottom-Clearance Allowance. Factors in addition to those presented
above that must be considered in determining the clearance between the
maximum vessel draft and the bottom are vessel operation, type of
bottom material, and a factor of safety.
Vessel operation. Vessel handling and maneuverability
become sluggish at low bottom clearances.
Bottom material. Soft bottom material can be displaced and
shoaled by passing vessel propeller action. Hard bottoms with
sharp outcroppings can cause severe damage to vessels upon
grounding. In active sedimentation areas, bottom shoals can
also occur during relatively short periods of storm activities.
Factor of safety. A clearance of 0.6 m (2 ft) between maximum
vessel draft and the bottom must be provided for all vessels
transiting a channel at any given time. Maximum vessel draft is
that value which considers low water levels, vessel static
conditions (i.e., salinity, trim, and list), wave motions, and squat.
If a channel is not sufficiently deep to accommodate a deep-
draft vessel at extreme low tides, then this factor must be
evaluated depending on operational criteria.
current also influence harbor site selection. Entrance widths should be adequate to
reduce currents to acceptable values. The maximum allowable current in an entrance
channel is a function of the type of ship or ships to be accommodated. It is only under
special circumstances that the current exceed 4 knots.
If the entrance is not constrictive, as is shown in Figure 5-11 and the
following conditions are met:
the basin is relatively short and deep; that is
gd (T )
basin length [ft]
average basin depth [ft]
the bay water area is relatively constant