CETN I-63
March 1999
the direction of prevailing winds, surface currents, vessel speed, and position relative to channel
banks all change quickly. Crosscurrents, winds, waves, and channel shoaling can be particularly
troublesome at the start of entrance channels. A vessel in a confined channel does not respond as
quickly to rudder and engine commands because of channel bank effects, reduced speed, and
confined propeller/rudder dynamics. Vessel captains know that they have to slow down well
before approaching the berth or terminal areas, relinquishing the control and maneuvering of
their crafts to tugs at speeds less than 4 knots. Tugs may be in full control for positioning vessels
against the dock and mooring them. When vessels arrive at the port entrance with drafts
exceeding the channel depth, they might fully or partially offload their cargo to lighters to reach
an acceptable draft before proceeding to port.
The density of ship traffic further complicates the channel transit, and inclement weather, charter
schedules, and ship rerouting may also affect port access. The degree of complication resulting
from these various factors to the vessel transit should dictate whether the channel design is one-
way or two-way. A two-way channel may significantly reduce or eliminate the amount of time
ships must queue while waiting. The obvious advantages of two-way channels must be balanced
against their higher dredging costs and increased safety risk.
The maximum speed of a vessel in confined, shallow-water channels is significantly less than
that in deep and open seas (SNAME 1988). The maximum speed in restricted channels is known
as the Schijf limiting speed (USACE 1995) and is a function of the blocking factor (the ratio of
channel to ship cross-sectional areas). As an example, speed reductions of 50 percent or more
can occur in highly confined waterways. Both the proximity of channel banks and channel
overbank depths also influence vessel hydrodynamics and maneuvering. These effects are
difficult to quantify and are not directly accounted for in the present USACE practice.
CHANNEL DEPTH ALLOWANCES BECAUSE OF WAVES AND SHIP SPEED:
Channel depth typically is chosen on the basis of economic optimization to meet the present need
and, if possible, anticipated traffic requirements. This is a key factor in the cost and usage of a
navigation channel. In a cost-effective project, the depth of a channel, for either deep-draft and
shallow-draft navigation projects, does not have to be constant throughout. Channel depth can,
and often does, vary in segments of the channels to allow the design vessel to make safe and
efficient transit. There may be different factors affecting the vessel underkeel clearance in
different channel segments, and these influences must be determined as a depth increment and
added to the design vessel draft to determine the required channel segment depths. USACE
refers to this as the authorized channel depth (USACE 1984, 1995, 1999), which is less than the
dredged (or contract) depth considering potential sedimentation. The permitted depth is the
extreme dredging depth allowed by regulators.
PIANC (1997) provides a step-by-step procedure for estimating the contributions of numerous
factors that determine the entrance channel depth. Procedures describing the USACE guidance
are specified in USACE (1984, 1995, 1999). USACE guidance states:
"Maximum heave of a vessel due to waves with wavelength twice the vessel length is
approximately a fifth of wave height; maximum pitch and roll magnitudes are half of wave
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