CETN I-63
March 1999
Deep-draft channels are typically designed to provide safe and efficient passage for a selected
vessel under specified transit conditions. Both the design ship and transit conditions must
represent the most adverse combination of conditions under which the project would be expected
to maintain normal operations. It then would be reasonable to assume that the project would
perform adequately for smaller vessels under the same transit conditions. In practice, harbor
pilots often transit vessels larger than the design ship(s) under conditions of (a) high tide,
(b) milder wave conditions, (c) reduced speed, and (d) tug assistance. The first three of these
factors increase the effective depth of the channel, and the last factor increases horizontal control
of the vessel.
Channel width is tied to horizontal vessel motions, and it is reasonable to assume that a larger
dimension vessel with the largest motions will impose the most severe limitations. By similar
reasoning, the maximum draft vessel would produce the minimum underkeel clearance
throughout the channel. The combinations of hydrodynamic response, speed, heading, and wave
direction for various design vessels using the channel may very well dictate otherwise. Because
ships have different wave-response characteristics, the required channel depths may vary at
different locations along the channel. However, it is not necessary to design a deep-draft channel
for extreme or rare events, as vessel operators and port authorities usually suspend operations
during these conditions. For cost-efficient transits, it is prudent to include wave statistics into the
design process from the onset by eliminating the highest waves to reduce the transit downtime.
by USACE to affect the vessel transits in channels includes the following:
"Wind, wave, and current conditions; visibility (day, night, fog, and haze), water level (including
possible use of tidal advantage for additional water depth), traffic conditions (one- or two-way,
pushtows, cross traffic), speed restrictions, tug assistance and pilots, underkeel clearance, and
ice" (USACE 1984, 1995, 1999).
Tides and/or water-level fluctuations may enter into entrance channel design, reducing the depth
requirements for vessels that would otherwise be restricted at low water. However, if channel
usage were limited to high tide, port access would be adversely affected, resulting in large
economic loss and higher vessel-operating costs. Care should be exercised when incorporating
water-level variability in the channel-depth specification. If water levels were included in the
design, a water-level probability analysis would be necessary to determine the safest channel
depth for optimum operability.
Harbor pilots provide the local knowledge and expertise necessary for safe ship transits. The
pilot assumes control of the vessel during channel transit, issuing rudder and engine commands
to steer the vessel and uses the navaids to maintain ship alignment safely within the channel.
One or more tugs also may accompany the ship to assist in tug-aided phases of transit and
docking.
Transits involve going through a series of channel segments, straight or curved. Channel turn
angles in excess of 30 deg pose maneuvering challenges even to experienced mariners because
5
Previous Page
