Quantcast Design water level

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b. Design water level the distance above the datum from which the dredge depth is calculated:
(1) Outer channels use 0 feet. The design water level should equal the datum to provide enough water
depth and ensure that the ship can transit when required.
(2) Inner channels and turning basins use a water level that ensures safe passage to the berth or basin.
This water level should be selected by the Activity based on optimizing cost and operation. Ship
operators generally accept some minor operational restrictions and transit shallow channels at mid to
high tide levels. Therefore, the design water level should be selected so that the carrier can transit
from deep water to the berth, or vice versa, as frequently as expected without encroaching on the
minimum water depth requirement noted in Attachment (a). The user should identify for the planner
the expected ship transit speed and desired days of accessibility, realizing that slow transits at low tide
levels result in excessive dredge depths.
An example of this procedure is as follows:
(a) Determine level of accessibility; e.g. minimum of 339 days per year of access to homeports and
300 days per year of access to shipyards and ports of call. These accessibility levels equate to
operational restrictions of approximately 2 consecutive days per month of encroachment on the
underkeel clearance for homeports and 5 consecutive days per month for shipyards and ports of
call, respectively.
(b) Based on local traffic and regulations, assume an average ship speed through channel; e.g. 5
knots.
(c) Using assumed transit speed and navigational charts, calculate the time required to accomplish
the transit from the outer channel to the turning basin or berth.
(d) Using the calculated transit time, days of accessibility, and the charts in attachment (a),
determine the channel depth requirement.
(e) Subtract the water depth requirement from the channel depth requirement to obtain the design
water level. This number will usually be negative and thus result in a design water level above
MLLW.
(3) All berths, except turning basins use 0 feet. Since 6 feet of clearance is provided, as noted later, the
design water level may equal the datum (MLLW).
c. Squat the downward displacement of a vessel while underway. Attachment (a) incorporates squat for
infinitely wide channels with ship speeds equal to or less than 10 knots. Ship squat greatly increases when
CVNs transit channels less than 600 feet in width or move at speeds faster than 10 knots. To determine
squat for conditions other than those addressed in Attachment (a), use the method contained in NAVFAC
Design Manual (DM) 26.1, "Harbors" dated July 1981. All channels normally used by aircraft carriers in
the U.S., except the Southern Branch, Lower Reach, of the Elizabeth River, Norfolk, VA (450 feet at
narrowest point) and the Entrance Channel, Mayport, FL (500 feet at narrowest point), are wide enough to
be considered infinitely wide. For these narrow channels, the squat increases by 2 feet. The water depth
requirement determined above should be modified to incorporate any difference in calculated squat.
(1) Outer channels assume ship speed of 15 knots.
(2) Inner channels base on local traffic and regulations, as a minimum assume an average ship speed
through channel of 10 knots and include the effects of narrow channels as noted above.
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