Figure 5. Effects on drawdown due to changes in variables from a basic
case.
be more significant than a 10-ft change in vessel
bidity, an estimate of the surge (increase in water
beam, vessel draft or channel depth. The basic
level) that often follows the drawdown phase, and
case in Figure 5 is a ship with a 25-ft draft and
an estimate of shore structure damage potential
100-ft beam traveling in a rectangular channel 35
for basic structural categories were included. They
ft deep and 2000 ft wide. The ship velocity rela-
also found that drawdown is very sensitive to
tive to the water is 12 ft/s. This corresponds to
the position of a vessel within the channel. The
the central point on Figure 5.
capability to predict the magnitude of the surge
that follows the drawdown phase is important
Vessel speed and water velocity are of equal
in evaluating whether an onshore bluff will be
importance in the calculations, but due to its
attacked or whether nearshore flooding during
greater range of variation, vessel speed is more
high-water periods will be exacerbated, whereas
significant in practical applications. An increase
the values of turbidity are significant in the po-
in vessel draft is more significant than an equal
tential for biological impacts.
increase in beam simply because it geometrically
Hochstein and Adams (1985b) adapted a model
adds more cross-sectional area to the ship. Simi-
that considers drawdown and the effects of pro-
larly, an increase in water depth (with discharge
peller wash to the St. Marys River. A subsequent
unchanged) would more than offset the cor-
report (Hochstein and Adams 1986) added treat-
responding increase in allowable draft since the
ment of ship-generated waves. Their model is at-
increase in flow area across the river is much larger
tractive for assessing environmental effects in that
than the change in the wetted area of the ship.
it makes quantitative predictions of the distribu-
The net effect is a decrease in the blockage of the
tion of water velocity, suspended solids and bed
channel.
load across a river cross section based on prop
Hodek et al. (1986) and Alger and Hodek (1986)
wash, waves and drawdown. The numerical for-
further refined the one-dimensional approach and
mulation they employed is a quasi-two-dimen-
developed an interactive, user-friendly program
sional treatment in that it conducts hydraulic cal-
that could be run on a desktop computer system.
culations in one dimension and then superimposes
This model allows rapid computation and com-
assumed distributions for the cross-channel vari-
parison of various scenarios of fleet mix and site
ations of both ambient and ship-influenced flow
characteristics. The database for ship effects was
variables. While in simple channel shapes this ap-
considerably expanded by monitoring hydraulic
proach may provide useful additional detail, ex-
conditions at five new sites on the St. Marys River
tension to the complex channel shapes and flow
in addition to those documented under previous
distributions present in the St. Marys River is un-
studies. New topics such as vessel-generated tur-
7
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