ERDC/CHL CHETN-IV-34
June 2001
Example 1: Material trapped by or crossing a channel during (a) typical surf conditions,
(b) and during a storm.
Given: Channel width W = 30 m; channel depth hc = 4.5 m; average ambient depth ha = 3 m;
sediment grain size = 0.2 mm. During typical surf conditions, Ua =0.1 m/s and nearshore wave
height is 1 m; during stronger storms, Ua =1 m/s and nearshore wave height is 4 m.
Find: The relative percentages of suspended material deposited into and bypassing the channel
for that portion of the channel located in the surf zone.
Solution: (a) Typical wave condition: Waves of 1-m height will break in water depth of about
1.3 m, or in depths well shoreward of the area of interest at 3-m depth and seaward of the jetties.
However, because of the wave current interaction and the presence of the ebb shoal in the
vicinity of the channel, wave breaking is observed to occur intermittently, suspending sediment.
Therefore, we proceed. Consulting Table 1, for 0.2-mm sand at 20-deg C, we have Vf =
0.025 m/s, giving:
Fr = V f / gha = 0.025 / 9.8 3 = 0.0046 ,
and
hcWV f /(haU a ) = 4.5 30 0.024 /(9 0.1) = 3.75
2
Calculating Equation 7 or entering Figure 2, one finds that p >1 for any value of Fr, so p =
100 percent, and all the material falls into the channel for this condition.
(b) Storm condition: Waves of 4-m height will be breaking all around the channel. For this
situation:
Fr remains as previously described, 0.0046, and
hcWV f /(haU a ) = 4.5 30 0.025 /(9 1) = 0.375
2
Calculating Equation 7 or entering Figure 2 for the curve Fr = 0.005, p = 46 percent, which
means that 1-p = 54 percent of the material passes the channel. It is again noted that not all
suspended material that falls into the channel will remain there. Some portion will be
resuspended and carried out of the channel, thereby increasing the percent bypassed.
Example 2: Determine the possibility that a sediment plume viewed in an aerial
photograph consists of sand bypassing an inlet.
Given: An aerial photograph at an inlet indicates that a sediment cloud or plume, evident by
brown color observed on the background of green and white turbulent water, sometimes appears
in the surf zone under typical wave conditions (wave height less than 1 m) that follow after a
storm. The plume bypasses both jetties of the inlet entrance and continues alongshore for a
distance of 200 m past the updrift jetty, where the plume originates. The water depth near the
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