EM 1110-2-1100 (Part III)
30 Apr 02
EXAMPLE PROBLEM III-2-7
FIND:
Cross-shore distribution of longshore sediment transport using Equation 2-23.
GIVEN:
Waves in 10-m (32.8-ft) depth have an rms wave height of 2.0 m (6.6 ft), angle of 10 deg to the
shoreline, and wave period 8.5 sec. The beach profile is as given below. Assume the wave height to
water depth ratio for incipient breaking κ = 1.0, the stable wave height to water depth ratio for wave
re-formation Γstb = 0.40, the energy flux dissipation rate γ = 0.15, the lateral mixing coefficient Λmix = 0.30,
and bottom friction coefficient Cf = 0.01. The K parameter (for use in longshore transport relationships)
was calculated as 0.60.
Beach profile for Example III-2-7
No.
Distance
Depth
Offshore (m)
(m)
1
100.0
-0.60
2
111.0
0.10
3
132.0
1.00
4
145.0
1.50
5
156.0
1.70
6
169.0
1.90
7
173.0
1.95
8
186.0
1.85
9
190.0
1.80
10
195.0
1.70
11
199.0
1.75
12
207.0
1.81
13
214.0
2.00
14
246.0
3.10
15
340.0
4.00
SOLUTION:
Part II-3 presents relationships for nearshore wave transformation and Part II-4 discusses the
cross-shore distribution of nearshore currents. Alternately, the PC-based numerical model NMLONG
(see Part II-4 for a complete description) may be used to calculate the cross-shore distribution of total
water depth and longshore current speed over an irregular bottom profile (Kraus and Larson 1991),
which can be used in application of Equation 2-23. Entering the given data set, with 100 computation
points, a cross-shore spacing of 2.0 m (6.6 ft), no wind, and a tidal reference elevation of 0.0, the
cross-shore distribution of waves and currents as shown in Figure III-2-22b is obtained. A reduced
listing of the NMLONG output, chosen to represent the peaks and minima of the wave height and
longshore current distributions, is presented in the first four columns of the following table. The
predicted qx(y)/kq is shown in the last column and in Figure III-2-22a.
99Example Problem III-2-7 (Sheet 1 of 4)
III-2-44
Longshore Sediment Transport
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