EM 1110-2-1100 (Part III)
30 Apr 02
EXAMPLE PROBLEM III-2-6
FIND:
Calculate the potential longshore transport rate using each of the seven wave height bands given in
Table III-2-2.
GIVEN:
Data in Table III-2-2, and information in Example III-2-5.
SOLUTION:
For each wave height band in Table III-2-2, the associated weighted average peak wave period Tp is
calculated. Using the criteria discussed previously, the wave data are checked to ensure that Hsig ~ Hmo.
The percent occurrence for that wave condition is determined by dividing the total number of occurrences
in the wave height band in the given 450 directional band by the total number of records (58,440) in all
directional bands. The results are shown in the table below as the "input" conditions. The breaking
conditions may be computed directly by the program WISTRT or by Equations 2-14 and 2-16 in the same
manner as described in Example 5, above. The potential longshore sediment transport from the 45o
direction band centered approximately 33o to the right of shore-normal (looking seaward) was calculated
to be -1,014,200 m3/year (-1,325,500 yd3/year) (directed to the left) for RAL2 Station 72.
Data for Example Problem III-2-6
Input Conditions
Program Output
T
α
αb
QR
Hs
Hb
Wave
Depth
Percent
(m3/year)
(m)
(sec)
(deg)
(m)
(deg)
Condition
(m)
Occurrence
1
0.5
8.3
-33
27
22.0
0.73
-7.8
-66,500
2
1.5
8.5
-33
27
8.17
1.8
-12.0
-355,000
3
2.5
9.5
-33
27
1.94
2.9
-14.2
-320,000
4
3.5
10.4
-33
27
0.493
4.0
-16.0
-198,800
5
4.5
11.5
-33
27
0.077
5.1
-17.4
-61,200
6
5.5
11.3
-33
27
0.005
6.0
-19.0
-6,500
7
6.5
13.0
-33
27
0.003
7.1
-20.0
-6,200
Total (directed to the left)
-1,014,200
Note that a similar approach can be utilized to find an answer using the CEDRS database (see
Part II, Chapter 8) which provides "percent" occurrences (rather than number of observations) in
22.5E energy bands. Note that engineering judgment must be utilized when assessing actual
transport rates. Values of longshore sand transport calculated with Equations 2-14 and 2-16 are
based on a limitless supply of sand available for transport, which is often not the case (e.g., if
structures are present, geologic features control sand movement, or sediments other than sand
are in the transport region). In addition, longshore transport rates calculated using WIS data
have been found to be larger than accepted rates (Bodge and Kraus 1991).
For this example calculated using waves offshore of the New Jersey coast, generally-accepted
transport rates might be used to adjust the calculated values. Inspection of Figure III-2-3
indicates that net longshore transport rates are between 153,000 and 275,000 cu m/year for this
portion of the New Jersey shore. This "calibration" implies that longshore sand transport rates
calculated with WIS data and Equations 2-14 and 2-16 might be reduced from 15 to 27 percent
of their calculated values to have application for this part of the coastline. Future calculations
using WIS data for this portion of the coast could use this calibration for adjustment.
Longshore Sediment Transport
III-2-25
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