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 *H*sig ~ *H*mo.

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.

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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