EM 1110-2-1100 (Part III)
30 Apr 02
Example Problem III-2-5 (Concluded)
(d) A weighting procedure using wave period bands similar to that used for the wave height bands
can be used to calculate an average peak spectral wave period Tp = 8.4 sec; that is,
Tp = [(4)(1146) + (6)(2955) + (8)(7951) + (10)(4898) + (12)(1812) + (14)(330) + (16)(14)] / 19106
= 8.4
(e) Compute the wave celerity C1 and group celerity Cg1 for the offshore wave data in water depth
d = 27 m and for the wave period Tp = 8.4 sec; and find C1 = 12.2 m/s and Cg1 = 7.6 m/s. Use Newton-
Raphson iteration (or a similar technique) to solve Equation 2-16 for the breaking wave height Hb
using the offshore wave height value H1 = 0.93 m and angle α1 = -33o, and find
Hb 1.2 m (3.9 ft)
The breaking wave angle is then computed from Equation 2-14:
αb = -8.8 deg
(f) Compute the potential longshore sediment transport rate for Ksig = 0.39 using Equation 2-7b:
Q = (0.39) [ (1025)(9.81)0.5 / (16)(2650-1025)(1-0.4) ] (1.2)5/2 sin(2(-8.8)) = -0.038 m3/s
Convert the result to annual equivalent transport and multiply by the percent annual occurrence of this
event, 32.7%:
Q = (-0.038 m3/sec) (3600 sec/hr) (24 hr/day) (365.25 day/yr) (0.327)
= -393,000 m3/yr (-514,000 cy/yr) (directed to the left).
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.
III-2-24
Longshore Sediment Transport
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