8. Design of Coastal Structures
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Problem II
If a smooth faced vertical wall rests on a rubble base of height 2.7 m is
considered in a water depth of 3 m for the above problem, find the force
and moment on the wall with the wave characteristics considered in the
Problem I. Height of rubble foundation b = 2.7 m, Hi = 1.5 m, d = 3 m,
î1 = 6 sec or 10 sec. Find the force and moment on the given wall on a
rubble foundation.
Solution
We know yc = 5.5 m and yt = 2.5 m
For T = 6 sec
b
2.7
b
2.7
- = — =0.49, - = — = 1.08 >1.0
yc 5.5
yt 2.5
From Fig. 8.9 for T = 6
(1 — ry) = 0.26; (1 — rTO) = 0.52 and
- > 1.0; (1 - rt) = 0.0; (1 - rm) = 0.0
ÿt
F” = 0.26 x Fc and F” = 0.0Ff
Fc and Ft are 101.7 and 17.1 as calculated. Hence F" = 26.5 kN/m and
F'' = 0.
Similarly moments at point A,
[M’ {]c = 0.52 x 163.8 = 85.2 kN-m/m
[M'^t = 0.0(11.8) = 0
The overturning moment at point B (refer Fig. 8.10 for details)
[M'£]c = 85.2 - 2.7(26.5) = 13.7 kN-m/m
[Mgjt — 0 kN-m/m
Similarly for T = 10 sec it can be calculated.
Problem III
At vertical wall 4.3 m high is sited in seawater with ds — 2.5 m. The wall
is built on a bottom slope of 1:20 (m = 0.05). Reasonable wave periods
range from T = 6 sec to 10 sec. The breaking wave height for 6 sec wave