8. Design of Coastal Structures
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Crest élévation
Crest Elévation is calculated as (dtotai+ Run-up + Free board).
The wave run up is calculated as a function of
(= 7-51784 x 10-3).
R
Rom CEM (2006), — = 0.8, R = 3.776 m.
Hence, fix, say crest élévation at 15.5 m from sea bed with a free board
of 0.5 m.
Cross section of the trunk portion at 8m water depth
Wfl Armour layer - two layer of
13.5 T to 22.5T stones at 3.78m thick
Core
(80kg - 180kg of stones)
Filter layer,
10 mm -10 kg of 0.3m thick
H
Underlayer,
1.2T- 1.8Tof 1.7m thick
S
Toe mound,
1.2T- 1.8Tof 1.7m thick
Problem VII
Design a rubble mound breakwater section in a water depth of 18 m for
a major port. The highest high tide level is +1.2 m and the storm surge
at the location is 1.0 m. The offshore design wave height (Hs) is given as
7.0 m. The seabed is of coarse sand to rocky strata. Adopt the unit weight of
rubble stones as 2.55 t/m3, Concrète as 2.4 ton/m3 and water as 1.03 t/m3.
Solution
Given: High tide level (HTL) — 1.2 m, Storm surge = 1 m, Offshore design
Wave, H offshore ~ 7 m.
Total water depth (dtotai) in front of the breakwater is estimated as,
dtotai = d + HTL + Storm surge =18 + 1.2 + 1 = 20.2 m.
Hence, maximum sustainable wave height, Hmax = 0.78 x dtotai = 15.8 m.
=> Sustainable significant wave height = T7max/l-8 = 8-75 m >
HOff = 7 m
Thus, the offshore design wave would not break on the structure.