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142 HARBOUR ENGINEERING.
action is most potent. The upper part of a wave, therefore, falls dead upon
the flat slope above, or, at the worst, upon the masonry apron in front of the
shelter wall, in neither case capable of producing any deleterious results.
0 10 20 30 40 50 60 70 00 90 100 200
L _J__L _J _J _J _«1 _J __L __1__J ____________________________________________i
Scale oF Feet.
Fig. 116. —Section of Grand Jetée, Marseiiles.
The parapet thus receives no appreciable shock, and spray alone passes, at
times, over its crest to fall upon the interior quay.
The blocks, forming the flattened slope referred to, are huge monoliths,
rectangular in shape, with a length twice as great as their width, having a
volume of 500 cubic feet and a weight of about 33 tons a-piece. They are
0 10 20 30 40 50 60 70 60 30 100 300
I___I__I___I__I___I___I__I I I I
Scale 06 Feet.
Fig. 117. -Section of Breakwater Extension, Grand Jetée, Marseiiles.
A. Rubble stone up to 2 cwts. a-piece.
B. Natural blocks from 2 to 25 cwts. a-piece.
C. ,, 25 to 75
D. „ over 75 ,,
E. Artificial blocks of 33 tons.
deposited so as to lie longitudinally in the direction of the onset of the waves.
The external profile of the breakwater has proved to be extremely stable
and is kept up at a very trifling expense in the way of repairs. For a length
of 1200 yards, constructed prior to 1865, the annual cost of maintenance is