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COASTAL WORKS
24;
measures to maintain and deepen the new channel of the
Thames.
The chief difficulties of an estuarine port are the silting
of its channel and shoaling at the entrance. The channel
may be maintained by dredging or by the automatic scour of
the tidal current. On an open coast the only available
method of harbour construction may be a costly breakwater,
which, if ill-designed, is ineffective. Thus the breakwater
at Ceara in Brazil (Fig. 62) was connected with the shore by
a viaduct, in the hope that the stream under it would scour the
harbour ; but in 20 years sand had filled the basin and carried
the foreshore almost to the end of the breakwater.
Sga-Warrs—Masonry may be weakened by the chemical
action of sea-water on concrete. for the sulphate and chloride
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Fic, 62.—THE CEARA BREAKWATER, BRAZIL.
Shoaling of the Ceara Breakwater, Brazil, built 1884. The four dots at
the lower end of the Breakwater represent the piers of the viaduct.
S.L. 1884, and S.L. 1903 represent the shore-lines in 1884 and 1go3.
LW. 1884 and L.W. 1003, low water at the same dates.
of magnesia alter any unstable form of lime into calcium
sulphate and chloride, which are removed in solution. These
reactions happen if the concrete contains an excess of lime,
and ‘the trouble can be avoided by preventing the entrance
of sea-water, or by the addition of material such as trass or
brickdust or ground slag, which will combine with the free
lime (D. B. Butler, Portland Cement, 3rd edition, 1913, p. 374).
Sea-walls, if perpendicular, are struck with the full force
of an advancing wave and may be undercut. If the foot
projects in a curve the wave is checked gradually ; a slope of
one vertical in ten horizontal is often adopted as it is usual
in shingle banks. A projection along the top of a sea-wall
is sometimes used to throw back the spray; but as this
coping is subject to heavy upward blows it is liable to crack
rhe wall.
