he
superstructure was only preserved from being undermined by frequent
deposits of stone along the sea face.

The south-west breakwater at Colombo Harbour, constructed in
1876-1884, facing the seas raised by the south-west monsoon, extends
into a depth of 39 ft. at low water, where the rise of tide is only 2
ft. at springs, and was built with a superstructure founded upon a
rubble mound at a depth of 20 ft. below low water, but raised only 12
ft. above this level without any parapet, and protected along its sea
face by an apron of concrete in bags. In this case, not only was the
depth of the sea much less than at Alderney, but the small elevation
of the superstructure above low water enabled a portion of the waves
in storms to pass over it without materially impairing the shelter
inside. These circumstances reduced the shock and recoil of the waves;
and the greater depth of the foundations and the protection of the toe
of the superstructure greatly diminished the danger of undermining.
Consequently, the Colombo breakwater has been preserved from the
injuries to which the outer part of the Alderney breakwater succumbed.
Nevertheless, in subsequently constructing the north-west detached
breakwater, less exposed to the south-west monsoon, but in somewhat
deeper water (see COLOMBO), the experience of the action of the sea on
the south-west breakwater led to the laying of the foundations of the
superstructure on the rubble mound at 30-3/4 ft. below low water (fig.
9).

[Illustration: FIG. 9.--Colombo North-West Breakwater.]

The breakwater for sheltering Peterhead Bay, where the rise of springs
is 11-1/4 ft., was begun in 1888, and designed to extend into a depth
of 9-1/2 fathoms at low water (see HARBOUR). It was built as an
upright wall upon the rocky bottom for 1000 ft. from the shore; but
owing to the increase in depth it was decided to construct the outer
portion with a rubble base, surmounted by a superstructure originally
designed to be founded 30 ft. below low water. As, however, during a
storm in October 1898, the recoil of the waves from the breakwater,
which is provided with a promenade wall rising about 35 ft. above low
water, disturbed rubble to a depth of 36-1/2 ft., the superstructure
has been founded 43 ft. below low water on the rubble base; and its
outer toe is protected from being undermined by two rows of concrete
blocks on the r