ght walls, because the greater depth of
water in which such composite breakwaters are built causes them to be
exposed to larger waves under similar conditions.

The superstructures of composite breakwaters erected by the United
States for harbours on the shores of Lake Superior were formerly in
some cases composed of timber cribs floated into position and sunk by
filling them with rubble stone. On account of the cheapness of timber
several years ago in those regions, this simple mode of construction
was also economical, even though the rapid decay of the timber in the
portions of the cribs where it was alternately wet and dry involved
its renewal about every fifteen years on the average. Owing, however,
to the fact that the price of timber has increased considerably,
whilst that of Portland cement has been reduced, durable concrete
superstructures are beginning to be substituted for the rapidly
decaying cribwork structures.

With the exception perhaps of the Alderney breakwater, which, owing to
its exceptional exposure and the unparalleled depth into which it
extended, had its superstructure so often breached by the sea that,
owing to the cost of maintenance, the inner portion only has been kept
in repair, the composite breakwater of Bilbao harbour has probably
proved the most difficult to construct on account of its great
exposure. The original design consisted of a wide rubble mound up to
about 16-1/2 ft. below low water, a mound of large concrete blocks up
to low water of equinoctial spring tides, and a solid masonry
superstructure well protected at its outer toe by a projection of
masonry, and raised several feet above high water, forming a quay
sheltered by a promenade wall. The rise of equinoctial spring tides at
the mouth of the river Nervion is 14-3/4 ft. In carrying out the work,
however, the superstructure built in the summer months was for the
most part destroyed by the following winter storms; and, accordingly,
the superstructure was eventually constructed on a widened rubble
base, so as to be sheltered to some extent by the outlying
concrete-block mound already deposited, a system subsequently adopted
in rebuilding the damaged portion of the North Pier at Tynemouth under
shelter of the ruins of the previous work. The modified superstructure
of the Bilbao breakwater was founded on the extended rubble mound at a
depth of 16-1/4 ft. below