299
OF ENGINEERING FORMULÆ,
Breakwaters—continued.
Proportion of interstices in breakwaters to the
contents of the breakwater as it stands:—
3rd-class rubble or quarry rubbish, | interstices.
2nd „ „ I to 2 tong, | „
1st ,, ,, 2 ,, 5 „ j „
Beton blocks, 15 „ 25 „ | »
At Cassis, when the water is deep outside,
blocks of 15 cubic metres were found insufficient
to resist the action of waves.
Breakwaters with vertical walls or faces of an
angle less than 1 to 1, will reflect waves without
breaking them. Waves of oscillation have no
effect on small stones at 22 feet below the surface,*
or on stones from 1| to 2 feet, 12 feet below the
surface.
A roller 20 feet high will exert a force of about
1 ton per square foot.
Greatest force observed at Skerryvore, 3 tons per
square foot.
Ditto, ditto, Bell Rock, 1J ditto.
The action of waves is most destructive at low-
water line.
Waves of the 1st order are nearly as powerful
at a great depth as at the surface.
At Madras harbour laterite rubble, 150 lbs. per
cubic foot, in blocks varying from 5 lbs. to 2 cwt.,
were removed at depths exceeding 40 feet by
cyclonic ground swell; at Wick harbour a masa
of 1300 tons was bodily removed by the waves
without breaking up.
* ‘Min. Inst. Civ. Eng.,’ vol. xviii.