COASTAL WORKS 239 
and are about 50 feet high. In the North Atlantic the maxi- 
mum is about 40 feet. Vaughan Cornish (Waves of the Sea, 
1910, p. 53), during a strong gale in December, 1900, measured 
many waves 29 feet high, and some of 43 feet. The height 
depends upon the * fetch,” i.e. the width of open water to 
windward ; if the fetch is more than 39 miles the height 
of the waves (H) in feet is one and a half times the square 
root of the fetch (D), i.e. H = 1-54/D; if the fetch is less 
than 39 miles H = 1-5 (2:5 — 4¢/D). The heights of waves, 
according to the formulas, are as follows :(— 
a 
Fetch. Wave Height. 
Fetch. Wave Height. 
A 
Fetch. Wave Height. 
L, 
— 
= 
I mile = 3 ft. 
10 miles = 5 ft, 6 in. 
20 ,, =2"%ft. 1in. 
30 miles = 8 ft. 4 in. 
go ,, = oft 5in. 
50 ., ==I0ft.6 in. 
100 miles = 15 ft. 
200 °,, =2I ft.5in. 
200 ,, = 26 ft. 
The waves in Lake Geneva are 8 feet high where they have 
a fetch of 40 miles, those on Lake Superior 20-25 feet high 
with a fetch of over 300 miles. 
The depth of disturbance of a wave is equal to its length ; 
the maximum length of ordinary waves in the Atlantic is 
600 feet, and they disturb fine sediment to the depth of about 
600 feet or 100 fathoms. The action diminishes rapidly 
with depth. The displacement of water particles at a depth 
equal to the length of the wave is only zly and at double 
that depth is only sggyy of that of the surface. At special 
points waves and currents move material far below the 
ordinary limit of wave action.. Lobster pots in the English 
Channel are sometimes filled with coarse shingle at the depth 
of 180-200 feet. Seaweeds which live not less than 200 feet 
deep are washed ashore with stones attached to their roots, 
and must have been torn from the sea-floor by waves. The 
cover of telegraph cables is cut by drifting sand at the depth 
of 600 feet, and silt is moved at greater depths. 
The transport of beach material depends on the angle at 
which waves strike the shore. A wave which rushes obliquely 
up the beach returns by the shorter steeper course at right 
angles to the shore; it carries material along a zigzag course. 
The backwash may be concentrated and strike a more power- 
ful blow than the oncoming wave: thus at Dunbar a wave