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BREAKWATER DESIGN. II?
as the height, in feet, of the wave, and F as the length, in miles, of the fetch,
he found that approximately
H=r5^ .................(a)
or, more closely, for short fetches of less than 30 miles,
11= 1'5 s/F + (2-5-4 x/F). ... (0)
The following instances afford a comparison of the results of calculation
based upon these formulæ, with the values obtained from actual observation.
Height of Waves.
Place of Observation. Length of Fetch, Nautical Miles. Observed Height of Waves in Feet. Calculated Height in Feet.
Formula (a). Formula (0).
Scalpa Flow 1-0 4-0 1-5 3-0
Firth of Forth . 1-3 1-8 1-8 3'2
Lough Foyle 7-5 4-0 4-1 4’96
Clyde. 9-0 4-0 4’5 5-25
Colonsay . 9-0 5-0 4-5 5'25
Lough Foyle 11 0 5-0 5-0 5-7
Anstruther 24-0 6-5 7-5 7'7
Lake of Geneva . 30-0 8-2 8-2 8-37
Buckie 40'0 8-0 9’5
Douglas, I.O.M.. 65-0 10-1 12-0
Kingstown. . . 114'0 15-0 16-0
Sunderland 165-0 15-0 19-3
Peterhead . 400-0 22-6 30-0
It is manifest, however, that waves cannot attain their full development
where there is inadequate depth. No wave can have a height greater than
the depth of water through which it passes. Consequently, the intervention
of shoals in the path of a wave serves to limit its size. Reefs and sandbanks,
even though entirely submerged, materially reduce the range of undulation,
and the length of fetch must be gauged accordingly. Thus the effective
length of open sea may be much less than the apparent length.
But, even with this restriction, the fetch is far from being an exact or
reliable indication of wave height in every locality. It is true that the
maximum height attainable can be calculated therefrom with some approach
to accuracy ; but the faet must not be overlooked that winds may not always,
or often, or indeed ever, blow along the line of maximum fetch. There is a
stretch of 500 miles of open sea leading to the harbour of Kurrachee with
unrestricted depth, yet the highest waves—those from the south west—are
said not to exceed 15 feet. Other instances might be adduced—such as
Peterhead in the preceding table—to show that the maximum fetch alone is
by no means an infallible criterion of wave height. There is also another
point of no slight importance. Not only is it possible for the severest gales
to blow from some other quarter of the compass than that lying in the direction