A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR DESIGN 21 extent conjectured. The velocity of a wind may be gauged more or less accurately by an anemometer, if one is available ; in other cases it is estimated. But the velocity is fitful, and, as a measure of pressure, by no means an ideal standard. The range of intensity is usually divided into twelve sections, forming a scale, known as Beaufort’s scale, which is given below. Beaufort Scale for Wind. 0 denotes Calm Velocity in miles per hour =0 1 ,, Light Air ,, ,, 7 2 „ Light Breeze ,, „ 14 3 ,, Gentle Breeze „ ,, 21 4 ,, Moderate Breeze „ ,, 28 ,, Fresh Breeze ,, ,, 35 6 „ Strong Breeze ,, ,, 42 7 ,, Moderate Gale „ „ 49 8 ,, Fresh Gale ,, ,, 56 9 ,, Strong Gale ,, „ 63 10 „ Whole Gale „ ,, 70 11 ,, Storm ,, ,, 77 12 ,, Hurricane ,, „ 84 Coastal Change.—The influence of the wind in relation to the magnitude of waves will be more fully considered in another chapter. At present, having regard to the general outlines of barbour design, we will simply notice its bearing upon the coastal contour in the vicinity of any artificial works. That the seacoast is undergoing a graduai change must be evident to the most superficial observer. In certain districts, notably the borders of York- shire and East Anglia, there are manifest signs of sea encroachment. Every year witnesses the retrogression of some extent of shore frontage, and, in the course of a few centuries, whole tracts, such as the Goodwin Sands and districts including villages and townships, have disappeared. On the other hand, in other quarters there has been a graduai gain and accretion. South- port in Lancashire, formerly, as its naine implies, situated at the water’s edge, now lies at a perceptible distance inland. At Dungeness in Kent, a headland of shingle is accumulating at something like the rate of 200,000 tons per annum. Instances of both kinds might be multiplied indefinitely. The essential point to consider is the probable effect of any artificial projection from the coast in accentuating or mitigating the natural process of mutation. This is not altogether an easy matter to determine, owing to the predominating influence of local circumstances, quite apart from the faet that the causes of coastal denudation and accretion are but imperfectly under- stood. The carriage of material from one point to another is assigned by one school of engineers entirely to wave-action, and by another school, mainly to current flow. It seems, on the whole, not improbable that both agencies are involved, in varying degree : the breaking of waves on a beach serves to stir up the sand and shingle, the former of which the water, in its troubled state, retains in suspension long enough for it to be projected some distance along the shore by the resolved component of wave force in that direction,