A Treatise On The Principles And Practice Of Harbour Engineering

28 HARBOUR ENGINEERING of harbour design. At a later stage it will be necessary to take it into detailed considération from the point of view of its influence on structural features. For the moment, however, we are only concerned with it in its general aspect, that is, in so far as it affects the important relationship existing betweeu the area of a harbour and the width of its entrance. The determination of area is the primary considération. Obviously, small harbours will be more appreciably affected by external commotion than large harbours, assuming the inclosed areas in each case to be equally well pro- tected, for it is easier to transmit agitation to a small body of water than to a large one. But, on the other hand, large harbours, unless most effectively screened and sheltered, are themselves liable to act in some degree as wave generating areas. Hence some discrimination is necessary, and the question of area is more likely to be determined by other considérations than those immediately connected with exposure. The required accommodation, the dictates of convenience to navigation, and the adaptability of natural features, in faet, have foremost place in the determination of area. As regards entrance width, there can be little doubt that the narrower the entrance, the more effectually is the interior secured from the ingress of disturbing elements. In addition thereto, a narrow inlet very materially re- duces the power of those waves which do find an entrance. On the other hand, an entrance must have adéquate width for vessels entering not only singly and in calm water, but also when driven in groups under stress of weather. Accordingly, the entrance bears a double relationship to the harbour, viz., (a) as regards shelter, and (i) as regards accommodation. From the first of these points of view, Stevenson has evolved an empirical formula to connect the extent of the reduction in the height of waves with the width of the inlet and the width of the sheltered area. Thus, calling H the height of the unrestricted wave at the mouth of the harbour having an entrance width b, the reduced height of the wave, h, within the harbours at a distance, D, from its mouth and at a point where the breadth of the harbour is B, is given by the expression J "K7;;-<^'+x/|)}- For example, when 1) = 256', b 100', B 400', and H 10', we obtain A= 3'8'. The entrance width, however, is subject to other and further considéra- tions. In tidal harbours there is the outrun of the ebb tide with the cumulative effect of the discharge of any upland waters, all tending to pro- duce a rapid current in a narrow waterway. And while the scour induced by this means is beneficial within certain limits in maintaining a deep channel, yet, carried to excess, it is likely to prove prejudicial to the stability of walls and piers by undermining their foundations, and, moreover, the rate of flow may be such as to interfere with and possibly prevent safe navigation. To be precise, a velocity of from 3| to 4 knots per hour should be looked upon as the maximum current permissible.