A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR DESIGN. 37 of about one foot annually. But lately it has been found, since the Port authorities have become possessed of a modern 1000-ton suction dredger, that a month’s work in the year easily overtakes the annual silting. “ When the old entrance is closed in 1911, the sand will undoubtedly tend to flow in the current, past the closed entrances and northwards to the end of the new sheltering arm, there to be dropped in the still water formed by that arm near the new entrance. But there seems no reason to fear that modern dredging methods will not be equal to coping with the difiiculty.” Whitby Harbour. — The town of Whitby, lying at the mouth of the river Esk, affords an instance of a fishing harbour maintained chiefly by tidal scour. There are two piers at the entrance, the west pier originally projecting considerably further seaward than the east pier. The meeting of the flood- tide, however, with the river current, produced an eddy just within the west pierhead, leading to slack water and shoaling. Thus, a vessel desirous of entering the harbour had to go nearer the east pier in order to avoid the bar ; in so doing it was in danger of losing steerage- way, owing to the strength of the flood-tide (which flowed eastward at a considerable rate), and tended to drift beyond the east pierhead before making the entrance. In order to minimise the trouble of the bar, some large stones were placed N.N.W. of the west pierhead below the water level, at which ships might safely enter the harbour. These stones acted as a groyne, tending to prevent the formation of the bar by arresting the eastward progress of the sand deposit.1 The remedy ultimately adopted consisted in prolonging the east pier until both pierheads were in a line parallel to the set of the tide. By this means the width between the pierheads was diminished by nearly one-half, the bar disappeared for a time, and heavy seas which formerly entered the harbour were perceptibly reduced. The projection of the east pier, however, caught the waves from the north-west, instead of allowing them to pass the entrance as before. These waves, passing into shallow water, stiried up the sandy bottom, becoming heavily charged with material. They swept along until they struck the inner face of the east pier extension, whence they rebounded within the harbour, and, reaching slack water, the sand which they carried was deposited. Besides silting up the harbour, the decreased width of the mouth made the entrance exceedingly dangerous, as, owing to the rapid cross-flow of the tide, a vessel had great difiiculty in shooting in between the pierheads when running before a north-west wind. She might strike the east pier-end or drift on to the rocks beyond; or, if she effected an entrance, she might collide with the inner face of the extension. The state of affairs was, therefore, far from satisfactory, and a further scheme of improvement has been decided upon and is about to be undertaken. Messrs J. Watt Sandeman and Son, the engineers, have favoured the author with the following observations, both in regard to the defects of the present harbour and the proposed remedial measures. 1 Vide Austen on Whitby Harbour, Afin. Proc. Inst. C.E., vol. clvi. p. 264.