GB877057A - Improvements in navigable waterways - Google Patents

Abstract

877,057. Groins control of silt deposition. CHRISTIANI & NIELSEN A. S. Nov. 19, 1958 [Dec. 2, 1957], No. 37294/58. Class 68(2). A means for influencing the movement of solid matter along the sea floor comprises a device situated on the seabed which produces eddies and increases the depth of the region immediately above the sea bottom in which solid matter is suspended due to the oscillatory motion of the water. Two devices are described comprising (a) Non-oriented artificial roughness in which staggered rows of rectangular blocks 1 (Fig. 1) are placed on the sea bed with their longitudinal axes transverse to the wave direction. Fig. 2 is a section through Fig. 1 and shows the formation of an eddy 9, due to the motion of a wavecrest 4 and hence the body of water under the wave, in the direction of the arrow 25. The velocity distribution of the water particles is indicated by the lines 5 and 6. When the following trough passes the velocity distribution will be as shown by the arrows 11 and 12 in Fig. 3, new eddies 14 will be formed and the eddies 9 will move upwards and backwards and at the same time expand so that the depth of the turbulent region 15 is increased to that shown at 16. (b) An oriented artificial roughness in which unsymmetrical blocks 17 are used. This construction results in the formation of eddies only on the forward motion of the wave in the direction of the arrow 25 (Fig. 7). The arrows 21, 22, 23 and 24 which indicate the direction of water movement show that the top most layers of water 21 move against the direction of wave propagation and the lowermost layers 22 normally move in the direction of wave propagation, but due to the action of the blocks 17 a water movement opposite to the direction of wave propagation is induced which results in material being deposited at 26. A number of shapes of block which will produce these effects are described and illustrated and also the device shown in Fig. 25 (not shown) in which a pivoted plate 53 swings to the position shown, when the water moves in the direction of the arrows and results in the formation of the eddy 9, but pivots to a nearly horizontal position on reverse movement of the water and allows suspended matter to pass through. An arrangement of oriented and non-oriented artificial roughness is illustrated in which the blocks are disposed so as to keep a harbour mouth free of silt (not shown, see Fig. 27).