EFFECT OF WAVES ON SAND TRANSPORT BY CURRENTS

Abstract

The effect of waves on the transport of sand in suspension by currents can be especially important in the outer regions of estuaries and tidal inlets. Two series of field experiments are therefore being carried out to investigate the transport of sand under the combined action of waves and currents. In the first series of tests, measurements were taken from a specially constructed tower located just off the Maplin Sands in the outer Thames Estuary, England, (Fig.l). The tower stands in about 1.5m of water at lowest tide levels, and the mean spring tidal range is about 4.5m. Average tidal velocities are about 0.6m/s, with peak values up to about 0.9m/s. The tower has a completely open structure (Fig.2) below the water line, with the exception of the four corner piles penetrating the sea bed, and a vertical trackway which stops approximately lm above bed level. A series of instruments were fixed to a trolley which could be winched up and down the trackway, thus allowing measurements at any height above bed level. The annual wave climate at a waverider buoy position some 900m from the tower is summarised in Fig.3: Fig.3a shows the percentage occurrence of different wave heights, and it can be seen that the tower is at a fairly sheltered site, with less than 2 per cent of the waves exceeding lm in significant height. Fig.3b shows the percentage occurrence of given wave periods which, as one would expect, are correspondingly short, with less than 2 per cent exceeding 4.5s mean-zero crossing period. The second series of field tests are presently being carried out at the seaward end of Boscombe Pier, located in Poole Bay, England, (Fig.4). Water depths at this site are about 2.5m at low tide, with a mean spring tidal range of about 1.7m. Maximum tidal velocities are only about 0.3m/s, although during prolonged storms the tidal currents are enhanced by strong wind-induced currents. This site is significantly more exposed than Maplin, which was the reason for the choice of this second location. Annual wave statistics are not available for Boscombe Pier itself, but a waverider buoy some 3000m from the pier (Fig.4) has been operated by the University of Southampton for several years, and Fig.5 shows the percentage occurrence of given wave heights for the year 1977. Some 17 per cent of waves were greater than lm significant height, and 2 per cent greater than 2m. Fig.6 shows the percentage occurrence of given wave periods: 5 second waves are the most common, but with periods of longer than 10 seconds being recorded. Despite initial appearances, Fig.7, Boscombe Pier is a relatively open structure below the water line, with only vertical columns 0.4 x 0.4m, spaced at 5.5m centres. As at Maplin, a vertical trackway is provided, and an instrument trolley can be winched up and down.