MORPHODYNAMICS OF INTERTIDAL DUNES - A YEAR-LONG STUDY AT LIFEBOAT-STATION-BANK, WELLS-NEXT-THE-SEA, EASTERN ENGLAND

Sand is driven clockwise by tidal currents round a diamond-shaped sand shoal in the entrance to the harbour at Wells-next-the-Sea. A populat ion of dune bed-forms occupying the ebb-dominated channel on the weste rn side of the shoal was monitored for a year (October 1975 to October 1976) in terms of shape (height, wavelength, superimposed immature du nes), movement and bedform composition. Individual dunes and the popul ation of dunes as a whole changed in response to sediment transport on astronomical tidal scales varying from the semi-diurnal to the equino ctial and in response to seasonal meteorological forcing. Most change occurred during the autumn and winter, when strong winds and gales cre ated surges which, in some cases very significantly, enhanced the sedi ment transport due purely to the astronomical tide. Dune height was on average greatest during the winter, when the sea temperature was low, and least during the period of summer warmth. Dune height also varied substantially on a spring-neap tidal scale, an increase in height wit h the onset of many of the springs being followed by a gradual lowerin g. Dune wavelength showed little response to the spring-neap variation of sediment transport, but decreased significantly between winter and summer. Varying degrees of time-lag accompanied all changes in bedfor m characteristics in response to hydraulic change. Although individual dunes had surprisingly large lifespans and ebb-directed excursions, s ome change was noted in the composition and statistical attributes of the bedform population as the result of appearances and disappearances . The more vigorous episodes of sediment transport created immature (n ascent) dunes, some of which grew large enough to become incorporated as new members into the population of mature forms. Internally, the du nes were dominated by ebb-oriented cross-bedding, complicated by a var iety of intra-set discontinuities commonly associated with mud drapes. Some recorded the smoothing of crests during tidal reversal and other s the immobility of the bedform over a number of tidal cycles (occasio nally many). Other discontinuities expressed the 'capture' of immature dunes by the main bedform within the duration of a single ebb-tide. B ecause of the frequent perturbation of the (astronomical) tidal sedime nt transport by meterological events, little order to the horizontal a rrangement of discontinuities within the sets was detected, in contras t to other reported cases.