Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Abstract

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 population of dune bedforms occupying the ebb-dominated channel on the western side of the shoal was monitored for a year (October 1975 to October 1976) in terms of shape (height, wavelength, superimposed immature dunes), movement and bedform composition. Individual dunes and the population of dunes as a whole changed in response to sediment transport on astronomical tidal scales varying from the semi-diurnal to the equinoctial and in response to seasonal meteorological forcing. Most change occurred during the autumn and winter, when strong winds and gales created surges which, in some cases very significantly, enhanced the sediment transport ue 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 with the onset of many of the springs being followed by a gradual lowering. Dune wavelength showed little response to the springneap variation of sediment transport, but decreased significantly between winter and summer. Varying degrees of time-lag accompanied all changes in bedform characteristics in response to hydraulic change. Although individual dunes had surprisingly large lifespans and ebb-directed excursions, some change was noted m 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 (nascent) dunes, some of which grew large enough to become incorporated as new members into the population of mature forms Internally, the dunes were dominated by ebb-oriented cross-bedding, complicated by a variety of intra-set discontinuities commonly associated with mud drapes Some recorded the smoothing of crests during tidal reversal and others the immobility of the bedform over a number of tidal cycles (occasionally many). Other discontinuities expressed the ‘capture’ of immature dunes by the main bedform within the duration of a single ebb-tide. Because of the frequent perturbation of the (astronomical) tidal sediment transport by meterological events, little order to the horizontal arrangement of discontinuities within the sets was detected in contrast to other reported cases.