Contemporary accumulation of marine sand in a macrotidal estuary, southwest Wales

Abstract

The River Taf forms a small macrotidal estuary impounded by a coastal barrier and opening into a shallow Carmarthen Bay (southwest Wales). Periodic estimates of the sand budget over a ten-year period show that sand is accumulating in the estuary at a mean vertical rate of 0.13 m or more per year. The rise of sand level is two orders of magnitude greater than the present-day rise of sea level. Abundant textural and some mineralogical evidence proves this sand to be of marine origin. The rate of accumulation is related to the frequency of westerly waves which move sand landwards from Carmrthen Bay. This sand is distinctive in its uniformity of texture but multiplicity of sedimentary structures. Neither the mean grain size nor the majority of structures reflect the high energy of the environment. Although the direction of net transport is landwards, most of the small-scale structures face seawards. The tide within the estuary is markedly asymmetrical, the flood being shorter than the ebb. Because sand is infilling the estuary, a spring tide must rise up to 2 m from low water mark before it can enter, and this takes several hours, by which time the flood is well advanced in the bay. Fast flood currents (up to 1.8 m/s), amplified at the constricted estuary mouth, are generated as the tide pours in. Fast ebb currents also develop, particularly when river flow is high, as shoals become exposed and water is confined to the main channels. Circulation varies from moderately stratified to well-mixed. Hence storm waves from the Atlantic are pushing sand into the estuary from Carmarthen Bay, and the accumulating sand in turn dictates the unusual estuarine dynamics.