SEDIMENT TRANSPORT PATTERNS AT THE ESSEX RIVER INLET EBB-TIDAL DELTA,MASSACHUSETTS, USA

Essex River Inlet is located along the Northern Massachusetts barrier chain between Castle Neck to the north and Coffins Beach to the south. The position of the inlet as well as the geometry of the backbarrier system are controlled by a pre-existing drainage system which, in turn , is strongly influenced by the bedrock topography. The inlet is front ed by a well developed ebb-tidal delta. The hydraulics, sediment trans port patterns and morphological changes of the ebb-tidal delta have be en investigated through the documentation and analysis of tidal- and w ave-generated currents, grain size distributions, bedform migrational trends, swash bar development, and historical shoreline changes. The i nlet throat is increasingly dominated by ebb-tidal currents and seawar d sediment transport as tidal range increases from mean towards spring tides. A similar trend exists in the marginal flood channels with inc reasingly stronger flood than ebb-tidal currents with increasing tidal range. These flow asymmetries explain the seaward- and inlet-oriented bedforms (sandwaves and megaripples) that door these channels, respec tively. The swash platform is dominated by landward currents and the o nshore migration and coalescence of swash bars. The period of time bet ween swash bar formation in the terminal lobe region and their eventua l attachment to the landward beaches is approximately 5 to 7 years.The channels and swash platform are parts of clockwise (updrift half of e bb-tidal delta) and counterclockwise (downdrift half of ebb-tidal delt a) sediment gyres that circulate sand within the ebb-tidal delta and a ccount for the sand that bypasses the inlet. Sediment transport rates determined using a variety of means (i.e. MADDOCK'S (1969) equation, s wash bar migration data, morphological changes) indicate that the main ebb channel can easily remove the sand supplied from the marginal flo od channels and across the channel margin linear bare. Moreover, calcu lated sand transport rates in the main ebb channel infer an order-of-m agnitude, more sand is moved across the swash platform than is indicat ed by the migration of the swash bars. This suggests that far more san d is circulated within the sediment gyres than bypasses the inlet.