Sand Transport and Sand Bypassing at Selected Inlets

Introduction This chapter describes sand transport patterns and sand bypassing at seven inlets; five of these are located on the east coast of the USA (Price Inlet, Breach Inlet, Captain Sam's Inlet, Mason Inlet and Wachapreague Inlet), one inlet is located in the Bay of Plenty on the North Island of New Zealand (Katikati Inlet) and another is part of the Dutch Wadden Sea coast (Ameland Inlet). The inlets are selected because they are still in their natural state and have been extensively studied. Emphasis is on the mode of bypassing, location stability and their relationship with the P / M ratio. In judging the results, it should be pointed out that estimates of longshore sand transport have limited accuracy. Price Inlet Price Inlet (Fig. 4.1) is located on the coast of South Carolina. Tides are semidiurnal with a mean tidal range of 1.5 m and a spring tidal range of 2.1 m. The annual average deep water significant wave height is 0.6 m. The mean tidal prism is 14×10 6 m 3 and the spring tidal prism is 20×10 6 m 3 . The throat cross-sectional area is 894 m 2 . From this, maximum cross-sectionally averaged velocities are 1.1 m s -1 for mean tide conditions and 1.56 m s -1 for spring tide conditions. The ebb delta has a volume of 6 × 10 6 m 3 and extents approximately 800 m offshore. The gross longshore sand transport is 0.25 × 10 6 m 3 year -1 and is predominantly from the north. Tide- and wave-generated currents carry the sand through marginal flood channels and across the channel margin linear bars to the main ebb channel. The lateral inflow of sand causes the channel to meander on timescales of decades. With ebb currents stronger than flood currents, most sand deposited in the ebb channel is ultimately transported to the seaward portion of the ebb delta. At low tide, waves break on the seaward edge of the delta and transport sand along the periphery of the delta towards the downdrift beaches and onto the ebb delta platform.