NATURAL BAR-BYPASSING OF SAND AT A TIDAL INLET

Abstract

Captain Sam's Inlet, a shallow unstable inlet, periodically migrates up to three kilometers (km) alongshore over a 30- to 40-year period. As the inlet migrates, sediment accumulates at the seaward terminus of the tidal inlet to form a wave-modified ebb-tidal delta that trends downdrift. Continued sediment accumulation on the tidal-delta shoal and subsequent lengthening of the tidal channel in a downdrift direction results in an unstable channel configuration. The ebb delta is eventually breached on its updrift side, releasing a sediment package for inlet bypassing. The sediment-bypassing process was initiated at Captain Sam's Inlet after the final landfall of Hurricane DAVID in September 1979. Initially, the newly formed updrift hurricane channel scoured 1.1 m, migrated updrift, and became the predominant tidal channel at the inlet with eventual abandonment of the prehurricane, main ebb channel. These two channels outlined a portion of the ebb-tidal delta that was freed for bypassing. The initial sediment volume contained in the bypassing shoal, above the -0.6 m (-2 ft) MSL contour, was 47,000m3. The sediment volume of the bypassing shoal did not change significantly until final attachment to Seabrook Island. This channel dominance and the wave-induced migration of the bypassing sediment package aided the bypassing of sand at the inlet. Immediately after filling of the prehurricane, main ebb channel on the delta, the downdrift beaches began accreting. The accretion continued throughout the bypassing process. The tidal prism and cross-sectional area reflected little change during the bypass, showing evidence of the system's overall stability.