Sediment transport processes in the nearshore waters adjacent to Galveston Island and Bolivar Peninsula

Abstract

The net longshore transport of sand along Bolivar Peninsula and Galveston Island during 1975 was toward the southwest at rates that varied from 1.52 x 10 ⁵ yds³/yr at Sea Isle on Galveston Island to 5.4 x 10 ⁴ yds³/yr at Gilchrist on Bolivar Peninsula. Net longshore transport was toward the southwest during months of fall, winter and spring and generally toward the northeast only during three months of summer. An excellent correlation exists between the longshore transport direction and several factors including the frequency of the observed wave approach, speed and direction of the longshore current and the wind direction and velocity. Fluorescent sediment tracer studies revealed that a significant portion of the longshore transport showed a tendency to move parallel to the shoreline or offshore toward the breaker zone under most surf conditions. The seaward transport of sand in the surf zone was characteristically along paths nearly parallel to the angle of wave approach until reaching the breaker zone. Transport parallel to the shore at a significant distance shoreward of the primary breaker zone was usually attributed to entrainment in moderate- to high-velocity longshore currents, transport on the crests of bars or channelized flow in the runnels. Sediment tracer analyses conducted along the north and south jetties revealed that transport along these structures was controlled primarily by wind direction, wave action and the strength and direction of the prevailing tidal currents. Strong northerly winds create intense wave action which vigorously churns the sediments and batters the north jetty. Enhanced ebb-tidal currents transport the sandy sediments seaward along the jetty, through the small boat passage into the outer bar channel and ultimately around the seaward extremity of the jetty..