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




Hall, G.L.

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Texas A&M University.


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 to the 5th yards cubed per year at Sea Isle on Galveston Island to 5.4 x 10 to the 4th yards cubed per year 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. Flourescent sediment trace 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 or 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. Sediment transport is prodigeous along the south jetty during a southernly wind regime. Waves are retracted around the jetty extremity and approach and strike the jetty farther shoreward at an acute angle or nearly perpendicularly. This phenomenon causes sediments (sand) to be lifted off the bottom and subsequently transported in the direction of the prevailing current. Sediment may be moved into a shadow zone of the jetty where it will either be trapped or simply moved to and fro along the jetty. During a norther, the area south of the jetty is a leeward side and thus is not affected by the intense wave action that batters the north jetty during such periods. This results in weak sediment transport, primarily in a seaward direction, in response to enhanced ebb-tidal currents that reach a maximum velocity of 0.5 knot. Sediment transport in the Galveston Bay Entrance Channel was evaluated using theoretical considerations of critical bed shear stress and critical erosional velocities. Measured near-bottom current velocities generated by separate tidal regimes varied from 1.6 to 0.2 knot. During a 21-hour anchor station, the values of bed shear stress calculated from vertical velocity profiles constantly exceeded the value required to initiate transport of particles sand size and smaller. Competence of the flow never fell below one mm. It is shown that bottom currents of sufficient magnitude to erode particles of sand size were recorded 86 percent of the time. Never did the bottom current fall below the level to cause sand to be deposited. Sediment transport theory predicts that the channel should be stable and one in which deposition and shoaling is minimal; however, this is not the case. Flocculated silts and clays behave hydrodynamically as much larger particles and are deposited during the lowest transitional flows. The cohesive clays are rapidly buried by the subsequent rain of flocculated masses of silt and clay. The dominant flood tidal cycle will result in net sediment transport landward and accumulation in the channel. Sediment transport processes in the dredged material disposal site (lower shoreface) were also evaluated using theoretical considerations of bed shear stress calculated from monthly vertical current profiles. Critical erosional velocities were determined from a 36-day sustained current profile measured approximately 3-feet above the bottom. The net sediment transport in the lower shoreface was toward the southwest and tends to parallel Galveston Island and the net longshore transport direction. Principal sediment transport agents are the near-bottom currents generated by the tides and superimposed on a semi-permanent current flowing toward the southwest. Wave action appeared to have little effect on bottom sediment movement. Sandy material placed in the dredge material disposal area has little chance to ever return to the channel and will probably enter the longshore transport system and nourish beaches farther down the Texas coast.


325 p., Dissertation


sediment transport, longshore sediment transport, tracers, tidal currents, dredge spoil, erosion, waste disposal sites