38 



Hayes (1967c) studied Hurricanes Carla and Cindy in the Gulf of Mex- 

 ico to examine the direct effects of storm processes and sediment trans- 

 port. They recorded cross-shelf thicknesses and textures of Hurricane 

 Carla beds to a depth of -35 m off Padre Island, Texas, along 50 km of 

 coast. Hayes (1967c) documented that sediment was transferred between 

 the beach and the inner shelf in both the onshore and offshore directions. 

 Before and during Hurricane Carla, mollusk shells, coral blocks, and other 

 materials were transported onshore from water depths between 15 m and 

 25 m and deposited on the beach. Storm surge seaward-directed turbidity 

 currents carried the sediment offshore. After the storm passed, offshore- 

 directed currents associated with hurricane-generated channels deposited 

 a 1.25-cm to 3.75-cm layer of sand over preexisting mud out to depths of 

 -18 m. In addition, a graded layer of fine sand silt and clay (known as a 

 turbidite) was deposited. 



Summary 



Green et al. (1988) document sediment transport changes according to 

 different phases of the storm. During fair-weather conditions, although 

 the waves were asymmetric in an onshore direction, the reversing tidal cur- 

 rents and resulting mean flow controlled inner shelf sediment transport. 

 During the early phase of the storm, sediment transport was controlled by 

 wind-driven jet-like flow (mean flow) with an offshore component. Dur- 

 ing the progression and towards the end of the storm, the waves were 

 more organized and highly skewed in a onshore direction, thus enabling 

 the highly skewed wave-orbital velocities to transport sediment in an on- 

 shore direction against the mean flow. Storm flow was dominated by sus- 

 pended load, which accounted for 75 percent of the sediment volume. 



In summarizing the findings of Green et al. (1988) and Wright et al. 

 (1991), mean flows, interpreted to be related to tides, were dominant over 

 incident waves in generating cross-shore sediment fluxes across the inner 

 shelf. Cross-shore mean flows during fair-weather conditions were negli- 

 gible, while these flows were greater than 20 cm/sec during storm condi- 

 tions. Oscillatory flows associated with waves were 10 cm/sec and 

 100 cm/sec during fair-weather and storm conditions, respectively. Sus- 

 pended sediment concentrations 10 cm above the bed were less than 

 0.1 kg/m^ and 1-2 kg/m^ during fair-weather and storm conditions, 

 respectively. 



Storm Sedimentation IVIodels 



Modeling of storm sedimentation is limited to the models of Dott and 

 Bourgeois (1982); Walker (1984); Brenchley (1985); Duke (1985); and 

 Duke, Arnott, and Cheel (1991), who base their models on the following 

 parameters: 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



