Flume Experiments on Sand, Silt and Clay Mixtures from the Offshore Dredged Material Disposal Site Galveston, Texas




Moherek, Anthony J.

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U.S. Army Corps of Engineers Waterways Experiment Station


Flume experiments were performed using four sand, silt, and clay mixtures sampled from the offshore Galveston dredged material disposal site in order to determine the critical erosion velocity, shear stress, and modes of sediment transport for each mixture. Also, an analysis of the hydrographic regime for offshore Galveston was performed based on meter logic and oceanographic data collected between February 1975 and June 1976. Results of the flume experiments and hydrographic analysis were extrapolated to sediment transport processes believed operative in the offshore disposal site. The results of the flume experiments indicated that the four sediment mixtures erode similarly as evidenced by the same critical bed shear necessary to induce massive bed load transport and rapid suspended sediment increases. Significant deposition of suspended fine silt and clay occurred at low bead shears. Extrapolation of these erosion and deposition bed shears to recorded offshore bottom current speed measurements over month-long periods suggests that bed load erosion occurs much more frequently near the northern margin of the disposal site than at the southern end. In addition, progressive vector diagrams indicate a net down-coast and/or offshore-directed bottom drift. These net drift directions presumably result from strong prevailing onshore winds that generate onshore-directed flow near the surface and down-coast to offshore flow at the bottom. Maximum bottom velocities occur during these wind-generated bottom flows. This suggests that net bed load transport of disposal mound sediment is oriented away from the Galveston bay entrance channel, and therefore material will not likely return as shoaling sediment.


204 pgs.


dredging, environmental aspects, flumes, sediment transport