Alcatraz Disposal Site Investigation

Source: https://erdc-library.erdc.dren.mil/jspui/ Dredged material from San Francisco Bay has been dumped at the Alcatraz Island open-water disposal site for many years. Since the disposal site lies in an extremely high energy area, historically it has been assumed that the vast majority of the disposed material moves with the current past the Golden Gate Bridge and is then carried out into the ocean. However, recent surveys of the disposal site have shown that a large mound of material, which creates navigation problems, has formed. Since a new dredging project in the area is expected to create approximately 5 million cu yd of new material to be disposed at the site over the next 2 years, a major concern is how much of the new material will remain within the disposal site and contribute to the existing mound. To address the problem, a numerical model has been applied that computes the behavior of a dredged material dump through three phases; namely, convective descent, during which the dump cloud falls under the influence of gravity; bottom collapse, occurring when the descending cloud impacts the bottom; and passive transport-diffusion, commencing when the material transport and spreading are determined more by ambient currents and turbulence than by the dynamics of the disposal operation. The model accounts for land boundaries, depth variations, ambient current variations in three dimensions and in time, several sediment classes within the dredged material, and variations of ambient density profiles in time. A major limitation of the model is that erosion and subsequent redeposition of material deposited on the bottom are not modeled. Therefore results from the numerical model were only used to provide the initial amount and distribution of material deposited within the disposal site. Analytic techniques were subsequently employed to analyze the erosional characteristics of the bottom deposits within the site. Results of the study show that the dumping of consolidated clumps of clays and silts will contribute to further mounding or accumulation of material within the disposal site since sufficient energy to erode these types of materials at the rate required to avoid mounding does not exist at the site.