Alcatraz Disposal Site Investigation. Report 3. San Francisco Bay- Alcatraz Disposal Site Erodibility

See also Report 1, ADA168170 and Rept. no. 2, ADA174609

Sediments from San Francisco Bay were subjected to laboratory erosion testing to provide information for the management of the Alcatraz disposal site, specifically to reduce the retention of disposed materials at the site. Erosion test results were used to estimate the erosion capacity of the site for a number of disposed material densities using data developed by this study. Erosion testing concentrated on fine-grained, cohesive sediment which makes up the bulk of the material disposed and retained at the Alcatraz site. Since the erodibility of such sediments is known to depend on interparticle cohesion, characterization testing was used to describe the cohesive nature of the sediments and their settling behavior. Appendix A describes 18 erosion tests performed on remolded sediment sections in a 24.4-m recirculating saltwater flume at the US Army Engineer Waterways Experiment Station. Two sediment materials were tested, each with three sand contents and three bulk densities. Flume flows were increased hourly during tests until appreciable erosion occurred. The critical shear stress for the onset of erosion tau sub c was found to be highly dependent on the specific weight of the fine-grained sediments present. The addition of up to 40 percent sand had little effect on tau sub c. Additional erosion and characterization testing, discussed in Appendix B, was performed by the University of Florida at Gainesville using an annular flume. A sediment from Richmond Harbor on San Francisco Bay was used for these tests. One erosion test was performed on remolded sediment material, and four other tests were performed on deposited sediment beds. The settled beds were found to be more erodible than remolded beds.