San Leandro Marina Alts Study_Final

Home

SECTION 3 Dredged Materials Disposal Concept This section describes a Dredged Materials Disposal Concept and its association with the options being considered for the Harbor Basin. The dredged materials disposal concept is a means of maintaining approximately 185 slips at the existing Marina while the long-term reuse plans for the Marina are being developed. It would provide a method for beneficially reusing the dredged materials that are excavated from the recurring dredging of the Harbor Basin and Channel. Required dredging for the concept was evaluated assuming a reduced maintenance depth and width as described above for the Marina Park Alternative, i.e. dredge the Channel and the Marina berthing area to -5 feet MLLW and reduce the federal Channel 1 width (see Figure 4-2) to 120 feet from the present 200 feet). As noted above, dredging of 105,000 cubic yards (CY) per cycle would be necessary approximately every four years to maintain the harbor, with an additional 10,000 CY for the berthing areas every alternate episode (total of 115,000 CY every 8 years). However, since the berthing area has not been dredged since 1997, this area will require additional dredging over and above that which would be required in future 8 -year cycles, The first episode is therefore expected to be about 125,000 CY of dredging (105,000 CY for the Channel and 20,000 CY for the berthing area). Dredged material from the Channel and Marina basin could be used to create high -marsh refugia in the Harbor Basin, consistent with Alternative 1, as well as within the South Basin (both are shown in Figure 3-1, on the following page). The refugia in the South Basin would consist of one mass about 14 acres in size or two or three refugia totaling the same amount, separated by a small channel, and the island in the North Basin would be about 5 acres in size. The South Basin refugia is able to accommodate about 204,000 CY assuming that present depths are about 1.5 feet below MLLW. The concept includes placing dredged material to an elevation of about Mean Higher High Water (MHHW). The North Basin refugia is able to accommodate about 89,000 CY assuming that present depths are about 4 feet below MLLW.5 Considering the high costs associated with off -haul of material from the DMMS, it may be also possible to place some of the dredged material from the first episode to the DMMS, after which it could be permanently converted to a shorebird habitat or managed marsh or seasonal wetland (see Section 5 for a discussion of the DMMS). Using the 60,000 CY estimate to convert the DMMS to 5 There would be some amount of sea level rise. However, sediment will continue to deposit in the area offsetting any increase in depth. San Leandro Marina Harbor Basin 3-1 ESA / 210461 Alternatives Study March 2011