San Leandro Marina Alts Study_Final

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4. Alternatives use of the existing Marina, the Aquatic Park Alternative would include a water trail between the islands created in place of the Marina, and the Nature Park Alternative would include a diverse wetland area in place of the Marina. As mentioned above, sedimentation is a major problem for the existing Marina and has resulted in an expensive maintenance dredging program. The potential amount of dredging for future uses of the Marina provide an understanding of the potential maintenance associated with each concept. The historic dredging practice has been to dredge the 200 foot wide federal Channel and Marina berthing area to -7 feet10 MLLW. This maintenance activity was completed by the US Army Corps of Engineers on atypical 4 -year schedule for the Channel, dredging an estimated 160,000 CY, and by the City on a typical 8 -year schedule for the Marina berthing area, dredging an estimated 20,000 CY. The Channel was partially dredged in December of 2009 (by the Corps) to -5 feet MLLW, and the berthing area was last dredged in 1997 to -7 feet MLLW. Figure 4-2 provides a summary graphic of Channel designations and maintenance dredge depths. Marina Park Alternative The Marina Park Alternative assumes a reduced maintenance dredging scenario. The reduced maintenance would be to dredge the Channels (1, 3, 4, and 4a) (see Figure 4-2 for locations of dredge Channels) and the Marina berthing area to -5 feet MLLW and reduce the Channel 1 width to 120 feet, from the present 200 feet. Although this will not restore the authorized depth of 7 ft at MLLW, it allows continued use of a portion of the harbor for navigation. Based on historic surveys the Channel would likely need to be dredged on a 4 -year dredging cycle to maintain access to the berthing area. This results in a dredging volume of 105,000 CY per 4 -year cycle in the Channel. The Marina berthing area, reduced to the eastern portion only, would be dredged on an 8 -year cycle, as sedimentation patterns indicate that lower shoaling rates persist in the berthing areas. This would result in an estimated dredge volume of 10,000 CY per 8 -year cycle in the berthing area. These values assume that only half of the Marina basin, which includes a portion of the federal Channel and the Marina berthing area, is being dredged. Aquatic Park Alternative After the initial work to create high marsh or island areas, no on-going maintenance dredging would occur under this option. 10 All dredged areas require "overdredging" to compensate for the inaccuracies in dredging in tidally varying water. All depths in this report are reported as required depths. Add 1 foot overdredge to all depths for volume computations. San Leandro Marina Harbor Basin 4-9 ESA / 210461 Alternatives Study March 2011