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4. Alternatives <br />Marina Park Alternative <br />With the continued use of the Marina in this alternative, sedimentation plays a significant role in <br />maintaining vessel access to the Marina and boat launch (Channels 1, 3, 4, and 4a). The <br />recommended dredge cycle for this alternative is based on the amount of shoaling which occurs <br />over 4 years and what size vessels have limited access as a result. Considering the reduced <br />maintenance dredge depth of -5 feet MLLW, the entrance will shoal to an estimated -3 feet <br />MLLW or shallower in approximately 4 years. According to Moffatt and Nichol (2008)13, this <br />would limit access at low tide for all sailboats and powerboats greater than 28 feet in length. An <br />additional year would reduce depths in the entrance by another 0.5 to 1.0 feet. <br />Changes in the circulation patterns and basin depths are expected to occur due to the creation of <br />the refugia area and realignment of the entrance Channel (Channel 3). These changes are <br />expected to reduce exchange and increase residence time, but should not produce water quality <br />problems. The refugia area will be maintained by natural sedimentation patterns and likely <br />expand toward the deeper entrance Channel. This portion of the Harbor Basin is expected to reach <br />dynamic equilibrium within five to 10 years where cyclic deposition and erosion occurs, in <br />response to the amount of tidal and wave activity, but in the long-term there would essentially be <br />little to no change in depths. <br />Aquatic Park Alternative and Nature Park Alternative <br />With the creation of island refugia, the Harbor Basin will eventually reach a depth at which wave <br />and tidal energy is in dynamic equilibrium with the water depths. The presence of the breakwaters <br />and resultant creation of a tidal circulation system between the bay and Harbor Basin will likely <br />result in an equilibrium condition that may prevent the formation of large areas of emergent tidal <br />marsh, naturally. The most likely condition is that of a mud flat between the refugia areas. The <br />slopes of the refugia areas would be designed and constructed to encourage high marsh <br />vegetation. However, over time there would be a period where mud flats may form between the <br />refugia areas prior to their natural transitioning to high marsh habitats. <br />The creation of the refugia areas with dredge material would depend on availability of material. <br />Once the necessary volume of material has been obtained it would likely be several years before <br />vegetation such as pickleweed and cordgrass colonize the island, unless they are seeded (or <br />planted). Due to changes in the circulation patterns and basin depths, for these two alternatives, <br />reduced exchange and increased residence time will occur, but should not produce water quality <br />problems. It is likely that the entrance Channels (1 and 4) will shoal to an equilibrium depth <br />similar to the elevation of the mudflats on either side of the Marina. The natural sedimentation <br />patterns will likely result in portions of the Channels (1 and 4) shoaling up to low tide elevations, <br />resulting in complete entrance blockage at monthly extreme low tides. <br />13 Moffatt & Nichol. 2008. Report on Maintenance Dredging Needs, San Leandro Marina. Prepared for John <br />O'Driscoll. January 6, 2008. <br />San Leandro Marina Harbor Basin 4-13 ESA / 210461 <br />Alternatives Study March 2011 <br />