San Leandro Marina Alts Study_Final

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4. Alternatives TABLE 4-3 CONCEPTUAL REFUGIA AREA DIMENSIONS AND VOLUME Dimension Approximate Acreage South Basin Harbor Basin Existing depth (feet below MLLVV) 1.5 4.0 Area (Acres) 14 4.5 Final top elevation (feet MLLVV) 6.8 (MHVV) 6.8 (MHVV) Material consolidation (CY) 31,250 10,900 Total Volume (CY) 204,250 89,500 Hydrology and Sedimentation Circulation in the vicinity of the Marina is driven by tidal and wind wave induced currents. The orientation of the Channel and Marina impacts circulation within the basin. The orientation of the Marina basin lends itself to good flushing within Channels 1, 4, and 4A, tidal currents providing exchange of tidal waters due to the relatively straight path of the Channels. Winds from the WSW result in wave driven circulation that helps with flushing. No Action Alternative The basin areas, which are protected by the breakwaters to reduce wave energy, would see reduced exchange and longer residence times as the entrance of the interior Channel shoals. Due to the sedimentation patterns, reduced flushing and longer residence time will occur, but should not reduce enough to produce water quality problems. If maintenance dredging ceases, some exchange will still continue within the basin limiting potential water quality problems. Natural sedimentation patterns will likely result in portions of the basin shoaling up to low tide elevations, resulting in complete entrance blockage at monthly extreme low tides. The areas of high sedimentation will eventually reach a depth at which wave and tidal energy is in dynamic equilibrium with the water depths. Dynamic equilibrium is where cyclic deposition and erosion occurs, in response to the amount of tidal and wave activity, but in the long-term there is essentially little to no change in depths. It is difficult to estimate the time to equilibrium because of the number of factors that influence sediment deposition and erodibility. Moffatt and Nichol estimated the entrance area of Channel would likely shoal up to approximately 0 to 1 feet MLLW and would likely be the shallowest portion. The presence of the breakwaters and resultant creation of a tidal circulation system between the bay and marina basin will likely result in a equilibrium condition that may prevent the formation of large areas of emergent tidal marsh. The most likely condition is that of a mudflat with varying elevations within the basin. San Leandro Marina Harbor Basin 4-12 ESA / 210461 Alternatives Study March 2011