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The use of shallow barges that would sit on the bottom of Oyster Bay Slough during low tide would not be expected to affect existing drainage patterns of the slough. The project would not alter the course of any stream or river. Construction of the trail portion of the project could slightly increase the volume of surface runoff, since the trail would add impervious surface to the top of the levee north of Oyster Bay Slough. The amount of increased runoff would be considered a less-than-significant impact. Water surface elevations in the channel are controlled by tide level in San Francisco Bay. In the vicinity of the project site, the FEMA mapped 1 DO-year flood hazard zone is contained within the banks of Oyster Bay Slough (FEMA, 2002). The only portion of the project that could affect the flood zone is the foundation support piers for the bridge. No portions of the proposed trail connection would be located within the 1 DO-year flood hazard zone. Therefore, the 1 DO-year flood elevation and the elevation of the I DO-year high tide are the hydraulic characteristics of the system of greatest concern when evaluating the elevation of the existing and proposed bridge components. The bridge structure would be supported by three piers consisting offour-foot (1.2-meter) diameter cast-in-steel-shell concrete piles. The prefabricated bridge truss would come with steel decking that would provide the formwork for the concrete deck. After the trusses are in place, lightweight concrete would be poured in the steel decking and finished in place. The total floodplain encroachment proposed is placement of these three four-foot (l.2-meter) diameter piles in the tidal inlet. Since the channel hydraulics are dominated by tidal levels in the Bay (and not backwater effects associated with storm water runoff), water levels in the channel will continue to remain in relative equilibrium with tidal levels in the Bay, regardless of encroachments in the channel. Under existing conditions, there are no substantial inputs to the channel (other than tidal inflow). As shown in Table 2, the 1 DO-year flood hazard level and the 1 DO-year high tide elevations are substantially below the surface elevation of the proposed bridge deck. Table 2: Elevations of Proposed Bridge, 1 00- Year Flood and Tide 13.51 FEMA lOO-year Flood Hazard Elevation (feet NA VD 88) 9.692 Corps 1 DO-year High Tide Elevation (feet NA VD 88) 9.593 Lowest Bridge Underside Elevation (feet NA VD 88) NA VD = North American Vertical Datum 1 Mark Thomas and Company, Inc., 2005, Vertical Datum data page, Bay Trail Bridge, City of San Leandro, 25 January. 2 National Flood Insurance Program, 2000, Flood Insurance Rate Map, Community Panel Number 060013 OOOIC, 9 February. 3 U.S. Anny Corps of Engineers, 1984, San Francisco Bay Tidal Stage vs. Frequency Study, October. Y4204IS.OO693.doc - 4/5107 -29-