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SAN LEANDRO SHORELINE DEVELOPMENT DRAFT EIR <br />CITY OF SAN LEANDRO <br />GEOLOGY, SOILS, AND SEISMICITY <br />Near the beginning of the Holocene age (approximately 11,000 years ago), the rising sea reentered the <br />Golden Gate, and sediments accumulated rapidly beneath the rising San Francisco Bay and on the <br />surrounding floodplains. Being geologically recent, these surface deposits are generally less dense, <br />weaker, and more compressible than the deeper, well -consolidated Pleistocene -age soils that predate the <br />last sea -level rise. <br />Site Geology <br />The Project site lies along the eastern margin of San Francisco Bay on the low-lying coastal plain and <br />adjacent filled portions of the bay. An 1878 topographic map of the area, shows that prior to fill <br />placement the original coastline was located east of Monarch Bay Drive as illustrated on Figure 4.5-1. 6 <br />The entire Project site is relatively low-lying with elevations ranging up to about 10 feet above mean sea <br />level. Roughly, the western two-thirds of the project site, including the existing marina and the <br />surrounding buildings, has been constructed by filling on the shallow margin of San Francisco Bay. Review <br />of historic aerial photographs covering the time period from 1947 to 2012 indicate that some fill was <br />placed periodically during the first half of the twentieth century. By 1947, a low levee had been <br />constructed along the existing coastline. Some small structures were present at Mulford point at that <br />time; however, the site was largely undeveloped. By 1959, the Marina Golf Course in the eastern portion <br />of the site had been constructed. By the early 1960s, the existing marina fill had been placed and <br />development began in the Marina area. By 1968, fill for the Marina Park to the south of the project site <br />had been placed and the park was under construction. <br />The marina construction included dredging to increase water depth and provide access for boats. <br />Dredging was performed in the marina area and in the channel that leads to San Francisco Bay. Additional <br />dredging has been performed periodically to maintain boating access. Currently there are three <br />maintained channels associated with the Marina: a main access channel leading from San Francisco Bay <br />into the Marina; an auxiliary access channel along the southern side of the southern dike leading to the <br />boat -launching ramp; and, an interior channel leading to the boat berthing area within the marina. <br />Dredging and filling plans for the marina indicate that the marina fill was constructed by first constructing <br />a series of clamshell dredged dikes (rock dikes were specified at the tips of the western and northern <br />dikes) around the perimeter of the areas to be filled and then filling the interior with either hydraulically <br />placed fill and/or clamshell dredged material.' The fill was placed using both hydraulic and clamshell <br />methods and consists of material derived from Bay Mud and the underlying alluvial sediments. <br />West of the original coastline, the fill was placed over soft estuarine deposits referred to as Bay Mud. <br />Borings drilled as part of the marina dredging plan development indicate that within the project site the <br />Bay Mud is relatively thin, ranging up to about 10 feet in thickness. The Bay Mud is underlain by older, <br />better consolidated, alluvial and estuarine deposits of Pleistocene and Holocene age. <br />6 Thompson and West, 1878, "Map Number Three, Alameda County Farm Map", Oakland, California. <br />7 Indenco Engineers, 1962, San Leandro Small Craft Harbor Stage II Dredging and Filling Plans, April 9. <br />4.5-4 DECEMBER 2014 <br />