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7.B. - Page 140 <br /> Comment No. 1-3 <br /> Currently the creekside wall is designed to prevent overtopping of a 100-year stillwater <br /> elevation. (Stillwater is defined as the flood level not including the effects of waves or <br /> tsunami, but includes storm surge and astronomical tide.) The 100-year stillwater <br /> elevation is 9.7 ft NAVD, and the proposed top of wall elevation is 10.0 ft NAVD, so <br /> there is about 3.6 inches freeboard, based on current data. <br /> The San Francisco Bay Conservation and Development Commission (BCDC) predicts a <br /> sea level rise of 10 to 17 inches by 2050. This range of potential increase would not <br /> raise the water surface elevation high enough to overtop the creekside wall during <br /> normal daily tide fluctuations. However, in extreme stillwater events, this rise could <br /> potentially raise the water surface elevation to levels that could overtop the wall. <br /> Table 1 presents the National Oceanic and Atmospheric Administration (NOAA) data for <br /> highest recorded tide elevations at the Redwood Creek (gauge No. 9414523) during the <br /> most recent 12 months, corrected to NAVD. Table 1 includes 10-inch, 17-inch, and 55- <br /> inch increases in these elevations, corresponding to the range of projected rises in seas <br /> levels predicted for 2050 and the maximum BCDC predicted increase for 2100. <br /> As shown in Table 1 on the following page, at the lower end of the predicted range, a <br /> 10-inch increase in the sea level in 2050 would result in two events of overtopping the <br /> creekside wall annually. At the upper end, a 17-inch increase in sea level would result in <br /> 12 events that overtopped the wall annually. Increases in sea level greater than 11.0 <br /> feet, such as may occur after 2050, would exceed the elevations of the adjacent levees <br /> and result in flooding along Main Street and other portions of Redwood City, requiring <br /> more widespread protection measures. <br /> Safety measures such as access gates and emergency call boxes have been included into <br /> the design. Access gates will be closed on both sides of the path leading under US 101 <br /> by City maintenance staff to prevent usage of the path during extreme events. This <br /> closing would allow the tide to subside, the path to drain out back to the creek and <br /> maintenance crews to clean and provide any miscellaneous maintenance to the path. <br /> The City is aware of the impacts of sea level rise to the project and would continue to <br /> monitor it on an annual basis. The City may raise the creekside wall in the future should <br /> sea level rise significantly increase the frequency of overtoppings or substantially <br /> decrease the use of the undercrossing. The City would determine this need based on a <br /> benefit cost analysis to raise the wall versus performing increased regular maintenance. <br /> Sea level impacts should not have any adverse impacts to the bridge structure as a <br /> result of the proposed pedestrian path. <br /> 11 <br />