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Flood Presence and effects of levee systems in San Mateo County are not reflected on the DFIRM, meaning that areas, structures, and populations vulnerable to failures of those levees cannot be determined. However, because the County estimates that the levees in their current state could not withstand a 1-percent-chance annual flood, reflections of effects of the levees on the DFIRM would not be reliable anyway. The 2016 preliminary DFIRMs do account for estimated sea level rise; however, because not yet finalized, these maps could not be utilized to contribute to vulnerability estimates of flooding within leveed areas. Following approval of the 2016 DFIRMs, San Mateo County will consider the extent to which the levees must be updated as a future mitigation action item, and consider protection from sea level rise. Action may not be considered until the next hazard mitigation plan update, and levee vulnerability will also be explored in further detail. Water and Sewer Infrastructure Water and sewer systems can be affected by flooding. Floodwaters can back up drainage systems, causing localized flooding. Culverts can be blocked by debris from flood events, also triggering localized urban flooding. Floodwaters can enter and thus contaminate drinking water supplies. Sewer systems can back up, spilling wastewater into homes, neighborhoods, rivers, and streams. 11.3.3 Environment Riparian areas, the zones along the edge of a river or stream that are influenced by or are an influence upon the water body, are particularly exposed to the flood hazard. The exposed environment includes wildlife that relies on riparian areas. 11.4 VULNERABILITY The vulnerability assessment indicates estimated damage for the 1-percent and 0.2-percent-annual-chance flood hazards. Detailed results by jurisdiction are included in Appendix E; countywide summaries are provided below. 11.4.1 Population Vulnerable Groups Vulnerable populations are all populations within the floodplain whose abilities to escape the area before floodwaters arrive are limited. This population includes all categories identified for the SoVI rating (see Section 7.2.2). Impacts on persons and households for the 1-percent and 0.2-percent-annual-chance flood hazards were estimated through the Level 2 Hazus analysis. Countywide results are provided in Table 11-9. Table 11-9. SoVI Index Population Distribution for the 1-Percent and 0.2-Percent-Annual-Chance Flood SoVI Rating 1% Annual Chance Flood Zone 0.2% Annual Chance Flood Zone Population Exposed % of Exposed Population Population Exposed % of Exposed Population Very High 11,700 29.58 14,930 16.8 Relatively High 19,397 49.04 36,802 41.41 Relatively Moderate 1,830 4.63 10,288 11.58 Relatively Low 3,260 8.24 21,220 23.88 Very Low 3,370 8.51 5,637 6.33 Total 39,557 100 88,877 100 11-23