City of Irvine
2020 Local Hazard Mitigation Plan
do not account for increased groundwater withdrawal, Irvine and the greater Orange County region could experience subsidence as a result.
Flooding (Sea Level Rise)
Description
A flood occurs when land that does not normally have bodies of water becomes suddenly inundated with water. Flooding can occur after periods of heavy rainfall, whether it occurs as a single extreme episode or as a series of storms. Drainages and stream courses may flood their banks and shores if their capacity is exceeded by rainwater. When heavy rainfall hits an area where the ground is already saturated, the risk of flooding is high. In developed areas, the presence of pavement and other impervious surfaces means that the ground is less able to absorb water. As a result, rainwater must be carried away in storm channels or waterways. Floods pose several threats to communities and public safety. Flooding can cause property damage, destroy homes, and carry away vehicles or other large debris. Topsoil and vegetation can be swept away by floodwaters, leading to erosion. Floodwaters may impede the movement of victims fleeing a flood or of first responders attempting to reach people in need of help. As an inland community that has coastally influenced drainage infrastructure, another concern that should be addressed is the potential influence of sea-level rise on flood control infrastructure. In the case of Irvine, a small portion of the flood control infrastructure in the northwestern corner of the City may be influenced by sea-level rise in the future.
Location and Extent
Flood events are measured by their likelihood of occurrence. For instance, a 100-year flood is a flood that has a 1 in 100 (1.0 percent) chance of occurring in any given year. A 500-year flood is a flood that has a 1 in 500 (0.2 percent) chance of occurring in any given year. The 100-year flood has been designated as the benchmark for major flood events, and thus 100- year floods are referred to as “base floods.” Floodplains are areas that experience frequent flooding. While it is possible for areas outside of these designated floodplains to experience flooding, the most likely locations to experience future flooding are low-lying areas near bodies of water. FEMA is the governmental body responsible for designating which areas of the United States can be classified as floodplains. The three most common designations are: • Special Flood Hazard Area: The area within a 100-year floodplain. • Moderate Flood Hazard Area: The area outside of the 100-year floodplain but within the 500-year floodplain. • Minimum Flood Hazard Area: The area outside of the 500-year floodplain. Within these three designations, FEMA has multiple floodplain categories for each unique environment.
Table 3-10 shows these detailed floodplain categories.
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