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b. Flooding Riverine flooding occurs when the flow of rainwater runoff exceeds the carrying capacities of the natural drainage systems. During extended periods of heavy rainfall, certain low-lying neighborhoods within the county are subject to considerable flood damage and isolation caused by the inability of natural and mechanical drainage systems to effectively remove the water. Heavy rainfalls can cause considerable damage to the county infrastructure of roadbeds, bridges, drainage systems and the water supply. The buildup of uncontrolled sediment contributes to the problem of inadequate drainage in natural and mechanical drainage systems. When a storm produces an overwhelming amount of storm water runoff, the accumulation of loose sediment causes flooding by clogging the drainage systems. In comparison to riverine flooding, coastal flooding is usually the result of a severe weather system such as a tropical storm or hurricane. The damaging effects of coastal floods are caused by a combination of storm surge, wind, rain, erosion, and battering by debris. All coastal property and inhabitants are subject to severe damage and loss of like resulting from floods caused by hurricane associated storm surge. Some coastal property, road arteries, and bridge approaches are subject to severe flooding caused by rare astronomical tides as well. In Indian River County, several variations of flood hazards occur due to the different effects of severe thunderstorms, hurricanes, seasonal rains, and other weather related conditions. For the majority of the county, the primary causes of flooding are hurricanes or tropical storms. However, the county's low-lying topography, combined with its subtropical climate, makes it vulnerable to riverine flooding. Indian River County Comprehensive Emergency Management Plan Basic Page 10