Wake County Hazard Mitigation Plan - January 2020


4.5.5 Flood

Hazard Background Flooding is defined by the rising and overflowing of water onto normally dry land. As defined by FEMA, a flood is a general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties. Flooding can result from an overflow of inland waters or an unusual accumulation or runoff of surface waters from any source. Flooding is the most frequent and costly of all natural hazards in the United States, and has caused more than 10,000 death(s) since 1900. Approximately 90 percent of presidentially declared disasters result from flood-related natural hazard events. Taken as a whole, more frequent, localized flooding problems that do not meet federal disaster declaration thresholds ultimately cause the majority of damages across the United States. Sources and Types of Flooding Flooding within Wake County can be attributed to two main sources as noted below. Riverine Flooding: The primary riverine flooding sources in Wake County are the Black River, Contentnea River, Haw River, Cape Fear River, and Neuse River, according to the 2015 Preliminary Flood Insurance Study for Wake County. These rivers and their tributaries are susceptible to overflowing their banks during and following excessive precipitation events. Though less common, riverine flood events (such as the “100 - year flood”) will cause significantly more damage and economic disruption for the area than incidences of localized stormwater flooding. Wake County has an Effective FIRM is dated May 2, 2006 and a Revised Flood Insurance Study (FIS) dated November 17, 2017. The FIS summarizes the principal flood problems in the county as follows: “Flooding problems in the unincorporated areas of Wake County have been mostly attributed to the inefficient removal of runoff from highly developed areas. The extent to which development in this area has affected flooding problems can be seen by comparing a flood in May 1957 with one in February 1973. The 1957 flood resulted from approximately 5.7 inches of rain and was considered to have an average frequency of once in 7 years. The 1973 flood reached higher levels in the floodplain but resulted from only 3.5 inches of rain, or a storm predicted to occur once in every 2 to 5 years. This increase in flood potential, caused partially by the intense development which has taken place in the area, has resulted in reduced crop yields and lowered land values and caused more frequent property damage.” Flash Flooding: A flash flood occurs when water levels rise at an extremely fast rate as a result of intense rainfall over a brief period, possibly from slow-moving intense thunderstorms and sometimes combined with rapid snowmelt, ice jam release, frozen ground, saturated soil, or impermeable surfaces. Ice jam flooding is a form of flash flooding that occurs when ice breaks up in moving waterways, and then stacks on itself where channels narrow. This creates a natural dam, often causing flooding within minutes of the dam formation. Flash flooding can happen in Special Flood Hazard Areas (SFHAs) as delineated by the National Flood Insurance Program (NFIP) and can also happen in areas not associated with floodplains. Flash flood hazards caused by surface water runoff are most common in urbanized areas, where greater population density generally equates to more impervious surface (e.g., pavement and buildings) which increases the amount of surface water generated. Flash flooding is a dangerous form of flooding which can reach full peak in only a few minutes. Rapid onset allows little or no time for protective measures. Flash flood waters move at very fast speeds and

Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019


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