4Risk Assessment
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- Impact on Existing and Future Development
- Past Occurrences
- Frequency/Likelihood of Future Occurrence
4.2.3Flood 100/500-YearHazard ProfileAccording to the State of Mississippi Standard Mitigation Plan, flooding causes 90% of all natural disaster damages. Most of the 288 deaths in Hurricane Katrina were flood related. Flooding can also cause extensive property damage, contributing to significant structural damage and in some cases structural failure from velocity associated with moving water and from saturation from flood waters. In nearly all cases of structural flooding unless personal property is relocated above the anticipated flood stage, it is also at risk. Flooding is a natural and inevitable occurrence. Floods occur seasonally with torrential rains associated with tropical storms that later drain into river basins exceeding the natural capacity of the stream. Rivers and other bodies have always overflowed their normal beds to inundate nearby land. The low lands lying adjacent to these bodies of water are referred to as the floodplain. Floodplains are made when floodwaters exceed the capacity of the main channel or escape the channel by eroding its banks. When this occurs, sediments (including rocks and debris) are deposited that gradually build up over time to create the floor of the floodplain. Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream. In Bay St. Louis, all flooding can be defined as coastal, drainage or flash flooding. Most drainage related flooding results from intrusion of tide water into drainage outlets that prevent drainage structures from operating as efficiently as they are designed to do during heavy rainfall. Coastal (Tidal) Flooding: All lands bordering the Mississippi Sound, bays, estuaries or lakes are prone to tidal affects/flooding. Coastal land such as sand bars, barrier islands and deltas provide a buffer zone to help protect human life and real property relative to the sea much as flood plains provide a buffer zone along rivers and other bodies of water. Coastal floods usually occur as a result of abnormally high tides or tidal waves, storm surge and heavy rains in combination with high tides, tropical storms and hurricanes. (see Section 4.2.2 for more details) Flash or Rapid Flooding: Flash flooding is the result of heavy, localized rainfall, possibly from slow-moving intense thunderstorms that cause small streams to overflow. In Bay St. Louis, flash floods are most common when rain fall on built-up areas where impervious surfaces, gutters and storm sewers speed up the flow of run-off. These floods can become raging torrents of water that rip through streambeds, streets, and coastal section, sweeping everything in their path. Drainage: Drainage flooding occurs primarily in urban or developed areas when the volume of runoff exceeds the capacity of the drainage system. Flooding of this nature can be the result of increased development, inadequate drainage structures, riverine flooding, coastal flooding or a combination of these causes. Flooding can occur in coastal Bay St. Louis year-around but is most frequent in late spring, summer, and winter. The summer months often bring persistent thunderstorms and late summer the heavy rains associated with tropical storms and hurricanes moving ashore from the Gulf of Mexico are more prevalent. Winter storms originating in the Gulf also account for flooding events. Mean annual rainfall is 61 inches along the Mississippi Coast. The City of Bay St. Louis defines flooding through its Ordinance No. 521 as “a general or temporary condition of partial or complete inundation of normally dry land from:
Bay St. Louis has sustained flood events severe enough to warrant federal disaster declarations. Table 4.9. shows the date, cause, designation, and funds extended as result of the declaration.
Source: Mississippi Emergency Management Agency – Public Assistance Program, State of Mississippi Standard Hazard Mitigation Plan, FEMA Flood prone areas are identified within the City of Bay St. Louis from the Flood Insurance Study (FIS) and associated DFIRM Maps developed by the FEMA and the NFIP and adopted by ordinance on October 16, 2009. The DFIRM and Ordinance assist the City to manage activity within the floodplain and reduce loss of life and property damage from flooding. The following Flood Insurance zones are identified by the DFIRM and defined in the ordinance:
The National Flood Insurance Program (NFIP) utilizes the 100-year flood as a basis for floodplain management. The Flood Insurance Study (FIS) defines the probability of flooding as flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 100 year period (recurrence intervals). Or considered another way, properties within a 100 year flood zone have a one percent probability of being equaled or exceeded during any given year. Elevations in Bay St. Louis average between 5 and 21 feet National Geodetic Vertical Datum (NGVD). Elevations drop to below 10 feet NGVD in the northern and northwestern area of the city and average about 5 feet NGVD along the seawall on South Beach Boulevard. Elevations in the annexed area average between 4 and 8 feet. There entire city is vulnerable to inundation from storm surge flooding during major hurricanes. For this reason, the entire City of Bay St. Louis should be considered at risk for flooding and identified as a flood hazard area. Mortgage lenders require that owners of properties with federally-backed mortgages located in Special Flood Hazard Areas (SFHA) purchase and maintain flood insurance policies on their properties. Consequently, newer and recently purchased properties in the community are insured against flooding. Due to the risk of flooding from hurricanes, the City of Bay St. Louis has encouraged all owners of property located in the City, even if the property is not located in a SFHA, to purchase and maintain flood insurance policies. Bay St. Louis has achieved a Class 7 flood insurance rating through participation in the Community Rating System. The present Class 7 rating rewards all National Flood Insurance Program policyholders in Bay St. Louis with a 15 percent reduction in their flood insurance premiums. Bay St. Louis annually participates in an outreach program involving mailing a brochure to every address in the city. The brochure addresses flood related subjects including flood safety, flood warning, flood hazard areas, drainage system maintenance/stream ordinance, property protection measures, flood protection assistance, floodplain development regulations, substantial improvements/damage requirements, the National Flood Insurance Program, the natural and beneficial functions of wetlands, and flood zone descriptions. A Hurricane Evacuation Zone Map and a Hurricane Tracking Map with instructions concerning hurricane safety are included in the mailing. The instructions address sheltering, helpful survival hints, family disaster planning, flood hazards in Bay St. Louis due to hurricane storm surge, and definitions of terms used by forecasters.
The positive impact is that in the months following Katrina FEMA responded with temporary Advisory Base Flood Elevation (ABFE) maps to guide recovery and initial reconstruction and within three years, completed new DFIRM for Bay St. Louis and Coastal Mississippi. These tools gave Bay St. Louis and other coastal communities authority to require higher elevation of new structures and require better construction techniques. The entire coastal community, including Bay St. Louis is being rebuilt safer and stronger and more able to withstand future flooding and storm surge because mitigation methods are designed into building plans. With expansion of VE Zones into areas previous A Zones came building and development requirements mandating higher elevations and stronger foundations than were previously required in most waterfront areas. The end result will be far more sustainable structures than the ones destroyed by Katrina. With the annexation also came the need to revise the city’s land use ordinance. Without existing development, an opportunity was available to look at land use and other development tools in a different way than would have been possible without the widespread destruction of development that existed when the annexation petition was filed in 2004. While it is expected that waterfront property will continue to be the most preferred property in the community it is also expected that future development will be stronger and far more sustainable than in the past. Past OccurrencesTable 4.10. shows the flood events from causes other than hurricanes reported by the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC) since 1995:
Source: NOAA National Climatic Data Center – Extreme Events The following chart indicates flood losses by property classification in Bay St. Louis and flood damage payments to owners from the NFIP.
Source: NFIP Frequency/Likelihood of Future OccurrenceHighly Likely—The City of Bay St. Louis has a 1 percent chance of a 100-year or significant flood being equaled or exceeded in any given year. Tidal flooding from tropical storms and depressions can be expected to occur every other year. Due to the low elevations, flat terrain and tidal influence on drainage unusually heavy rainstorms and any tropical storm, hurricane or other storm-tide producing event will flood unmitigated properties in Bay St. Louis. Due to its location on the shores of the Mississippi Sound, the area’s susceptibility to hurricanes and other tropical disturbances originating in the Gulf of Mexico, and the high rate of annual rainfall, there is a 100 percent chance that flooding will continue to occur from time to time. Directory: documents documents -> Concept stage documents -> Concept stage documents -> The great global switch-off documents -> Nonprofit grant application documents -> The Truth about the Rwandan Genocide documents -> Annual InStructional unit plan documents -> Chaos equipment included Download 0.88 Mb. Share with your friends:
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