Section A. Flooding in Mississippi
Mississippi can flood in any season. Floods are generally caused by localized storms and rain over several days on saturated ground or by storm surge. Over the last decades, a significant flood has occurred somewhere in the state each year, often associated with a hurricane. Many of these events received a state or federal disaster declaration.
Overbank flooding: The most common and most damaging floods occur along the state’s rivers and streams. This is referred to as overbank flooding. Most floods are caused by too much precipitation in the watershed. Large rivers respond more slowly to rain and runoff than do the small streams. When floods do occur on the large rivers, they can last for days or weeks.
Impact of flooding: Most of the flooding within the state is slow moving and shallow. However, this does not mean that floodwaters are safe. A car will float in less than two feet of moving water and will be swept downstream into deeper waters. Six inches of fast-moving water will knock a person off his feet.Electrocution is a cause of flood deaths, claiming lives in flooded areas that carry a live current created when electrical components short out. Floods also can damage gas lines, floors, and stairs, creating secondary hazards such as gas leaks, unsafe structures, and fires.
Health: Floodwaters carry whatever was on the ground that the upstream runoff picked up, including dirt, oil, animal waste, and lawn, farm and industrial chemicals. Pastures and areas where cattle and hogs are kept can contribute polluted waters to the receiving streams. Overloaded sewer lines can back up into low lying areas and some homes. Even though diluted by flood waters, raw sewage can be a breeding ground for bacteria, e-coli, hepatitis, and other disease causing agents.
Another type of health problem comes after the water is gone. Stagnant pools become breeding grounds for mosquitoes, and wet areas of a building that have not been cleaned breed mold and mildew. A building that is not thoroughly and properly cleaned becomes a health hazard, especially for small children and the elderly. Another health hazard occurs when heating ducts in a forced-air system are not properly cleaned after inundation. When the furnace or air conditioner is turned on, the sediments left in the ducts are circulated throughout the building and breathed in by the occupants.
Buildings: Due to the relatively low velocities and shallow flood depths in some areas, the most common type of building damage inflicted by a flood is caused by soaking. When soaked, many materials change their composition or shape. Wet wood will swell and, if dried too quickly, will crack, split or warp. Plywood can come apart. Gypsum wallboard will fall apart if it is bumped before drying out. The longer these materials are wet, the more moisture, sediment, and pollutants they will absorb.
Soaking can cause extensive damage to household goods. Wooden furniture may become so badly warped that it cannot be used. Other furnishings such as upholstery, carpeting, mattresses, and books usually are not worth drying out and restoring. Electrical appliances and gasoline engines will not work safely until they are professionally dried and cleaned.
In short, while a building may look sound and unharmed after a flood, the waters can cause a lot of damage. To properly clean a flooded building, the walls and floors should be stripped, cleaned, and allowed to dry before being recovered. Several feet of drywall and insulation must also be removed. This can take days or weeks and is expensive. It is better to be prepared and prevent flood damage.
Other Types of Flooding:
Flash flooding: Occurs where impervious surfaces, gutters and storm sewers increase rain to the receiving stream. Flash floods also can be caused by dam failure or the collapse of a debris dam.
Drainage problems: Flooding can also occur in streets when rainwater cannot flow away from the house or when the sewers or culverts are blocked. These problems are usually caused by heavy local rains and are often not related to overbank flooding or floodplain locations.
Section B. Flood Protection
New development in floodprone areas, which include hurricane prone areas increases the amount of development exposed to damage and can aggravate flooding on neighboring properties.
Development outside a floodplain can also contribute to flooding problems. Stormwater runoff is increased when natural ground cover is replaced by urban development, such as large roofed areas and parking lots. Development in the watershed that drains to a river can aggravate downstream flooding, overload the drainage system, cause erosion, and impair water quality. Remember, we all live downstream. Accordingly, most communities have enacted several ordinances to protect people from activities that may cause flooding or drainage problems.
Before you build on, fill, alter, or re-grade your property, always check with your building department. A permit is probably needed to ensure that such projects do not cause problems on other properties.
Do not dump or throw anything into the storm sewers, inlets, ditches, or basins. Dumping in ditches, storage basins and wetlands is a violation of federal and state regulations and local codes.
Every piece of trash can contribute to flooding. Even leaves, grass clippings and branches can accumulate, plug storm drain inlets and channels, or kill vegetation and contribute to erosion. If your property is next to a ditch or storage basin, do your part and keep the banks clear of brush and debris.
If you see dumping or debris in the ditches or basins, filling or construction near property lot lines, or filling or construction in a mapped floodplain without a permit placard posted, contact your building department or community floodplain manager. The debris or project may cause flooding on your property.
In accordance with your community ordinance, new construction in the floodplain must be protected from flood damage. Local regulations require that the lowest floor (including basement) of new buildings must be elevated above the base flood elevation. There are additional restrictions on filling, grading, storage of materials or building in a mapped floodway.
Local codes also require that substantial improvements to a building be treated as new construction. A substantial improvement takes place when the value of an addition, alteration, repairs or reconstruction project equals or exceeds 50 percent of the value of the existing building. In the case of an addition, only the addition must be protected if it equates to less than a 50 percent improvement. In the case of an improvement of more than 50 percent to the original building, the entire building must be protected to or above the base flood elevation.
For example, a house in the floodplain is flooded, has a fire, is hit by a tornado, or is otherwise damaged. If the value of the repairs equal or exceeds 50 percent of the value of the building before the damage, then the house must be elevated to or above the base flood level depending on the local flood damage prevention ordinance. In some communities, improvements are cumulative, so small projects add up to 50 percent over time.
These regulations are designed to protect you and your neighbors. By keeping the drainage system clear and getting the proper permits before you build, you can help prevent flooding and other drainage problems for both you and your neighbors.
