Types & Causes of Flooding

Floods are one of the most common hazards in the United States and can occur anywhere. A flood is a general or temporary condition where two or more acres of normally dry land area or two or more properties are inundated by water or mudflow. Floods can be caused by many different conditions including hurricanes or storms, storm surge, heavy rain, prolonged rain, failure of dams, above average rainfall, or inadequate drainage. The impact can be small, only affecting a few properties, or very large, affecting entire communities. Some floods develop slowly, over a period of days or even weeks, while others like flash floods, can develop as quickly as within a few minutes.

Flooding in Leon County is usually caused by one or more of the following factors.

  • Hurricanes & Tropical Storms With a combination of soaking rain, flying debris, high winds, and tidal surges, hurricanes and tropical storms can pack a powerful punch. Besides causing extensive damage in coastal areas, hurricanes and tropical storms often bring flooding hundreds of miles inland, placing communities that normally would not be affected by the strongest hurricane winds in great danger.
  • Heavy Rains An excessive amount of rainfall can happen anytime throughout the year, putting your property at risk. With an average rainfall of approximately 60 inches per year, Tallahassee is very prone to this type of flooding. Summer months often bring heavy rains to the area as warm air and moisture from the gulf move inland. Cresting rivers, backed-up storm drains or saturated ground can cause significant, widespread floods during these times.
  • Flash Floods A flash flood is a rapid flooding of low-lying areas in less than six hours, which is caused by intense rainfall from a thunderstorm or several thunderstorms. Flash floods can also occur from dam or levee failure or other cases where a sudden release of water occurs. In the United States, flash floods are the number one weather-related killer since they can roll boulders, tear out trees, and destroy buildings and bridges.
  • Dams and Levees Dams provide recreation, flood control, irrigation, water supply, and hydroelectric power but they can also be breached with little warning. Levees are designed to protect against a certain level of flooding; however, they can and do decay over time and can also be overtopped or breached during large floods. Dam and Levee breaches can result in catastrophic flooding.
  • Closed Basin Flooding A closed basin is an area that drains to a depression (low area) or sinkhole, from which water is lost only by evaporation, percolation, through a sinkhole, or eventually "popping off" into another basin. It is a basin without a surface outlet (e.g., a stream or river) for stormwater runoff. Flooding occurs in closed basins when the amount of water accumulating in the low area is larger than the amount of water leaving the closed basin. This often occurs as a result of increased rainfall, high groundwater, or saturated soil. Often times, water levels continue to rise in the low area of the closed basin until they eventually "pop off" into a different basin. Flood water in closed basins may even stay at flood stage for weeks, months, or years.
  • Sheetflow Flooding Sheetflow is essentially a broad sheet of water that flows along the ground surface until it reaches a conveyance system such as a swale, storm drain, or watercourse. Sheetflow flooding often occurs in areas where structures are not constructed high enough off the ground to prevent water from entering the structure, or on sites that are not graded to drain water away from a structure.
  • Overland Flooding The most common type of flooding event is overland flooding, which typically occurs when waterbodies overflow their banks as a result of rainwater, or a possible dam or levee breach, and cause flooding in surrounding areas. Overland flooding can also occur when rainfall exceeds the capacity of underground pipes, or the capacity of streets and storm drains designed to carry flood water away from urban areas.

All areas throughout Leon County have the potential to flood, but some areas have a higher probability of flooding than others. The areas around the waterbodies of Leon County such as streams, rivers, lakes, and wetlands are all susceptible to overland flooding. Many of the waterbodies in Leon County have been mapped in a special flood hazard area (SFHA) according to the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs).

Closed basins occur throughout Leon County and play a large role in area flooding. Homes in many of the closed basins are built below the level where water can "pop-off" into another basin, making them more at risk for flooding. Unlike the rivers and lakes in Leon County, many of the low areas and areas prone to closed basin flooding are not shown on the FIRMs.

In some of the County's more urban areas, stormwater systems can become overtaxed or blocked with debris, leaving no space for excess water to enter the system. When the stormwater is unable to enter the stormwater system, it can cause localized flooding, standing water, block roadways, or cause sheetflow or overland flow. Do you know that nothing but rainwater is allowed to drain into the storm drains located in streets, curbs, and right-of-ways? Help us keep the waters and streets of Leon County clean! If you witness dumping of any liquid or materials into storm drains or waters of the County, please call Leon County Code Enforcement at 606-1300 or visit the Leon County webpage to report these activities.

To protect yourself, it is important to learn what flood threats affect our community and your property.

  • Are there are rivers or creeks near you that flood frequently?
  • Is your home located in a low-lying area?
  • What is your home's elevation relative to the surrounding ground?

Be aware of flood hazards no matter where you live or work, but especially if you are in low-lying areas, near water, behind a levee, or downstream from a dam. Even very small streams, gullies, creeks, culverts, dry streambeds, or low-lying ground that appear harmless in dry weather can flood. If your property is in the SFHA, the odds are that someday your property will be damaged by flooding.

Just because you haven't experienced a flood in the past, doesn't mean you won't in the future. Flood risk isn't just based on history, it's also based on a number of factors: rainfall, river-flow and tidal-surge data, topography, flood-control measures, and changes due to building and development.