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Flash Flood in Florida

Flash flood introduction

The incursion of large volumes of water into a normally dry area is a situation of great risk to public health and property. The tremendous destructive force of fast-moving water and the dangerous presence of long-standing stagnant water creates a very hazardous condition. This is referred to as flooding, one of the most common natural disasters in the United States.

A flash flood is a type of flood that builds up very rapidly, submerging a previously dry area within minutes or hours. This duration is what categorizes a water influx as a flash flood, as the rise in water level occurs within six hours. Regular floods typically take days of continuous water build-up. The sudden occurrence of flash floods is what makes them more dangerous than regular floods, as these can catch the public unawares. Flash floods are often unpredictable, but an understanding of their mechanics of occurrence provides insight into this dangerous event.

Flash Flood Science

Floods generally occur as a result of the large differential between the rates of water inflow and outflow in a region. This leads to the accumulation of water in sufficient volumes to cause an elevation in the localized water level and submerge the area. The flow dynamic is primarily balanced by two forces, the source of the intruding water and its drainage means. The water source is the origination point that releases large volumes of water, usually over a short duration. This includes:

  • Clouds: Rainfall from clouds can rapidly introduce large volumes of water into an area. A lot of moisture is localized within large clouds and storm systems; moisture that when released produces heavy rains. This causes a severe downpour such as that experienced during a slow-moving thunderstorm, tropical storm, or hurricane. The high-intensity rains with long durations produced in these instances can easily cause a flood

  • Water channels: The path through which water routinely flows can become a source of floodwater under certain conditions. The first is when an obstruction occurs. Natural or man-made structures such as dams, debris, or ice can block a waterway, interrupting the water’s progression. This can lead to the water piling up behind the blockage and eventually jumping the channel’s banks or constructed levees before spilling onto the adjoining land. This is flooding upstream. Flooding downstream happens when water in a channel jumps its boundaries due to the sudden rupture of a blockage or an increase in the volume of water in the channel. A volume increase typically comes from a diversion of one channel into another, heavy rainfall, or snowmelt

  • Water bodies: Large water bodies such as seas and oceans are also a source of floodwater, especially in coastal regions. Intense offshore low-pressure systems, such as tropical storms, are accompanied by high-speed winds that push water inland, producing storm surges. This is in addition to global environmental conditions such as climate change causing glacial melts, and consequently a rise in the level of ocean water that encroaches on coastal land.

Drainage describes the characteristics of a region that promotes the removal of water from the area. This factor governs the outflow of water, and high drainage severely impedes the occurrence of a flood, and quickly dissipates it when it happens. Two major components are involved in the mechanics of drainage: infiltration and run-off. Infiltration refers to the seepage of water into the soil where it joins the groundwater at the water table, while run-off refers to the transport of water on the surface to the nearest water body or channel. The interaction of these components with the characteristics of the region affects the drainage.

  • Soil type: The soil type heavily determines how much water can be absorbed by the surrounding land before it becomes saturated. Non-cohesive soil types allow for a lot of infiltration while cohesive soil types retain water

  • Topography: This describes the gradient of the land as this dictates the direction of water flow. Moving water seeks equilibrium, and tends to congregate in locations in which it can remain stationary. Flat land and low-lying areas are highly susceptible to this, as water flowing from higher land will move here and gather there

  • Water table level: The closeness of the groundwater flowing beneath the soil to the surface also impacts infiltration. The closer the water table, the slower the infiltration

  • Ground cover: The properties of the land surface also impact drainage. Vegetation has a trapping effect, delaying water from running off. More common in urban areas are concrete coverings and roads through which water cannot infiltrate, here drainage is achieved using man-made drainage systems and evaporation to eliminate floodwater

  • Terrain: The general geographical features of the land can also impede run-off. Regions with depressed areas or other features into which water can pool will be unable to rely on run-off to get rid of floodwater

  • Man-made drainage: Storm drains are designed to facilitate water run-off and remove stormwater from urban areas. A failure of these to adequately drain an area leads to the elevation of the localized water level.

Floods typically develop when one or more of the sources above introduces large volumes of water into a region much faster than the available drainage can eliminate it. The occurrence of flash floods, on the other hand, is most influenced by the source of water, and much less by drainage. Flash floods begin just like regular floods, with a water source introducing large volumes of water into a normally dry area and submerging it. In the case of flash floods, this occurs very rapidly, within as little as a minute, but usually under six hours. The influx of water can be sudden, as is possible during a storm surge, dam break, or channel obstruction; or it can be extremely rapid, as occurs during intense rainfall. For sudden water inputs, the drainage of a region will do little to nothing to deter the occurrence, but will determine how quickly the water will dissipate.

Flash Flood Impacts

The consequences of a flash flood are multiple and varied, most of which are extremely negative to human life and property. The threat of a flash flood, as with regular floods, lies in the large-scale movement of water and the lingering water level. These conditions coupled with the suddenness of flash floods impact humanity and the environment in the following ways:

1. Floodwaters
The introduction of large volumes of water into a previously dry area is the hallmark of a flash flood. The rapid accumulation of water in the affected region leading to its submersion is called inundation. Floodwaters are capable of reaching heights of several feet, and in storm surges, can be as high as 30 feet. The presence of such high levels of water can lead to the drowning of persons and widespread water damage to structures, building interiors, and property. As flash floods are sudden and often unpredictable events, it may be very difficult to take protective action between the start of the flood and when it hits.

Flash floods that begin suddenly, such as with dam breaks and storm surges, are usually characterized by fast-moving floodwaters. This is a massive amount of water rushing at high speeds from the point of origination straight through the surrounding area. This onrush is often powerful enough to sweep clear and destroy everything in its path, including people, buildings, cars, trees, power lines, roadways, bridges, and sewerage systems.

The water introduced into an area by a flash flood may persist for several days depending on the drainage of the area. The prolonged presence of the floodwater may displace residents, make transportation facilities unusable, and hinder the response of emergency services. Even after receding, areas touched by floodwater may remain wet, and be more prone to mold growth.

2. Waterborne diseases
The movement of water in a flood transports bacteria and pathogens that spread diseases through the population affected by the flood. The high chances of damage to sewerage systems during a flood also introduces the probability of the presence of human sewage in the floodwaters. The outbreak of diseases such as typhoid, cholera, hepatitis, and diarrhea is very likely following a flood. The prevalent presence of dirty stagnant water may also encourage the growth of vectors such as mosquitos that may spread different types of diseases such as malaria and dengue fever.

3. Landslides
Floods can also initiate landslides by disturbing the stability of a slope. The heavy saturation of the ground produced by floodwaters may produce a flow of water-saturated earth downhill. This mass movement of mud and rocks is destructive enough to severely damage or destroy cars and structures in its path.

4. Deposition of materials
Several materials are transported by the movement of water during a flood. In floodplains, sediments such as clay, silt, and sand are deposited on the adjoining land by the overflowing river. The transport of mud may also occur, blanketing an area in several inches of wet soil material. The threatening aspect of this material transport is the high probability of the presence of pesticides from farmlands, industrial chemicals, heavy metals, and sewage in the floodwaters.

Florida Flash Flood Threat Profile

The state of Florida is located in the southeastern region of the United States, bordered by the Atlantic Ocean to the east, the Gulf of Mexico to the West, to the north by Georgia, and to the northwest by Alabama. The location of Florida, its geographical properties, and its infrastructure make it susceptible to flooding from different sources. First, being a coastal state, Florida is highly vulnerable to flooding events engendered by the presence of large water bodies. The situation of the state between two prominent water bodies: the Atlantic Ocean and the Gulf of Mexico, exposes it to the influence of both. Tropical storms and hurricanes originating from either water body can impact the state during hurricane season, bringing along storm surges and torrential rainfall likely to produce flash floods.

The geographical features of the state of Florida include several rivers which can produce flash floods if they become obstructed or swollen due to heavy rains. Areas adjacent to these water channels may be at risk of flash floods. The infrastructure of Florida also introduces dams and levees respectively designed to hold back water and prevent rivers from overflowing. The overflow of a dam, structural failure of a dam, or a levee failure will see the release of water highly likely to produce flash floods.

A majority of Florida’s regions are flat low-lying areas, but the flood risk for each area is affected by multiple drainage factors. The most accurate representation of the flood hazard for each region is presented in the national flood map maintained by the Federal Emergency Management Agency (FEMA). Through this tool, the flood hazard for each community can be properly gauged.

Preparing for Flash Flood in Florida

Flash floods, as the name implies, appear swiftly and most often unpredictably. These combined characteristics make it difficult to execute protective actions to preserve life and property when a flash flood hits. Within a few minutes to six hours, an area that was previously dry will become submerged. In this scenario, every second is important to prevent the loss of lives and destruction of property. While some flash flood situations are very perilous, completing some preliminary actions and planning for such an event will have a significant impact on the damage caused by a flash flood.

Preliminary steps:

  • The first action to perform when considering setting up residence in an area is to obtain information about the region’s flood risk. This information can be obtained from the national flood map produced by FEMA which collects local data to update the flood hazard of a region. While there are no regions in which floods cannot occur, there are locations that have low to moderate chances of becoming flooded

  • The next preliminary step comes during the construction of buildings. As no location is immune from flooding, decisions made while constructing the house should take a potential flood into account. While it is almost impossible to completely fortify a house from an onrushing deluge, it is possible to prevent damage in smaller floods. Building features such as sump pits equipped with battery-powered sump pumps, water-proofed basements, elevated electrical connections, and heavily secured supply systems of flammable substances should feature prominently in the building’s design

  • Obtain flood insurance to handle the economic impact of a flood. FEMA manages the National Flood Insurance Program (NFIP) to help communities required to adopt floodplain management regulations recover from the financial implications of a flood. In other communities, flood insurance may not be a default part of homeowner’s insurance policies, but getting one may be a means to reduce severe loss when a flash flood occurs

  • Locate all available shelters in the region and identify the closest and most viable choice. The operating policy of shelters differ from county-to-county, so it is advisable to find out what the local policy is. Some shelters only open their doors for certain emergencies, some do not provide resources and expect the occupants to bring their own, and some do not have accommodation for persons with special needs or pets. It is always best to anticipate flood response as evacuation of the vulnerable area, but knowing the location of a shelter can save lives in a desperate situation.

Preparing helps save time during a time of emergency and reduces the uncertainty of action during such a period. The following preparatory actions can save lives and prevent property damage during a flood:

  • Identify where to go when a flood hits. It may be necessary to move to higher ground or out of the vulnerable area when a flood is imminent. Take note of evacuation routes and possible residences to move to. Communicate this information to family members, and set up a meeting point if needed

  • Prepare an emergency kit to tend to needs during the duration of a flood. The emergency kit should contain enough food for at least three days, water to last at least three days (1 gallon of water per person per day), prescribed medication, copies of important documents (house deeds, car papers, insurance documents, personal identification, a recent utility bill) sealed in water-proof containers, cell phones with chargers, flashlights, a portable radio, spare batteries, personal hygiene supplies, pet food, pet medication, cash, and a first-aid kit. This emergency kit should be stored in the house, but should be easily accessible if stuck in the house or required to evacuate

  • When preparing for the potential emergency, include special considerations for persons with special needs. This includes. children, the disabled, elderly persons, persons who do not speak the native language, and persons with cultural or religious restrictions. Persons such as these may require more time or assistance to respond adequately to an emergency situation

  • As soon as a flash flood watch is issued, make preparatory actions to leave. Fill the gas tank of evacuation vehicles, bring outdoor fixtures such as patio furniture indoors, turn off propane tanks to reduce the chances of a fire, and remove cars from along canals and other waterways

  • If there is still sufficient time following a watch notification, move valuable items and furniture to a higher building level unlikely to be flooded, turn off utilities if required to do so by the authorities, and disconnect small appliances to prevent damage to them due to potential power surges.

Flash Flood Warnings and Alerts

Notifications about potential or imminent flash floods are issued to the general public by the National Weather Service (NWS), a dedicated weather monitoring agency. These notifications advise citizens on actions to take to protect themselves and their property. To clarify the intended message, there are five different types of warnings that can be issued for flood-related events:

  • Flood watch: A flood watch is issued when conditions favorable for flooding persist in a region. Citizens are advised to prepare as a flood may occur.

  • Flood advisory: Flood advisories are meant to notify the public that a minor flooding event is going to occur and that caution should be exercised

  • Flood warning: A flood warning announces that flooding is imminent or already occurring and that the public should take protective action

  • Flash flood watch: This warning is similar to that of a flood watch, but instead warns of favorable conditions for the development of a flash flood, such as an approaching tropical storm

  • Flash flood warning: This type of warning is specific to flash floods as it announces that a flash flood is imminent or already occurring. This warning is meant to encourage immediate action as flash floods appear very swiftly. Emergency plans should be activated immediately and flood safety practiced, as a flood watch might not even precede this notification.

Flash flood warnings are usually issued through the Wireless Emergency Alerts (WEA), as text messages appearing on cell phones and other mobile devices. The National Oceanic and Atmospheric Administration (NOAA) weather radio and other local alerting services also issue alerts updating the information about an ongoing flash flood. Several counties in Florida have personalized alert systems residents can sign up for to receive flood warnings through their mobile devices.

Flash Flood Risk Factors

The risk of occurrence of a flash flood in a region is tightly linked to the region’s location, the topographical layout, drainage system, water source, and seasonality. The location of a region presents a risk factor due to its nearness to a water source such as a river or ocean. Coastal areas and floodplains are most evident of this, being susceptible to surges of water from the adjoining water body. The other factors are heavily intertwined and can cumulatively alter the flood risk of an area.

The topographical layout describes the land gradient and influences water flow. Water will flow from a higher area to a lower one, presenting the increased safety of higher ground from floods. This geographical advantage serves as drainage for the area, but as increased flash flood risk for low-lying areas and the base of mountainous regions. The drainage of an area, either natural or man-made expresses the outflow of water in the region. A high enough drainage can prevent the formation of a flash flood respective to the water source. The water source determines the water volume released over an area to form a flash flood. This aspect of the water source is often tied to seasonality. Hurricanes are highly likely during hurricane season, heavy rains can be released by slow-moving thunderstorms during spring and summer, and snowmelt in rivers occurs during spring. A combination of these factors dictates the flash flood risk of a region.

When Caught in a Flash Flood

At the issuance of a flash flood warning or the visual evidence of an occurring flash flood, an emergency requiring precise action is apparent. An adequate response is dependent upon the situation and the information available. Evacuation orders are typically issued for imminent flash floods, advising residents to move out of the area. This most frequently occurs for slow-building flash floods. Sudden flash floods may make it impossible to evacuate and force affected residents to remain sheltered in place. Depending on the situation and available information, observe the following precautions to lessen the danger of the flood event:

When sheltering indoors:

  • Turn off the power supply and water mains if instructed to do so.

  • Move to higher floors. Do not go into the basement and avoid ground floor rooms. If the water level in the building keeps rising, move to the roof and signal for help if there is nowhere else to go. Avoid entering attics to prevent being trapped by floodwaters. If trapped, call 911.

  • Consume food and water stored in emergency kits until floodwaters subside. Avoid eating food or drinking water that has come in contact with floodwaters. Floodwaters are highly likely to contain sewage or other materials that might contaminate them. If there is no longer a supply of clean water, boil tap water before drinking it until the water supply is declared safe.

  • Avoid physical contact with floodwaters to reduce the chances of disease, infection, or injury from physical, biological, or chemical substances in the water. If there is contact with floodwater, wash the wetted area with soap and clean water. If these are not available, use alcohol-based wipes or sanitizers. Seek medical attention if necessary as soon as possible, especially if there are open wounds.

  • Do not use candles or other open flames as illumination during a power outage. These are prominent starters of house fires during historic floods.

  • Do not use electrical appliances or gas lines that have been flooded. If electrical cords or outlets have been submerged by water, the water may be electrified. Stay away from water if there are visible sparks, buzzing, crackling, or popping noises.

  • Pay attention to the local radio, TV stations, or the NOAA weather radio for continuous updates on the situation outside.

When caught outdoors or when evacuating:

  • Do not walk, swim or drive through floodwaters. The signature term “Turn Around, Don’t Drown”, applies to all three of these actions. Floodwaters may have very powerful currents; two feet of rushing water is powerful enough to sweep away a large vehicle, and just six inches of water can knock a person off their feet. The depth of the water and the condition of the roadway beneath may also be very difficult to gauge. Attempting to move across floodwaters is one of the most common causes of death during flooding events.

  • If caught outside, immediately move towards higher ground and remain there. If a vehicle stalls while moving, exit the vehicle and proceed on foot unless floodwaters are already present.

  • If trapped in a vehicle by the water, remain in the vehicle. Climb to the roof of the car and signal for help if water starts rising in the vehicle

  • Do not drive around barricades as these warn of flooded areas ahead or washed-out roads.

  • Stay off bridges during a flash flood. Bridges may be washed away without any warning.

  • Stay away from beaches and riverbanks during a flash flood warning.

  • Avoid areas that can easily become flooded such as underpasses, dips, canyons, and arroyos.

After the Flood

At the end of a flood, the floodwaters will begin to dissipate. Based on the capability of the drainage available, this can take between a few hours to several days. Within this period, and in the days following the full dissipation of floodwaters, there are several measures and precautions to take to protect oneself, property, and to recover losses.

  • Pay continuous attention to information being issued by the local authority. Do not leave the place of shelter or return home until it is declared safe to do so.

  • Stay off the roads, either as a pedestrian, swimmer, or driver. Only operate a vehicle if necessary, until the roads are cleared as safe. Beware of cautionary road signs while moving on the road.

  • Maintain distance from lingering floodwaters as these may contain harmful materials or be electrically charged by downed power lines.

  • If supplying electricity with a generator or other gasoline-powered machine, keep the machine outside and away from windows.

  • Beware of animals such as snakes and insects in the house, around it, or inside floodwaters. Wear protective clothing such as gloves and boots when cleaning up.

  • Do not use flooded gas lines or electrical installations until they have been inspected by a professional.

  • Begin cleanup operations. It is necessary to sanitize food contact surfaces and repair water-damaged sections. Items that have absorbed water and cannot be cleaned or disinfected should be thrown out. This includes mattresses, carpeting, baby toys, and stuffed animals. Food, beverages, and medicines that have been touched by water should all be thrown out.

  • Take pictures of the damaged home and its contents, then contact an insurance agent to discuss the damages to insured property.

At the end of any flooding event, always remember that while flash floods are very dangerous, their impact can be mitigated with the right planning, well-informed actions, and a good recovery procedure.