Flood Hazards

Flooding in Texas is probably the most common natural hazard in the state, if not the most intense. Flooding can be caused not only by a hurricane, but also by a tropical storm, tropical depression, or other weather systems that produce heavy rain. Flooding can build up gradually over a period of days, or occur suddenly in a few minutes (this is commonly known as a flash flood). In addition, coastal flooding and wave inundation can be produced by a hurricane or high-surf event with waves generated by local storms or by those that are hundreds of miles from the Texas coast. Flooding most often occurs near a body of water such as the Gulf of Mexico or a stream, river, or reservoir. Of Texas’ 171 million acres, an estimated 20 million are flood-prone, more than any other state.

You can determine if you are in a high-risk flood area by looking at FEMA’s flood insurance rate maps. These maps show what areas are susceptible to flooding and high velocity wave action (for those near coastal areas). Copies of the maps can be obtained digitally from the FEMA Map Service Center at https://msc.fema.gov/portal/search. Copies may also be available for viewing at your city or county building departments.

Coastal Flooding

Coastal flooding results from storm surge and wave action and is usually associated with hurricanes or tropical storms. As described earlier, the low pressure inside a storm’s eye sucks up a dome of ocean water near the center of the storm. As the storm approaches land, the storm’s strong winds push the dome of water ashore as storm surge. An intense hurricane can have a dome of water that is many miles wide and more than 25 feet high as it hits the coast. In addition, with this temporary increase in sea level rise, breaking waves and floating debris have access to areas and structures that were not designed to withstand the pounding of ocean waves. These battering waves are responsible for most beach erosion and extensive damage to coastal structures, including buildings, roads, bridges, marinas, piers, boardwalks, and sea walls.


Flash Floods

Flash floods typically develop within less than six hours after a heavy rainfall event. Other characteristics that define flash floods are a rapid rise in water, high-velocity water flow, and large amounts of associated debris. These characteristics can make a flash flood more dangerous, and even fatal, compared to other types of flooding and require a rapid response to safeguard life and property. Major factors that contribute to the severity of flash floods are the intensity and duration of rainfall and the steepness of the area’s terrain.

Impervious surfaces, which do not absorb water well, can cause excessive runoff during heavy rainfall and bring about flash flooding. Rocky terrain, certain types of soil, and land hardened by long periods of drought are examples of impervious surfaces in the natural environment and in rural communities. However, with vast road systems, parking lots, and concrete buildings, cities also contain large areas of impervious surface. Stormwater systems are built to control runoff during severe rain events but may not keep pace with the rate of urban development. If these systems become overwhelmed, they can release stormwater back into the community in the form of flash flooding.

Low water crossings exist in areas where it is inefficient to build typical bridges or culverts, such as creek beds that remain dry for extended periods of time and roads that do not get a lot of traffic. Yet during extreme rainfall, water can quickly rise above the crossing and flash floods can even wash away the crossing structure. Permanent warning signs may be placed at low water crossings and roads may be blocked off during flash flood events to protect the public.

Dams, levees, and floodwalls are structures built to contain water and to protect communities. They can pose a danger if they become overwhelmed during heavy rain events, especially tropical storms and hurricanes. If they are overtopped or undergo structural failure, flash flooding could occur with little to no warning.

Examples of Low Water Crossing Signs

Source: retrieved April 24, 2018, from www.roadtrafficsigns.com

Flood Zone Definitions

In addition to knowing about the different types of flooding, it is also important to understand your personal risk from flooding. FEMA has defined zones that are outlined on Flood Insurance Rate Maps (FIRMs). The maps show in which zone your property lies and what type of risk you face. FEMA answers homeowners’ frequently asked questions about flood hazard maps and risks on their website. The definitions and descriptions of the flood zones are:

Zone A: This zone is also known as a Special Flood Hazard Area (SFHA). It was previously known as the 100-year floodplain, but that terminology led people to believe that flooding in their area would only occur once every 100 years. This SFHA is currently referred to as an area susceptible to 1.0% annual chance of flooding, meaning that every year there is a 1.0% chance that it will flood. There are sometimes other letters or numbers that follow the “A.” The letters and numbers determine a base flood height and an elevation height for your house.

Zone B: This flood hazard area has moderate risk, or a 0.2% annual chance for flooding. It was previously called the 500-year floodplain. The base flood heights for this zone can be less than 1 foot.

Zone C: This hazard area has minimal risk to floods.

Zone D: This area has an undetermined risk, but it is still possible that flooding will occur.

Zone V: This zone is also referred to as a Coastal-V, or Coastal Velocity Flood Zone. It is susceptible to not only a 1.0% annual chance of flooding, but also wave action and storm surge from hurricanes. A number or letter after the “V” will determine the base flood height and the elevation height for homes.

Zone X: Newer FIRMs may show Zones B and C as Zone X.

Please be aware that some FIRMs are quite old, and FEMA has not updated all maps. Due to increased development, impervious surfaces, and sea level rise, the risk of flooding in your community is likely to be greater than appears on the FIRM. Inland properties may also be susceptible to flooding if there is poor localized drainage. Therefore, you do not need
to be in an official flood zone to be at risk for flooding and to be eligible for flood insurance. Another way to determine your risk is to observe and study your property. If your property floods during small rain events, then the problem will be greater during an intense storm or hurricane. You can protect yourself by improving the local drainage, making your house resistant to floods, and purchasing flood insurance.