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Frequently Asked Questions

What is a watershed?
All of the land area that drains water into a common point, usually through a stream, river or lake. Knowing your watershed helps you better prepare for heavy rain and flooding. To learn more about other frequently used flood and water-related terms, please visit:

Which watershed do I live in?
There are five watersheds within the San Antonio River Basin which encompass Bexar County: Cibolo Creek, Leon Creek, Salado Creek, Medina River and the San Antonio River. Some of these watersheds extend beyond the jurisdictions of both Bexar County and the City of San Antonio. Click here to learn which watershed you live in.

What is a floodplain?
The area along the edges of a stream or river where floodwaters deposit sediments and that is subject to flooding. Click here to determine if you live in a floodplain.

Why are floodplains regulated?
• Protect human life, health and property
• Ensure that Federal flood insurance and disaster assistance are available
• Minimize damage to public facilities and utilities
• Reduce future flood losses

What is the 100-year floodplain?
The 100-year floodplain is based on a statistical probability needed by the insurance industry as a standard upon which to base policies. Both the federal government and the private sector assist the insurance industry in gathering scientific measurements that are then used to generate a "best guess" of stream flow peaks over a time. All this information goes into a formula/statistical model that generates elevations on tracts of land throughout a watershed that have a “1 in 100 chance (1 percent) of occurrence of flooding in any given year or a ‘return period’ of once every 100 years.”

100-year floodplains are not arbitrary but they are:

  • Limited to the “best information at the time”
  • Not a determination of where and how frequently actual flood damage will occur
  • Subject to change

For more information on 100-year floodplains, click here:

What is a digital Flood Insurance Rate Map (dFIRM)?
The Standard dFIRM Database is a digital version of the FEMA flood insurance rate map that is designed for use with digital mapping and analysis software. The Standard dFIRM Database is designed to provide the user with the ability to determine the flood zone, base flood elevation and the floodway status for a particular location. It also has National Flood Insurance Program (NFIP) community information, map panel information, cross section and hydraulic structure information, Coastal Barrier Resource System information (if applicable) and base map information like road, stream and public land survey data. More details regarding dFIRM are available at:

What is the National Flood Insurance Program (NFIP)?
In 1968, Congress created the National Flood Insurance Program (NFIP) in response to the rising cost of taxpayer funded disaster relief for flood victims and the increasing amount of damage caused by floods. Nearly 20,000 communities across the United States and its territories participate in the NFIP by adopting and enforcing floodplain management ordinances to reduce future flood damage. In exchange, the NFIP makes federally backed flood insurance available to homeowners, renters and business owners in these communities. For more information regarding NFIP, click here:

How does development impact flooding?
Human activities encroach upon floodplains, affecting the distribution and timing of drainage and potentially increasing flood problems. The built up environment creates localized flooding problems outside natural floodplains by altering or confining drainage channels. This increases flood potential in two ways: it reduces the stream's capacity to contain flows and increases flow rates downstream. As land is converted from fields or woodlands to roads, parking lots and other types of impervious cover, it loses its ability to absorb rainfall. Urbanization increases runoff two to six times over what would occur on natural terrain.

However, current standards require proactive flood mitigation in order to permanently eliminate or reduce the long-term risk to human life and property and the negative impacts on natural and cultural resources that can be caused by hazards historically associated with land development and urbanization. Because of this proactive approach, there are a growing number of instances where downstream flooding is significantly reduced following the completion of a new development.

How is water quality associated with flooding?
Water may not be safe to drink, clean with, or bathe in after an emergency such as a flood. During and after a disaster, water can become contaminated with microorganisms, such as bacteria, sewage, agricultural or industrial waste, chemicals and other substances that can cause illness or death. As part of its water quality focus in the San Antonio River Basin, BRWM has developed a basin-wide Watershed Protection Plan (WPP). In an effort to enhance the San Antonio River Improvements Project and improve and protect water quality throughout the Planning Reach, the WPP is serving as a guide to address non-point source pollution. To learn more about the WPP, please visit:

What are the different methods of flood control?
Methods of flood control management and mitigation include low water crossings, enhanced conveyance, stormwater detention facilities (e.g. construction of levees, dams, reservoirs and floodways), stormwater outfalls and property buyouts.

How are Potential Projects identified?
Potential Projects are identified using many sources of data and information, including:

  • Hydrologic and hydraulic computer models
  • Identified and potential flood damage centers
  • Knowledge and expertise of local floodplain administrators
  • Reports of flooding problem areas from emergency responders, field personnel, elected officials and community representatives
  • Flood Insurance Rate Maps

How will the proposed Potential Projects be funded?
Local sources of funding include the Bexar County Flood Tax and the City of San Antonio General Obligation Bonds and Stormwater Revenue Bonds. Other funding sources include grants from state agencies and funding from federal agencies, including the U.S. Army Corps of Engineers, Natural Resource Conservation Service and Federal Emergency Management Agency.

What are the unique characteristics of the Bexar County area that attributes to flash flooding?
According to the USGS, Texas leads the Nation in flash flood fatalities and the state holds about half of the world record rainfall rates occurring in 48 hours or less. The National Weather Service has identified South Central Texas as one of the most flash-flood prone areas in the United States.

Both the geography and geology of this South Central Texas region, which includes Bexar County, allow for the formation of severe storms that can stall and produce torrential rain. For this reason, South Central Texas is called “Flash Flood Alley.” For more information on flooding in Texas, please visit:

What is the flood history of the Central Texas area (including Bexar County)?
There is a long and tragic history to flooding in Bexar County. Below is a short list of catastrophic floods that have occurred in the Bexar County and Central Texas area over the last 30 years. For a full history of major and catastrophic storms in Texas from 1853 to 2002, please visit:

  • June 30 – July 7, 2002: As much as 35 in. of rain fell during this eight-day storm event. The heaviest depths occurred in the Texas Hill Country northwest of San Antonio, with flooding affecting about 80 counties in Texas. Record flood stages occurred at sites on the Medina River, San Antonio River, Sabinal River and Nueces River. The floods caused 12 deaths and damage to about 48,000 homes, with total damages totaling close to $1 billion. Nearly 250 flood rescue calls were reported, more than 130 roads were closed and thousands of homes and businesses lost electrical power and telephone service.
  • October 17 – 18, 1998: Up to 30 in. of rainfall occurred in a two-day period. Thirteen streamflow-gaging stations in the Guadalupe and San Antonio River Basins recorded peak discharges equal to or greater than the 100-year peak and record-breaking peak discharges were recorded at 11 of the stations. Thirty-two lives were lost and property damage was estimated to be $500 million.
  • May 5, 1993: Up to 8 in. of rainfall in Bexar County produced large peaks on Olmos Creek and Salado Creek.
  • July 16 – 17, 1987: During the evening of July 16th and early morning of July 17th, storms produced flash floods across seven counties north and northwest of San Antonio. Flooding caused tragic loss of life when a church bus filled with 39 teenagers and 4 adults was swept into a raging river. Ten persons drowned and the remaining 33 were rescued by helicopter.
  • June 4, 1986: San Antonio reported 6.5 in. during 24 hours. Other unofficial amounts of about 10 in. caused widespread flash flooding. Subsequent river flooding lasted for several days along Medina and San Antonio Rivers. Local damage was estimated at $3 million.
  • August 1 – 4, 1978: Rain initiated by the remnants of Tropical Storm Amelia fell over Central Texas with rainfall totaling more than 48 in. near Medina in Bandera County establishing a U.S. record of extreme point rainfall for a 72-hour period. Major flooding occurred on the Medina and Guadalupe Rivers. Thirty-three lives were lost and total damages reportedly exceeded $110 million.

Source: USGS