Flash Flood Alley Segements

Flood Warning Terms / Technical Terms


Flood Warning Terms:

Flash Flood Watch - Indicates that flash flooding is a possibility in or close to the watch area. Those in the affected area are urged to be ready to take action if a flash flood warning is issued or flooding is observed. These watches are issued for flooding that is expected to occur within 6 hours after the heavy rains have ended.

Flash Flood Warning - A flood warning issued for life/property threatening flooding that will occur within 6 hours. It could be issued for rural or urban areas as well as for areas along the major rivers. Very heavy rain in a short period of time can lead to flash flooding, depending on local terrain, ground cover, degree of urbanization, amount of man-made changes to the natural river banks, and initial ground or river conditions. Dam breaks or ice jams can also create flash flooding.

Flash Flood Statement - A Flash Flood Statement is issued to inform the public about current flash flood conditions. These statements usually contain river stage information if major streams or rivers are involved.

Flood Hazard - The potential risk to life and limb and potential damage to property resulting from flooding. The degree of flood hazard varies with circumstances across the full range of floods.

Flood Potential Outlook - A late winter product used to provide information about expected hydrological conditions during the spring thaw.

River Statement - Used to provide information about significant with-in river rises or for river stages nearing the listed flood stage. Also used to provide information about minor ice jams. They frequently contain river stage forecasts.

River Summary - Contains routine hydrological information. Also used to provide a summary of ongoing flooding events.

Urban/Small Stream Flood Advisory - Alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.

Floodplain Terms:

100-year flood: Defined by many agencies as the flood having a 1% or greater annual probability of occurring.
(NOTE: We highly recommend you see our explaination of this confusing term)

500-year flood: The flood having a 0.2% or greater annual probability of occurring.

Base Flood: Defined by FEMA as the flood having a 1-percent probability of being equaled or exceeded in any given year; also referred to as the 100-year flood.

Base Flood Elevation (BFE): Defined by FEMA as the height of the base (100-year) flood in relation to a specified datum, usually the National Geodetic Vertical Datum of 1929 or North American Vertical Datum of 1988. Generally speaking, this is the elevation of the 100-year floodwaters relative to "mean sea level". BFE is not depth of flooding. To determine depth of flooding, you would need to subtract the lowest elevation of a particular property from the BFE. For example, if the property's foundation was at an elevation of 125 feet and the BFE was 131 feet, then one might infer that the 100-year depth of flooding would be approximately 6 feet.

Design Flood - A hypothetical flood representing a specific likelihood of occurrence (for example the 100 year or 1% probability flood).   The design flood may comprise two or more single source dominated floods.   Typical works are filling of land, and the construction of roads, floodways and buildings.

Flood prone land - Land susceptible to inundation by the probable maximum flood (PMF) event. The flood prone definition should not be seen as necessarily precluding development. Floodplain Management Plans should encompass all flood prone land (i.e. the entire floodplain) flood-proofing, measures taken to improve or modify the design, construction and alteration of buildings to minimize or eliminate flood damages and threats to life and limb.

Technical Terms:

Average Annual Damage (AAD) - Depending on its size (or severity), each flood will cause a different amount of flood damage. The average annual damage is the average damage in dollars per year that would occur in a designated area from flooding over a very long period of time. In many years there may be no flood damage, in some years there will be minor damage (caused by small, relatively frequent floods) and, in a few years, there will be major flood damage   (caused by large, rare flood events). Estimation of the average annual damage provides a basis for comparing the effectiveness of different floodplain management measures (i.e. the reduction in the annual average damage).

Average Recurrence Interval (ARI) - The long-term average number of years between the occurrence of a flood as big as (or larger than) the selected event.   For example, floods with a discharge as great as (or greater than) the 20yr ARI design flood will occur on average once every 20 years. ARI is another way of expressing the likelihood of occurrence of a flood event.

Cubic feet per second (ft3/s, cfs) - The rate of discharge representing a volume of 1 cubic foot passing a given point during 1 second and equivalent to 7.48 gallons per second or 448.8 gallons per minute.

Discharge - The volume of water that passes a given point within a given period of time.

Drainage area - The area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream upstream from the specified point.

Drainage basin - The part of the surface of the Earth that is occupied by a drainage system with a common outlet for its surface runoff, consisting of a surface stream or a body of impounded water with all tributary surface streams and bodies of impounded water. Feeder bands - In tropical systems, feeder bands are the spiral bands of showers and thunderstorms that follow the outer bands and precede the center of the storm. They are often the location of severe weather and can produce copious rains. Circulation around tropical systems can bring the feeder bands over the same areas repeatedly, contributing to high rainfall totals.

Flood Storages - Flood plain areas that are important for the temporary storage of floodwaters during a flood.

Floodway - A flow path (sometimes artificial) that carries significant volumes of floodwaters during a flood.

Gage height - The water-surface elevation referred to some arbitrary datum. The gage height added to the elevation of the datum of the gage represents the water-surface elevation. For example, the elevation of the datum of the gage might be 100.00 feet, which, when added to a gage height of 12.50 feet, represents a water-surface elevation of 112.50 feet.

Isohyetal - Line of equal precipitation.

Low-level jet - A relatively fast-moving (20 to 60 miles per hour) layer of air that forms 1,000 to 3,000 feet above the surface. The low-level jet is a summertime nocturnal event, forming above the nighttime inversion as the air near the surface cools bringing low-level stratus clouds (late night and morning low clouds) to south-central Texas. It is also associated with flow off the Gulf of Mexico during spring through fall, bringing moisture rapidly back into Texas and the Plains as high pressure systems move eastward behind departing cold fronts. This low-level jet provides much of the moisture needed for thunderstorm development in this area.

Peak flood level, flow or velocity - The maximum flood level, flow or velocity occurring during a flood event.

Precipitable water (PW) - The total atmospheric water vapor contained in a vertical column extending between any two specified levels, generally from the ground to the top of the upper-air sounding, expressed in terms of the height to which the water would stand if completely condensed and collected in a vessel of equal cross section as the column. Two inches is a very moist, tropical atmosphere capable of producing copious amounts of rain. In central Texas 2-inch PWs are routinely seen only in association with inland tropical activity.

Runoff - The part of the precipitation that appears in surface streams.

Runoff in inches - The depth to which the drainage area would be covered if all the runoff for a given period were uniformly distributed on it.

Stage hydrograph - a graph of water level over time.

Steering winds - The flow exerting influence over the movement of a disturbance, such as a thunderstorm. Steering winds for thunderstorms typically extend from 10,000 to 20,000 feet.

Streamflow - The discharge that occurs in a natural channel.

Tropical/barotropic - Barotropic describes the condition of the atmosphere when lines of constant temperature are parallel to lines of constant pressure through the depth of the atmosphere. True barotropic conditions are rarely achieved but come closest in tropical weather systems. Wind shear, the change of wind speed and (or) direction with height, is weak in barotropic systems making them conducive to the production of heavy rain.

Tropopause - The boundary between the troposphere and the stratosphere. "Weather," as we know it, occurs within the troposphere, that part of the atmosphere closest to the Earth's surface (6.2 to 12.4 miles deep). Temperature decreases with height within the troposphere. The tropopause is marked by an abrupt change of lapse rate (the change of temperature with height).

Upper-level diffluence - Diffluence is the rate at which adjacent flow is diverging along an axis normal to the flow at the point in question. Upper-level diffluence (between 15,000 and 30,000 feet), a spreading out of the airflow, places lower-speed winds in the region critical to thunderstorm movement. This slow movement leads to higher rainfall totals as storms remain over a location longer and contributes to storms developing and moving continually over the same areas.

Velocity - The speed at which the floodwaters are moving. Typically, modeled velocities in a river or creek are quoted as the depth and width averaged velocity, i.e. the average velocity across the whole river or creek section.

Watershed - The divide separating one drainage basin from another. However, over the years, the term has evolved to represent the drainage basin.