Home Warning Plan Maintenance Flood Prediction 2002 Research Report



The 1976 Big Thompson Canyon flood disaster, which occurred just north of the Denver area, claimed 145 lives and made the dangers of flash floods very real to the residents of Colorado. This event contributed directly to the birth of the Urban Drainage and Flood Control District's Flash Flood Prediction Program in 1979, which employs the services of a private meteorological firm to provide predictions of flash flood potential directly to 35 local governments. The program focuses specifically on flash flood problems affecting the 1,600 square mile Denver/Boulder metropolitan area.

There are many complexities involved in evaluating and disseminating timely flash flood information and in making the ultimate decision to warn. Reliable quantitative precipitation forecasts are necessary to prompt early preparedness actions by local government agencies. Depending on the magnitude of the flood potential, alerting public may not be required and "cry wolf" perceptions can be avoided. Local officials involved with the District’s program are trained concerning the probabilistic nature of these early forecasts and are prepared to manage the information internally.

Problem identification and decision aid development are important first steps in preparing an effective flood warning plan. By taking measures to insure that accurate, timely and understandable communications occur both before and during a flood, the potential for loss-of-life can be substantially reduced. The District developed its first basin specific flood warning plan in 1977 and currently supports seven such plans which are updated and exercised annually.

District supported plans address the following three basic flood warning components:

early flood threat detection and evaluation
warning dissemination

The response component involves two phases: the pre-emergency or pro-active response of technical personnel and public safety officials; and the public response to the warning. The communications flow, hydro-meteorological support, data sources, standard operating procedures, and a certain amount of technical information are described in each plan to familiarize all participants with the responsibilities of their flood warning partners.

The District's program works in close cooperation with the National Weather Service (NWS) and many local government agencies. It is important to note that the NWS and local governments are ultimately responsible for warning the public and the District's role is to provide technical support to aid in that decision. The electronic news media (local television and radio) is the primary vehicle for warning the public but other methods such as NOAA Weather Radio, sirens and outdoor public address systems are also used.

Written agreements and procedures define individual responsibilities and prescribe how coordination is to occur. The NWS Weather Forecast Office at Denver works closely with the District in developing and refining their internal procedures for interacting during potential flood situations. Likewise, input from the NWS and participating local governments is considered critical for improving operational procedures used by the District and it’s contract meteorologist.

The District's program disseminates three message types:

Internal notifications of low to moderate flash flood potentials
NWS flash flood watches or their equivalent indicating high risk potentials
NWS flash flood warnings or their equivalent

Two scenarios are possible with regard to issuing flash flood watches and warnings. The first involves a NWS decision to issue a watch or warning to the public, and the second would be a decision by the District's meteorologist to take an "equivalent" action prior to an NWS issuance but direct that communication only to emergency contact points, not the general public. When the NWS initiates the process, the District meteorologist responds by immediately notifying affected emergency communications centers and passing along any additional information or recommendations pertinent to local authorities. If the latter scenario occurs, the local government(s) affected must make their own decisions concerning public warning. It should be noted that strict technical criteria has been established for when this type of "equivalent" action is warranted. In reality, it is very unlikely that a unilateral warning message would be issued by the District meteorologist. In both cases, written procedures require coordination to occur between District and NWS meteorologists before taking such actions unless the need for immediate public warning exists or communication is not possible.

The District meteorologist issues flood potential advisories known as "internal alerts" on a relatively frequent basis independent of any NWS action. The internal alert is not intended for public dissemination but is used to inform local authorities of the potential for flood problems later in the day and to keep them advised regarding the status of this threat. By maintaining frequent contact with key local officials, a high degree of confidence in the program has resulted. When communication problems occur, the opportunity exists to immediately resolve any problem in a manner which best serves the needs of the local government.

Internal communications and message fan-out procedures for a single local or regional authority can be quite complex and involve many contacts. In making initial notifications of flood potential and disseminating flood warnings, the use to technical jargon should be avoided. Key phrases like "RED FLAG" or some other appropriate "wake up" message can be very effective at prompting the desired emergency or pre-emergency responses. Emergency management professionals in the Denver area have provided valuable assistance over the years by recommending changes addressing how and when meteorologists communicate with emergency dispatchers. When the time comes, we believe that our continued emphasis on communications will improve the chances for a successful flood response.

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