FLOOD WARNING SYSTEMS &
EARLY NOTIFICATION PROCEDURES
IN DENVER, COLORADO

Kevin G. Stewart, P. E.

Introduction

An effective flood warning system design must include a flood threat recognition system to detect and evaluate the threat as it develops, and alert local authorities concerning potential dangers. With the prediction/forecast technical support in place, internal communications should be considered the most important component of the flood warning system and are often identified as the weakest link when systems fail. By developing plans which emphasize early notification procedures and communications, public safety officials can be prompted to take appropriate preparedness actions and involve additional support personnel with technical evaluation and warning decisions before flood damages or deaths occur. The total flood warning system must also provide for disseminating flood warnings to the public. The success of the system will ultimately be judged by the public’s response to the warning, which local officials can least control. This paper describes the flood warning system for Denver, Colorado and the surrounding region. The program’s early notification procedures, custom products and supporting technology have evolved from 17 years of providing flash flood predictions to emergency managers and public works agencies within the Urban Drainage and Flood Control District. The District (UDFCD) serves six county governments including 30 cities and towns, and operates one of the largest automated flood detection networks in the United States (Figure 1). The District also routinely conducts flash flood exercises and training programs for those involved with preparing for and responding to flood emergencies.

Background

During the late evening of July 31 and early morning hours of August 1, 1976, a disastrous flash flood cascaded out of the Big Thompson Canyon between Estes Park and Loveland, Colorado claiming 145 lives and causing extensive property damage. This historic event occurred less than 50 km north of the Denver/Boulder metropolitan area and increased public awareness of mountain flash flood dangers to an all-time high. For residents of Boulder, Colorado this near-miss was especially frightening with Boulder Creek flowing through the heart of the city and having its canyon mouth at the city’s western edge. Boulder Creek has long been considered Colorado’s most dangerous flash flood stream with the highest potential for loss-of-life.

Very soon after the Big Thompson flood, the hazards research community investigated the warning process used for Big Thompson, made projections concerning the impact of a similar flood on Boulder Creek, and recommended the development of a local flood warning system with a meteorological support component tailored for Boulder. The District's Flash Flood Prediction Program (F2P2) was created in 1979 in direct response to this research. This comprehensive program uses a private meteorological service to provide heavy precipitation forecasts and predictions of flash flood potential directly to Boulder and other local governments within the District. The program focuses specifically on flood problems and functions in cooperation with the National Weather Service (NWS), which is the federal government agency responsible for providing public warnings and forecasts of dangerous weather and floods.

The success of the District's program can be attributed to the direct involvement and commitment of many local government officials. Each year the District Board of Directors, comprised of 15 locally elected officials and two engineers, commits funds for continued meteorological support, acquisition of weather data and maintenance of critical equipment for telephone, radio and satellite communications; displaying radar and satellite data; accessing computers; and measuring rainfall, stream/reservoir levels and weather conditions in real-time. District staff maintains close coordination with the various flood response agencies and volunteer organizations. Also, the District routinely seeks input from those most closely involved with field operations in an effort to continually improve services.

Flood Warning Plan Development

A written plan should be a required component for every local flood warning program. The District developed its first drainage basin specific flood warning plan in 1977. Since then, this basin planning approach has been used to develop a total of seven plans including the one for Boulder Creek. All District-supported flood warning plans are reviewed, updated and exercised annually. Problem area identification and decision aid development are important first steps in preparing an effective flood warning plan. Special efforts are made to insure that accurate, timely and understandable communications occur both before and during a flood emergency. With this emphasis, the opportunity for a successful emergency response is greatly enhanced.

Flood warning plans address the three basic elements of early detection and evaluation, warning dissemination, and response. Experience and research have shown that reliable heavy precipitation forecasts and corresponding predictions of flood potential are necessary to prompt early preparedness actions by public safety officials. Additional technical support personnel such as hydrologists can also be called to an increased state-of-readiness based on this meteorological information. These actions should occur well before the public becomes aware of a possible flood concern. The response component can be looked at in two ways: 1) the pre-emergency or pro-active response of technical personnel and public safety officials; and 2) the public response to the warning. Although the warning plan cannot completely address or control the public response, knowledge of human behavior is considered when planning how to issue flood warnings and conduct emergency field operations. With communications generally representing the weakest link in any warning program, Figure 2 is provided on the following page to illustrate how internal communications are designed for one of the District-supported flood warning plans.

Figure 2

Flood Warning Dissemination Procedures

As stated previously, the District's program works in close cooperation with the NWS and many local government agencies. It is important to understand 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/NWS Weather Radio, fixed location public address/siren systems, cable television audio override, emergency vehicle loudspeakers, and door-to-door notifications are also used.

Written agreements and standard operating procedures define individual responsibilities and prescribe how coordination should occur. For example, the Denver NWS Forecast Office has written procedures describing how forecasters will coordinate with the District's F2P2 meteorologist. District input was obtained by the NWS in developing these procedures. Similarly, internal procedures used by the District were developed with recommendations provided by the NWS and participating local governments.

District F2P2 message dissemination may be categorized in two ways; 1) early internal notification of flash flood potential and 2) internal notification of NWS flash flood watches and warnings or their equivalent. The second category generally involves the NWS disseminating information to the public but the F2P2 meteorologist is only responsible for contacting local authorities and providing a more detailed interpretation for the jurisdiction. Therefore, dissemination of watches and warnings are still considered internal to the program. Figure 3 provides a brief description of each type of standardized message issued by the F2P2 meteorologist.

Figure 3

With regard to flash flood watches and warnings (i.e. Message 2 and Message 3), two possibilities exist. The first case involves a decision by the NWS to issue either a watch or warning. Coordination between F2P2 and NWS meteorologists would normally occur before the NWS releases this information for public broadcast. The F2P2 meteorologist responds by immediately notifying affected emergency communications centers and passing along any additional information or recommendations pertinent to local authorities (e.g. anticipated flood problems, specific geographic areas and streams affected, estimated severity and probability of occurrence, predicted precipitation amounts, available field observations, etc.). Once notified, each emergency communications center will relay the message according to their respective dissemination procedures. This action will initiate the mobilization of appropriate field resources and key personnel from various response agencies. Each agency has their own emergency plan which they follow. The F2P2 meteorologist has the opportunity to concur or disagree with the NWS decision, but must issue the appropriate message. The second case would be a decision by the F2P2 meteorologist to issue a Message 2 or 3 (flash flood watch or warning equivalent) without concurrence of the NWS. The same procedure described for the first case is followed and coordination with the NWS is required. Strict technical criteria has been established for when this type of action would be warranted. In reality, it is very unlikely that a unilateral warning message (Message 3) would be issued by the F2P2 meteorologist. Such an occurrence has never happened. However, the flexibility to do so has been requested by local emergency management officials since they are the ones ultimately responsible for warning their residents of any imminent danger, including a flash flood.

The internal alert, or Message 1 (M1), is by far the most common message disseminated by the F2P2 and might best be classified as a flood potential advisory. Rainfall amounts meeting M1 criteria would likely cause relatively minor flood problems and represent a low to moderate threat to life and property. The F2P2 meteorologist is solely responsible for the decision to issue a Message 1. The NWS is notified of this decision and may choose to issue their own public statement in response. It is important to note that a Message 1 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. Between 20 and 30 M1s are issued every year and special care is taken to identify priority messages so that unnecessary communications do not occur. This assures that relatively frequent contact with local officials is routine, thereby helping maintain a high degree of confidence in the program. When a communication problem occurs, the opportunity exists to immediately resolve the problem in a manner which best serves the needs of the local government.

Internal communications and message dissemination procedures for a single local or regional authority can be quite complex and involve many contacts as illustrated in Figure 4. Clear, concise communications is a skill that must be acquired by technical personnel responsible for disseminating flood predictions and other related information to local decision makers. The use to technical jargon should be avoided as much as possible. 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 F2P2 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.

Figure 4

Conclusions and Recommendations

For flood warning programs to be effective at protecting lives and property, a proper perspective must be maintained by those responsible for evaluating and detecting threatening conditions. Communication barriers must be eliminated by avoiding excessive use of technical terms and codes. Standardized messages are needed to insure consistent relay of information.

Judgements must be made by local officials on when and how to disseminate public warnings. Decision makers must be willing to risk false alarms by recognizing that delayed warnings may cost lives. Specific responses can be targeted to known problem areas if reliable predictions and confirming reports are conveyed to the appropriate officials in a timely manner.

Those involved with local flood warning operations must anticipate public response and understand that people tend to seek confirmation before perceiving personal danger. Therefore, actions like barricading flooded road crossings must occur before a motorist or pedestrian makes the wrong choice. Police and other public safety officials need to know the locations of hazardous stream crossings and other problem areas. Floodplain residents must also be warned and may need to be evacuated. While meteorologists and hydrologists strive for "accurate" flood predictions and increased lead times, equal or greater importance needs to be placed on recognizing when initial actions should be taken to prepare for a possible flood and where to target emergency field resources.

References

Institute of Behavioral Science, University of Colorado, 1977: Warning for Flash Floods in Boulder, Colorado. Natural Hazard Research Working Paper 31, prepared by Thomas E. Downing, Boulder, Colorado, USA, July.

Institute of Behavioral Science, University of Colorado, 1977: What People Did During the Big Thompson Flood. Natural Hazard Research Working Paper 32, prepared by Eve C. Gruntfest, Boulder, Colorado, USA, August.

Stewart, Tucker, 1993: Flash Flood Prediction and Early Warning. Paper presented at the Republic of China Workshop on Natural Disaster Reduction, Taipei, Taiwan, June.

Urban Drainage and Flood Control District, 1977: Flash Flood Warning Recommendations for Front Range Communities. prepared by Thomas E. Downing, Institute of Behavioral Science, University of Colorado, Boulder, Colorado, USA, July.

Urban Drainage and Flood Control District, 1977: Early Flood Warning Planning for Boulder Creek. prepared by Leonard Rice Consulting Water Engineers, Inc., Denver, Colorado, USA, July.

Urban Drainage and Flood Control District, 1991: Flash Flood Prediction Program & Related Activities. Flood Hazard News, Vol. 21, No. 1, Denver, Colorado, USA, December.

Urban Drainage and Flood Control District, 1992: Flash Flood Prediction Program & Related Activities. Flood Hazard News, Vol. 22, No. 1, Denver, Colorado, USA, December.

Urban Drainage and Flood Control District, 1978 (Rev. 1994): Boulder Creek Flood Warning Plan. Denver, Colorado, USA, May.

Urban Drainage and Flood Control District, 1989 (Rev. 1995): Ralston Creek Flood Warning Plan. Denver, Colorado, USA, August.

The preceding paper was presented at "Current Issues in Total Flood Warning System Design," an international invitational workshop held at the Flood Hazard Research Centre, Middlesex University, London, England, UK, September 10-12, 1995.