A policy statement of the American Meteorological Society as adopted by the Executive Committee on 16 April 1993
Flash floods are distinguished from other types of flooding by the short time scales of their development and intensification. They are the leading cause of deaths from natural hazards in the United States. A majority of flash flood deaths are motor vehicle related. Flash floods are most frequent during the night, and damage annually is greatest in urban areas. In a natural setting, intense rain, soils that are of low permeability or are in a highly saturated state, impervious ground surfaces, steep slopes that facilitate rapid flood-wave movement, and possibly anchor convection are all conducive to the genesis of these disastrous events. Failure of small to medium-sized dams, including debris dams, and the sudden formation and breakup of ice jams are also responsible for increased damage associated with flash floods. In spite of continuing efforts of government and the private sector to improve observations and warning procedures, flash floods remain one of the elusive natural killers, as the recent disaster in Shadyside, Ohio, has painfully demonstrated. On the evening of 14 June 1990, 26 people lost their lives as rains estimated to be in the range of 3 to 5 inches fell on saturated soil, which generated flood waves in streams that reached tens of feet in height, destroying near-bank residences and businesses. Preceding months of above-normal rainfall had generated soil moisture contents of near saturation. As a result, moderate amounts of rainfall caused large amounts of surface and near-surface runoff. Steep valleys with practically vertical walls channeled the floods, creating very fast, high, and steep wave crests. Wave travel times from the stream sources to their outlets on the Ohio River were as short as 30 minutes, and 10-square-mile drainage basins generated up to an estimated 15 000 cubic feet per second of outflow.
As recreational activities and personal preferences continue to entice people to develop in flood plains, the number of flood-prone communities continues to increase and the need for more timely and accurate warnings becomes greater. Structural measures of protection (i.e., flood control reservoirs) can provide a solution to the problem for the larger streams, but continued efforts to monitor the safety of dams are essential for disaster mitigation. For the large number of small streams, however, it is too costly to use flood control dams, so it is equally important to reduce the population living in the flood plains by providing disincentives to encroachment, which is the principle of the national flood insurance program of the Federal Emergency Management Agency (FEMA). The bottom line is that there are too many people located along small streams that can easily flood. Therefore, the flood warning systems and community self-help programs provide the only practical safeguard against such disasters. Instrumentation and data communication systems, diagnostic/predictive models and attendant calibration procedures, warning dissemination systems, and proactive preparedness programs constitute the essential components of flash flood warning systems. Present needs in research and development are identified next for various components of the warning system. Improvement can only come from coordinated efforts from the individual's level up to the government level of agencies with a flood warning mandate.
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The deployment of the new weather surveillance radars nationwide will undoubtedly improve our capability for efficiently monitoring intense localized rainfall continuously in space over mesoscale areas. This new technology combined with satellite and other technologies should provide revolutionary advances in flash flood detection. Databases of flash flood-producing rainfall will be enhanced, allowing better studies of the physical character of such rainfall data and leading to the development of improved diagnostic and forecast models. Deployment of on-site sensors will still be important, however, for the local interpretation of the radar rainfall data, especially in mountainous terrain. In such terrain, interference of the vertical radar reflectively profiles is expected to be the subject of research efforts, and rainfall–lightning relationships need further exploration. Utilization of both remotely sensed and on-site recorded data for the production of "best" estimates of rainfall down to a few square kilometers in spatial scale remains an important hydrologic research and development theme. Enhancing our understanding of the microphysical processes and dynamics of flash flood-producing rainfall is a basic research effort that would also lead to improved radar rainfall estimates (e.g., issues associated with radar underestimation of "warm process" rain).
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Recognition of the coupled meteorological/hydrological nature of flash floods is becoming more and more evident in fundamental interpretive studies and in the development of predictive models. Such hydrometeorological efforts are expected to dominate research and development of diagnostic/predictive models in this area. With respect to basic research, issues of convection and advection fundamentals on hydrologic scales, including their predictability, remain important research challenges for meteorologists. Fundamentals of small-catchment hydrology/hydraulics that include the genesis and development of shock waves in steep valleys and the effects of the initial spatial distribution of soil moisture on the development of surface runoff are important research themes. The development of predictive models for local rainfall prediction remains a challenge. The feedbacks from the near-surface soil water to the atmosphere and their influence on the development of conditions favorable to the occurrence of flash floods are now beginning to be explored. The availability of the new weather radar data is expected to enhance rainfall forecast reliability, at least for short forecast lead times. Dependence of flash flood development on the local nature and dynamics of rainfall, and the incompletely understood, wide spectrum of spatial and temporal scales that such dynamics span, necessitates using statistical–dynamical predictive rainfall models. Due to the nature of flash floods and the fact that a single storm can generate several such disasters, hydrologic/hydraulic models with spatially distributed structure and parameters are necessary to predict the development of the spatial distribution of flooding. Ways of linking such local distributed models with larger-scale hydrologic models that are used operationally by the river forecast centers of the National Weather Service require further exploration. The influence of the spatial scales of soil moisture on flash flood modeling is central to supporting basic research efforts. The data requirements of such models necessitates the utilization of geographic information systems (GIS) for the determination of catchment geometrical properties pertinent to surface runoff development. Enhanced resolution down to a horizontal length of 30 meters or better is essential for reliable estimation of flooding location and time. Continued research efforts are needed toward the development of reliable distributed models for simulating the hydrologic response of urban areas to enhance the predictive capability for flash floods.
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Dissemination of flood warnings has improved in recent years with the enhanced coverage and attention of the local commercial media. Further improvement that would enhance public response to warnings would result when individual streams are identified in the warning message. Also, enhanced NOAA Weather Radio coverage of flash flood-prone areas over the United States would undoubtedly contribute to smaller disaster impact. Proactive preparedness programs remain indispensable for loss-of-life and flood-damage reduction. Continued efforts by the NWS, FEMA, the media, and state and local emergency management agencies to educate the public regarding the occurrences and destructive force of flash floods are essential. Improved community monitoring, detection, and warning programs with emphasis on individual warning responses are a must. Real-time feedback from local designated persons or authorities to the National Weather Service Forecast Offices as to the hydrological and meteorological aspects of flood development will make for a more effective warning system.
In summary, present-day flash floods are calamities with the potential for very high death toll and huge losses of property. Integrated hydrometeorological approaches that are based on sound science and new technological advances are necessary for reliable and timely predictions. Coordinated dissemination and preparedness programs that involve individual and government initiatives are essential for effective flash-flood hazard mitigation.
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