flash floods Flash floods are a significant threat to lives and property in many parts of the world. Unlike ‘slow-rise’ floods, they result from too much water in too little time. The amount of lead time varies from place to place. In some localities 6 hours' warning can be given, but in arid, mountainous, or canyon regions only half an hour's notice is possible. Loss of life generally presents a larger problem than property losses. In 1996, for instance, more than 800 people were killed in flash flooding in South Africa, Morocco, and Afghanistan.
Flash floods occur in both rural and urban areas. Steep topography, sparse vegetation, and infrequent but intense thunderstorms typify many flash-flood hazard areas. Flash floods on alluvial fans are an increased threat as the population living in hazardous areas continues to rise. Urban environments, where vegetation has been removed, where bridges and culverts constrict flow, and where building and paving have added greatly to impermeable surfaces, also present an increasingly serious flash-flood problem. Urban drainage problems compound the effects of severe thunderstorms with catastrophic results. Many of these storms, if they occurred in uninhabited areas, would pass without any notice or impacts. Levee breaks, dam breaks, and ice jams can also result in serious flash floods.
Two flash-flood case studies
Rapid City, lying at the foot of the Black Hills in South Dakota (USA), has developed along Rapid Creek. On 9 June 1972, heavy rain began to fall just after 6 p.m. Up to 15 inches (38 cm) of rain fell in less than 6 hours. The spillway at Pactola Dam, upstream, plugged up with cars and house debris, causing the water level to rise 3.6 m (12 feet). A dam then failed at about the same time as the natural flood crest occurred, unleashing a torrent of water on the city. The death toll was 238 people.
On 31 July 1976 the Big Thompson Canyon, near Estes Park, Colorado (USA), was filled with residents and visitors. That Saturday night a flash flood ravaged the canyon, and 140 people lost their lives. Heavy rain fell over an area of 70 square miles (180 km
2) in the central portion of the Big Thompson watershed between 6.30 and 11.00 p.m. The most intense rainfall, between 12 and 14 inches (30 and 36 cm), fell on slopes in the western end of the canyon. (The flood washed out all stream and rain gauges, and accurate measurements were thus not possible.) If people had climbed to higher ground, rather than driving their cars or staying in their homes, fewer lives would have been lost.
Reducing the threat: warning systems and advances in forecasting
Structural control measures are not effective because of the local nature of flash floods and the short lead times. Because of the threat to humans, most efforts at mitigation focus instead on the development of detection and response warning systems. The burgeoning number of ALERT (automated local emergency response in real time) systems has been the most dramatic type of hazard reduction effort since 1980. Successful warning systems must include detection and response elements, but often only detection is provided. Detection alone does not save lives unless mechanisms are in place to warn the populations at risk.
ALERT systems are also being used extensively for water-quality monitoring, water-supply decision making, fine-weather forecasting, and air-quality monitoring. ALERT use for reservoir management and watering of urban golf courses saves cities millions of dollars a year and pays for the cost of the system. Internet capability enables officials and flood-plain residents to monitor gauge conditions 24 hours a day in some places, such as Maricopa County, Arizona. Knowing the conditions in real time is a step towards a warning but it is not an effective mechanism for ensuring that peo-ple are notified and take appropriate action in a timely fashion.
Flash-flood forecasting is becoming more reliable. Doppler radar and geographical information systems, often combined with ALERT data, help to pinpoint the severe storms. How-ever, because of the impacts of fires, local land-use patterns, and the limitations of the technology, some of the worst flash floods are not recognized until after the damage has occurred.
In the USA and in Europe most flash-flood deaths occur in cars. People underestimate the power of water. Flowing water less than a metre deep can carry away a car. People often feel a false sense of security in their cars. For instance, in 1996 in Pennsylvania, police closed flooded roads. Then they stood in the water and gave 50 tickets to motorists who tried to drive through the barriers on the closed roads. If flash-flood deaths are to be reduced, motorists in particular must be educated to heed warnings and interpret environmental cues (the length of a storm, the soil moisture, and the sound of the river), which often indicate when to climb to high ground. Nevertheless, some flash-flood victims die downstream of the rain, in ‘dry’ areas.
Technological advancements in forecasting make it more likely that major flash floods will be predicted in advance, but recent research on the seasonality of flash floods has made prediction more complicated, and forecasting rare events, especially when they take place outside ‘rainy’ seasons, is extremely difficult. In the 1990s, for example, summer flash floods took their toll on tourists: 79 people were swept to their deaths by a raging river in Spain in 1996, and 11 tourists died in a narrow canyon in Arizona in 1997. In this respect, hydrological science and meteorology are still far from being able to predict, before the rains come, which thunderstorms will turn out to be the worst killers.
E. Gruntfest
Bibliography
Gruntfest, E. and and Huber, C. J. (1991) Toward a comprehensive national assessment of flash flooding in the United States. Episodes, 14(1), 26–35.
