Weather Whys; Flooding

This is weather-wise. Flooding is one of the most common dangers presented by the weather. It does an average of $1 billion worth of damage to property in the United States every year and poses a serious threat to human life. In fact, some of the worst weather disasters in history have been flood-related. For instance, on May 31st, 1889, a devastating flood swept through Johnstown, Pennsylvania. It killed over 2,000 people and caused terrible damage in the Kanema Valley. But that event doesn't even compare with the 1972 flooding in Bangladesh that was caused by a tropical cyclone. 300,000 lives were lost there. Floods arise when the ground becomes too saturated to hold any more water, or when the water comes too fast for the ground to absorb it, or for urban storm drains to accept it. The excess water often runs into rivers and streams, and depending on the amount of

runoff, those waterways may overflow onto the land around them. That sort of flooding may occur well down stream from where the heavy rains took place, and the water level may peak days after the rain is gone. Flash flooding occurs when large amounts of water rush into an area very quickly. For instance, if a slow-moving thunderstorm dumps heavy rain in a mountainous region, the runoff will be channeled through ravines and other crevices, where large amounts of water can build up and rush down on the towns below. One example of that phenomenon is the flood in Big Thompson Canyon, Colorado, on July 31, 1976. In that case, a severe thunderstorm stalled over some nearby mountains and dumped as much as 12 inches of rain in a four-hour period. The ensuing flood killed over 130 people. Though ample warning can often be given for the flooding of rivers and streams, there may be little or no warning of an imminent flash flood. For weather-wise, I'm Drew Barlow.