Small River Basin Floods

The Yellow, Yangtze, and Mississippi Rivers exemplify the large river basins that periodically endure floods. In contrast, the Arno River in Italy shows that even relatively small river basins may endure devastating floods. The Arno is a short stream, running 241 kilometers from the Apennines Mountains to the Mediterranean Sea.

It is not particularly beautiful; the color of its water is often brown, and it is little more than a mud flat in dry seasons. Yet, sitting astride the Arno in the shadow of the Apennines is the magnificent city of Florence. The Arno has flooded the city many times. Significant damage occurred in 1117, 1333, 1547, 1666, and 1844, but other floods were nearly as costly in lives and property.

On average, floods occurred once every twenty-six years. Leonardo da Vinci (1452-1519) once proposed to divert the Arno around Florence, using a large retention basin, a canal, tunnel, and floodgates, but the city fathers ignored his plans. Although Florence lived under the constant threat of floods, the flood that hit the city in 1966 caught the city unprepared. In November of that year floodwaters from heavy rains poured down the steep ravines to the west of the city.

The lower slopes of the ravines were densely populated. Two upstream dams produced hydroelectric power but had not been built for flood control (the two purposes are fundamentally in opposition: hydroelectric power requires full reservoirs to supply water to the turbines; flood control requires low water elevations to provide space for upstream floodwater). The city had made no attempt to dredge the river so that it could accommodate more water in its channel.

The flood largely resulted from extraordinary downpours. During two days at the beginning of November, the upper Arno River basin, already saturated with October rain, received 48 centimeters of rain. Yet, human error probably contributed to the tragedy. In the weeks preceding the flood, the operators of the Penna hydroelectric dam, about 48 kilometers upstream from Florence, had held water behind the dam rather than gradually releasing it over an extended period of time.

During the downpour, late on 3 November, they determined that they had to release a significant amount of water, presumably to relieve pressure on the dam. The wall of water from Penna descended on the lower dam, Levane, just 6.4 kilometers above Florence. The engineers there believed that they had no choice; they, too, released water in order to accommodate the additional flow. A wall of water descended on the city of Florence.

The flood overwhelmed Florence's inadequate defenses, rendered the city without power, and devastated the city's ancient churches and invaluable art collections. Water swirled around the basement stacks of the Biblioteca Nazionale Centrale, Italy's largest library, damaging or destroying over 1 million books and manuscripts. It licked the tombs of Michelangelo, Galileo, Rossini, and Machiavelli in the church of St. Croce. Five of the sculptor Ghiberti's ten bronze panels came loose from their portals. More than one hundred thousand persons sat marooned on their rooftops and on upper floors in the darkened city.

Only thirty-five people lost their lives in the flood, but thousands became homeless, and hundreds were injured. As the water receded, the world came to Florence's aid. Ghiberti's doors were found in the muck; Michelangelo's statues and Donatello's sculpture of Mary Magdalene were painstakingly cleaned; the Museum of Archaeology's unsurpassed Etruscan collection was repaired, although many pieces were badly damaged; and the enormous work, still not completed more than thirty-five years later, of rescuing Florence's books and manuscripts began.

The 1966 flood of Florence was, of course, both dramatic and tragic. Yet, similar floods can be found around the world. Lynmouth, England, for example, sits at the confluence of the West and Fast Lyn Rivers in north Devon.

The two streams plunge some 457 meters over a distance of only 6.4 kilometers before joining for a short stretch to the sea. In mid-August 1952 torrential rains blackened the skies and swelled both the Fast and West Lyn Rivers. After a brief lull, the rain began again. Water poured down into Lynmouth from both rivers, flooding out the city's hydroelectric plant. During the peak of the flood, about 521 metric tons of water per second formed a wall of water 9.1 meters high that all but demolished the town.

Ninety-three houses were demolished, seventeen bridges destroyed, 132 vehicles washed out to sea, and at least thirty-four people lost their lives. In some places, mud and silt deposits reached a depth of 7.6 meters.

In the United States, one of the deadliest flash floods in the country's history occurred within the confines of the 40-kilometer-long Big Thompson Canyon, just east of Estes Park, Colorado. In that canyon. Big Thompson River descends 762 meters before joining the South Platte River. Thunderstorms beginning in the late afternoon of 31 July 1976, turned the usually shallow river into a torrent. Within two to three hours, a wall of water 5.7 meters high swept through the canyon, destroying motels, cars, and just about everything else in its path.

Between 139 and 145 people died; 418 houses and 152 businesses were destroyed; and an-other 138 houses were damaged. Subsequent investigations showed that much of this tragedy was preventable. Hampered by budget restrictions, the local weather service reporting system had fallen into disarray, and the one automated rain gauge in Big Thompson Canyon malfunctioned. The result was that flash flood warnings appeared far too late; indeed, they appeared after much of the destruction had already occurred.

Moreover, people caught between the canyon walls did not know what to do. Some sought shelter in their cars—precisely the wrong response—or tried to drive out of the canyon instead of climbing the canyon walls. Valuable lessons were learned but at a terrible price.