Understanding Floods

Floods are not as costly nor are they as destructive, on average, as earthquakes.

At the same time, they are the deadliest of all environmental hazards. In the course of the twentieth century, worldwide, close to seven million died as a result of floods, one million were injured and over one hundred million were rendered homeless. The majority of the deaths occurred in China, almost all of them from the Yellow River, or Huanghe as it is known today, often and appropriately named the “River of Sorrow.” In its long journey from the high ground of Northwest China the Yellow River flows through territory bordered by hundreds of miles of deep deposits of loess, a very light soil that is easily eroded and added to the river. By the time the Yellow River reaches the lowland areas near its mouth and slows down, its water level is high because of the load of yellow loess deposits. All it takes at that stage to create a crisis is an unexpected heavy rainfall. Such an event can raise the river level above the levees, making it overtop the banks and flood the farmland below.

Again and again, in the more than 2,000 years since levees were first built on the banks of the Yellow River, overtopping has happened and hundreds of thousands of the farmers who lived and worked on the fertile land beside the river were drowned. In response to each of these tragedies, the levees were built up a bit higher than they had been in order to forestall another flood. As a result, over time, the river flows along high above the villages below. When rainfall triggers another overtopping the destruction is greater than that of previous similar occasions because of acceleration in the flow of water from the higher elevation of the river. Additionally, because it is a slow-moving river as it reaches its mouth, alternative channels are carved out in the delta area. On one occasion, late in the nineteenth century, the river carved out a channel farther south than it had ever done before, linking it with the other great river of China, the Yangtze, so that both rivers flowed together into the East China Sea.

The Mississippi River drainage basin is the largest in the world and certainly the largest in North America. Flood risks are annual threats and, historically, flood events were left to states to resolve. There was an assumption that nothing could be done on a collective, national, basis to anticipate and prevent flood damage. All of that kind of thinking changed after the massive flood of 1927 when 246 lost their lives, about 137,000 buildings were flooded, and 700,000 lost their homes. Pressure on the national government led to the Flood Control Act in the year following the big flood. Levees, two thousand miles of them, together with a number of floodwalls and floodways, were installed by joint state and federal authorities. In 1993 there was a much worse flood than the one in 1927. The 1993 flood affected more than half of the total area of the river basin. A rare combination of highs and lows raised rainfall levels from the Dakotas to the upper areas of Mississippi. The loss of life from this event was considerably lower than in 1927, thanks to the precautions taken after 1927.Nevertheless, about 74,000 people were left homeless and 1,000 levees collapsed.

The tragedy of Hurricane Katrina and the flooding that ensued may have led to new concerns about the relationships between hurricanes and floods. Whether it was because of Katrina or otherwise, the National Oceanic and Atmospheric Administration (NOAA) decided in 2005 to document the details of hurricanes that made landfall in the contiguous United States and, in the course of their first two or three days, caused nine or more inches of rainfall in their landfall areas. There is another aspect of Katrina that should be noted because they have national implications—the insurance claims. Residents in New Orleans who had taken out insurance policies against damage from hurricanes were refused benefits from their insurance companies because, in the opinion of the insurance companies, the damage elements that residents suffered and that had been caused by flooding was not covered. Far away from the United States, on the other side of the earth, in Australia, there are new concerns and many public protests over the failure of the national government to protect people from floods. On this normally very dry continent major floods in 1973 and 1990 did extensive destruction and state and federal authorities were totally unprepared to deal with them.