Tales Of Our Times: A 500-Year Flood Apportions Risk

Tales of Our Times 
New Mexico Citizens
for Clean Air & Water
A 500-Year Flood Apportions Risk
Good sense keeps working best when we notice it least. A case in point is the 500-year flood. The term seems to tell the last time and the next time such a bad flood will hit. In reality, the term is a beautiful tool that describes and manages flood risk.
The special term works in major ways to measure and divide the risk of flooding among government, citizens and the free market. Dividing up risk is a central reason societies organize.
Technically, a 500-year flood is the highest flood water we expect to see every 500 years on average. The 500-year flood is more accurately referred to as the 0.2 percent flood, since it is a flood that has a 0.2 percent chance of coming in any given year.  
Bad floods also come in the smaller, “100-year flood” variety. They have a 1 percent chance of striking in any single year. That is a five-times higher chance. For any water level in a given locale, engineers can map out the predicted extent of flooding. This map of the flood plain has a big part in building permits, environmental regulations and flood insurance.
The statistical risk of flooding is used to debate and decide how high, thus how safe, levees should be built. The higher the levee, the less chance it will be overtopped by flood waters. But the higher the levee, the higher the cost. The U.S. Army Corps of Engineers uses taxpayer money to design and build levees. How high they build them is set by public policy, say to handle the big 500-year flood or the smaller, but more frequent, 100-year kind.
How much money, from whose pocket, is wisely spent to guard against how small a chance of harm from what? Having zero risk in everything is a normal wish, but it is not in the cards and does not get the most good from resources. Better policies merge technical know-how and public perspectives.
The system works as follows. Meteorologists study weather patterns and records—snowpack, spring runoff, rainfall, and hurricanes. Estimates are made of historical and projected water flows in a given drainage basin. Geographers study land features in the basin. From the knowledge of land features and projected water flows in an area, engineers estimate the chance the water level will rise above a certain height. “Chance” means chance per year. Thus, engineers compute the heights of a 500- and 100-year flood in a locale.
From there, public policy takes over. How much should be spent on levees? Of course, we want safe levees. We also want safe bridges, highways, air terminals, neighborhoods, schools, food, water supplies, prescription drugs, treatment options, and children’s toys. For any total dollars spent, how do we reduce total risk the most for us and others?
We fret through such questions in the crowded mazes of democracy. A council or legislature for some jurisdiction may specify levees will be built to handle a 100-year flood. The council may reject the higher public cost for levees able to withstand the rare 500-year flood. The decision puts the populace at risk from anything worse than a 100-year flood, which has a 1 percent chance of hitting next year. Maybe a sandbag brigade can stop it. Maybe not.
The remaining risk falls to personal business. Insurance companies sell insurance for flood damage. The rates they charge depend on the established maps of flood risk and the portion of the risk that is handled by levees in the area.
We do not easily notice when science, engineering, economics and people make good sense together.