U.S. Flood Damage: future expectations based upon historical trends

Floods cause billions of dollars worth of annual losses to physical property, crops, and public infrastructure. Annual flood losses, which have increased from 1929 to 2004, average approximately 7 billion U.S. dollars. A common assumption is that increasing flood damage over time is caused by increasing intensity and frequency of floods. Contrary to this assumption, research indicates that increasing flood damage is the result of a complex interplay between weather and the growing vulnerability of society to flood damage. Growing vulnerability, and thus growing damage, results from increasing population and wealth in locations exposed to flood impacts. To the extent that policy makers have misperceptions about the causes of growth in flood damage, decisions might be implemented based on incorrect information, leading to inefficient or ineffective flood policies.

To better understand the increase in flood damage, this research developed and refined a technique to normalize U.S. National Weather Service unadjusted yearly flood damage estimates from 1929 to 2004. “Normalization” refers to an approximation of how much damage a flood event from the past would cause under today's societal conditions. Adjusted data reflects not only inflation, but also population change, and change in wealth to arrive at an estimate of what damage floods of the past might cause under today's levels of development and floodplain occupancy. I observed that increased population and wealth convincingly influence the increase of flood damage from 1929 to 2003. Based on historical trends of U.S. flood damage, this research indicates the costs that we can expect from future flood damages as compared to projected changes in global precipitation from climate models used in the IPCC model projections. As recent upward trends of flood damage are to some degree, largely the result of human choice, this research begs policy questions regarding future treatment of societal vulnerability to flood damage.