Preliminary Long-term and Recent-term Trend Analysis Results for Floods and High Flows in Iowa

Floods have caused more damage annually in the United States than any other natural disaster. There is an increasing trend in flood damage. Most of the flood damage increase results from continuing development on flood plains. However, there is also a perception that extreme hydrologic events are increasing in frequency and/or magnitude as a result of climate change. Knowledge of trends in floods and high flows is essential for the economic planning and safe design of bridges, culverts, roadways, and hydraulic structures along rivers and streams relative to flood risk, scour, bank stability, and stationarity-based design – a major assumption of flood and high-flow probability analyses. Stationarity is the idea that natural fluvial systems fluctuate within an unchanging envelope of variability and is a fundamental concept that permeates practices in water-resource engineering. The U.S. Geological Survey is currently conducting Kendall’s Tau trend analyses for annual peak flows, highest annual 1-, 3-, 7-, 15-, and 30-consecutive day high flows, and annual high-flow durations for quantiles of 1-, 2-, 5-, and 10-percent for continuous-record stream gages in Iowa. The trend analyses are being conducted for the entire period of record through the 2013 water year and for the last 30-year period of record from 1984-2013 to evaluate both long-term and recent-term trends in floods and high flows in Iowa. Preliminary results will be compiled for approximately 60 stream gages in Iowa unaffected by regulation or diversion with continuous peak-flow and daily-flow records of at least 50 years. Results of the Iowa trend analyses may provide important information for hydraulic engineering practices regarding stationarity-based design