Paleoflood Hydrology and Flood Hydrology

To evaluate the safety of critical structures, decision makers need information on low probability floods. There are numerous models available to extrapolate historical observations to return periods that are many times the record length. However, all of these methods suffer the same critical shortcomings. They all rely on small data sets, short records, and incomplete knowledge of how flood runoff is generated. What if the direct record of past floods over thousands of years could be developed? A record that captures the exact information flood hydrologists need. Since the 1970’s, paleoflood hydrology has provided a means to directly evaluate the magnitude of large floods. However, the use of this direct information is still met with skepticism and many practicing flood hydrology are either unaware of it or choose to ignore it.
For more than 15 years, the Bureau of Reclamation has been incorporating paleoflood information into hydrologic hazard assessment and dam safety risk analysis across the 17 western states. This information has revolutionized the way the hydrologic hazards are assessed because it provides actual information on the magnitude of floods that are of the greatest concern for dam safety. The information that is employed includes the reconstruction of actual past floods and the development of physical limits on the magnitude of floods over a given time period. Both provide information about actual floods over thousands of years. The type of paleoflood information collected can be tailored to the concern of interest. Information can be collected for a particular stream, a series of tributaries in a drainage basin, or for a region. Since it incorporates numerous changes in climate, response to forest fires, and other natural phenomenon that have impacted the drainage basins over many thousands of years, it provides a useful tool for understanding the potential impact of climate change on flood magnitude and frequency throughout the West.
Paleoflood hydrology provides one of the most powerful pieces of information for flood hydrology; the actual record of the magnitude of peak discharges over thousands of years. This information may extend the record of floods two orders of magnitude beyond the longest stream gage or rainfall records. Furthermore, this flood record can be directly incorporated into hydrologic hazard assessment and can be used to evaluate the results of rainfall-runoff models and other extrapolations.
The actual history of floods is available across the western U.S. The time has come to collect this information and regularly incorporate it into flood hydrology. 8 pp. 33 references.