THE APPLICATION OF SOIL STRATIGRAPHY AND GEOMORPHOLOGY TO FLOOD HAZARD ASSESSMENT SAVES TAXPAYERS MORE THAN A BILLION DOLLARS

By the early 1990s the Bureau of Reclamation concluded that about two-thirds of its inventory of nearly 350 dams in the 17 western states could not accommodate the Probable Maximum Flood (PMF). The PMF is a hypothetical, deterministic, maximum flood with no associated probability. Modifying all of these dams for the PMF would potentially cost the taxpayers billions of dollars. Based on observations of flood plain soils and geomorphology, it was clear that there had not been floods equivalent to the magnitude of the PMF in the Holocene.

To test the hypothesis that the PMF had a very low probability, mapped inundation limits of the PMF were compared to terrace chronologies. The results showed that the probability of the PMF was very small relative to other natural hazards such as earthquakes, volcanic eruptions, and meteorite impacts. In fact, floods with a probability of 1 in 10,000 have peak discharges that are only 10% to 30% of the hypothetical PMF. The identification of this inequity in natural hazard assessment eventually helped lead Reclamation to the use of risk analysis in dam safety to more accurately portray the hazards and risk from floods and earthquakes.

With the aid of advances in hydraulic modeling, radiocarbon analysis, and statistical methods, it soon became clear that soils developed on stable geomorphic surfaces could establish limits on the peak discharge of floods through the Holocene. This insight led to the formulation of the concept of paleohydrologic bounds, which is a time interval during which a particular discharge has not been exceeded. The primary indicator of this bound is evidence for surface stability as recorded by soils developed on terrace alluvium. In this way, soil stratigraphy and geomorphology form the basis for understanding flood behavior throughout the West. These basic observations of landscape stability can then be quantified through the use of two-dimensional hydraulic models, radiocarbon ages, and flood frequency analysis capable of properly considering non-exceedance information.

This basic application of soils and geomorphology has revolutionized the understanding of extreme flood behavior in the western U.S. The application of this knowledge to dam safety issues at Reclamation has probably saved the taxpayers of the U.S. more than a billion dollars. Thanks Pete.