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WATER RESOURCES RESEARCH, VOL. 36, NO. 10, PAGES 2957-2984, OCroBER 2000 Regional interdisciplinary paleoflood approach to assess extreme flood potential Robert D. Jarrett ~ U.S. Geological Survey, Denver, Colorado Edward M. Tomlinson Applied Weather Associates, Monument, Colorado Abstract. In the past decade, there has been a growing interest oidam safety officials to incorporate a risk-based analysis for design-flood hydrology. Extreme or rare floods, with probabilities in the range of about 10-' to 10-7 chance of occurrence per year, are of continuing interest to the hydrologic and engineering communities for purposes of planning and design of structures such as dams [National Research Council, 1988]. The National Research Council stresses that as much information as possible about floods needs to be used for evaluation of the risk and consequences of any decision. A regional interdisciplinary paleoflood approach was developed to assist dam safety officials and floodplain managers in their assessments of the risk of large floods. The interdisciplinary components included documenting maximum paleofloods and a regional analyses of contemporary extreme rainfall and flood data to complement a site-specific probable maximum precipitation study [Tomlinson and Solak, 1997]. The cost-effective approach, which can be used in many other hydrometeorologic settings, was applied to Elkhead Reservoir in Elkhead Creek (531 km2) in northwestern Colorado; the regional study area was 10,900 km2. Paleoflood data using bouldery flood deposits and noninundation surfaces for 88 streams were used to document maximum flood discharges that have occurred during the Holocene. Several relative dating methods were used to determine the age of paleoflood deposits and noninundation surfaces. No evidence of substantial flooding was found in the study area. The maximum paleoflood of 135 m' S-1 for Elkhead Creek is about 13% of the site-specific probable maximum flood of 1020 m' S-I. Flood-frequency relations using the expected moments algorithm, which better incorporates paleoflood data, were developed to assess the risk of extreme floods. Envelope curves encompassing maximum rainfall (181 sites) and floods (218 sites) were developed for northwestern Colorado to help define maximum contemporary and Holocene flooding in Elkhead Creek and in a regional frequency context. Study results for Elkhead Reservoir were accepted by the Colorado State Engineer for dam safety certification. 1. Introduction Worldwide, floods are among the most destructive events related to meteorological processes. In the United States the average annuai death toll of 125 is accompanied by about $2.4 billion in damages from floods [Federal Emergency Manage- ment Agency (FEMA), 1997]. Poor understanding of floods contributes to unnecessary ioss of life and increased -flood damage in some cases and, conversely, leads to costly overde- sign of hydraulic structures located in floodplains for other situations [Jamtt, 1991, 1993; Baker, 1994J. Extreme or rare floods, with probabilities in the range of about 10-3 to 10-7 chance of occurrence per year, are of continuing interest to the hydrologic and engineering communities for purposes of plan- ning and design of structures such as dams [National Research Council (NRC), 1988]. However, estimating the magnitude and frequency of extreme floods is difficult because of relatively short streamflow-gaging station record lengths. For about the past 50 years the design criteria for construc- Copyright 2000 by the American Geophysical Union. Paper number 2000WR900098. 0043-1397/0012000WR900098S09.00 tion of structures such as dams has included an estimate of the probabie maximum flood (PMF) [Cudwol1h, 1989]. The PMF is an estimate of the maximum flood potential for a given drainage basin and is derived from an analysis of the probable maximum precipitation (PMP) [Cudwol1h, 1989]. Prior to"" about 1950, a variety of methods were used to obtain the magnitude of the design flood. The NRC [1988, 1994] recog- nized (1) the limited hydroclimatic data available to estimate PMP/PMF values for mountain basins less than about 1050 km2, (2) the subjectivity and variation ofPMP estimates among experienced meteorologists, (3) the critical need for regional analyses of extreme precipitation and flooding, (4) the need to use historic and paleoflood data, and (5) the potentiai use of probability-based methods for providing an alternative to the PMPIPMF approach. In an evaluation of extreme floods and the use of the PMF methodology to estimate design floods, the Interagency Advisory Committee on Water Data [1986] raised a major concern about PMFs being either dangerously small or wastefully large, and they emphasized the importance of accu- rately ,slimating the risk of extreme flooding. Thus the objec- tive of a flood study should be to generate as much information as practicable about the flood potentiai at a site [NRC, 1988J.