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<br />JARRETI AND TOMLINSON: REGIONAL INTERDISCIPLINARY PALEOFLOOD METHOD
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<br />components include documenting maximum paleofloods and , ing just the 162 high-risk dams in Coiorado to the PMP stan-
<br />regional analyses of contemporary extreme rainfall and flood"- dards [Hansen et aI., 1988] to be approximately $18{million.
<br />data in a basin and in a broader regional setting. Site~specific This modification cost appears low as the estimated modifica-
<br />PMP studies were conducted to better' understand extreme tion cost for proposed modifications of the Cherry Creek dam
<br />rainfall processes by anaiyzing the rainstonns with similar hy-. are as high as $250 million for Cherry Creek dam near Denver,
<br />
<br />droclimatic conditions [Tomlinson and Solak, 1997]. The ap-
<br />proach provides scientific information to help determine the
<br />delicate balance between cost of infrastructure and public
<br />safety.
<br />The approach was applied to Elkhead Reservoir on Elkhead
<br />Creek (531 Ian') near Craig in the Colorado Rocky Mountains.
<br />On~site paleoflood investigations to determine maximum pa-
<br />leoflood magnitudes and regional analyses of extreme rainfall
<br />and flood data in northwestern Colorado, primarily in the
<br />Yampa River and White River basins (10,900 Ian'), were con-
<br />ducted. Bouldery flood deposits and slack-water sediments,
<br />which are preserved for many thousands of years, were used to
<br />estimate flow depth of paleofloods. A variety of relative dating
<br />techniques (degree of soil development, surface-rock weather-
<br />ing, surface morphology, lichenometry, and bouider burial)
<br />were used to determine the paleoflood record length for pa-
<br />leoflood deposits and noninundation surfaces. Peak discharge
<br />for a paleoflood deposit was obtained primarily using the crit-
<br />ical-depth method, which had a discharge uncertainty of about
<br />25-30% for most study sites. Maximum paleofloods provide
<br />physicai evidence of an upper bound on maximum peak dis-
<br />charge for any combination of rainfall or snowmelt runoff in
<br />northwestern Colorado in at least the last 5000 to 10,000 years.
<br />Envclopc curves of maximum rainfall and flood data were
<br />devclopcd for contemporary data and for the paieoflood data.
<br />Maximum 24-hour rainfall for northwestern Colorado is about
<br />150 mm in about the past 100 years, which provides additionai
<br />support for the lack of flood and paleoflood evidence. Maxi-
<br />mum rainfall and flooding in northwestern Colorado is sub-
<br />stantially less than in eastern Colorado, which is suhject to
<br />some of the most extreme rainfall flooding in the United
<br />States. Large floods, if as hypothesized by transposition of
<br />large rainstorms into northwestern Colorado, would have left
<br />paleoflood evidence in at feast one of the streams studied.
<br />The envelope curve of paleoflood data is about 20 to 25% of
<br />the envelope curve deflned with contemporary data alone. This
<br />suggests that effects of climate change and other factors (wild-
<br />fire and vegetation changes) during the Holocene have not had
<br />a dramatic impact on maximum flooding in northwestern Col-
<br />orado. Flood-frequency analyses were made for eight gages
<br />with the expected moments algorithm, which makes better use
<br />of historical and paleoflood data. Frequency data were super-
<br />imposed on the envelope curves to heip place the contempo-
<br />rary and paleoflood data and associated envelope curves in a
<br />probability context. The maximum paieoflood of 135 m' S-1
<br />for Elkhead Creek at Elkhead Reservoir is about 13% of the
<br />site-specific PMF of 1020 m' S'I. The estimated 1O,000-year
<br />flood is about 170 m' S-1 at Elkhead Reservoir. The lack of
<br />substantial rainstorms and flood evidence in northwestern Col-
<br />orado probably is explained by high mountain barriers, which
<br />substantially reduce the available atmospheric moisture from
<br />the Pacific Ocean or Gulf of Mexico. The results of the site-
<br />specific PMP/PMF study and the regional interdisciplinary pa-
<br />leoflood sludy showed that Elkhead Dam would not be over-
<br />topped from the site-specific PMP. These results were
<br />accepted by the Coiorado State Engineer for dam safety cer-
<br />tification with no modifications to the existing structure.
<br />Changnon and McKee [1986] estimated the cost for modify-
<br />
<br />Coiorado(U.S. Anny Corps of Engineers, written communi-
<br />cation, 1997). There are over 10,000 dams in the RockY Moun-
<br />tain region that may need to be modifled for current PMP
<br />criteria during dam safety recertification. Thus, given the large
<br />differences in maximum paieoflood and PMF values in the
<br />Rocky Mountains, it seems prudent to conduct additionai hy-
<br />drometeorologic and paleoflood research to help reduce the
<br />uncertainty in estimates of maximum flood potential. This re-
<br />gionai interdisciplinary paieoflood approach, which is cost-
<br />effective, can be used in other hydrometeorologic settings to
<br />improve flood-frequency relations and provide information for
<br />a risk-based approach for hydroiogic aspects of dam safety.
<br />
<br />Acknowledgments. We gratefully acknowledge partial support and
<br />assistance from Ray Tenney and Dave Merritt, Colorado River Water
<br />Conservation District, and along with Jim Pankonin, City of Craig,
<br />Colorado, and Doug Laiho, Ayres Associates, help enhancing technol-
<br />ogy transfer. Helpful review comments from Charles Parrett, Pat
<br />Glancy, Gerhard Kuhn, Lisa Ely, and Dean Ostenaa are appreciated.
<br />Conversations with John England Jr. and Jery Stedinger were partic-
<br />ularly helpful with flood-frequency analysis; they provided exceUent
<br />technical review comments as well. Jenny Curtis and Gary D'Urso
<br />provided excellent suggestions with relative dating techniques and
<br />provided thorough reviews of the flood chronology section of the
<br />manuscript.
<br />
<br />References
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