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Theme for 2007:
Estimating Extreme Floods in California's Central Valley
April 13, 2007
California State University, Sacramento
The 2007 Special Recognition Award was presented to Maurice D. Roos, P.E.. See the award language and the presentation on the Symposium's Maurice D. Roos award page.
A Half Century of Watching Floods
Maurice (Maury) Roos, P.E.
Chief Hydrologist (part time)
Division of Flood Management
California Dept. of Water Resources
Sacramento, CA
Abstract:
The purpose of this talk is to present some personal observations on the big floods we have seen in northern California the past half century or so starting with 1950. There are at least three types of floods in California — winter season general floods, spring and early summer snowmelt floods, and strong thunderstorm floods. A personal look at the big floods of 1950, 1955, 1964, 1969, 1983, 1986, 1995, and 1997 and seeing the floods getting larger over the years. Flood forecasting has improved greatly over the past 30 years, largely from three factors: computer advances, data gathering, especially with the California Data Exchange Center (CDEC), and quantitative precipitation forecasts.
Extreme Flood Concepts, an Historical Perspective
Darryl W. Davis, P.E., D.WRE
Senior Advisor, Water Resources Engineering
Institute for Water Resources
U.S. Army Corps of Engineers
Davis, CA
Abstract:
The concept of what constitutes an ‘extreme flood’ depends on the perspective of those developing or
espousing their application. In early times, the concept was generally associated with the largest floods
that had occurred locally or at least within the nearby region. Over time, great floods impacted the
nation, policies for national investment in water management infrastructure evolved, and the desire to
wisely manage the nation’s floodplains stimulated the need for more refined and purposespecific
definitions of and methods for estimating extreme floods. Players in this arena include a better
informed public and stakeholders, policy makers, hydrologic scientists, engineers, statisticians, and the
occasional soothsayer. This presentation will offer a working definition of ‘extreme floods’, highlight
notable flood, legislation, and policy happenings that have resulted in major focus on particular concepts
of extreme events, and conclude with some reflections on current ideas and thoughts on the way
forward.
Flood Hydroclimatology: Insights into Mixed Flood Populations
Katherine (Katie) Hirschboeck, Ph.D.
Laboratory of Tree-Ring Research
University of Arizona
Tucson, AZ
Abstract:
Flood hydroclimatology (Hirschboeck 1988) is the analysis of flood events within the context of their history
of variation in magnitude, frequency, seasonality — over a relatively long period of time — and analyzed
within the spatial framework of changing combinations of meteorological causative mechanisms. It was first
proposed in the late 1980s as a conceptual framework within which to think about the underlying physical
reasons for flood variations, how these might be linked to climate variability, and why the most extreme
flood events and outliers in the upper tails of some flood distributions continue to confound practitioners of
standard flood frequency analysis (FFA). Flood hydroclimatology challenges the underlying "iid" assumption
that flood peaks are independently, identically distributed by re-examining flood time series to arrive at a
mechanistic understanding of long-term flooding variability and its probabilistic representation based on
hydroclimatically defined mixed populations. Strengths and weaknesses of the approach are illustrated with
an example from Arizona gaging stations and the potential for use of the approach to address Central Valley
FFA is addressed.
Flood hydroclimatology research to date has shown that:
- In regions where floods are produced by several types of meteorological events, different storm types may exhibit unique probability distributions.
- Unusually large floods in drainage basins of all sizes are likely to be associated with well defined circulation anomalies — hence such features are good candidates for mixed distribution categories.
- The interaction between storm properties and drainage basin properties may result in different combinations of mixed distributions.
- In the largest and most extreme floods studied, persistence was always a factor and served to bridge meteorological and climatological time scales.
Some implications of the approach are:
- The distributions of key subgroups may be better for estimating the probability and cause of extremely rare floods than the overall frequency distribution of the entire flood series.
- To preserve spatial homogeneity, basins can be grouped according to how their floods respond to different types of mechanisms and circulation patterns.
- The conceptual framework of climate-driven, time-shifting means, variances and/or mixed distributions provides a useful explanation for non-stationarity in flood times series which challenges the iid assumption.
- To address how flood frequencies might respond to a changing climate, such changes can be conceptualized as time-varying atmospheric circulation regimes that generate a mix of shifting streamflow probability distributions.
Extrapolating Frequency Curves, or Not
Beth Faber, Ph.D., P.E.
Hydrologic Engineering Center
U.S. Army Corps of Engineers
Davis, CA
Abstract:
[Abstract not available]
Pitfalls of Risk Analysis In Designing Flood Control Projects
Joseph D. Countryman P.E., D.WRE
President
MBK Engineers
Sacramento, CA
Abstract:
[Abstract not available]
Improved Tools for Estimating Extreme Floods
George H. Taylor
State Climatologist
Oregon Climate Service
Oregon State University
Corvallis, OR
Abstract:
Extreme precipitation information is of interest for a variety of purposes, including public safety, water
supply, dam design and operation, and transportation planning. Two common parameters calculated for
extreme precipitation purposes are probable maximum precipitation (PMP) and intensity-durationfrequency
(IDF). The definition of PMP is “theoretically, the greatest depth of precipitation for a given
duration that is physically possible over a given area at a particular geographical location at a certain time
of the year.” PMP estimates are used to calculate the probable maximum flood (PMF), which in turn is
used to evaluate the adequacy of hydraulic structures. IDF calculations are used in a variety of
precipitation-related tasks, including PMP.
Recent advances in geographical information systems (GIS) technology have enabled development new
opportunities for mapping extreme precipitation. Another important development has been PRISM
(Parameter-elevation Regressions on Independent Slopes Model), an expert system that uses point data
and a digital elevation model (DEM) to generate gridded estimates of climate parameters. PRISM is wellsuited
to mountainous regions, because the effects of terrain on climate play a central role in the model's
conceptual framework. It also works quite well in data-sparse regions.
In addition to improved mapping and geographical analysis techniques, improvements have been made
in statistical approaches. One exciting development has been “regional frequency analysis ” (RFA), a set
of statistical techniques based on work by Hoskins and Wallace. RFA works especially well in data sparse
regions by “trading space for time”: data from several sites are used in estimating event frequencies at
any one site using a method called “L-moments.” L-moments form the basis of an elegant mathematical
theory in their own right, and can be used to facilitate the estimation process in regional frequency
analysis. L-moment methods are demonstrably superior to those that have been used previously, and are
now being adopted by major organizations worldwide.
Comparing StatisticalApproaches to EstimatingFloods
Joe DeVries, Ph.D., P.E., P.H., D.WRE
David Ford Consulting Engineers
Sacramento, CA
Abstract:
Results from computing the flood frequency on the American River using 3-day discharge data using different methods is shown. The analyses were made only to provide comparisons between the various methods and not make any recommendations as to the most appropriate flood flow frequency method to use. Extrapolations are made for the 100-year, 200-year, and 500-year floods.
Some Ideas on Estimating the Reasonably Foreseeable Flood
Maurice (Maury) Roos, P.E.
Chief Hydrologist (part time)
Division of Flood Management
California Dept. of Water Resources
Sacramento, CA
Abstract:
We have heard a lot about statistical processes and risk. I think we need a simpler method to explain to folks what we are trying to accomplish in flood protection and then in measuring how well our designed systems are working. I would propose that we go back to an earlier concept, the standard project flood (SPF), which is modeled after recent events, which have been measured in the region. I am not sure we should call it that, because the Corps’ definition of the SPF is too close to that of the probably maximum flood (PMF). So maybe we call it the Maximum Regional Storm, MRS, or simply the Reasonably Foreseeable Flood, RFS. How would we estimate such a flood? For major rivers it would be transposing the largest measured event in the region.
Looking Toward an Urban Flood Standard
Ronald Stork
Senior Policy Advocate
Friends of the River
Sacramento, CA
Abstract:
The California legislature and the Congress are considering the adoption of new approaches to size floodwater management projects and thresholds for floodplain management regulations. Current "level of protection" assessment techniques conflate a series of facts and assessments into one number, obscuring the important individual aspects of system performance or floodplain characteristics — or methodological uncertainties. Potential flood magnitudes faced by communities may be better addressed by methodologies designed to provide realistic estimates of potential worst-case floods rather than flood magnitude "prediction" based on flood-magnitude probability distributions that rely on extrapolating existing stream-gage data. Uncertainties in either system performance, hydraulics, hydrology, topography, or channel stability also need to be addressed by extending floodplainmanagement regulations to areas behind levees or that could be flooded by reasonably foreseeable floods. Scientific and governmental attention to refining both the "level of protection" and "worst-case" flood methodologies is long overdue.
Panel Discussion: How Do We Estimate the Size of the "Reasonably Foreseeable Flood" From Which Urban Areas Should Be Protected?
Moderator:
Arthur Hinojosa, P.E.
Chief of Hydrology Branch
California Dept. of Water Resources
Sacramento
Panelists:
Maury Roos, P.E.
Chief Hydrologist (part time)
California Dept. of Water Resources
Sacramento
Ronald Stork
Former Member
California Floodplain Management Task Force
George H. Taylor
State Climatologist
Oregon State University
Corvallis
Joe Countryman, P.E., D.WRE
President
MBK Engineers
Sacramento
- American River Watershed Institute
- California State University, Sacramento — College of Natural Sciences and Mathematics
- EDAW, Inc.
- El Dorado Irrigation District
- HDR/SWRI
- Jones and Stokes
- Local Government Commission
- MBK Engineers
- Placer County Water Agency
- Regional Water Authority/Sacramento Groundwater Authority
- Sacramento Region Water Forum
- Sierra College Natural History Museum
- U.S. Bureau of Reclamation
Special recognition to Placer County Water Agency for their financial support of the 2007 California Extreme Precipitation Symposium.
Gary Estes
Phone: 530-889-9025
Email: gary32 @dg4135.us [remove space before @]
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