Theme for 2008:
Estimating and Forecasting Extreme Floods
University of California, Davis
Special Recognition Award
The 2008 Special Recognition Award was presented to California Data Exchange Center of the California Department of Water Resources. See the award language and the presentation on the Symposium's CDEC award page.
How to Extrapolate Frequency Curves — with No Regrets!!
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The design of flood control facilities is based on the utilization of design floods that (in most cases) are larger than any historic flood. Much effort has been expended in the estimation of 100-year, 200-year and now 500-year floods. Since many localities only have 50 to 100 years of record and that record has been subject to changing conditions (climate, reservoirs, levees, urban development, etc) it is problematic to assume that recorded data provide a clear indication of the magnitude of an extreme flood suitable for the use in design of flood protection works for an urban area. Although underestimation of the flood has obvious ramifications (high risk of failure of the structure), overestimation of the flood has significant consequences also (project feasibility, misallocation of public resources, environmental impacts, etc).
This paper provides information on how not to extrapolate statistically derived frequency curves. In particular, careful evaluation of the federally approved methods described in the Bulletin 17B guidelines is given. A new/old method that considers the physical limitations of a watershed to produce runoff was utilized. A new Monte Carlo method of establishing uncertainty around the derived curve was employed. Specifically, an example of integrating the Probable Maximum Flood (PMF) into the process of frequency curve development and extrapolation is described.
Updating Flood Frequency in California
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[Abstract not available]
Annual Exceedance Probability of Extreme Events
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The Corps of Engineers is currently evaluating its portfolio of dams with regard to risk and related maintenance. A draft Engineer Technical Letter "Risk Analysis and Assessment for Dam Safety" is under review. This ETL describes processes that will be used for screening projects for planning of corrective actions. One of the contributing factors that must be uniformly evaluated across all projects is the development of inflow frequency curves (peak flow and volume frequency) that define the frequency curve in the mid-range events (1 in 500 to 1 in 3000) and then extend out to the probable maximum flood level. Currently, no extension method is uniformly accepted. Multiple methods exist to facilitate extension. The current effort being undertaken by the Hydrologic Engineering Center is to present these variety of methods and provide a recommended method for extension that can be applied to the Portfolio Risk Assessment (PRA) effort. This presentation will provide an overview of the current status of the extension methodology.
Australian Methods for Estimating Large to Extreme Floods
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This paper broadly describes the range of flood estimation procedures used currently in Australia. It presents the context for the range of different approaches used, and discusses the overall rationale for transitioning from the use of flood frequency analyses for the estimation of frequent floods, through to the use of rainfall-based procedures for the estimation of extreme events. The importance of reconciling differences between the different approaches used is emphasised. The different frameworks used to undertake flood simulation are discussed, and the relative advantages between deterministic, joint probability, and continuous simulation approaches are briefly mentioned.
Updating California Precipitation Frequency Estimates
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The rainfall frequency atlases and technical papers published by the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) serve as de-facto national standards for rainfall intensity at specified frequencies and durations in the United States. This presentation reports on the status and schedule for updating frequency estimates for the part of California not currently included in NOAA Atlas 14 Volume 1. It includes a discussion of the user survey and decision to continue producing 1,000 year average recurrence interval estimates, the potential impact of climate change on the estimates, and a discussion of the status of Federal guidelines for probable maximum precipitation estimates for the United States.
Atmospheric Rivers and Their Role in Generating Heavy Orographic Precipitation and Flooding Along the U.S. West Coast
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The pre-cold-frontal low-level jet within oceanic extratropical cyclones represents the lower-tropospheric component of a deeper corridor of concentrated water vapor transport in the cyclone warm sector. These corridors are referred to as atmospheric rivers (ARs) because they are narrow relative to their length scale and are responsible for most of the poleward water vapor transport at midlatitudes. Using illustrative case-study examples and longer-term compositing strategies, this presentation will first briefly review the key structural and dynamical characteristics of ARs over the eastern Pacific Ocean and then comprehensively describe their hydrometeorological impacts upon landfall across westernmost North America.
Lower-tropospheric conditions during the landfall of ARs are anomalously warm and moist with weak static stability and strong onshore flow, resulting in orographically enhanced precipitation and unusually high melting levels. Hence, ARs are critical contributors to extreme precipitation and flooding events. Despite these deleterious impacts, ARs also replenish snowpacks and reservoirs across parts of the semiarid West, so they represent a key to understanding regional impacts of climate change on water resources. A winter-season analysis of quantitative precipitation forecasts during NOAA's Hydrometeorological Testbed (HMT) in California in 2006 reveals that the heavy precipitation associated with ARs is often challenging to predict, even though the heaviest areas of precipitation tend to be orographically anchored. These challenges arise due to hard-to-forecast frontal waves and differential rain shadowing between adjacent watersheds, among other factors.
Improving Observations of Coastal Storms
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Costal storms account for 71% ($7B) of U.S. disaster losses annually. In addition, over 50% of the U.S. population lives in coastal areas that account for less than 20% of the nation's land, and coastal development still is increasing. As a result, more homes, businesses and lives will be vulnerable to coastal storms and more accurate forecasts will be required to reduce risk associated with hazardous weather. Over the last decade, research conducted by the NOAA's Earth System Research Laboratory has begun to address these issues through a series of focused wintertime field experiments conducted along the U.S. West Coast.
These experiments collected new meteorological observations, both to support research on the physical processes related to coastal storms and to aid operational 0-48 h weather forecasting. In particular, a new bulk water vapor transport tool has been developed to help improve short-term (<~6 h) precipitation forecasts in mountainous terrain. The tool uses coastal Doppler wind profilers to measure the upslope component of the flow aloft and collocated GPS receivers to measure the integrated water vapor content. The product of these two variables produces a bulk water vapor flux. The flow at the surface often bears little resemblance to that at a controlling level (where the correlation between the water vapor flux and mountain precipitation is maximized) due to the ubiquitous presence of shallow terrain-blocked flows, thus highlighting the need to obtain upper-air wind measurements for this particular application. The tool is described in the context of the intense West Coast storm that inundated California with heavy precipitation in early January 2008. Numerical weather prediction results are also shown to highlight the deficiency of model and human forecasts of heavy precipitation enhanced by mountains.
Finally, improving the forecast lead time of coastal storms impacting the Western U.S. will require a winter storms reconnaissance program analogous to NOAA's hurricane reconnaissance mission. NOAA's Unmanned Aircraft System (UAS) Program will test and evaluate the use of unmanned aircraft to provide much needed offshore measurements in developing storms.
Quantitative Precipitation Forecasting and Estimating During the HMT Field Experiment: Ensemble Applications
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ESRL/GSD has been supporting the Hydrometeorlogical Testbed focused on precipitation in the American River Basin in the California Sierras during the last 3 winters. GSD's contribution has been to run an experimental, high-resolution (3-km) set of state-of-the-art, non-hydrostatic prediction models (the WRF-ARW and WRF-NMM in various configurations) to form an ensemble aimed at prediction of quantitative precipitation (QPF) and probabilistic QPF (PQPF). Products from these ensembles were made available to collaborating NWS WFOs over the last two years with variable levels of success using next generation workstation technology.
This talk will focus on two areas:
- A QPF-focused case study of the recent, powerful 4-7 January, 2008 storm that impacted central California, and summary results from the last two years of running the experimental ensemble; and
- An exploration of utilizing a running ensemble to improve the diagnosis or estimate of precipitation in real time
In the former (1) we found that the ensemble mean captured the time-evolving precipitation patterns with some onset and cessation error and more RMS error as the precipitation amounts increased. Ensemble mean ETS scores were clearly superior to any of the individual models and were significantly better than the model configuration of the current NWS NAM model, running at 3km resolution. Verification of the probabilistic products did show that the ensemble had some skill for predicting rain rates as high as 25.4mm/6hr, but did not show skill for higher rates.
For the second area (2), we will demonstrate that a 3-D variational approach where the needed error covariance can be specified fully in 2-D space, can improve the areal estimates of precipitation relative to the standard methods that interpolate rain gauges to a grid. By withholding random sets of rain gauges 183 times, we found that the 3-D var method that combines the model ensemble precipitation structure with precise measurements at gauge locations, can provide improved precipitation estimates relative to current methods.
Extreme Precipitation Analysis Tool (EPAT)
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In general, Probable Maximum Precipitation (PMP) estimates are an attempt to determine the physically possible greatest amount of precipitation for dam spillway size which is required for by the regulatory agency dam licensing. The goal of these estimates is to define the greatest volume of precipitation in the watershed upstream of the dam location that could naturally happen and hazard the capacity and spillway capability of the dam. A series of Hydrometeorological Reports (HMR) were developed by representatives of the Corps of Engineers, National Weather Service, and Bureau of Reclamation to develop a common standard of practice for the country in determining PMP. In recent decades the HMR-based PMP estimates have been supplemented and to a large degree superseded by both site-specific (SSPMP) and regional (RPMP) basin-specific studies.
The primary differences of these studies from the HMR-PMP estimates are: (1) SSPMP and RPMP are storm-based studies and incorporate recent research on storm characteristics while HMR studies provide more general and overly conservative estimates of PMP potential; and (2) SSPMP and RPMP studies consider the characteristics of specific basins while HMR studies rely on a general consideration of topographic impacts on precipitation.
Currently, SSPMP and RPMP studies have no designated standard of practice. Due to this problem, HDR with help from the Colorado Division of Water Resources developed an Extreme Precipitation Analysis Tool (EPAT).
The EPAT is an ArcGIS-based developed software that provides an automated objective application of accepted SSPMP and RPMP analysis techniques to a specific basin based on analyses of state approved extreme precipitation event climatology. This tool was created by using ArcObject programming functions.
The application works by inputting a basin into the software in shapefile format. The software then runs a number of calculations using stored historical storms over that basin based on location and elevation of the basin. In the end, the application determines the controlling storm that creates the greatest amount of volume due to the elevation change and spatial extent of the storm in the basin.
The EPAT essentially takes the guess work out of SSPMP and creates reproducible results. The output of the EPAT gives the user the watershed peak rainfall amounts, total volumes, and temporal distributions for both local storms and general storms. It also allows the user to save the controlling storms in shapefile format. The EPAT tool has become the new standard of practice for the State of Colorado for elevations above 5500 feet.