Theme for 2011:
Analyzing, Forecasting, and Managing Floods

The Problem: Estimating Extreme Floods — Too Much Statistics … No Common Sense

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Abstract:

Over time the development of design floods for urban areas has shifted from a physically-based methodology (Standard Project Flood) to a statistical-based methodology (200-year flood). This presentation discusses the problems associated with estimating extreme floods from the statistical analysis of a single historic sample. It provides an example of a frequency curve extrapolation utilizing various Probability Distribution Functions (pdfs) and a graphical approach. The problems encountered when statistical extrapolations are done without regard to physical constraints are discussed. In addition, the basis for Confidence Bound calculations is discussed. The paper recommends abandoning the confidence bound calculation because it is theoretically incorrect and provides unusable information.

New Regional Skew Values for California — Implications for Flood Frequency Analysis

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Abstract:

The U.S. Geological Survey recently updated flood-frequency information at 364 gaged sites in California. The flood-frequency information at each site was determined by fitting a Pearson Type 3 flood-frequency distribution to the logarithms of annual peak-discharge data using the new Expected Moments Algorithm (EMA) procedure. A Bayesian Generalized Least Squares (GLS) regression method was used to derive a regional-skew model based on annual peak-discharge data for 158 long-term (30 or more years of record) stations throughout most of California. The desert areas in southeastern California were excluded from the regional-skew analysis, because the few sites with long-term records exhibited extreme flow variability.

The new Bayesian GLS methodology produced a nonlinear function relating regional skew to mean basin elevation. The regional skew values ranged from -0.62 for a mean basin elevation of zero feet to 0.61 for a mean basin elevation of 11,000 feet. This relation between skew and elevation indicates a change in flood hydrology with an increase in elevation, primarily as a result of the increase in the effects of snow and snowmelt runoff. The equivalent record length for the new regional skew ranges from 52 to 65 years of record depending upon mean basin elevation, whereas the old regional skew map in Bulletin 17B reported skew values from -0.2 to 0.2 and an equivalent record length of only 17 years. Differences between flood-frequency estimates based on the old regional skew and the new regional skew are greatest for sites approaching the extremes in mean basin elevation and also having short record lengths.

Approximating the Probability of the Probable Maximum Flood

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Abstract:

The Corps of Engineers is performing an on-going evaluation of its portfolio of dams with regard to risk. One of the contributing factors that must be included in these evaluations is the development of frequency curves (peak flow and volume frequency) which define the flow values for the mid-range events (1 in 500 to 1 in 3000) and then extend out to the Probable Maximum Flood (PMF) level. Currently, the PMF has been assigned an annual exceedance probability (AEP) of 1 in 10,000. This value was set based on the opinion of several senior USACE engineers. The current effort being undertaken by the Hydrologic Engineering Center is to provide general guidance for curve extension to the level of the PMF and provide a simplified method to estimate the AEP of the PMF to help guide the upper portion of the frequency curve. In order to estimate the AEP of the PMF, the Hydrologic Engineering Center has developed a regional method that uses historic regional rainfall and the Probable Maximum Precipitation for the area of interest.

This talk summarizes the extension methodology and provides the details on computing the estimated AEP of the PMF.

Communicating Risk and Uncertainty of Extreme Weather and Flood Events

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Abstract:

Weather forecasting is a common form of environmental risk communication that is familiar to much of the public. Forecasts and warnings are also used by public officials, members of the public, and others to help reduce negative impacts of hazardous weather and flood events. This presentation will discuss research to improve communication and use of weather and flood-related forecasts and warnings, with a particular emphasis on communication of risk and uncertainty. It will focus on building empirical understanding of how people conceptualize weather and flood risk, how they interpret information about that risk, and how that information interacts with their decisions.

First, results will be presented from a nationwide survey examining people's perceptions, interpretations, and uses of uncertainty information in everyday weather forecasts. Next, results will be synthesized from a variety of studies of how people interpret hydrometeorological warnings and how they use those warnings in decisions. Finally, ongoing work will be described that analyzes people's warning decisions in extreme weather events, including mental modeling to understand experts' and laypeople's flash flood risk perceptions and a survey that includes questions on how people interpret and use flash flood warnings.

Forecast-Coordinated Operations: A Multi-Agency Flood Management Program Update

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Abstract:

[Abstract not available]

Orographic Precipitation Processes in West Coast Mountains

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Abstract:

For the past 15 years, the Water Cycle Branch of the Physical Sciences Division within NOAA's Earth System Research Laboratory (ESRL) has been studying orographic precipitation processes with the goal of better understanding and better forecasting extreme precipitation and flooding events. This talk highlights three orographic processes that impact California precipitation: moisture flux, orographic precipitation rain types, and Sierra barrier jets.

The first orographic process presented is the moisture flux. The moisture flux (upslope flow × integrated water vapor) is directly correlated to the coastal rain rate and amount when atmospheric rivers impact the terrain of California. Atmospheric river observatories and the Coastal Atmospheric River Monitoring and Early Warning System (aka "the flux tool") were developed to monitor the moisture flux and to provide enhanced situational awareness and forecast guidance for extreme precipitation events along the U.S. West Coast.

The second process described is non-brightband rain. Orographic rainfall can be partitioned into three rainfall types: non-brightband (NBB), bright band (BB), and hybrid rain. Of the three, NBB rain is found to contribute significantly (28%) to the total rainfall amount, and yet this key orographic precipitation process had not been documented until 2003. The characteristics of NBB rain are presented.

The final orographic process discussed is the Sierra barrier jet which significantly impacts the precipitation distribution in the Sierra Nevada. The Sierra barrier jet occurs when an airstream approaching the Sierra Nevada slows down and is deflected leftward as a result of a weakened Coriolis force when the Froude number is less than unity. A corridor of low-level blocked flow forms upstream and below the top of that barrier and usually contains a barrier jet paralleling the long axis of the high terrain. The presence of this orographically-induced jet tends to redistribute precipitation upwind of the barrier.

Improvements and Evolving Technologies at the California-Nevada River Forecast Center

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Abstract:

The California-Nevada River Forecast Center (CNRFC) is constantly seeking to improve its ability to generate and provide useful information and forecasts to its customers and partners. This presentation will cover three areas of investment:

1. CNRFC HAS Unit Quantitative Precipitation Forecasting
2. The Community Hydrologic Prediction System
3. The Hydrologic Ensemble Forecasting System

Forecast precipitation and temperatures represent the greatest sources of uncertainty in hydrologic forecasts. CNRFC hydrologic models are fairly well calibrated and if we knew the future weather, we could issue some scary hydrologic forecasts. Recognizing this, the CNRFC has invested heavily in developing meteorological skill in its HAS (Hydrometeorological Analysis and Support) Unit. The question is, "Has that investment paid off?" An analysis performed recently by Alan Haynes, CNRFC SCH, suggests it truly has. Verification information showing the level and pace of improvements will be provided.

Over the last 4 years, the NWS has been developing and deploying a new hydrologic forecasting architecture for River Forecast Centers. This new system called the Community Hydrologic Prediction System or CHPS is now operational at 5 of 13 RFCs. The process of implementation as well as the advantages of the new system will be described.

Few things in life are certain and hydrologic forecasts most definitely contain uncertainty. The question is, just how much. As we learn to estimate the uncertainty in our hydrologic forecasts, decision makers can begin to benefit from integrating "risk" into their mitigation and management efforts. The NWS has been focused on developing the ability to generate reliable ensemble-based hydrologic forecasts for the past five years. The new system, called the Hydrologic Ensemble Forecasting System or HEFS, has been designed and several key components are in prototype use. The presentation will describe the value of these new forecasts, some of the requirements, our status, and some early results.

Comparison of California and Pacific Northwest Atmospheric Rivers and Flooding

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Abstract:

Atmospheric Rivers (ARs) are long, narrow regions in the atmosphere (>2000 km and <1000 km, respectively) that are responsible for most of the horizontal water vapor transport outside of the tropics. ARs translate with extratropical cyclones and number four to six in the midlatitudes at any given time. Pacific Atmospheric Rivers are key phenomena that are highly correlated to extreme precipitation and winter floods along the West Coast of the US. Although most major flooding along the West Coast can be associated with atmospheric rivers, not all floods are caused by ARs nor do all ARs cause flooding. To be defined as an AR, the total column precipitable water has to be greater than 2 cm and winds in the lowest 1 to 2 km have to have a component normal to the terrain of at least 25 knots (28.8 mph).

Both the Pacific Northwest and California experience multiple land-falling ARs each winter season. They can contribute substantially to the winter snowpack water content as well as to seasonal rainfall. On rare occasions however, an AR does produce extreme rainfall and runoff leading to flooding. This dual presentation, by experts from both regions, will focus on flood producing AR storms. You will begin to understand the challenges of flood forecasting shared by water management professionals along the entire west coast of North America. You will learn there are common critical issues and some important differences between these two locations with respect to flood and non-flood producing ARs. By recognizing the differences, you may become more confident in evaluating your own flood risk management strategies.