Assessing the Probabilities of Extreme Flood Hazards—Workshop Proceedings Released
The results of a workshop prompted by extreme flooding during the 2011 Japanese tsunami have been published. Proceedings of the Workshop on Probabilistic Flood Hazard Assessment (PFHA) was released in September 2013 by the U.S. Nuclear Regulatory Commission (NRC).
The tsunami triggered by the Tohoku earthquake of March 11, 2011 (see "Japan Lashed by Powerful Earthquake, Devastating Tsunami," Sound Waves, March 2011), caused widespread damage and many thousands of deaths. It also flooded Japan’s Fukushima Dai-ichi nuclear power plant, knocking out backup power systems needed to cool the plant’s reactors. Several of the reactors underwent fuel melting, hydrogen explosions, and the most extensive release of radioactivity since the Chernobyl accident in 1986. Radioactive contamination from the plant forced the evacuation of communities as far as 25 miles away and affected as many as 100,000 residents. (Congressional Research Service, 2012, R41694, PDF, approx. 1 MB)
To address flooding events such as this one, with extremely low probabilities but extremely severe consequences, the NRC held a “Workshop on Probabilistic Flood Hazard Assessment (PFHA),” January 29–31, 2013, at NRC headquarters in Rockville, Maryland. Participants from Federal agencies and other organizations examined numerous flood-causing mechanisms—including extreme rainfall, flood-induced dam and levee failures, tsunami flooding, river flooding, extreme storm surge, and combined-events flooding—and shared information about the probabilistic assessment of these hazards.
Probabilistic hazard assessment entails creating and testing mathematical models of many possible sets of conditions that might cause or influence a hazard. In the case of earthquake-generated tsunamis, for example, a probabilistic assessment might model thousands of possible sets of earthquake parameters, generate thousands of possible tsunami scenarios, and report each of their probabilities. Probabilistic techniques have been used for many years in weather forecasting and now are coming into favor for assessing other hazards, such as tsunamis, dam failures, and extreme storm surge.
An earlier, complementary technique—deterministic hazard analysis—entails modeling an event with a specific set of parameters (commonly a “worst-case” event) and determining one or a few scenarios that will result. Deterministic assessments typically produce a single, vivid story that can capture public attention and assist agencies and communities in developing emergency-response plans; an example is the USGS SAFRR Tsunami Scenario described in “Experts Team Up on Tsunami Resilience in California,” this issue.
Probabilistic assessments produce numerous possible scenarios, reporting the probabilities not only of each scenario but also of each factor considered to affect the scenario—such as the tidal stage at the time a hurricane storm surge reaches land, the direction of the storm’s approach, the strength of its winds, and many additional variables. Probabilistic assessments are used by FEMA’s National Flood Insurance Program to set flood-insurance rates and by engineers to weigh the costs and benefits of designs for such structures as bridges, dams, seawalls, coastal highways, and nuclear power plants.
The workshop was organized and conducted by the NRC in cooperation with the U.S. Department of Energy (DOE), the U.S. Bureau of Reclamation, the U.S. Army Corps of Engineers, the U.S. Geological Survey (USGS), and the Federal Energy Regulatory Commission. Other contributors were the National Weather Service of the National Oceanic and Atmospheric Administration, the Federal Emergency Management Agency (FEMA), and invited industry, academic, and DOE national laboratory experts. Significant technical support was also provided by Deltares, an independent institute in the Netherlands for applied research in the field of water, subsurface, and infrastructure.
U.S. Geological Survey (USGS) researchers were involved in the workshop at all levels. Hydrologist Timothy Cohn (Reston, Virginia) and geophysicist Eric Geist (Menlo Park, California) served on the organizing committee and co-chaired panels on Tsunami Flooding (Geist) and State-of-the-Practice in Identifying and Quantifying Extreme Flood Hazards (Cohn). Hydrologist Robert Mason (Deputy Chief, USGS Office of Surface Water, Reston) was the USGS management liaison for the workshop. Geophysicist Uri ten Brink (Woods Hole, Massachusetts) and hydrologist Jim O’Connor (Oregon Water Science Center, Portland) gave presentations. Additional USGS contributors were geophysicist Daniel Brothers (Woods Hole), geologist Jason Chaytor (Woods Hole), geologist Thomas Cronin (Reston), hydrologist Julie Kiang (Reston), hydrologist Thomas Over (Urbana, Illinois), and geophysicist Tom Parsons (Menlo Park).
The full citation for the new report is:
All of the workshop presentation slides, a video of the workshop, the webcast, and the public meeting summary can be viewed at the NRC webpage.
in this issue:
Proceedings of the Workshop on Probabilistic Flood Hazard Assessment