Exploration of Tolerable Risk Guidelines for the USACE Levee Safety Program

U.S. Army Corps of Engineers (USACE) | January 10th, 2011

Risk assessment methodologies have been used around the world to provide decision makers with information on the likelihood of adverse outcomes and resulting consequences

Risk assessment methodologies have been used around the world to provide decision makers with information on the likelihood of adverse outcomes and resulting consequences. Governments are recently moving to broader flood risk management approaches that encompass structural measures and manage floodplain development while recognizing climate change, environmental functions and social factors. Such flood risk approaches consider the probability of a flood hazard occurring; the vulnerability of flood mitigation measures implemented to lessen flood consequences through preparation, response, recovery and mitigation; and the consequences that result from the mitigated flood event. Within that overarching flood risk management context, the U.S. Army Corps of Engineers (USACE) is considering risk management approaches for dams and levee systems in order to make better decisions, better prioritize and justify risk reduction measures, better communicate risks to decision makers and the public, and better understand and evaluate public safety risks in an environment of shared flood risk management responsibilities. USACE consideration of risk management approaches includes the potential use of a Tolerability of Risk (TOR) framework, originally developed in the United Kingdom and adapted elsewhere, as well as Tolerable Risk Guidelines (TRG). Tolerability of Risk was developed as a framework for reaching decisions with stakeholders that focuses on the most serious risks consistently, efficiently, and transparently. Tolerable Risk Guidelines categorize the nature of risks in ways that can assist in assessing their acceptability or non-acceptability, and in prioritizing actions for reducing risks. (Further information regarding TOR and TRG is available in the summary of Dr. Jean LeGuen’s presentation, in Section 3 of this report, and in his paper, included in Appendix D.) In March 2008, three Federal agencies - US Army Corps of Engineers (USACE), Department of Interior, Bureau of Reclamation (Reclamation), and Federal Energy Regulatory Commission (FERC) convened a workshop entitled “Workshop on Tolerable Risk Evaluation – A step towards developing tolerable risk guidelines for dams and levees”. With the formation of the USACE Levee Safety Program (USACE 2007), attention began to be directed toward adapting the newly developed risk-informed dam safety policies and methods for application to levee systems. The National Committee on Levee Safety (NCLS) authorized by the Levee Safety Act of 2007 published its draft recommendations in January 2009 (NCLS 2009) including Recommendation #5 that Tolerable Risk Guidelines (TRG) be developed for application in risk-informed, flood risk management (FRM) associated with levees. This March 2010 workshop constitutes one step in USACE engaging the flood risk management community to work collaboratively in developing policies, TRG, and methods to further levee safety for the nation. The workshop purpose was to examine the concepts and principles of tolerability of risk and tolerable risk guidelines and explore their application to, and use in, managing life, economic, and environmental risk associated with levee systems. The workshop scope encompassed national and international approaches to flood risk management, tolerability of risk, and tolerable risk guidelines as they would apply to the USACE levee safety program. The workshop was comprised of three parts: introductory plenary presentations that set a common base and vocabulary for subsequent discussions; three facilitated break-out sessions that deliberated on questions prepared in advance; and concluding panels and plenary sessions that would capture the sense of the participants in what was learned and how to proceed. Invited participants numbering about sixty were from USACE, FEMA, other Federal agencies, professional societies, NGO’s, and from The Netherlands, United Kingdom, Japan, and Spain.

Exploring scientific and management implications of upper trophic level food webs in the Delta

Delta Stewardship Council (Delta Council) | April 17th, 2024

The Sacramento-San Joaquin Delta (the Delta) has complex and evolving ecosystems characterized by multiple food webs that vary in time and space. Here, food webs are defi

The Sacramento-San Joaquin Delta (the Delta) has complex and evolving ecosystems characterized by multiple food webs that vary in time and space. Here, food webs are defined as the network of species interactions (competition, predation) and interactions with their environment that are related through the flow of energy through the ecosystem (e.g., Paine 1980). Physical modifications, in addition to the introduction of non-native species and the changing climate, have significantly altered the structure and energetic pathways of the food webs for decades. The science clearly suggests that predicting the impacts of habitat restoration, fisheries, changes in environmental drivers (e.g., climate, changes in nutrient loading, invasive species) on the ecological carrying capacity and productivity of the Delta, as well as the bioaccumulation of contaminants on species or the ecosystem, requires an understanding of food web processes.

Exploring the Origins of Snow Drought in the Northern Sierra Nevada, California

American Meteorological Society (AMS) | February 22nd, 2018

The concept of snow drought is gaining widespread interest as the climate of snow-dominated mountain watersheds continues to change. Warm snow drought is defined

The concept of snow drought is gaining widespread interest as the climate of snow-dominated mountain watersheds continues to change. Warm snow drought is defined as above- or near-average accumulated pre- cipitation coinciding with below-average snow water equivalent at a point in time. Dry snow drought is defined as below-average accumulated precipitation and snow water equivalent at a point in time. This study contends that such point-in-time definitions might miss important components of how snow droughts originate, persist, and terminate. Using these simple definitions and a variety of observations at monthly, daily, and hourly time scales, the authors explore the hydrometeorological origins of potential snow droughts in the northern Sierra Nevada from water years 1951 to 2017. This study finds that snow droughts can result from extreme early season precipitation, frequent rain- on-snow events, and low precipitation years. Late-season snow droughts can follow persistent warm and dry periods with effects that depend upon elevation. Many snow droughts were characterized by lower snow fractions and midwinter peak runoff events. These findings can guide improved evaluations of historical and potential future snow droughts, particularly with regards to how impacts on water resources and mountain ecosystems may vary depending on how snow droughts originate and evolve in time.

Exploring the Potential for Water-Limited Agriculture in the San Joaquin Valley

Public Policy Institute of California (PPIC) | July 21st, 2022

The rollout of California’s Sustainable Groundwater Management Act (SGMA) is altering the state’s agricultural landscape. As groundwater sustainability measures are

The rollout of California’s Sustainable Groundwater Management Act (SGMA) is altering the state’s agricultural landscape. As groundwater sustainability measures are implemented and water scarcity increases, at least half a million acres are projected to come out of irrigated production in the San Joaquin Valley, the state’s agricultural heartland. Rather than widespread land idling—which comes with unintended consequences such as dust, weeds, pests, and soil degradation — a switch from summer irrigated crops to winter crops produced with limited water (including winter cereals and forage crops, among others) might keep some of this land in production. Although water-limited agriculture faces large hurdles, some promising aspects warrant further exploration.

Exploring the Spatially Compounding Multi‐Sectoral Drought Vulnerabilities in Colorado's West Slope River Basins

American Geophysical Union (AGU) | November 9th, 2024

Exploring Water System Vulnerabilities in California's Central Valley Under the Late Renaissance Megadrought and Climate Change

American Geophysical Union (AGU) | April 24th, 2025

Exposure history determines pteropod vulnerability to ocean acidification along the US West Coast

Nature Portfolio (Springer Nature) | July 3rd, 2017

The pteropod Limacina helicina frequently experiences seasonal exposure to corrosive conditions (Ωar  < 1) along the US West Coast and is recognized as

The pteropod Limacina helicina frequently experiences seasonal exposure to corrosive conditions (Ωar  < 1) along the US West Coast and is recognized as one of the species most susceptible to ocean acidification (OA). Yet, little is known about their capacity to acclimatize to such conditions. We collected pteropods in the California Current Ecosystem (CCE) that differed in the severity of exposure to Ωar conditions in the natural environment. Combining field observations, high-CO2 perturbation experiment results, and retrospective ocean transport simulations, we investigated biological responses based on histories of magnitude and duration of exposure to Ωar < 1. Our results suggest that both exposure magnitude and duration affect pteropod responses in the natural environment. However, observed declines in calcification performance and survival probability under high CO2 experimental conditions do not show acclimatization capacity or physiological tolerance related to history of exposure to corrosive conditions. Pteropods from the coastal CCE appear to be at or near the limit of their physiological capacity, and consequently, are already at extinction risk under projected acceleration of OA over the next 30 years. Our results demonstrate that Ωar exposure history largely determines pteropod response to experimental conditions and is essential to the interpretation of biological observations and experimental results.

Extent of California's Perennial and Non-perennial Steams

California State Water Resources Control Board (SWRCB) | October 1st, 2011

The distribution of perennial and non-perennial streams in different regions of the State has direct bearing on several of the State Water Board’s current pri

The distribution of perennial and non-perennial streams in different regions of the State has direct bearing on several of the State Water Board’s current priority initiatives (e.g., biological objectives for perennial streams and rivers, and the Wetland and Riparian Area Protection Policy). However, currently best-available stream maps are replete with inaccuracies about the extent and classi cation of perennial and non-perennial streams. For example, a recent eld study in Southern California demonstrated that the ow status of greater than 50% of streams was misclassi ed. The Surface Water Ambient Monitoring Program’s (SWAMP) Perennial Streams Assessment (PSA) gives us the ability to assess the magnitude of resource mis-classi cation in different regions of California. This Management Memo summarizes key findings on the extent of perennial and non-perennial streams and highlights the need for more refined base maps to define the extent of the water resources protected by State and Federal regulations.

Extreme and compound events in lakes

Nature Portfolio (Springer Nature) | August 19th, 2025

Key points Lake heatwaves have increased in average intensity, average duration and total duration globally, at rates of 0.15 ± 0.3 °C per decade, 2.1 

Key points Lake heatwaves have increased in average intensity, average duration and total duration globally, at rates of 0.15 ± 0.3 °C per decade, 2.1 ± 3.1 days per decade and 8.8 ± 7.6 days per decade, respectively, since the 1980s. The probability of lake heatwave events is three times more likely in a 1.5 °C warming scenario and 25 times more likely in a 3.5 °C scenario compared with pre-industrial conditions. Half of the world’s largest lakes experienced declines in water storage between 1992 and 2020, accompanied by corresponding reductions in surface extent. Extreme events in lakes can cause considerable impacts, including the onset of hypoxia, harmful algal blooms, disruption of food webs and loss of ecosystem services. Basin-scale anthropogenic stressors, such as nutrient enrichment, land-use change and water withdrawal, interact with climate extremes to increase the frequency, intensity and ecological consequences of in-lake extremes.

Extreme and persistent drought in California and Patagonia during medieval time

Nature Portfolio (Springer Nature) | June 16th, 1994

STUDIES from sites around the world1–5 have provided evidence for anomalous climate conditions persisting for several hundred years before about AD 1300. Early workers

STUDIES from sites around the world1–5 have provided evidence for anomalous climate conditions persisting for several hundred years before about AD 1300. Early workers emphasized the temperature increase that marked this period in the British Isles, coining the terms 'Mediaeval Warm Epoch' and 'Little Climatic Optimum', but many sites seem to have experienced equally important hydrological changes. Here I present a study of relict tree stumps rooted in present-day lakes, marshes and streams, which suggests that California's Sierra Nevada experienced extremely severe drought conditions for more than two centuries before ad ∼ 1112 and for more than 140 years before ad ∼ 1350. During these periods, runoff from the Sierra was significantly lower than during any of the persistent droughts that have occurred in the region over the past 140 years. I also present similar evidence from Patagonia of drought conditions coinciding with at least the first of these dry periods in California. I suggest that the droughts may have been caused by reorientation of the mid-latitude storm tracks, owing to a general contraction of the circumpolar vortices and/or a change in the position of the vortex waves. If this reorientation was caused by mediaeval warming, future natural or anthropogenically induced warming may cause a recurrence of the extreme drought conditions.