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1. Risk as a process: a history informed hazard planning approach applied to the 2018 post-fire debris flows, Montecito, California

   Serra-Llobet, Anna ; Radke, John ; Kondolf, G. Mathias ; Gurrola, Larry ; Rogers, J. David ; Douvinet, Johnny ( July 2023 , Frontiers in environmental science)

   García-Ayllón Veintimilla, Salvador (Ed.)

   Historical information about floods is not commonly used in the US to inform land use planning decisions. Rather, the current approach to managing floods is based on static maps derived from computer simulations of the area inundated by floods of specified return intervals. These maps provide some information about flood hazard, but they do not reflect the underlying processes involved in creating a flood disaster, which typically include increased exposure due to building on flood-prone land, nor do they account for the greater hazard resulting from wildfire. We developed and applied an approach to analyze how exposure has evolved in flood hazard zones in Montecito, California, an area devastated by post-fire debris flows in January 2018. By combining historical flood records of the past 200 years, human development records of the past 100 years, and geomorphological understanding of debris flow generation processes, this approach allows us to look at risk as a dynamic process influenced by physical and human factors, instead of a static map. Results show that floods after fires, in particular debris flows and debris laden floods, are very common in Montecito (15 events in the last 200 years), and that despite policies discouraging developments in hazard areas, developments in hazard zones have increased substantially since Montecito joined the National Flood Insurance Program in 1979.We also highlight the limitation of using conventional Flood Insurance Rate Maps (FIRMs) to manage land use in alluvial fan areas such as Montecito. The knowledge produced in this project can help Montecito residents better understand how they came to be vulnerable to floods and identify action they are taking now that might increase or reduce their vulnerability to the next big flood. This science-history-centric approach to understand hazard and exposure evolution using geographic information systems (GIS) and historical records, is generalizable to other communities seeking to better understand the nature of the hazard they are exposed to and some of the root causes of their vulnerabilities, in other words, both the natural and social processes producing disasters. 

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2. Flood risk behaviors of United States riverine metropolitan areas are driven by local hydrology and shaped by race

   https://doi.org/10.1073/pnas.2016839118  

   Knighton, James ; Hondula, Kelly ; Sharkus, Cielo ; Guzman, Christian ; Elliott, Rebecca ( March 2021 , Proceedings of the National Academy of Sciences)

   Flooding risk results from complex interactions between hydrological hazards (e.g., riverine inundation during periods of heavy rainfall), exposure, vulnerability (e.g., the potential for structural damage or loss of life), and resilience (how well we recover, learn from, and adapt to past floods). Building on recent coupled conceptualizations of these complex interactions, we characterize human–flood interactions (collective memory and risk-enduring attitude) at a more comprehensive scale than has been attempted to date across 50 US metropolitan statistical areas with a sociohydrologic (SH) model calibrated with accessible local data (historical records of annual peak streamflow, flood insurance loss claims, active insurance policy records, and population density). A cluster analysis on calibrated SH model parameter sets for metropolitan areas identified two dominant behaviors: 1) “risk-enduring” cities with lower flooding defenses and longer memory of past flood loss events and 2) “risk-averse” cities with higher flooding defenses and reduced memory of past flooding. These divergent behaviors correlated with differences in local stream flashiness indices (i.e., the frequency and rapidity of daily changes in streamflow), maximum dam heights, and the proportion of White to non-White residents in US metropolitan areas. Risk-averse cities tended to exist within regions characterized by flashier streamflow conditions, larger dams, and larger proportions of White residents. Our research supports the development of SH models in urban metropolitan areas and the design of risk management strategies that consider both demographically heterogeneous populations, changing flood defenses, and temporal changes in community risk perceptions and tolerance.

    

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3. A progressive flow-routing model for rapid assessment of debris-flow inundation

   https://doi.org/10.1007/s10346-022-01890-y  

   Gorr, Alexander N. ; McGuire, Luke A. ; Youberg, Ann M. ; Rengers, Francis K. ( September 2022 , Landslides)

   Abstract Debris flows pose a significant hazard to communities in mountainous areas, and there is a continued need for methods to delineate hazard zones associated with debris-flow inundation. In certain situations, such as scenarios following wildfire, where there could be an abrupt increase in the likelihood and size of debris flows that necessitates a rapid hazard assessment, the computational demands of inundation models play a role in their utility. The inability to efficiently determine the downstream effects of anticipated debris-flow events remains a critical gap in our ability to understand, mitigate, and assess debris-flow hazards. To better understand the downstream effects of debris flows, we introduce a computationally efficient, reduced-complexity inundation model, which we refer to as the Progressive Debris-Flow routing and inundation model (ProDF). We calibrate ProDF against mapped inundation from five watersheds near Montecito, CA, that produced debris flows shortly after the 2017 Thomas Fire. ProDF reproduced 70% of mapped deposits across a 40 km 2 study area. While this study focuses on a series of post-wildfire debris flows, ProDF is not limited to simulating debris-flow inundation following wildfire and could be applied to any scenario where it is possible to estimate a debris-flow volume. However, given its ability to reproduce mapped debris-flow deposits downstream of the 2017 Thomas Fire burn scar, and the modest run time associated with a simulation over this 40 km 2 study area, results suggest ProDF may be particularly promising for post-wildfire hazard assessment applications. 

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4. Risky Development: Increasing Exposure to Natural Hazards in the United States

   https://doi.org/10.1029/2020EF001795  

   Iglesias, Virginia ; Braswell, Anna E. ; Rossi, Matthew W. ; Joseph, Maxwell B. ; McShane, Caitlin ; Cattau, Megan ; Koontz, Michael J. ; McGlinchy, Joe ; Nagy, R. Chelsea ; Balch, Jennifer ; et al ( July 2021 , Earth's Future)

   Losses from natural hazards are escalating dramatically, with more properties and critical infrastructure affected each year. Although the magnitude, intensity, and/or frequency of certain hazards has increased, development contributes to this unsustainable trend, as disasters emerge when natural disturbances meet vulnerable assets and populations. To diagnose development patterns leading to increased exposure in the conterminous United States (CONUS), we identified earthquake, flood, hurricane, tornado, and wildfire hazard hotspots, and overlaid them with land use information from the Historical Settlement Data Compilation data set. Our results show that 57% of structures (homes, schools, hospitals, office buildings, etc.) are located in hazard hotspots, which represent only a third of CONUS area, and ∼1.5 million buildings lie in hotspots for two or more hazards. These critical levels of exposure are the legacy of decades of sustained growth and point to our inability, lack of knowledge, or unwillingness to limit development in hazardous zones. Development in these areas is still growing more rapidly than the baseline rates for the nation, portending larger future losses even if the effects of climate change are not considered.

    

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5. The need for paleoflood investigations on the American reach of the Red River of the North

   https://doi.org/10.1177/09596836211060542  

   St. George, Scott ; Zeleznik, Joseph ; Avila, Judith ; Schlauderaff, Matthew ( December 2021 , The Holocene)

   Over the past century, the Red River of the North has been the least stationary river in the continental United States. In Canada, historical and paleoenvironmental evidence indicates severe floods were common during the early 1800s, with the record ce 1826 flood having an estimated peak discharge 50% higher than the second-most severe flood ever observed. Unfortunately, the recorded history of flooding upstream in the United States does not begin until seven decades after this event. If 1826 was an equally exceptional flood on American reach of the river, then current flood-frequency curves for the river underestimate significantly the risks posed by future flooding. Alternatively, if the American stretch did not produce a major flood in 1826, then the recent spate of flooding that has occurred over the past two decades is exceptional within the context of the past 200 years. Communities in the Fargo-Moorhead metropolitan area are building a 58-km long, $2.75 billion (USD) diversion channel that would redirect floodwaters westward around the two cities before returning it to the main channel. Because this and other infrastructure in North Dakota and Minnesota is intended to provide protection against low-probability, high-magnitude floods, new paleoflood investigations in the region would help local, state, and federal policy-makers better understand the true flood threats posed by the Red River of the North.

    

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