Urban growth and development opportunities are needed worldwide, but growth and development must be hazard-aware and should follow smart-growth principles. The $2.89 billion Fargo-Moorhead Area Diversion Project was designed to provide flood protection following damaging floods on the Red River of the North. But the project has quadrupled in cost and shifted from protecting existing infrastructure to promoting sprawl across similar to 200 km(2) of previously connected floodplain. We modeled three scenarios-offering divergent visions of growth-and found that expansion onto the floodplain is not required for continued development. All scenarios allowed for a doubling of the region's population, and both no-Diversion scenarios better served low-income, high-vulnerability portions of the Fargo-Moorhead area. Our scenarios do not reflect all available planning and policy tools, but they test key assertions in project justification and implementation. The Diversion project contrasts sharply with flood protection built upstream in Grand Forks, ND after 1997 flooding, which utilized wholesale buyouts, levee setbacks, and other "Room for the River" principles. Changes in the location, scope, and design of the Fargo-Moorhead project seem to reflect local political and economic self-interest, at the expense of bedrock principles of floodplain management and planning. The Diversion project did not begin this way, and we explore the potential role of "path dependence" in this and other suboptimal outcomes. Another pressing issue raised by this project is cost equity, where financial burdens are borne by all taxpayers, whereas benefits accrue to developers and jurisdictions that enabled that development. The Diversion project illustrates the thorny tradeoffs between opening development of floodplain land and, in the process, violating this "Prime Directive" of floodplain management.

The Sacramento River in California, USA, and the Rhine River in Europe both have histories of major flooding events and great efforts to manage flood risk. We compare these two watersheds with an interdisciplinary lens to explore the goals, approaches, outcomes, and “parallel evolution” of differing flood risk management paradigms.The two basins share hydrologic similarities, but each approach to managing floods reflects the basin’s unique historical, environmental, and governance context. The Sacramento basin is entirely within the state of California, whereas the Rhine is a transnational river that drains nine European countries. The Rhine basin is larger and has a much larger population compared with the Sacramento basin. The Sacramento basin has high interannual precipitation variability and receives most of its precipitation in the winter with significant mountain snowfall. The hydrology of the Rhine is also strongly influenced by mountain snowpack, but has precipitation that is more evenly distributed throughout the year. Flood-risk management on both the Sacramento and Rhine Rivers has evolved from ad hoc and local approaches, towards more systematic planning, culminating in significant state-level control in California, and state, federal, and transnational management on the Rhine. This transition was driven in recent years by the Central Valley Flood Protection Act and the European Floods Directive.Management of each basin has been shaped by an event-based evolution, in which disasters have driven management responses, tools, and approaches. Flood-risk paradigms in both basins include significant investment in engineering protection and, increasingly, soft-policy adaptations. Over time, flood management methods and objectives in each basin have become more diverse. For example, single-objective approaches have evolved towards multi-benefit projects. Both basins are expanding consideration of floodplain ecosystem importance and both now consider climate change to some in flood risk management. Flood-protection levels are higher on the Rhine than on the Sacramento. Some areas of the Rhine have 1000-year or better protection whereas a 200-year-level protection for urban areas is now required in the Sacramento basin.The Sacramento River and the Rhine River are geographically and hydrologically similar in surprisingly many ways, including in the flood risk they pose. But the flood-risk management paradigms in the two basins have evolved differently. We argue that the differences are a form of “parallel evolution,” reflecting historical and political contrasts between the two systems. Such contrasts present opportunities for alternative tools and lessons that can be explored and perhaps imported in both directions.

The Sacramento River in California, USA, and the Rhine River in Europe both have histories of major flooding events and great efforts to manage flood risk. We compare these two watersheds with an interdisciplinary lens to explore the goals, approaches, outcomes, and “parallel evolution” of differing flood risk management paradigms. The two basins share hydrologic similarities, but each approach to managing floods reflects the basin’s unique historical, environmental, and governance context. The Sacramento basin is entirely within the state of California, whereas the Rhine is a transnational river that drains nine European countries. The Rhine basin is larger and has a much larger population compared with the Sacramento basin.  The Sacramento basin has high interannual precipitation variability and receives most of its precipitation in the winter with significant mountain snowfall. The hydrology of the Rhine is also strongly influenced by mountain snowpack, but has precipitation that is more evenly distributed throughout the year. Flood-risk management on both the Sacramento and Rhine Rivers has evolved from ad hoc and local approaches, towards more systematic planning, culminating in significant state-level control in California, and state, federal, and transnational management on the Rhine. This transition was driven in recent years by the Central Valley Flood Protection Act and the European Floods Directive. Management of each basin has been shaped by an event-based evolution, in which disasters have driven management responses, tools, and approaches. Flood-risk paradigms in both basins include significant investment in engineering protection and, increasingly, soft-policy adaptations. Over time, flood management methods and objectives in each basin have become more diverse. For example, single-objective approaches have evolved towards multi-benefit projects. Both basins are expanding consideration of floodplain ecosystem importance and both now consider climate change to some in flood risk management.  Flood-protection levels are higher on the Rhine than on the Sacramento. Some areas of the Rhine have 1000-year or better protection whereas a  200-year-level protection for urban areas is now required in the Sacramento basin. The Sacramento River and the Rhine River are geographically and hydrologically similar in surprisingly many ways, including in the flood risk they pose.  But the flood-risk management paradigms in the two basins have evolved differently.  We argue that the differences are a form of “parallel evolution,” reflecting historical and political contrasts between the two systems.  Such contrasts present opportunities for alternative tools and lessons that can be explored and perhaps imported in both directions.

In response to growing threats of climate change, the US federal government is increasingly supporting community-level investments in resilience to natural hazards. As such federal programs become more widespread, evaluating their efficiency and equity is essential. The Community Rating System (CRS), which is part of the National Flood Insurance Program (NFIP), is a promising example of a federal policy designed to reduce flood losses by providing financial incentives for local climate adaptation. In exchange for community engagement in a range of risk communication and risk reduction activities, CRS provides discounts on NFIP premiums ranging from 5% to 45%. Using national-scale NFIP claims, policies, and CRS data between 1998 and 2020, we assess the program, asking whether it has been effective in reducing flood losses, how it can be improved, and what lessons it holds for similar programs. We find that participation in CRS is associated with reduced flood damage, with the percent reduction in claims roughly proportional to NFIP premium discounts. Among CRS activities, those related to ‘Flood Damage Reduction’ are most effective in reducing flood losses and are associated with a 20%–30% decrease in NFIP claims. Between 1998 and 2020, cumulative damage reductions attributable to CRS were $11.4 billion; over the same period, cumulative NFIP premium discounts were $12.1 billion. This close match is an endorsement of CRS historically and supports its future continuation. To improve the efficiency and equity of CRS, we recommend that Federal Emergency Management Agency: (a) reexamine the surcharge levied on NFIP premiums that cross-subsidizes premium discounts, and (b) allocate greater resources towards supporting participation among smaller, under-resourced communities. In general, CRS serves as an effective model for other federal market-based programs seeking to stimulate community-level investment in climate resilience.

Abstract This article analyzes residential property transactions to better understand the impact of urban flooding events and property distributions on the floodplain on real‐estate markets. We studied patterns before and after major fluvial flooding events in three counties that experienced such events between 2009 and 2013: Benton County, Oregon; Boulder County, Colorado; and Cass County, North Dakota. We tested for the presence and distribution of price discounting before and following these flood events using a hedonic difference‐in‐difference regression model. Floodplain discounts were detected in all three counties, over the full study period, including before and after flooding. However, only Boulder County exhibited a statistically significant price discount in the wake of the flooding event at the center of our analysis, with prices falling by 6.26% in the 100‐year floodplain until they rebounded after approximately 2–3 years. In Benton County, we were not able to detect a post‐flood price effect, but prices throughout the study period were 9.4% lower in the 100‐year floodplain compared to comparable properties outside the floodplain. Cass County experienced weaker discounting and only in the 500‐year floodplain, but a large flood control project was widely discussed after the 2009 flood event, which may have prevented widespread price discounting. The Boulder County case study confirms the phenomenon of post‐flood real estate discounting and subsequent rebound, as documented by other researchers. The other two case studies, interestingly, document that such discounting is not universal. We suggest that the difference seems to be explained by differing levels of pre‐flood local flood‐risk awareness, along with the magnitude of the triggering flood event. The new availability of nationwide real‐estate data allows for new and more detailed assessment of these important distinctions.

ABSTRACT Uplifted coastal terraces are present on Santa Catalina Island, but so poorly preserved that researchers have debated for more than a century whether they even exist. Morphometric analyses of LIDAR-based topography on the California Channel Islands suggest that the poor expression of terraces on Catalina Island is due a combination of (1) geological conditions less conducive to terrace formation than on the other Channel Islands and (2) high landslide susceptibility around Catalina’s coastal margin that is erasing terrace morphology and deposits before they can reach the island's relatively un-dissected interior. Both of these factors seem to be related to the mechanical properties of the island’s predominant rock type, the Catalina Schist. Several of the other Channel Islands – such as Santa Barbara, San Clemente, Anacapa, and San Miguel Islands – have topography that is dominated by their terrace morphology, mostly by virtue of the small size of those islands. Of the large islands, Santa Rosa has the most extensive terraces, apparently driven by the prevalence of shallowly dipping sedimentary rock. Looking at terrace preservation generally, subhorizontal sedimentary units seem to represent an optimum between promoting the initial creation of wide terrace platforms and resistance to terrace erosion thereafter.

Abstract Flood-loss estimates are needed for floodplain development and mitigation projects, for setting fair insurance rates, and for guiding climate adaptation policy. Currently, flood-loss models, including depth-damage functions (DDFs) widely used in the U.S., lack empirical validation commensurate with the geographic extent and diversity of structures and flood exposure over which these predictions are needed. Using data from 845,776 U.S. National Flood Insurance Program claims, we validate DDFs and create alternative models grounded in empirical data and validation. These alternative models more accurately predict average observed damages for many types of structures and hazard compared to current DDFs which omit important variables and interactions that drive observed losses. We find that a major bottleneck in flood-loss estimation is the development and validation of flood-loss models for both damaged and undamaged homes, a gap FEMA could help close.

Previous research found that National Flood Insurance Program (NFIP) premiums collected in some U.S. states, including California, have far exceeded damage payments. However, this finding raises the question of whether such an imbalance represents systematically good flood management or, instead, merely short‐term hydrologic good luck. This study investigated patterns in flood losses on structures that pre‐date and post‐date the first available flood maps (“pre‐Flood Insurance Rate Map [FIRM]” vs. “post‐FIRM”) in California, several peer states, and nationwide. California has a larger inheritance of pre‐FIRM structures than the national average, apparently reflecting development during the latter half of the 20th Century but before most Federal Emergency Management Agency (FEMA) flood maps. Pre‐FIRM properties are a disproportionate cost burden on the system, and the number of pre‐FIRM policies has declined over time, but only slowly. Local patterns in pre‐FIRM claims suggest targeted areas for enhanced mitigation efforts, including buyouts. Conversely, we find that claims on post‐FIRM policies are a reasonable metric of good floodplain management and enforcement, and California's 38% of post‐FIRM policies generated just 24% of the state's NFIP claims. Local “post‐FIRM claim hotspots” suggest areas to examine more closely. California continues to be a net payer into the National Flood Insurance Program, with $102 million in payouts 2009–2018 versus $2.3 billion in premiums collected, or 4.5 cents of premiums collected for every dollar of premiums. In California, its peer states, and nationwide, future management of flood risk depends on: (1) continued investment in flood control and mitigation of existing floodplain structures, and (2) prudent planning and limitations on new floodplain and coastal development.

Managed retreat involves the relocation of structures or abandonment of land to manage natural-hazard risk. In the USA, a number of towns have completed or are planning partial or complete relocations. This study focuses on a subset of these towns located in the US Midwest, each relocated out of FEMA designated flood zones to mitigate riverine flooding risk. Past community relocations hold hard-won lessons for future managed retreat projects. Three pragmatic and preliminary metrics were developed here: (1) flood losses avoided, (2) population trends, and (3) trends in home sales prices. Flood-loss modeling shows that community relocation can be cost-effective, reducing economic flood exposure by over 95%. Analysis of population data and residential real-estate sales shows that Midwestern floodplain towns have lagged non- floodplain towns, and homes on mapped floodplains have sold at a discount to non-floodplain homes since at least the early 1990s. Relocation off the floodplain removed the stigma of flooding and quantifiably reenergized those communities. Relocation projects cost more than buyouts without relocation but maintain social bonds and represent a tangible investment in future resilience. Future relocation projects, including retreat from both floodplains and coasts, should draw from the wealth of empirical data and lessons learned from past retreat efforts.

Marine terraces are widespread along California's coastline, including on all of the Channel Islands, with the possible exception of Santa Catalina. For over a century, the origins of subhorizontal surfaces and gravel deposits on Santa Catalina have been debated, with recent suggestions that Santa Catalina has no marine terraces and is subsiding. We mapped, measured, and described terrace deposits on Santa Catalina Island, including both in situ deposits and distributed gravel float. Rounded gravels and cobbles, locally pholad-bored, are present as float across low-relief surfaces in the Little Harbor area. We also mapped and described the Eagles Nest Gravels, an ∼8-m-thick package overlying a broad bedrock-cut platform at ∼200 m elevation and dipping 3.2° northward. The Eagles Nest Gravels contain rounded cobbles and boulders, many of which contain pholad borings. Two other platforms are inferred from concordant gravels with similar orientations but at lower elevations. Terrace deposits on Santa Catalina truncate underlying lithological units, including a narrow band of fossiliferous Miocene to Pliocene sands. Terrace deposits and gravel lag on Santa Catalina closely resemble older terrace deposits on other California Channel Islands. The terraces on Santa Catalina Island remain undated but document at least 200 m of net uplift, similar to the elevations of undated terraces on the other Channel Islands. While the timing of uplift of Santa Catalina is unclear, analysis of terrace deposits in the Little Harbor area confirms their marine origin and settles the debate regarding the presence of marine terraces on Santa Catalina Island.

Managed retreat refers to the relocation of population or infrastructure to address sea-level rise, climate-driven flood risk, and other threats. One variety of managed retreats involves the wholesale relocation of communities. The focus of retreat and relocation projects is to make the retreating communities more resilient to future losses; add-on benefits may include environmental enhancement and broad potential social goals such as promoting equity. Facing spiraling flooding and other climate-change impacts, the United States has been planning and implementing new retreat projects, but without full awareness of past relocations. This study reviews more than 50 relevant community relocations in U.S. history. These endeavors represent millions of taxpayer dollars and enormous investment of personal effort, leadership, triumph, and frustration by residents. And these case studies represent real-world, context-specific expertise needed to guide future U.S. retreat and relocations efforts. This study reviews U.S. relocation history as a resource for scholars of managed retreat, disaster management professionals, and local stakeholders contemplating retreat.

Improvements in modelling power and input data have vastly improved the precision of physical flood models, but translation into economic outputs requires depth–damage functions that are inadequately verified. In particular, flood damage is widely assumed to increase monotonically with water depth. Here, we assess flood vulnerability in the US using >2 million claims from the National Flood Insurance Program (NFIP). NFIP claims data are messy, but the size of the dataset provides powerful empirical tests of damage patterns and modelling approaches. We show that current depth–damage functions consist of disparate relationships that match poorly with observations. Observed flood losses are not monotonic functions of depth, but instead better follow a beta function, with bimodal distributions for different water depths. Uncertainty in flood losses has been called the main bottleneck in flood risk studies, an obstacle that may be remedied using large-scale empirical flood damage data.