Acknowledging residual risk behind dikes: Examples from the USA, Quebec (Canada) and France

Abstract

Although hydraulic infrastructure such as levees remain important for flood risk management in the USA, France, and Quebec (Canada), there is increasing emphasis on non-structural measures, such as regulatory flood maps, to reduce exposure and vulnerability, e.g., preventing people from building in high hazard areas. One key concept related to areas protected by levees is that of "residual risk", i.e., the risk from floods greater than the design standard of the levees (levee overtopping), and from levee breach. In this article, we review the legislative framework for regulatory flood maps in the US, France, and Quebec (Canada), compare restrictions on land use within the designated high-hazard zones, and compare how residual risk behind protective structures is taken into account (or not) in regulatory flood maps. Comparing the Flood Insurance Rate Maps (FIRMs) in the USA with the "Plans de Prévention du Risque d'Inondations" (PPRi) in France and the "Cartes de Zones Inondables" in Quebec, we find big differences in how the USA, France and Canada manage residual risk behind the levees.

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

“…This area is, in some countries, used for regulatory purposes, and it is sometimes further divided into different zones each with its own implications for insurance, building codes, emergency management, risk communication, etc. (Serra-Llobet et al, 2022). These "official" maps typically do not depict assets that are exposed to floods (e.g., buildings), aging infrastructure (e.g., levees) or the residual hazard area.…”

“…The reduced area mapped as the SFHA cannot be explained by the construction of hydraulic infrastructure. In the US, lands protected from the 100-year flood (by levees, dams, or other hydraulic infrastructure) are not depicted as part of the SFHA in the FIRMs Frontiers in Environmental Science frontiersin.org Frontiers in Environmental Science frontiersin.org (Serra-Llobet et al, 2022). However, no such infrastructure large enough to control the 100-year event has been constructed in Montecito.…”

“…While the concept might strike one as an obvious thing to do-to learn from the past, to avoid building/rebuilding in harms's way-in fact it would be a revolutionary approach to flood management in the US, where, unlike many countries in Europe, historical information is commonly overlooked in assessing flood risk (Keller and Capelli, 1992;Serra-Llobet et al, 2022), in deference to standardized hydraulic models. Historical information on floods includes historical maps, drawings, photographs, and historical accounts (both formal records and informal journals, correspondence, or press accounts) (Grabowski and Gurnell, 2016).…”

Managing residual flood risk behind levees: Comparing USA, France, and Quebec (Canada)

“…After Storm Xynthia in 2010, France began to take climate change into account more systematically, especially in coastal areas (MTES, 2018a;Serra-Llobet et al, 2021). In 2019, the government defined once and for all the different concepts and principles and integrated them into the Environmental Code, with the 2019 Decree Concerning Risk Prevention Maps for Riverine and Coastal Flooding (MTES, 2019), in the context of the European Floods Directive (2007).…”

“…Biological information is also used to define the "zone littoral" (riparian zone) (2-y flood), an area that should be preserved to ensure the connectivity of the river with the immediate floodplain. In Quebec, clear guidelines to account for climate change when defining future flood prone areas have yet to be defined (Serra-Llobet et al, 2021).…”