Ulysse Pasquier

In their commentary “Flooding is not like filling a Bath” in Earth's Future ( https://doi.org/10.1029/2024EF005164 ) Sanders and colleagues highlight biases and uncertainties in the application of bathtub modeling for assessing flood risks, specifically for coastal assessments at broad scales (world‐regional to global scales). This is a laudable and timely undertaking. Sanders et al., however, also draw a number of far‐reaching conclusions including a general call to abandon the bathtub model together with a claim that the bathtub model introduces unacceptable levels of uncertainty, and that its broad scale applications have contributed to a “climate hype” exaggerating the impacts of climate change. In our opinion, these conclusions are misleading and don't follow from the author's analysis, because the authors take an overly narrow perspective on flood risk. Addressing this issue, this comment characterizes the system problem of assessing current and future coastal flood risk at broad scales, discusses the roles that bathtub and physics‐based models play in addressing this problem, as well as the implications for communicating and receiving information on future flood risk at broad scale. Three conclusions are drawn. First, there is no one‐size‐fits‐all model for assessing coastal flood hazard at broad scales and the bathtub model and its variants still have a role to play. Second, physics‐based models are computationally too expensive to deliver the multitude of simulations needed for future flood risk and adaptation. Third, future work needs to create a framework to most effectively utilize physics‐based models, emulators of them and bathtub‐based models for broad scale assessments.

Despite the growing demand for continental to global perspectives on coastal flooding, broad-scale inundation modeling remains restricted by computational resources and data availability. While fast and practical, the simple static approach to inundate cells below a water level – much like a “bathtub” – has shown to lack accuracy compared to more complex dynamic models. Various improvements to the bathtub method have been proposed but are yet to be assessed at broad scales. We compare three static inundation models with increasingly complexity in representing the attenuation of flood propagation. Using results from a pan-European 2D modeling effort as a benchmark, this study finds that a non-linear attenuation of water levels can help improve the representation of both flood depth and extent with negligeable computational cost. We also find that in “narrow” coastal floodplains, a bathtub model can yield similar results to dynamically generated flood maps, emphasizing the need to carefully consider floodplain characteristics when choosing an inundation modeling method.

Global sea‐level rise is intensifying pressures on coastal regions, increasing the need for adaptation strategies (e.g., protect, retreat, accommodate). At the same time, decision makers require a better understanding of the available responses to address the widening adaptation implementation gap. Structural measures aimed at reducing the impacts of coastal hazards as part of the accommodation strategy have received limited attention in the coastal adaptation literature with few studies looking at how it is currently considered to address sea‐level rise. We first advance a conceptual framework that separates structural from non‐structural accommodation, recognizing that this distinction is essential to accurately define the adaptation “solution space.” Building on this framework, we synthesize scientific and gray literature, conduct a multilevel review of policy and technical documents, and draw on expert input to not only evaluate the current state of structural accommodation in Europe but also to highlight generic lessons for its potential implementation. This includes consideration of its advantages and disadvantages. Uptake remains fragmented and highly localized, embedded mainly in municipal spatial planning rather than national adaptation agendas, and is hampered by financial, institutional, and technical constraints. We argue that stronger policy integration and dedicated financial incentives could overcome these barriers and harness accommodation's value as a flexible option capable of reducing risk and avoiding long‐term lock‐in. This study improves our understanding of how this strategy can contribute to coastal resilience in Europe and beyond.

The substantial risks posed to Venice and its lagoon by ongoing and projected sea-level rise require unprecedented long-term adaptation strategies. With present insufficient greenhouse gas mitigation policies, the current open lagoon strategy, with mobile barriers and multiple accommodation measures, is likely to encounter hard limits within the current century. Follow-up strategies could be based either on ring-dikes, isolating the city from the rest of the lagoon, or on a closed lagoon with permanent coastal dams separating it from the sea. Relocation of monuments to a suitable inland area and abandonment of Venice would be the only remaining strategy for sea-level above 10-15 m. Indicative adaptation costs increase with sea level rise from less than 1 to more than 30Ge. Abandonment might become unavoidable in the 22nd century under current climate policies and an Antarctic ice-sheet collapse, a risk that could still be avoided by a rapid reduction of greenhouse gas emissions.

The 1997 New Year's flood event was the most costly in California's history. This compound extreme event was driven by a category 5 atmospheric river that led to widespread snowmelt. Extreme precipitation, snowmelt, and saturated soils produced heavy runoff causing widespread inundation in the Sacramento Valley. This study recreates the 1997 flood using the Regionally Refined Mesh capabilities of the Energy Exascale Earth System Model (RRM‐E3SM) under prescribed ocean conditions. Understanding the processes causing extreme events informs practical efforts to anticipate and prepare for such events in the future, and also provides a rich context to evaluate model skill in representing extremes. Three California‐focused RRM grids, with horizontal resolution refinement of 14 km down to 3.5 km, and six forecast lead times, 28 December 1996 at 00Z through 30 December 1996 at 12Z, are assessed for their ability to recreate the 1997 flood. Planetary to synoptic scale atmospheric circulations and integrated vapor transport are weakly influenced by horizontal resolution refinement over California. Topography and mesoscale circulations, such as the Sierra barrier jet, are better represented at finer horizontal resolutions resulting in better estimates of storm total precipitation and storm duration snowpack changes. Traditional time‐series and causal analysis frameworks are used to examine runoff sensitivities state‐wide and above major reservoirs. These frameworks show that horizontal resolution plays a more prominent role in shaping reservoir inflows, namely the magnitude and time‐series shape, than forecast lead time, 2‐to‐4 days prior to the 1997 flood onset.

The 1997 New Year’s flood event was the most costly in California’s history. This compound extreme event was driven by a category 5 atmospheric river that led to widespread snowmelt. Extreme precipitation, snowmelt, and saturated soils produced heavy runoff causing widespread inundation in the Sacramento Valley. This study recreates the 1997 flood using the Regionally Refined Mesh capabilities of the Energy Exascale Earth System Model (RRM-E3SM) under prescribed ocean conditions. Understanding the processes causing extreme events inform practical efforts to anticipate and prepare for such events in the future, and also provides a rich context to evaluate model skill in representing extremes. Three California-focused RRM grids, with horizontal resolution refinement of 14km down to 3.5km, and six forecast lead times, 28 December 1996 at 00Z through 30 December 1996 at 12Z, are assessed for their ability to recreate the 1997 flood. Planetary to synoptic scale atmospheric circulations and integrated vapor transport are weakly influenced by horizontal resolution refinement over California. Topography and mesoscale circulations, such as the Sierra barrier jet, are prominently influenced by horizontal resolution. The finest resolution RRM-E3SM simulation best represents storm total precipitation and storm duration snowpack changes. Traditional time-series and causal analysis frameworks are used to examine runoff sensitivities state-wide and above major reservoirs. These frameworks show that horizontal resolution plays a more prominent role in shaping reservoir inflows, namely the magnitude and time-series shape, than forecast lead time, 2-to-4 days prior to the 1997 flood onset.

Decades of urbanization have created sprawling, complex, and vulnerable cities, half of which are located in water-scarce areas. With the looming effects of climate change, including increasing droughts and water shortages, there is an urgent need to better understand how urbanization impacts the water cycle at city scale. Impervious surfaces disrupt the natural flow of water, affecting groundwater recharge in water-scarce cities, such as Los Angeles, looking to harness local water resources. In the face of growing water demand, informing on opportunities to maximize potential groundwater recharge can help increase cities’ resilience. WRF–Hydro, a physics-based hydrological modeling system, capable of resolving atmospheric, land surface, and hydrological processes at city scale, is adapted to represent urban impervious surfaces. The modified model is used to assess the hydrological implications of historical urbanization. Pre- and post-urban scenarios are used to quantify the impacts of impervious surfaces on the local water budget. Our results show that urbanization in LA has vastly decreased the potential for groundwater recharge, with up to half of the water inflow being redirected from infiltration in highly urbanized watersheds, while doubling surface runoff’s share of the city’s water budget, from ~15% to 30%. This study not only sheds light on the role of imperviousness on groundwater recharge in water-scarce cities, but also offers a robust and transferable tool for the management of urban land and water resources.