In a context of global warming characterized by a mean sea level rise and extreme meteorological events, the study of the causes for coastal flooding is essential to protect communities and ecosystems. Densely urbanized, and rather unprotected cities in developing countries such as the historic Saint Louis city in Senegal are particularly vulnerable to coastal flooding and sea hazards. From satellite-derived high resolution DEM and global ocean reanalyses, here we quantify the extreme coastal water level in order to identify the neighborhoods and places of particular socio-economical interest of Saint-Louis potentially vulnerable to flooding. The results reveal that the most severe levels have the potential to flood up to almost half of this low-lying river mouth plain. Social, economic and heritage stakes are exposed, and the artisanal fisherman district of Gueth Ndar, is particularly vulnerable to coastal flooding because of its low elevation and situation on the sand barrier. The co-occurrence of high tides and wave-induced runup contributes most to ECWL but they do not present a significant trend over the study period. The results show that over the period 1994-2015, potential flood risk increased by nearly one day per year, primarily due to sea level rise, sounding a warning signal to take countermeasures to protect communities and infrastructure.

/pdf/extreme-coastal-water-levels-with-potential-flooding-risk-at-2k4ljdhi.pdf

Extreme coastal water levels with potential flooding risk at the low-lying Saint Louis historic city, Senegal (West Africa)

Flood risk assessments commonly use event-based approaches to reduce the number of scenarios required to be run through computationally intensive physical process models. Often the return period of the response variable (e.g., a fluvial water level or overtopping discharge) generated by an event (e.g., upstream/downstream water level or set of sea state variables) does not match that of the event itself; a limitation of event-based approaches which can lead to the misspecification of flood risk. We present a transferable hybrid statistical - hydraulic modeling framework for rapidly locating transition zones; river reaches where extreme water levels are driven by both upstream riverine discharge and downstream sea level. Instead of an event-based approach the framework utilizes a surrogate model to reduce computational expense of the hydraulic model. The surrogate-based approach allows the empirical estimation of response-based along-river return levels from on a large number of plausible discharge – coastal still water level events simulated from the statistical model. We assess the robustness of the event-based approach by comparing the associated return levels with the response-based return levels. The framework is applied to the Suwannee River in Florida (United States). Three surrogate models are evaluated, highlighting the enhanced ability of non-linear models to accurately capture discharge-sea level interactions along the river. The along-river return levels of the “most-likely” design event is found to lie within the range of variability of the response-based return levels for most of the transition zone. 

/pdf/a-hybrid-framework-for-rapidly-locating-transition-zones-a-u7luh4ol.pdf

A Hybrid Framework for Rapidly Locating Transition Zones: A Comparison of Event‐ and Response‐Based Return Water Levels in the Suwannee River FL

Increasingly, it is reported that the coastline of the Dakar region is affected by coastal flooding due to extreme water levels during wave events. Here, we quantify the extreme coastal water levels as well as the different factors contributing to coastal flooding during the period 1994–2015. Severe water levels reach values of 1.78 m and increase by 8.4 mm/year. The time spent above this threshold has already increased by 1.7 over the study period and will increase by 2100 to 8 times with 0.4 m mean sea level rise and up to 20 times with 0.8 m in the IPCC low and high greenhouse gas emission scenarios, respectively. Tide is the main contributor to the extremes when combined with large wave runup, due to wave breaking which contributes to 38% of the increase in extreme events while sea level rises to 44%. Our results show that because of its prominent location, Dakar region is affected by waves coming from the Northern and Southern Hemispheres with contrasted evolutions: wave runup events increase faster (7 mm/year) during austral winter due to a maximum of the South Atlantic storm activity, and have a decreasing trend (−3 mm/year) during boreal winter (December, January, February) driven by the evolution of corresponding climate modes.

https://www.mdpi.com/2225-1154/11/1/6/pdf?version=1673398321

Extreme Coastal Water Levels Evolution at Dakar (Senegal, West Africa)

The occurrence of an extreme atmospheric or oceanic event depends on there being a favourable initial state, and on the influence of large-scale drivers, positive local feedbacks and stochastic processes. Extremes occur and change over time, due to a combination of internal variability and external forcing. Changes in extremes that are driven by global-scale thermodynamic processes are relatively well understood compared to changes that are driven by dynamic processes, or by regional and local forcing, including regional and local thermodynamic processes. Compound extremes emphasise even further the need to consider multiple drivers when discussing the initiation and evolution of weather, climate and ocean extremes. The interdependence of these drivers is often of critical importance. 

Drivers of past and future changes in weather, climate and ocean extremes

http://www.cell.com/article/S2589004223001074/pdf

Consideration of compound drivers and impacts in the disaster risk reduction cycle

Abstract. Recent studies have drawn special attention to the significant dependencies between flood drivers and the occurrence of compound flood events in coastal areas. This study investigates compound flooding from tides, river discharge (Q), and specifically waves using a hydrodynamic model at the Breede Estuary, South Africa. We quantify vertical and horizontal differences in flood characteristics caused by driver interaction and assess the contribution of waves. Therefore, we compare flood characteristics resulting from compound flood scenarios to those in which single drivers are omitted. We find that flood characteristics are more sensitive to Q than to waves, particularly when the latter only coincides with high spring tides. When interacting with Q, however, the contribution of waves is high, causing 10 %–12 % larger flood extents and 45–85 cm higher water depths, as waves caused backwater effects and raised water levels inside the lower reaches of the estuary. With higher wave intensity, the first flooding began up to 12 h earlier. Our findings provide insights on compound flooding in terms of flood magnitude and timing at a South African estuary and demonstrate the need to account for the effects of compound events, including waves, in future flood impact assessments of open South African estuaries. 

https://nhess.copernicus.org/articles/22/187/2022/nhess-22-187-2022.pdf

Investigating the interaction of waves and river discharge during compound flooding at Breede Estuary, South Africa

Abstract. Hydrodynamic models are increasingly being used in recent years to map coastal floodplains on local to continental scales. On regional scales, however, high computational costs and the need for high-resolution data limit their application. Additionally, model validation constitutes a major concern, as in-situ data are hardly available or limited in spatial coverage to small parts of the study region. Here we address these challenges by developing a modelling framework, which couples a hydrodynamic coastal inundation model covering the German Baltic Sea coast with a hydrodynamic coastal ocean model of the western Baltic Sea, to produce high resolution (50 m) regional scale flood maps for the entire German Baltic Sea coast. Using a LiDAR derived digital elevation model with 1 m horizontal resolution, we derive and validate the elevation of dikes and natural flood barriers such as dunes. Using this model setup, we simulate a storm surge event from January 2019, a surge with a return period of 200 years and two sea-level rise scenarios for the year 2100 (200-year event plus 1 m and 1.5 m). We validate the simulated flood extents by comparing them to inundation maps derived from Sentinel-1 SAR satellite imagery, acquired between 1.5 and 3.5 hours after the peak of the 2019 surge, covering a large part of the study region. Our results confirm that the German Baltic Sea coast is exposed to coastal flooding, with flood extent varying between 118 km2 and 1016 km2 for the 2019 storm surge and a 200-year return water level plus 1.5 m of sea-level rise, respectively. Hotspots of coastal flooding are mostly located in the federal state of Mecklenburg Western Pomerania. Our results emphasise the importance of current plans to update coastal protection schemes along the German Baltic Sea coast over the course of the 21st century in order to prevent large-scale damage in the future. 

/pdf/a-new-modelling-framework-for-regional-assessment-of-extreme-18rffjwe.pdf

Kathy Jetñil-Kijiner

Papers

Investigating the interaction of waves and river discharge during compound flooding at Breede Estuary, South Africa

A new modelling framework for regional assessment of extreme sea levels and associated coastal flooding along the German Baltic Sea coast