Global warming is expected to drive increasing extreme sea levels (ESLs) and flood risk along the world's coastlines. In this work we present probabilistic projections of ESLs for the present century taking into consideration changes in mean sea level, tides, wind-waves, and storm surges. Between the year 2000 and 2100 we project a very likely increase of the global average 100-year ESL of 34-76 cm under a moderate-emission-mitigation-policy scenario and of 58-172 cm under a business as usual scenario. Rising ESLs are mostly driven by thermal expansion, followed by contributions from ice mass-loss from glaciers, and ice-sheets in Greenland and Antarctica. Under these scenarios ESL rise would render a large part of the tropics exposed annually to the present-day 100-year event from 2050. By the end of this century this applies to most coastlines around the world, implying unprecedented flood risk levels unless timely adaptation measures are taken.

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Global probabilistic projections of extreme sea levels show intensification of coastal flood hazard.

https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=1802&context=vimsarticles

Seamless cross-scale modeling with SCHISM

One of the main consequences of mean sea level rise (SLR) on human settlements is an increase in flood risk due to an increase in the intensity and frequency of extreme sea levels (ESL). While substantial research efforts are directed towards quantifying projections and uncertainties of future global and regional SLR, corresponding uncertainties in contemporary ESL have not been assessed and projections are limited. Here we quantify, for the first time at global scale, the uncertainties in present-day ESL estimates, which have by default been ignored in broad-scale sea-level rise impact assessments to date. ESL uncertainties exceed those from global SLR projections and, assuming that we meet the Paris agreement goals, the projected SLR itself by the end of the century in many regions. Both uncertainties in SLR projections and ESL estimates need to be understood and combined to fully assess potential impacts and adaptation needs.

/pdf/understanding-extreme-sea-levels-for-broad-scale-coastal-2rx0xpvmth.pdf

Understanding extreme sea levels for broad-scale coastal impact and adaptation analysis

Storm surges are an important coastal hazard component and it is unknown how they will evolve along Europe’s coastline in view of climate change. In the present contribution, the hydrodynamic model Delft3D-Flow was forced by surface wind and atmospheric pressure fields from a 8-member climate model ensemble in order to evaluate dynamics in storm surge levels (SSL) along the European coastline (1) for the baseline period 1970–2000; and (2) during this century under the Representative Concentration Pathways RCP4.5 and RCP8.5. Validation simulations, spanning from 2008 to 2014 and driven by ERA-Interim atmospheric forcing, indicated good predictive skill (0.06 m < RMSE < 0.29 m and 10 % < RMSE < 29 % for 110 tidal gauge stations across Europe). Peak-over-threshold extreme value analysis was applied to estimate SSL values for different return periods, and changes of future SSL were obtained from all models to obtain the final ensemble. Values for most scenarios and return periods indicate a projected increase in SSL at several locations along the North European coastline, which is more prominent for RCP8.5 and shows an increasing tendency towards the end of the century for both RCP4.5 and RCP8.5. Projected SSL changes along the European coastal areas south of 50°N show minimal change or even a small decrease, with the exception of RCP8.5 under which a moderate increase is projected towards the end of the century. The present findings indicate that the anticipated increase in extreme total water levels due to relative sea level rise (RSLR), can be further enforced by an increase of the extreme SSL, which can exceed 30 % of the RSLR, especially for the high return periods and pathway RCP8.5. This implies that the combined effect could increase even further anticipated impacts of climate change for certain European areas and highlights the necessity for timely coastal adaptation and protection measures. The dataset is publicly available under this link: http://data.jrc.ec.europa.eu/collection/LISCOAST
 .

/pdf/projections-of-extreme-storm-surge-levels-along-europe-3v4g5c80cn.pdf

Projections of extreme storm surge levels along Europe

Rising extreme sea levels (ESLs) and continued socioeconomic development in coastal zones will lead to increasing future flood risk along the European coastline. We present a comprehensive analysis of future coastal flood risk (CFR) for Europe that separates the impacts of global warming and socioeconomic development. In the absence of further investments in coastal adaptation, the present expected annual damage (EAD) of €1.25 billion is projected to increase by two to three orders of magnitude by the end of the century, ranging between 93 and €961 billion. The current expected annual number of people exposed (EAPE) to coastal flooding of 102,000 is projected to reach 1.52–3.65 million by the end of the century. Climate change is the main driver of the future rise in coastal flood losses, with the importance of coastward migration, urbanization and rising asset values rapidly declining with time. To keep future coastal flood losses constant relative to the size of the economy, flood defence structures need to be installed or reinforced to withstand increases in ESLs that range from 0.5 to 2.5 m. Climate change is the main driver for future coastal flood risk in Europe. However, in the absence of increased flood protection, damages may rise by two to three orders of magnitude by the end of the century.

Climatic and socioeconomic controls of future coastal flood risk in Europe

This study investigates the contributions of short waves in storm surges through the hindcast of two storms that hit the central part of the Bay of Biscay recently. Despite displaying comparable wind speed and directions in the study area, these two storms induced different storm surges and sea states. Xynthia (27–28th of February 2010) was characterized by large (up to 7 m significant wave height H s ) and short-period waves and induced an exceptional storm surge, locally larger than 1.6 m. The second storm, Joachim (15–16th of December 2011), was characterized by very large (up to H s >10 m) and long-period waves but only induced a storm surge almost two times lower. To investigate these differences, a new unstructured grid and fully coupled modeling system is applied, with a spatial resolution fine-enough to adequately represent the surf zones over most of the study area (25 m). The analysis of the modeling results and the available field observations reveals firstly that the exceptional surge during Xynthia originated from young and steep waves, enhancing surface stress. This particular sea-state is explained by the abnormal track of Xynthia, which restricted the fetch to a few hundred km. The wave radiation stress gradient locally induced setup larger than 0.4 m along the coastlines fully exposed to ocean waves, while wave setup in the range 0.1–0.2 m was also shown to develop regionally and to propagate in sheltered harbors. Comparatively, wave-enhanced bottom stress appears to be a second-order process and has a more limited impact on storm surges.

The contribution of short-waves in storm surges: Two case studies in the Bay of Biscay

A modeling-based analysis of the flooding associated with Xynthia, central Bay of Biscay

http://www.lnec.pt/fotos/editor2/dha/DHA%20PDFs/fortunato_imum2014.pdf

Determination of extreme sea levels along the Iberian Atlantic coast

The design, implementation and demonstration of a novel and generic computational forecast framework for multi-scale prediction of extreme sea levels and associated flooding is presented. Denoted Water Information Forecast Framework (WIFF), it integrates process-based models for waves, tides and surges from regional to local scales, predicting the flooding of coastal areas, and supporting the routine and emergency management of coastal resources. WIFF manages the simulations and the real-time monitoring data, archives the data and makes the information available through a WebGIS that targets users with distinct access privileges. Additionally, the web component of WIFF adapts automatically and transparently to any device. WIFF also provides ways to assess the model accuracy and generates tailored products based on model results and observations. WIFF is demonstrated in the prediction of extreme water levels in the Portuguese coast, simulating processes at different scales: at basin scales, waves a...

Operational forecast framework applied to extreme sea levels at regional and local scales

During storm events inducing extreme sea levels, coastal floods may occur. Water volumes that flood low-lying coastal zones may be important and significantly impact the sea levels along the coast. This effect is expected to be particularly significant in tidal bays or estuaries. This phenomenon was recently illustrated during storm Xynthia (February 2010) on the French Atlantic coast; volumes that flooded Charente-Maritime and Vendee departments corresponded to a non-negligible fraction of the tidal prisms of the Pertuis Charentais Sea (area between Re and Oleron Islands and the continent); the order of magnitude is 10 %. The impact of the flooding associated with Xynthia is investigated by means of numerical modelling in the Pertuis Charentais carried out by LIENSs. Within the framework of risk mapping, CREOCEAN conducted flood studies in two sites of the eastern English Channel: Authie Bay and Dives Estuary, for which simulations of assessed future extreme events were run. Although these studies used different modelling systems at sites displaying distinct features, they all suggest that flooding can impact water level along the coastline up to more than 0.5 m. Limitation of high tide levels, related to flooding phenomena, are mapped and analyzed. Recommendations are provided to define a relevant methodology, accounting for such a limitation process, for submersion and risk studies.

Kai Li

Papers

The contribution of short-waves in storm surges: Two case studies in the Bay of Biscay

A modeling-based analysis of the flooding associated with Xynthia, central Bay of Biscay

Determination of extreme sea levels along the Iberian Atlantic coast

Operational forecast framework applied to extreme sea levels at regional and local scales

Limitation of High Water Levels in Bays and Estuaries During Storm Flood Events