Storm Xaver Research Articles

The impacts of coastal flooding and sea level rise on critical infrastructure: a novel storyline approach

ABSTRACT This study presents an event-based storyline framework to assess the influence of future climatic and socioeconomic conditions on coastal flood impacts to critical infrastructure. The framework combines well-established quantitative methods of sea level rise, coastal inundation, and critical infrastructure (CI) physical damage assessments into an integrated modelling approach. We apply our approach to re-imagine three historic events: storm Xaver, storm Xynthia , and a storm surge event along the coast of Emilia Romagna (Italy). Our results indicate that northern Germany would benefit mostly from coordinated adaptation action to reduce the flood impact, whereas the southwestern coast of France would find the highest damage reduction through asset-level ‘autonomous’ adaptation action. Our approach helps to improve the scientific understanding of how coastal flood risk are assessed and best managed, and forces a distillation of the science into an accessible narrative to support policymakers and asset owners to make progress towards more climate-resilient coastal communities.

How will climate change impact North Atlantic storms?

North Atlantic storms present one of the major weather risks for North West (NW) Europe. Their associated strong winds and heavy precipitation can result in wind damage and both coastal and inland flooding. Notable recent examples include the St Jude's Day storm of October 2013 which brought hurricane-force winds to NW Europe, including record-strong winds in Denmark and the loss of 17 lives. In November and December of that year, intense rainfall associated with a series of storms led to widespread UK flooding which was particularly severe over the Somerset Levels, and the timing of storm Xaver on 5 December 2013 generated a storm surge which led to coastal flooding around both the North and Irish Seas. Understanding and predicting how North Atlantic storms might respond to climate change is therefore essential for assessing future weather risks and informing climate change adaptation strategies. This briefing focuses on the North Atlantic storm track which traverses the North Atlantic Ocean and extends over NW Europe, with the strongest impacts usually occurring in autumn and winter. Over most of the Northern Hemisphere, climate models project that the midlatitude storm tracks will, on average, shift further poleward under climate change. Coupled with the shift is a general decrease in the number of storms. If this pattern occurred in the North Atlantic, one might expect a reduction in the number of storms impacting NW Europe. This is indeed the case in summer, but the wintertime North Atlantic storm track does not follow this simple picture; instead, it is projected to extend more strongly eastward over NW Europe resulting in an enhanced storm risk in this region. Comprehensive analyses of wintertime North Atlantic storms across a wide range of global climate models suggest an increase in storm occurrence of 2–4% by the end of the century (Zappa et al., 2013). The quoted range represents scenario uncertainty associated with two different future greenhouse gas emissions pathways, corresponding to global warming levels of approximately 2–4 degC respectively. There are additional uncertainties not accounted for, particularly due to internal variability of the climate system. A full analysis of North Atlantic storms in the most recent generation of climate models (CMIP6) has yet to be completed, but initial studies indicate broadly similar changes per degree of global warming (Harvey et al., 2020). On the level of individual storms, substantial and robust increases in precipitation are expected. The same suite of global climate model simulations quoted above project a 9–18% increase in the average precipitation per storm from wintertime storms impacting NW Europe. Although the strongest winds associated with storms are expected to weaken, wintertime North Atlantic storms again do not follow the simple picture with a modest 0.4–0.9% increase in the strongest near-surface wind speeds expected by the end of the century. Increases in the occurrence and severity of coastal flooding are also expected, predominantly due to rising sea levels (Murphy et al., 2018). North Atlantic storms are generated by the large-scale contrast in temperature that exists between the tropics and the Arctic. Climate change is expected to warm the Arctic more than the tropics through a process known as polar amplification, acting to generally reduce storminess in the midlatitudes. However, the intensification of the wintertime North Atlantic storm track over NW Europe bucks this trend and the reasons for this are not fully understood. It may partly arise from a region of slower ocean warming in the central North Atlantic, associated with changes in the ocean circulation, resulting is a local increase of the north–south temperature gradient over the ocean surface and stronger potential for storm growth there (Gervais et al., 2019). Alternatively, more hemispheric-scale dynamical changes may be playing a role (Ciasto et al., 2016). On the level of individual storms, the projected increases in precipitation are consistent with the increased availability of moisture associated with increased air temperatures. One key research question currently being addressed is the extent to which the current generation of climate models accurately represent North Atlantic storms. For example, small-scale atmospheric processes not fully resolved by these models, including sting jets and embedded convective precipitation, have the potential to locally amplify both precipitation and wind impacts in a manner not currently captured by global climate projections.

\u201cGrey swan\u201d storm surges pose a greater coastal flood hazard than climate change

In this paper, we show that over the next few decades, the natural variability of mid-latitude storm systems is likely to be a more important driver of coastal extreme sea levels than either mean sea level rise or climatically induced changes to storminess. Due to their episodic nature, the variability of local sea level response, and our short observational record, understanding the natural variability of storm surges is at least as important as understanding projected long-term mean sea level changes due to global warming. Using the December 2013 North Atlantic Storm Xaver as a baseline, we used a meteorological forecast modification tool to create “grey swan” events, whilst maintaining key physical properties of the storm system. Here we define “grey swan” to mean an event which is expected on the grounds of natural variability but is not within the observational record. For each of these synthesised storm events, we simulated storm tides and waves in the North Sea using hydrodynamic models that are routinely used in operational forecasting systems. The grey swan storms produced storm surges that were consistently higher than those experienced during the December 2013 event at all analysed tide gauge locations along the UK east coast. The additional storm surge elevations obtained in our simulations are comparable to high-end projected mean sea level rises for the year 2100 for the European coastline. Our results indicate strongly that mid-latitude storms, capable of generating more extreme storm surges and waves than ever observed, are likely due to natural variability. We confirmed previous observations that more extreme storm surges in semi-enclosed basins can be caused by slowing down the speed of movement of the storm, and we provide a novel explanation in terms of slower storm propagation allowing the dynamical response to approach equilibrium. We did not find any significant changes to maximum wave heights at the coast, with changes largely confined to deeper water. Many other regions of the world experience storm surges driven by mid-latitude weather systems. Our approach could therefore be adopted more widely to identify physically plausible, low probability, potentially catastrophic coastal flood events and to assist with major incident planning.

North Sea Infragravity Wave Observations

Coastal safety assessments with wave-resolving storm impact models require a proper offshore description for the incoming infragravity (IG) waves. This boundary condition is generally obtained by assuming a local equilibrium between the directionally-spread incident sea-swell wave forcing and the bound IG waves. The contribution of the free incident IG waves is thus ignored. Here, in-situ observations of IG waves with wave periods between 100 s and 200 s at three measurement stations in the North Sea in water depths of O(30) m are analyzed to explore the potential contribution of the free and bound IG waves to the total IG wave height for the period from 2010 to 2018. The bound IG wave height is computed with the equilibrium theory of Hasselmann using the measured frequency-directional sea-swell spectra as input. The largest IG waves are observed in the open sea with a maximum significant IG wave height of O(0.3) m at 32 m water depth during storm Xaver (December 2013) with a concurrent significant sea-swell wave height in excess of 9 m. Along the northern part of the Dutch coast, this maximum has reduced to O(0.2) m at a water depth of 28 m with a significant sea-swell wave height of 7 m and to O(0.1) m at the most southern location at a water depth of 34 m with a significant sea-swell wave height of 5 m. These appreciable IG wave heights in O(30) m water depth represent a lower bound for the expected maximum IG wave heights given the fact that in the present analysis only a fraction of the full IG frequency range is considered. Comparisons with the predicted bound IG waves show that these can contribute substantially to the observed total IG wave height during storm conditions. The ratio between the predicted bound- and observed total IG variance ranges from 10% to 100% depending on the location of the observations and the timing during the storm. The ratio is typically high at the peak of the storm and is lower at both the onset and waning of the storm. There is significant spatial variability in this ratio between the stations. It is shown that differences in the directional spreading can play a significant role in this. Furthermore, the observed variability along the Dutch coast, with a substantially decreased contribution of the bound IG waves in the south compared to the northern part of the Dutch coast, are shown to be partly related to changes in the mean sea-swell wave period. For the southern part of the Dutch coast this corresponds to an increased difference with the typically assumed equilibrium boundary condition although it is not clear how much of the free IG-energy is onshore directed barring more sophisticated observations and/or modeling.

Storm Xaver over Europe in December 2013: Overview of energy impacts and North Sea events

Abstract. Storm Xaver on 5–6 December 2013 was a serious winter storm in northern Europe with important impacts on societal and energy infrastructure. The storm's low pressure centre passed eastward north of Scotland, across the North Sea and southern Scandinavia, and into the Baltic region. The trajectory resulted in strong northwest winds and a cold air outbreak southward across the North Sea. The resultant convection system was associated with powerful wind gusts and freezing precipitation that impacted the UK, Belgium, the Netherlands, Germany, Poland, Denmark, Sweden, and Norway. The storm caused coastal flooding that was comparable with the most serious North Sea surge events of the 20th century. The primary impact for energy meteorology was a large scale electrical power loss in the northern part of the British Isles, Sweden, Poland, and parts of Germany. Petroleum production was reduced as offshore platforms were evacuated ahead of the storm. For wind energy, a number of onshore turbines were damaged by the gust field. Other societal impacts included travel and transport interruptions, building damage, forest damage, and coastal erosion. Because of the high water levels and sea state in the North Sea, the storm was important for offshore wind energy. The wind energy research tower FINO1 sustained unexpected damage during the storm, similar to previous wave strikes during Storm Britta (2006) and Storm Tilo (2007). A closer analysis is made of the tide gauge records across the North Sea to understand the progression of the storm surge and identify high amplitude, short-period features that may be linked to unusual seiches, meteotsunamis, or infragravity waves. Similar to previous storms, there is an indication that large infragravity waves during Storm Xaver may have had an impact on North Sea transport and energy infrastructure as well as coastal erosion. The review of information from different sources permits the met-ocean conditions and resultant societal/energy impacts to be related in time and space.

Impact of storm propagation speed on coastal flood hazard induced by offshore storms in the North Sea

Storm propagation speed (SPS) can noticeably impact coastal floods around semi-closed basins influenced by extratropical offshore storms. As a case study, the SPS impact on potential flood hazards due to extreme water levels along the UK east coast was studied using a numerical shelf sea model (FVCOM). The storm Xaver, which caused the largest North Sea surge over the past 60 years, was studied as a base scenario. Halving/doubling the SPS results in a smaller surge and a longer/shorter surge duration. Hence, the largest peak water level was found at actual speed, while the largest potential flood hazard occurred at half speed. Tide–surge interaction tends to reduce the M2 tide along the coast and advance its propagation for all SPS. A three-dimensional semi-analytical model, including a time-periodic wind forcing, was used to investigate the dominant mechanisms behind the surge dynamics, where wind duration is directly related to the SPS. Long wind durations correspond to small SPS, and vice versa. The semi-analytical model was applied to the North Sea. The model reproduces the spatial features of the North Sea surge and its dependence on SPS, confirming the surge induced by offshore storms is primarily associated with wind set-up. Model results suggest the SPS of Xaver is likely to have contributed greatly to the occurrence of the largest North Sea surge due to wind-generated resonance. The impact of the SPS on the surge and tide–surge interaction are of great importance to coastal flood hazard assessment.

Extreme Coastal Water Levels Exacerbate Fluvial Flood Hazards in Northwestern Europe

Compound flooding, such as the co-occurrence of fluvial floods and extreme coastal water levels (CWL), may lead to significant impacts in densely-populated Low Elevation Coastal Zones. They may overstrain disaster management owing to the co-occurrence of inundation from rivers and the sea. Recent studies are limited by analyzing joint dependence between river discharge and either CWL or storm surges, and little is known about return levels of compound flooding, accounting for the covariance between drivers. Here, we assess the compound flood severity and identify hotspots for northwestern Europe during 1970–2014, using a newly developed Compound Hazard Ratio (CHR) that compares the severity of compound flooding associated with extreme CWL with the unconditional T-year fluvial peak discharge. We show that extreme CWL and stronger storms greatly amplify fluvial flood hazards. Our results, based on frequency analyses of observational records during 2013/2014’s winter storm Xaver, reveal that the river discharge of the 50-year compound flood is up to 70% larger, conditioned on the occurrence of extreme CWL, than that of the at-site peak discharge. For this event, nearly half of the stream gauges show increased flood hazards, demonstrating the importance of including the compounding effect of extreme CWL in river flood risk management.

Describing Storm Xaver in disaster terms

The aim of this paper is to understand the relationship between the types of losses incurred in the context of Storm Xaver, and the use of the term ‘disaster’ for a winter storm that occurred in 2013. This understanding is important as regards disaster risk reduction policy.This case study of the social-economic impact of Storm Xaver and the criteria that defined the 'disaster' in Poland, Germany, the UK and the Netherlands allows us the opportunity to understand and assess whether such a term is justified.This investigation reveals that affected populations are key when it comes to justifying the ‘disaster’ term.This study looks into those hidden meanings within the description for those affected by such a disaster, with quoted figures provided for the numbers of individuals affected subject to correction. However, it soon became clear the importance of separating the impact of such an event over the short and long-term as regards the study of disaster risk reduction on these groups.On the one hand, this case study reveals an imprecise use of the term ‘disaster’, and on the other misinformation in the numbers of those affected, which in turn leads to a misinterpretation in data and misleading optimism.If the approach is focused on "affected people" and the consequential effects of living in the area, then this can be used as a tool to put together more responsible activities for action towards Disaster Risk Reduction, e.g. an allocation of budget funds in the regions and locally.

Study of the wind drag coefficient during the storm Xaver in the German Bight using data assimilation

Wind stress is the most important driving force for storm surge in coastal waters. Currently, the parameterization of wind stress is still not well developed and is the most uncertain parameters for storm surge models. In this paper, a storm surge model for the German Bight is developed using the model Delft3D. The data assimilation method, adjoint-free 4Dvar, which is relatively easier to be implemented than the adjoint method, is used to assimilate measured water level data into Delft3D. The storm surge prediction is more accurate when we adjusted the drag coefficient using the adjoint free data assimilation method, than in a companion forward model. The drag coefficient is consistent with results from previous studies. The wind drag coefficient is also calculated using results of a wave model. It has been found that the relation between the drag coefficient and wind speed is approximately linear in the deep water. But in the shallow water, wind drag coefficient shows larger variability due to the wave shoaling effect. The linear relation adjusted by data assimilation agrees well with that derived from a wave-related formula. It is also found that wave-related parameterization scheme is necessary for shallow waters.

Analysis of physical factors of the windstorm Xaver in Poland: ­post‐hazard review

The windstorm Xaver, a severe wind cyclone that occurred in December 2013 caused widespread damage in many regions of Europe, including Poland where the maximum speed of wind reached 38ms−1. This paper aims to study a post event taking into consideration the forecasts and meteorological data. The analysis of the effects of the storm Xaver indicates the importance of prevention of the impacts caused by extreme natural events.

Variations in the response of the dune coast of northern France to major storms as a function of available beach sediment volume

AbstractA series of airborne topographic LiDAR data were obtained from May 2008 to January 2014 over two coastal sites of northern France (Bay of Wissant and east of Dunkirk). These data were used with wind and tide gauge measurements to assess the impacts of storms on beaches and coastal dunes, and particularly of the series of major storms that hit western Europe during the fall and early winter of 2013. Our results show a high variability in shoreline response from one site to the other, but also within each coastal site. Coastal dune erosion and shoreline retreat occurred at both sites, particularly on the coast of the Bay of Wissant where shoreline retreat up to about 40 m was measured. However, stability or even shoreline advance were also observed despite the occurrence of an extreme water level with a return period >100 years during the storm Xaver in early December 2013. Comparison of shoreline change with variations of coastal dune and upper beach volumes revealed only weak relationships. Our results nevertheless showed that shoreline behavior seems to strongly depend on the initial sediment volume on the upper beach before the occurrence of the storms. According to our measurements, an upper beach volume of about 30 m3 m−1 between the dune toe and the mean high water level is sufficient at these sites to protect the coastal dunes from storm waves associated with high water levels with return periods >10 years. The identification of such thresholds in terms of upper beach width or sediment volume may represent valuable information for improving the management of shoreline change by providing an estimate of the minimum quantity of sand on the upper beach necessary to ensure shoreline stability in this region. Copyright © 2016 John Wiley & Sons, Ltd.

Coastal flooding: impact of waves on storm surge during extremes – a case study for the German Bight

Abstract. This study addresses the impact of wind, waves, tidal forcing and baroclinicity on the sea level of the German Bight during extreme storm events. The role of wave-induced processes, tides and baroclinicity is quantified, and the results are compared with in situ measurements and satellite data. A coupled high-resolution modelling system is used to simulate wind waves, the water level and the three-dimensional hydrodynamics. The models used are the wave model WAM and the circulation model GETM. The two-way coupling is performed via the OASIS3-MCT coupler. The effects of wind waves on sea level variability are studied, accounting for wave-dependent stress, wave-breaking parameterization and wave-induced effects on vertical mixing. The analyses of the coupled model results reveal a closer match with observations than for the stand-alone circulation model, especially during the extreme storm Xaver in December 2013. The predicted surge of the coupled model is significantly enhanced during extreme storm events when considering wave–current interaction processes. This wave-dependent approach yields a contribution of more than 30 % in some coastal areas during extreme storm events. The contribution of a fully three-dimensional model compared with a two-dimensional barotropic model showed up to 20 % differences in the water level of the coastal areas of the German Bight during Xaver. The improved skill resulting from the new developments justifies further use of the coupled-wave and three-dimensional circulation models in coastal flooding predictions.

Effects of wave-induced forcing on a circulation model of the North Sea

The effect of wind waves on water level and currents during two storms in the North Sea is investigated using a high-resolution Nucleus for European Modelling of the Ocean (NEMO) model forced with fluxes and fields from a high-resolution wave model. The additional terms accounting for wave-current interaction that are considered in this study are the Stokes-Coriolis force, the sea-state-dependent energy and momentum fluxes. The individual and collective role of these processes is quantified and the results are compared with a control run without wave effects as well as against current and water-level measurements from coastal stations. We find a better agreement with observations when the circulation model is forced by sea-state-dependent fluxes, especially in extreme events. The two extreme events, the storm Christian (25–27 October 2013), and about a month later, the storm Xaver (5–7 December 2013), induce different wave and surge conditions over the North Sea. Including the wave effects in the circulation model for the storm Xaver raises the modelled surge by more than 40 cm compared with the control run in the German Bight area. For the storm Christian, a difference of 20–30 cm in the surge level between the wave-forced and the stand-alone ocean model is found over the whole southern part of the North Sea. Moreover, the modelled vertical velocity profile fits the observations very well when the wave forcing is accounted for. The contribution of wave-induced forcing has been quantified indicating that this represents an important mechanism for improving water-level and current predictions.