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In this study, the decadal evolution of a washover fan on the west coast of Denmark is examined from its initial generation in 1990 until 2015. Since its inception, the bare and flat washover fan surface has recovered and accreted slowly due to re‐activation by overwash during surges and due to aeolian activity and dune formation, stimulated by vegetation growth. The volume of sand on the washover has increased steadily at an average rate of about 23 m3/yr per unit length of shoreline, and a total of 175,000 m3 of sand is now deposited on the fan, while at the same time the shoreline has receded by some 250 m. The evolution can be divided into three stages: 1) An initiation phase when storm surge levels and energetic wave conditions caused a breach in the foredunes and overwash processes formed a washover fan with a relatively low elevation above mean sea level; 2) An initial recovery phase during which waves supplied sand to the fan during frequent overwash activity and winds transported this sand into marginal dunes surrounding the fan; and 3) A later recovery phase when the surface of the fan had accreted to a level where vegetation could survive and trap sediment into new foredune growth across the fan. The rate of accretion has been overall linear but scales with neither annual overwash frequency, nor with aeolian transport potential. Instead, the linear accretion is more closely related to the steady onshore migration of nearshore bars that weld to the beach and provide a sand supply for transfer to the fan. The fan evolution demonstrates the importance of washover fans in preserving barrier resilience during transgressional phases caused by increasing mean sea level. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Coastal lagoons and beach ridges are genetically independent, though non-continuous, sedimentary archives. We here combine the results from two recently published studies in order to produce an 8000-year-long record of Holocene relative sea-level changes on the island of Samsø, southern Kattegat, Denmark. The reconstruction of the initial mid-Holocene sea-level rise is based on the sedimentary infill from topography-confined coastal lagoons (Sander et al., Boreas, 2015b). Sea-level index points over the mid- to late Holocene period of sea-level stability and fall are retrieved from the internal structures of a wide beach-ridge system (Hede et al., The Holocene, 2015). Data from sediment coring, georadar and absolute dating are thus combined in an inter-disciplinary approach that is highly reproducible in micro-tidal environments characterised by high sediment supply. We show here that the commonly proximate occurrence of coastal lagoons and beach ridges allows us to produce seamless time series of relative sea-level changes from field sites in SW Scandinavia and in similar coastal environments. [ABSTRACT FROM AUTHOR]

ABSTRACT Beach-ridge systems are important geo-archives providing evidence for past wave climate including catastrophic storm flood events. This study investigates the morphological impacts of the 1872 Baltic storm flood on a beach-ridge system (sandy spit) in south-eastern Denmark and evaluates the frequency of extreme storm flood events in the area over a longer time perspective. This paper combines field studies of morphology and sedimentary deposits, studies of historical maps, digital terrain model, ground-penetrating radar profiles, and luminescence dating. Sea water reached 2.8 m above mean sea level (amsl) during peak inundation and, based on studies of the morphological impacts of the 1872 storm flood, the event can be divided into four phases. Phase 1: increasing mean water levels and wave activity at the beach brought sediments from the beach (intertidal bars and normal berm) higher up in the profile and led to the formation of a storm-berm. Phase 2: water levels further increased and sediment in the upper part of the profile continued to build up the storm-berm. Phase 3: water levels now reached the top of the dune ridge and were well above the storm-berm level. Sea water was breaching the dune ridge at several sites and wash-over fans were generated until a level where the mean water level had dropped too much. Phase 4: the non-vegetated wash-over fans functioned as pathways for aeolian sand transport and relatively high dunes were formed in particular along the margins of the fan where aeolian sand was trapped by existing vegetation. The studied beach-ridge system records about 4500 years of accumulation; the storm flood sediments described are unique suggesting that the 1872 Baltic storm flood event was an extreme event. Thus studies of beach-ridge systems form a new source for understanding storm surge risk. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Geological studies of past and present sea level rely on valid and robust features marking sea level in geological sections. Present sea-level markers around the upper shoreface to beachface transition are detected in a beach-ridge plain formed in a microtidal regime. These sea-level markers identify specific relative sea levels at the time of formation. Ground-penetrating radar (GPR) data collected across the youngest part of the beach-ridge system of Feddet, Denmark are compared with independent coastal morphological and sedimentological data of the active strand plain and interpreted in relation to sea-level data. The data show consistency between dip values of the present beachface and upper shoreface compared with dip values of interpreted beachface and upper shoreface GPR reflections. A clear change in dip value is observed between beachface and upper shoreface deposits in both data sets. Within few centimetres, this break point coincides with actual sea level and is interpreted to correspond to downlap points observed in the GPR reflection data. Furthermore, our observations may indicate that downlap points of deposits, formed under both relatively high and low water levels, are preserved and may be identified in GPR reflection data. Downlap points identified in GPR data across microtidal beach-ridge systems from other localities can also constitute markers of palaeo-sea level at the time of deposition. Records of these sea-level markers can be used to reconstruct the local relative sea-level history during the Holocene. [ABSTRACT FROM AUTHOR]

Kroon, A., Kabuth, A.K. and Westh, S., 2013. Morphologic evolution of a storm surge barrier system. Morphologic evolution of a barrier island system on the southern shore of Zealand, Denmark shows regular inlet breaching and inlet closing over the last centuries. At the same time, the barrier slowly moves in a landward direction at its most exposed stretches. Alongshore wave-driven processes cause shoreline alignment and closure of inlets. A combination of high-energy waves and high water levels induces inlet breaching and overwash processes over the barrier crests. However, local high water levels are often not correlated with storm events in the inner Danish coastal waters. The water level fluctuations along the southern shore of Zealand are strongly influenced by the water level fluctuations in the Kattegat due to wind set-up and seiches in the Baltic Sea; local wind set-up and local wave set-up on the southern shore of Zealand are often of secondary importance. Statistics on extreme water levels based on measured water levels over the last century are often used to express the risk for flooding in a coastal environment. The present study indicates that this approach cannot directly be used in case of coastal erosion and inlet breaching. Extreme water levels and associated exceedance curves may overestimate these risks in cases of de-coupled extreme water levels and storm events. [ABSTRACT FROM AUTHOR]

From November 12th to 13th in 1872, an extreme coastal flood event occurred in the south Baltic Sea. An unusual combination of winds created a storm surge reaching up to 3.5 m above mean sea level, which is more than a meter higher than all other observations over the past 200 years. On the Danish, German, and Swedish coasts, about 300 people lost their lives. The consequences of the storm in Denmark and Germany were more severe than in Sweden, with significantly larger destruction and higher numbers of casualties. In Denmark and Germany, the 1872 storm has been more extensively documented and remembered and still influences local and regional risk awareness. A comparative study indicates that the collective memory of the 1872 storm is related to the background knowledge about floods, the damage extent, and the response to the storm. Flood marks and dikes help to remember the events. In general, coastal flood defence is to the largest degree implemented in the affected areas in Germany, followed by Denmark, and is almost absent in Sweden, corresponding to the extent of the collective memory of the 1872 storm. Within the affected countries, there is local variability of flood risk awareness associated with the collective memory of the storm. Also, the economic dependency on flood-prone areas and conflicting interests with the tourism industry have influence on flood protection decisions. The processes of climate change adaptation and implementation of the EU Floods Directive are slowly removing these differences in flood risk management approaches. [ABSTRACT FROM AUTHOR]

The morphological and sedimentological impacts of an extreme storm event (‘Bodil’ in December 2013) were investigated at a mixed sand and gravel beach (Vesterlyng) in a micro-tidal environment along the Belt Sea, Denmark. The storm response was characteristic for mixed sand and gravel beaches: gravelly storm berms with sandy to gravelly washover fans were formed at the upper beach where swash processes dominated during extreme water levels in combination with overtopping and overwash. In addition, storm scarps and gravel flats were formed. The elevation of the storm berm crest was identical to a summation of the still-water level (1.60 m) in the Belt Sea during the storm flood and the wave run-up (0.75 m) at the shore. The crest of 2.35 m is therefore a precise measure of the storm flood level. This storm flood was classified as a large-scale extreme event. In a Holocene perspective it is suggested that storm berm (beach ridge) heights above contemporaneous sea level can be used as a proxy of past flood events along micro-tidal shores. [ABSTRACT FROM AUTHOR]

Abstract: The formation of berms and their transformation into beach ridges in a micro-tidal environment is coupled to wave run-up and overtopping during extreme sea levels. A straight-forward comparison between extreme sea levels due to storm-surges and active berm levels is impossible in the semi-enclosed bays along the Baltic Sea. Quite often, the maximum water levels do not coincide with the maximum intensity of the wave driven processes because of seiches in the Baltic. In this paper, we look into the joined distribution of extreme water levels and high-energetic wave conditions at Feddet, a sandy prograding spit on the south-eastern Baltic shores of Zealand, Denmark. The modern, sandy beach at this location consists of a beachface with a shallow incipient berm, a mature berm, and a dune-covered beach ridge. It borders a beach-ridge plain to the west, where more than 20N–S oriented beach ridges and swales are present. Measured water-level data from 1991 to 2012 and topographical observations, carried out during fair weather period and during a storm event, provided the basis for a conceptual model exhibiting berm formation and transformation into the local beach-ridge system. The character of extreme sea level events is identified using thirty-three well described extreme events throughout a period of 15years. Analysis of the meteorological conditions during these events revealed that berm formation only occurred during 20% of all extreme events when onshore winds, high-energy wave action and elevated water coincided. These berm-forming events had a mean return period of once every three years. The average age between successive beach ridges on the beach-ridge plain was about 180years, which means that the transformation of a berm into a single beach ridge is the result of a large number of storm episodes with high-energy waves and extreme water levels. [Copyright &y& Elsevier]