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Published in Enzo Pranzini, Allan Williams, Coastal Erosion and Protection in Europe, 2013
not related to tides, essentially storm surges and waves (their amplitude and duration), play an important role in shoreline formation. Most storms occurring during low sea levels produce seabed and beach changes, but do not affect the dunes or cliffs, as dune or cliff erosion is a result of `significant storms'; these usually happen once every 10-12 months and are associated with very high storm surges (Furman 'czyk and Dudzin 'skaNowak, 2009). The majority of winds in the southern Baltic are south-westerly, blowing from the offshore sector, but due to coastal exposure to the north-east and north-west, the most severe storms come from those directions (Figure 5.1). Another important factor influencing coastal erosion is a result of North Sea water inflow to the Baltic Sea. Inflow occurs when a deep depression travels slowly above the Danish Straits, increasing the water volume that flows from the North Sea to the Baltic Sea. Such a situation occurs once every several years and high sea levels continue for many weeks. When an inflow is accompanied by a strong storm surge, the shore suffers catastrophic erosion; in such situations, the possibility of flooding is also very high. The highest storm surges associated with the North Sea inflows were recorded in 1874, when the ' Swinoujscie tide gauge showed 1.96 m above mean ' sea level, and in 1913/14, when it indicated 1.95 m. This resulted in major erosion over almost the entire coast; dunes were torn apart and storm surge flooding ensued (Majewski et al., 1983; Zeidler et al., 1995; Wisniewski and Wolski, 2009). ' When sea level drops in post-storm conditions, the beach rebuilds, but the pace of shore rebuilding is five times slower than that of erosion (Musielak, 1980). Due to wind activity, beach sand accumulates to form foredunes or to enlarge an existing main dune. The extent of dune baseline changes caused by one major storm may be similar to the total of longterm changes occurring within a 40-year period (Furman 'czyk, 1995). Therefore, if a series of storms occurs within a short time period, each shore reconstruction response time is short and total erosion changes are at their largest (Ferreira, 2005).
Conceptual and detailed design
Published in Dominic Reeve, Andrew Chadwick, Christopher Fleming, Coastal Engineering, 2018
Dominic Reeve, Andrew Chadwick, Christopher Fleming
The characteristic behaviour of a dune will depend upon its stage of evolution. A number of broad types can be recognised (Jay et al. 2003): Embryonic dunes – represent the first stage in the development of dune ridges and are formed by the deposition of sand along the high tide mark. They are low-lying mounds of sand and are often vegetated by salt-tolerant species; they are easily overwashed and removed during storms, releasing sand back to the beach.Foredunes – continuous or semi-continuous ridges of sand, often vegetated, which lie at the back of the beach and parallel to the shoreline. Parallel dunes can be modified during storm surges when wave overwash and breach may occur resulting in sand being swept landward in the form of fans or sheets. The height of foredunes is dependent upon the wind strength and sediment supply. Foredunes may become cliffed at their seaward margin during storms and undercutting at the dune toe can cause collapse and failure of the dune cliffs. Foredunes are also vulnerable to overwash (depending upon height) and breaching, particularly where the ridge is narrow and/or characterised by a series of blow-outs (see below).Climbing dunes – occur on some cliffed coasts; there are sand dunes either piled against the cliff, forming climbing dunes or at the top of the cliff. Where dunes have spilled inland and become separated from any source of sand, they have become relict cliff-top dunes, such as observed at locations along the Cornish coast.Relict dunes – are also present where there are no contemporary sources of sand or where the link between the beach and dunes has been broken, for example, on shingle beach ridges such as at Blakeney Point, Norfolk (Steers 1964).Blow-outs and parabolic dunes – generally form where dunes are unstable, possibly due to a lack of stabilising vegetation cover. There are two main ways in which they form: (1) where natural gaps or storm-damaged cliffs in the foredune ridge are exploited by winds and (2) by erosion processes, for example, the deflation of a poorly vegetated terrain. The movement of blow-outs and parabolic dunes is dependent upon the direction, frequency and strength of the onshore winds.Transgressive dunes – mobile dune forms which develop where sand blown inland from a beach has been retained by vegetation or where previously vegetated dunes become unstable and the numerous blow-outs merge to form an elongate dune (Bird 2000).
A preliminary assessment of machine learning algorithms for predicting CFD-simulated wind flow patterns over idealised foredunes
Published in Journal of the Royal Society of New Zealand, 2021
Sarah J. Wakes, Bernard O. Bauer, Michael Mayo
Foredune systems play an important role in the protection of coastal lands as well as the communities that live close to the shoreline (Carter 1991; Davidson-Arnott et al. 2019). Mitigating the threat of sea level rise, urban development on the coast, and the likelihood of more frequent, larger storm events requires time, money, and a sound knowledge of the dynamic nature of these complex beach-dune systems. In order to predict how coastal dunes respond to a range of stressors, it is critical to understand wind flow behaviour in the vicinity of dunes. Site-specific observational measurements are invaluable, but they are weather dependent, resource intensive, time consuming, and ultimately, limited in scope and coverage. Thus, model simulations of wind flow over beach-dune systems provide a complementary opportunity to explore a fuller range of conditions than might be possible with field experimentation (e.g. Jackson et al. 2013).