Quantification of morphodynamic variability and sea state damping of plates at the nearshore area in the East Frisian North Sea

Highlights

• ETD sandbank dynamics explain on average 14% of the sea state damping variation.

• Damping variation due to ETD changes almost independent of the boundary conditions.

• Sea state damping was strongly influenced by incoming wave height.

Abstract

At the nearshore area of Norderney, the ebb-tidal delta (ETD) sandbanks exert a huge impact on the local wave climate, which is of particular interest for coastal protection. Besides an eastward migration of the sandbanks with a mean velocity from 435-491 m/y, depending on the plates position, the ETD sandbanks had a high variability in shape and size. Therefore, the influence of those morphodynamic changes on the sea state damping effect was quantified by dynamic bathymetries using validated SWAN simulations. The impacts of the bathymetries from the years 1995, 2005, 2006, 2008, and 2015 with highest morphological variability were considered for five different boundary conditions, representing storm surge events of the Norderney nearshore wave climate. A mean variation of 14 $\pm$ 2% of the relative sea state damping effect was found for the different bathymetries of the ETD sandbanks at the various boundary conditions. Swell waves showed a range of 16 $\pm$ 2% for the damping effect, which increased with increasing incoming wave height. Buoy data off the Norderney coast from 1992 to 2017 were investigated and a mean buoy-measured damping effect of 0.37 was determined over the entire period. General results of SWAN modeling were also found in the in-situ measurements.

Introduction

Coastal areas are of great interest as present and future climate change impacts, such as sea level rise, will put increasing pressure on humans living in those regions. Also the possibility of an increase in extreme storm events, which may damage coastal areas, is conceivable (Weisse et al., 2012; Bengtsson et al., 2009). For the North Atlantic and the North Sea, various studies that have analyzed the wave climate identified an increasing trend in ocean wave heights (Bacon and Carter, 1991; Weisse and Gunther, 2007; Grabemann and Weisse, 2008; Mori et al., 2013; Vanem and Walker, 2013). To sufficiently assess coastal management risks and evaluate human interferences, detailed knowledge about the wave climate is of particular interest, not only for coastal protection, but also for offshore structures and operations (Wang et al., 2012; Kaiser and Niemeyer, 2007; Teich et al., 2018).

For such purposes, the state of the art third generation wave model SWAN (Simulating WAves Nearshore) is often applied for modeling design waves, wave climate, and special events in shelf seas, coastal, and nearshore areas (Booij et al., 1999; Hoque et al., 2019; Vieira et al., 2020; Sebastian et al., 2014). In many cases, static bathymetries are used for those applications (Hoque et al., 2019; Akpinar et al., 2016; Kutupoğlu et al., 2018; Sierra et al., 2017). But SWAN modeling with dynamical bathymetries is also an important and promising approach in coastal risk assessment. Thus, some studies estimated morphological impacts on the wave climate in coastal and nearshore areas (Dallas and Barnard, 2011; Eshleman et al., 2007). Due to local changes in the morphodynamic conditions and the interaction of numerous different influences, global wave climate issues are very heterogeneous on a local scale (Wang et al., 2012; Kaiser and Niemeyer, 2007). Various studies analyzed those morphological conditions and sediment dynamics of ebb-tidal deltas (ETD) in coastal areas (Hansen et al., 2013; Anthony, 2013; Bergillos et al., 2016). ETDs consist of sediments from ebb-tidal currents, deposited seawards of the tidal inlet (Dallas and Barnard, 2011). They are subject to rapid changes by migration, erosion, and accretion (Castelle et al., 2007).

The ETD sandbanks off the Norderney coastline have a major impact on the local wave climate, since they are a natural barrier against incoming waves, especially during extreme events for which only few in-situ buoy data are available (Niemeyer, 1979; Niemeyer and Kaiser, 1999). Wave breaking causes the sea state energy to be dissipated on the sandbanks before reaching the islands beaches, while many influences affect the damping, whose complexity is not yet entirely known even due to missing measurements (Niemeyer and Kaiser, 1999). In order to consider local morphodynamic influences on the sea state, small-scale and high-resolution analyses are necessary. Until now, no quantification of this damping effect, especially for the specific impact on swell waves, has yet been done. In particular, the influence of specific small-scale morphological changes or rather gaps in the ETD sandbanks on the sea state in this area is of great interest for coastal protection, as the ETD sandbanks are constantly exposed to morphological changes.

Thus, the main objective of this study was to quantify the influence of the ETD sandbank dynamics on the sea state and to analyze their damping effect by validated SWAN simulations for representative storm events at Tidal High Water (THW) and for swell in specific.

In chapter 2 the research area and dataset are presented. In chapter 3 the ETD sandbank dynamics (migration velocities and changes in size and shape) are analyzed with GIS based on five different bathymetries from 1995, 2005, 2006, 2008, and 2015. Chapter 4 presents the methods for data analysis and numerical modeling with SWAN. Based on the previous analyzes, chapter 5 focuses on the modeling results of the nearshore sea state of Norderney with dynamic bathymetries in SWAN to isolate and quantify the ETD influences on the sea state damping. Therefore, five representative storm cases are simulated and the respective damping effects are discussed. The Norderney nearshore wave climate is also characterized in this chapter to distinguish natural variability from morphological influences. Further evaluations transfer some general findings of the modeled sea state damping effect to the buoy measurements, unaffected and affected by the morphodynamic of the ETD sandbanks off the Norderney coast. Chapter 6 then provides appropriate conclusions.