IC1_II

As part of the INTERCOAST program, the IC1 project focuses on the morphodynamic properties of coastal areas, in particular in the German Bight and in the Bay of Plenty-Coromandel (New Zealand). In order to understand the potential impact of the human activity on the coasts nearby, the morphodynamics behaviour of the local beaches must be characterized. Therefore it must be understood what controls the equilibrium profile of a beach and what is the influence of seasonal variations in the climate or of extreme punctual storm events.
At first sight, Northwest Germany and New Zealand coastal systems seem totally different. The first location exhibits a sequence of numerous barrier islands protecting the Wadden Sea environment from the North Sea highly energetic incident wave climate. Meanwhile the North Sea side of these islands exhibit logically multi-barred dissipative beaches resulting from the significant wave energy dissipation, the Wadden Sea is controlled by the tidal variations of the water level and shows large tidal flats composed of mixed sand and mud and housing a rich biodiversity. Conversely, the Bay of Plenty-Coromandel exhibits an open shore subject to the Pacific swell and to occasional winter storm energetic waves. Rocky coasts are found at the extremities of the bay that are intersected by numerous sandy embayed pocket beaches (for instance Hot Water Beach, Otara Bay, Te Karo Bay, Tairua in the north, Whangaparaoa Bay, Te Rangiharu Bay in the south). In the deep part of the bay, from Waihi Beach to Opape, the coast is a long sandy beach intersected by river estuaries. The embayed sand beaches like Tairua are found to show more frequently beach states close to the reflective state (Almar et al., 2008; Gallop et al., 2011) compare to the sand beaches of the North Sea.
Sylt where takes place the first field site is one of the numerous Wadden Sea barrier islands. Located at the border with Denmark in Schleswig-Holstein, the northernmost region of Germany, Sylt separates the Wadden Sea (east coast) from the North Sea (west coast). In order to face the recurrent issue of erosion, the behaviour of the North Sea beach face needs to be better understood. The beach clearly exhibits a doubled-barred profile with a submerged bar located approximately 500m from the shoreline and an inner bar located in the intertidal area. To assess the behaviour of these bars, the Center for Marine Environmental Sciences MARUM from the University of Bremen runs two beach video cameras on the west coast in the south of the island in cooperation with Karin Bryan (Waikato University, NZ and Helmholtz Center Geestacht (HZG).
Tairua and Pauanui are located in the north of the Bay of Plenty, along the east coast of the Coromandel peninsula. Smith and Bryan (2007) investigated beach volumes using video imaging techniques in Tairua. According to this paper, Tairua and Pauanui are generally exposed to north-easterly to easterly waves. The mean significant wave height is 1.4m and can reach 6m offshore during storm conditions. Tairua beach is in an embayment of around 1.2km wide delimited by two rocky headlands. Similarly, Pauanui beach takes place between two rocky headlands approximately 2.7km away from each other. However, at the northern extremity of the beach is located the estuary of the Tairua river which influences significantly the morphology. According to Smith and Bryan (2007), the characteristic profile at Tairua typically exhibits one longshore bar on a steep 0.06 averaged bed slope with a well-sorted medium sand (d50 = 300 μm). It can also be observed on satellite images that the Pauanui beach profile shows two sandbars parallel to the shoreline. The distance between the bars and the shoreline increases from the southern to the northern extremity of the beach where the bar somewhat merges with the ebb delta sandbanks. In addition, beach cusps develop regularly along the shoreline of both beaches. Video cameras from the New Zealander Cam-Era network (http://www.niwa.co.nz) are installed on the headland between Tairua and Pauanui (Paku Hill) and on the headland south of Pauanui. Thus similarly to Sylt, images from both beaches are available for study. Moreover, Christopher Daly, former IC1 PhD student, performed an extensive annual field campaign in 2011, gathering a large number of beach profiles and sediment samples along both Tairua and Pauanui.
Beaches exhibiting alongshore sandbars are commonly observed at sandy and wave-dominated coasts. Nearshore sandbar dynamics is studied mainly because bars influence strongly the surfzone circulations and the transport of sediments nearshore; as well they are directly linked to the occurrence of rip currents. Therefore, the understanding of their behaviour is fundamental regarding the recurrent coastal issues of erosion, ecological conservation and recreational safety. The better understanding of beach cusps behaviour and especially of their formation would also significant

Usually, numerical models are intensively used in the domain of engineering in order to characterize and find solutions to the problems specific to a study case (decision support). Ideally in coastal engineering, they are used to simulate erosion and sedimentation processes for coastal protection or harbour maintenance. They are also used to understand the propagation of long waves in harbour and to prevent resonance effects likely to damage mooring ships. However, they can also be used in order to characterize and understand natural processes, and their nonlinear interactions. Indeed thanks to sensitivity analysis on the parameters controlling the processes they describe, it is possible to understand phenomena that are not easy to observe and to monitor in nature. Numerical models become then interesting in the study of storm impacts, of long-term beach behaviours or of a specific process that occur generally in a complex combination with other processes. They will therefore be used in the present study in order to understand and characterize the morphodynamics of the German Bight and the Bay of Plenty-Coromandel areas.
Given the present background, the aim of the present project is to describe the different processes responsible for the development of the coastal morphological features that are the nearshore sandbars and the beach cusps. At the present state, the research questions are:

The methods to answer these questions are as follow:

Multiple sediments with different properties can be simultaneously used in a Delft3D model, allowing for the study of the different behaviours of fine and coarse sand. Therefore, the Delft3D morphological model of Tairua and Pauanui will be used to characterize the beach sedimentology during selected morphodynamics beach transitions. The results will be compared to the analysis of the sediment sampling survey performed by Christopher Daly.