Understanding sediment bypassing processes through analysis of high-frequency observations of Ameland Inlet, the Netherlands

Abstract

Ameland inlet is centrally located in the chain of West Frisian Islands (the Netherlands). A globally unique dataset of detailed bathymetric charts starting in the early 19th century, and high-resolution digital data since 1986 allows for detailed investigations of the ebb-tidal delta morphodynamics and sediment bypassing over a wide range of scales. The ebb-tidal delta exerts a large influence on the updrift and downdrift shorelines, leading to periodic growth and decay (net erosion) of the updrift (Terschelling) island tip, while sequences of sediment bypassing result in shoal attachment to the downdrift coastline of Ameland. Distinct differences in location, shape and volume of the attachment shoals result from differences in sediment bypassing, which can be driven by morphodynamic interactions at the large scale of the inlet system (O(10 km)), and through interactions that originate at the smallest scale of individual shoal instabilities (O(0.1 km)). Such shoal instabilities would not be considered to affect the ebb-tidal delta and inlet dynamics as a whole, but as we have shown in this paper, they can trigger a new sediment bypassing cycle and result in complete relocation of channels and shoals. These subtle dynamics are difficult, if not impossible, to capture in existing general conceptual models and empirical relationships. These differences are, however, essential for understanding tidal inlet and channel morphodynamics and hence coastal management.

Measurements of hydrodynamics, sediment, morphology and benthos on Ameland ebb-tidal delta and lower shoreface

“…Over the last centuries, multiple large-and small-scale interventions, such as coastal defence works, closure dams, dikes, sea walls, land reclamations and closing of the Middelzee around 1600, have reduced and essentially fixed the basin dimensions and kept the barrier islands in place. As a result, a geomorphic transition in morphodynamic behaviour of Ameland Inlet occurred around 1926 as the main ebb channel migrated from an updrift to a downdrift position in the inlet gorge, encroaching on the western side of Ameland (Elias et al, 2019). This channel has retained this position since then, partly due to extensive coastal protection works at the tip of the island.…”

Measurements of Hydrodynamics, Sediment, Morphology and Benthos on Ameland Ebb-Tidal Delta and Lower Shoreface

“…Over the last centuries, multiple large-and small-scale interventions, such as coastal defence works, closure dams, dikes, sea-walls, and land reclamations, closing of the Middelzee around 1600, have reduced and essentially fixed the basin dimensions and kept the barrier islands in place. As a result, a geomorphic transition in morphodynamic behavior of Ameland Inlet occurred around 1926 as the main ebb-channel migrated from an updrift to a https://doi.org/10.5194/essd-2020-13 downdrift position in the inlet gorge, encroaching on the western side of Ameland (Elias et al, 2019). This channel has retained this position since then, partly due to extensive coastal protection works at the tip of the island.…”

“…These datasets are an addition to the regular bathymetric monitoring conducted here and follow similar protocols. Ameland Inlet has a long history of bathymetric surveying (Elias et al, 2019). Since 1985, bathymetric data are collected systematically by Rijkswaterstaat, which is part of the Ministry of Public works and Infrastructure, following the Vaklodingen protocol (De Kruif, 2001).…”

Sediment Connectivity: A Framework for Analyzing Coastal Sediment Transport Pathways

“…The lateral boundaries within the Wadden Sea are considered closed in these simulations. Ameland Inlet has a tidal range of between 1.5-3 m, and tidal prism of 400-500 Mm 3 (Elias et al, 2019). The tide drives currents of approximately 1 m/s in the main channel of the inlet at ebb and flood.…”

“…We adopted a morphostatic (fixed bed) modeling approach, but permitted sediment exchange between the bed and water column. We ran the model for 6 months (360 tidal cycles), which ensures that the modeled timescale is smaller than the timescale of observable morphologic change at the chosen spatial scale, based on annual bathymetric surveys (Elias et al, 2019). This is also long enough to ensure that the network is well connected with few separate subsystems or components.…”