Dynamic preservation of Texel Inlet, the Netherlands: understanding the interaction of an ebb-tidal delta with its adjacent coast

Abstract

Tidal inlets and the associated ebb-tidal deltas can significantly impact the coastal sediment budget due to their ability to store or release large quantities of sand. Nearly 300 million m3(mcm) of sediments were eroded from Texel Inlet's ebb-tidal delta and the adjacent coasts following the closure of the Zuiderzee in 1932. This erosion continues even today as a net loss of 77 mcm was observed between 1986 and 2015. To compensate, over 30 mcm of sand has been placed on the adjacent coastlines since 1990, making maintenance of these beaches the most intensive of the entire Dutch coastal system.Highly frequent and detailed observations of both the hydrodynamics and morphodynamics of Texel Inlet have resulted in a unique dataset of this largest inlet of the Wadden Sea, providing an opportunity to investigate inlet sediment dynamics under the influence of anthropogenic pressure. By linking detailed measurements of bathymetric change to direct observations of processes we were able to unravel the various components that have contributed to the supply of sediment to the basin, and develop a four-stage conceptual model describing the multi-decadal adaptation of the ebb-tidal delta.Prior to closure of the Zuiderzee a dynamic equilibrium state (stage 1) existed with a stable ebb-tidal delta. The largest morphological changes occurred in roughly the first 40 years since the closure, and were dominated by the rotation and scouring of large tidal channels and landward retreat of the Noorderhaaks ebb shoal (stage 2; adaptation). Between 1975 and 2001 the general layout of main channels and shoals was stable, but large sediment losses continued to occur (stage 3; equilibrium erosional state). Since 2001, the erosion rates have significantly reduced to 2 mcm a−1(stage 4; stabilisation).Twenty-five years of data on ‘Dynamic Preservation’ prove that sand nourishments are well capable of keeping the coastlines adjacent to the Texel Inlet in place. Moreover, the abundant supply of sediment may also have compensated for the sediment losses on the larger scale of the southern part of the ebb-tidal delta, resulting in a recent stabilisation of its volume. This response illustrates the potential benefits of Dynamic Preservation not only for coastline resilience but also on the larger scale of the inlet system. Such knowledge is essential for future preservation, management and maintenance of inlet systems in the scope of climate change and accelerated sea-level rise.

Sediment budget and morphological development of the Dutch Wadden Sea: impact of accelerated sea-level rise and subsidence until 2100

“…2) have been the major sources of sand for the Dutch Wadden Sea over the last 85 years (Elias et al, 2012). Whether this will be the case in the future is unclear since the morphodynamic changes in these areas are reaching a near-equilibrium state (see Elias & Van der Spek, 2017, for an analysis of Texel Inlet). The erosion of the North Sea shorelines of the barrier islands, especially the tips of the islands if not protected, and the Noord-Holland coast is an additional sediment source; the sand is predominantly transported into the tidal inlets by littoral drift.…”

“…Sand nourishment along the North Sea coasts of the majority of the barrier islands since 1990, to ensure the long-term safety against flooding, introduces an artificial sediment supply to the system. This additional sediment source could have influenced the morphological development so far (Van der Spek & Lodder, 2015; Elias & Van der Spek, 2017) and may have an increasing impact in the future as larger nourishment volumes are most likely needed. In the last century, local human activities have been more important than SLR in driving the morphological development of the Dutch Wadden Sea system (Elias et al, 2012).…”

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

“…The above studies all demonstrate that it is essential to develop a comprehensive understanding of the underlying inlet dynamics and sediment transport processes before generalizing in a conceptual model description. Such understanding cannot be obtained solely by analysing (usually sparsely available) bathymetric data, but must be obtained through applying process-based models (Elias, 2006, Herrling and Winter, 2017, 2018 and/or detailed analysis of field data such as morphological changes, spatial grain-size patterns, and internal sedimentary structures (Son et al, 2011;Elias, 2006, Elias andVan der Spek, 2017). Based on these studies, we may conclude that the existing sediment bypassing concept provides a useful framework to describe channel and shoal formation and migration on the ebb-tidal delta, but it may not describe the (full) process of sediment bypassing.…”

Storm-induced sediment supply to coastal dunes on sand flats

“…The Marsdiep inlet is classified as a mixed-energy wave dominated inlet, with a gorge width of 3 km and a channel depth up to 50 meter (Elias and Spek, 2017). The inlet has an asymmetric ebb-tidal delta that is mostly conditioned by side-effects of a past large-scale engineering project (i.e.…”

“…The inlet has an asymmetric ebb-tidal delta that is mostly conditioned by side-effects of a past large-scale engineering project (i.e. construction of the "Afsluitdijk") (Elias and Van Der Spek, 2006;Elias and Spek, 2017). The sand flat is exposed to wind most of the time and is flooded only during storm surge events.…”