Ameland Inlet

Ameland, Friesland, the Netherlands

Implementing the world’s first-ever ebb-tidal delta nourishment. Ebb-tidal deltas are large volumes of sand on the seaward side of tidal inlets. They are an important and integral part of barrier coasts because they provide sand for barrier islands and shelter back-barrier basins from high-energy swells. Many of the world’s ebb-tidal deltas are at risk because of structural erosion, dredging, and sand mining. This risk will increase with climate change and the associated increase in coastal erosion. In 2019, Rijkswaterstaat—the operational agency of the Dutch Ministry of Infrastructure and Water Management—undertook a five million cubic meter sand nourishment at the ebb-tidal delta of Ameland Inlet, in the north of the Netherlands. This first-ever nourishment scheme at an ebb-tidal delta was implemented within the framework of the Coastal Genesis 2.0 research program. The goal of the scheme was threefold: to enhance understanding of the ecological and morphological dynamics of tidal inlet systems and ebb-tidal deltas, to gain operational insights into the possibilities of nourishing ebb-tidal deltas and the technical design of this type of nourishment scheme, and to feed the coastal system with sediments in line with Dutch coastal policy.

Article Cover: Aerial photo of the Wadden Sea (back-barrier basin) in the direction of the North Sea open coast, where the ebb-tidal delta is located. (Photo by Rijkswaterstaat)

Producing Efficiencies

The dredging contractor collaborated closely on the technical design to ensure the safe construction of the nourishment scheme was feasible in this highly dynamic environment. Rijkswaterstaat partnered with research institutes Deltares and Wageningen Marine Research to track and evaluate the development of ebbtidal delta nourishment during and after construction. Dutch universities like Delft University of Technology and Utrecht University collaborated closely with Rijkswaterstaat to establish the academic research program SEAWAD (SEdiment supply At the WAdden sea ebb-tidal Delta) to investigate hydrodynamics, sediment transport, and benthic species at Ameland Inlet by collecting data through a field campaign, regular surveys, and numerical modeling.

Using Natural Processes

Technical design included placing five million cubic meters of sand against an outer ebb-shield along the rim of the ebb-tidal delta, where natural currents and waves redistributed the sand throughout the delta. The sand assimilated in the surrounding area as typical morphological features like underwater ridges formed, and the sand largely dissipated into the adjacent ebb shoal. Benthic communities were hardly affected because the nourishment site is subject to strong tidal currents and large waves. This showed that carefully selecting nourishment locations will not alter autonomous coastal morphodynamics or ecological processes and can restore the natural functions of ebb-tidal deltas.

The ebb-tidal delta nourishment plan with delineated nourishment area. Natural features, such as ebb-tidal deltas, require sediment import to sustain over time. When no natural source of sediment is available, periodic maintenance by means of sand nourishments may be a solution.

(Image by Rijkswaterstaat)

Broadening Benefits

Off-shore nourishment at ebb-tidal deltas provided economic benefits over traditional beach nourishment due to possible cost reductions. It can also reduce greenhouse gas emissions by requiring less material and fewer barge and boat trips to the placement site. In the Netherlands, sand is extracted offshore, seaward of the long-term depth of closure. Dynamic environments subject to high waves and strong tidal currents, ebb-tidal deltas are resilient and able to recover from disturbances, such as nourishment. Moreover, to sustain ebb-tidal deltas over time, sediment import is required. When no natural sediment source is available, dredged sediments or beneficial use of dredged material can feed ebb-tidal deltas.

Satellite image and satellite-derived bathymetry, respectively, of the ebb-tidal delta with the encircled nourishment site.

(Images by Sentinel Hub and Rijkswaterstaat)

Promoting Collaboration

In parallel with the Coastal Genesis 2.0 research program investigating sand nourishment, the universities of Delft, Utrecht, and Twente, along with other partners, established SEAWAD in close collaboration with Rijkswaterstaat. SEAWAD was a five-year scientific program (2016–2021) to investigate hydrodynamics, sediment transport processes, and benthic species in Ameland Inlet by collecting data through a field campaign, regular surveys, and numerical modeling. SEAWAD and Coastal Genesis 2.0 reinforced each other and resulted in the wide dissemination of new scientific and applied knowledge about the dynamics of tidal inlet systems and ebb-tidal deltas.

Bird’s eye view of the measured bathymetry of the nourishment site.

(Image by Thijs van Rhijn)