The seabed rarely consists solely of bare sand: often other materials, such as shells are present. They can influence sand transport by armoring the bed and modifying its roughness. Biogenic shells come in different shapes and sizes, depending on the mollusc species that produce them. To understand how changes in bivalve species composition affect sediment transport, we need a mechanistic understanding of how shell content and shell shape influence the near-bed flow and sand transport. We performed experiments in a racetrack flume, testing the effect of elongated (Ensis leei) versus rounded (Spisula subtruncata) shells on unidirectional current-driven sand transport. For both types of shells, a higher depth-averaged flow velocity was needed for initiation of motion and a decrease in bedload transport of sand was found. At a shell content of 20%, the threshold of motion of sand increased up to 75%, and bedload transport was reduced by up to 50%. These effects are explained by a balance between roughness-induced turbulence and bed armoring. Compared to a bare bed, shells decreased bed roughness by reducing ripple formation; rounded shells lowered roughness more than elongated shells, which formed roughness elements themselves, but also covered a larger fraction of the bed. However, there was no clear difference between round versus elongated shells on the overall sand transport; only shell content was key for the overall effect. Our results imply that sediment transport is likely overpredicted when a high number of shells is present in the seabed.

There is a relative lack of research, targeted models and tools to manage beaches in estuaries and bays (BEBs). Many estuaries and bays have been highly modified and urbanised, for example port developments and coastal revetments. This paper outlines the complications and opportunities for conserving and managing BEBs in modified estuaries. To do this, we focus on eight diverse case studies from North and South America, Asia, Europe, Africa and Australia combined with the broader global literature. Our key findings are as follows: (1) BEBs are diverse and exist under a great variety of tide and wave conditions that differentiate them from open-coast beaches; (2) BEBs often lack statutory protection and many have already been sacrificed to development; (3) BEBs lack specific management tools and are often managed using tools developed for open-coast beaches; and (4) BEBs have the potential to become important in nature-based management solutions. We set the future research agenda for BEBs, which should include broadening research to include greater diversity of BEBs than in the past, standardising monitoring techniques, including the development of global databases using citizen science and developing specific management tools for BEBs. We must recognise BEBs as unique coastal features and develop the required fundamental knowledge and tools to effectively manage them, so they can continue providing their unique ecosystem services.

A field campaign was carried out at a sheltered sandy beach with the aim of gaining new insights into the driving processes behind sheltered beach morphodynamics. Detailed measurements of the local hydrodynamics, bed-level changes and sediment composition were collected at a man-made beach on the leeside of the barrier island Texel, bordering the Marsdiep basin that is part of the Dutch Wadden Sea. The dataset consists of (1) current, wave and turbidity measurements from a dense cross-shore array and a 3 km alongshore array; (2) sediment composition data from beach surface samples; (3) high-temporal-resolution RTK-GNSS beach profile measurements; (4) a pre-campaign spatially covering topobathy map; and (5) meteorological data. This paper outlines how these measurements were set up and how the data have been processed, stored and can be accessed. The novelty of this dataset lies in the detailed approach to resolve forcing conditions on a sheltered beach, where morphological evolution is governed by a subtle interplay between tidal and wind-driven currents, waves and bed composition, primarily due to the low-energy (near-threshold) forcing. The data are publicly available at 4TU Centre for Research Data at: https://doi.org/10.4121/19c5676c-9cea-49d0-b7a3-7c627e436541 (Van der Lugt et al., 2023).