Sand nourishments on beaches and ebb-tidal deltas are examples of strategies that relocate sediment with the aim of accumulating sediment along the coast to mitigate flooding. The Water, Climate and Future Deltas hub assesses the impact of these interventions on the sediment dynamics and budget of the Rhine-Meuse delta.
Protection against erosion
Like deltas, sandy coasts are highly dynamic (Follow the sediment). Over decades to centuries an imbalance in sand exchange between the submerged and emerged parts of the beach can arise, which results in either growth or erosion of the beach. Erosion is especially problematic in light of future sea-level rise and changing storm conditions, which may further increase erosion. Sand nourishment involves adding large amounts of sand to the coastal system which is gradually distributed along the coast and dunes by winds, currents and tides to protect the beach and dunes against erosion.
Between 1997-2008, ~25% of the nourished sand ended up in the dune systems
Along the Dutch coast, ~12 million m-3 of sand is artificially supplied each year to counteract the erosive trend. "Measurements show that the total volume of sand along the coast has increased. Between 1997-2008, ~25% of the nourished sand ended up in the dune systems”, says Timothy Price, assistant professor of coastal dynamics, fluvial systems and global change at Utrecht University. However, it remains unclear how the sand from individual nourishments finds its way to the beach and dunes and becomes part of the larger system. It is also unknown how repeated or large-scale nourishments affect the development of the bio-geomorphological coastal system.
Increasing the volume
Further research is needed to fully understand the consequences of nourishments for the bio-geomorphological system, especially if nourishment volumes are increased in the future to allow the coast to keep up with the effects of climate change. “If sea levels rise by 0.8 m by the end of this century, we need 4-5 times as much sand as currently is applied annually”, says Marjolijn Haasnoot, associate professor of water management and climate adaptation. “The required amount of sand can even rise to 20 times the current volumes if sea-level rise further accelerates (Storyline: The future of the Rhine-Meuse delta)”.
Sand nourishment on ebb-tidal deltas
Positive sand budget
Ebb-tidal deltas are large underwater bodies of sand in front of tidal inlets, deposited by outflowing tides (i.e. ebb) and reshaped by waves. The Wadden Sea, a dynamic area in the south-eastern part of the North Sea, continuously exchanges sediment between its tidal basins, the North Sea and the ebb-tidal deltas (Follow the sediment). Currently the Wadden Sea has a positive sediment budget, which is why it can keep up with sea-level rise and compensate for human interventions. The sediment volume in the area increased by ~600 million m-3 over the period 1935-2005, primarily due to erosion of the ebb-tidal deltas and the nearshore zones of the barrier islands.
Nourishments on ebb-tidal deltas
Rijkswaterstaat, who is responsible for the design, construction, management and maintenance of the main infrastructure in the Netherlands, currently nourishes the beaches and nearshore zone of the barrier islands to compensate for erosion. In light of potentially accelerating sea-level rise, they are now also considering nourishing ebb-tidal deltas. However, an ebb-tidal delta nourishment is very different from beach nourishment, which occurs in wave-dominated environments. Ebb-tidal deltas are strongly influenced by tides, and the dynamics of flow and sediment transport are far more complex.
Improving models with field data
Maarten van der Vegt, associate professor of coastal morphodynamics, together with colleagues from Utrecht University, TU Delft and University of Twente, was involved in the SEAWAD project. Part of the project was a large-scale field campaign at the Ameland Inlet in 2017. Maarten van der Vegt explains, "We use the unique dataset obtained from the field campaign to design and improve modelling tools to predict and design nourishments on ebb-tidal deltas". Using the improved numerical models, Klaas Lenstra found that if the nourishment is placed in a shallow part of the ebb-tidal delta, it enhances the sediment supply to the Wadden Sea and the Wadden Islands by accelerating natural dynamics.