The urgent need for sediment management strategies and upscaling to larger-scale projects to prevent deltas from drowning requires future scientific attention. Moreover, the need to improve collaboration with upstream countries and adopt a more holistic management strategy for deltas emphasizes the importance of an integrated approach.
Understanding the decreasing sediment delivery to deltas requires further investigation of the processes related to sediment in the upstream catchment. Tatjana Edler, PhD researcher studying the contemporary supply and provenance of suspended sediment in the Rhine river, explains, "Sediments often travel several thousands of kilometers. What and how sediment is impacted on the way, naturally and anthropogenically, still needs to be better understood". Besides, the impacts of decreasing river sediment delivery to deltas worldwide remain difficult to predict. Impacts will differ for deltas across the globe and depend on the measures taken in the delta itself to protect land from flooding, or to re-establish sedimentation.
Urgent need for flexible and adaptable strategies
Considering future relative sea-level rise, there is an urgent need for efficient and socially feasible sediment management strategies in deltas worldwide to prevent them from drowning. For these strategies to be successful and sustainable, it is crucial that sediment delivery to relevant locations is maximized e.g. by removing hard engineering solutions, such as dams, dikes and seawalls, which prevent sediment accretion. With decreasing amounts of sediment delivered by rivers, marine sand and mud may be the main material remaining for sediment management. However, “If we do not keep the global temperature increase well below 2°C, the resulting sea-level rise of the next century may be too extreme to adapt to by sedimentation strategies (Storyline: Flood risk management)”, says Maarten Kleinhans, professor of rivers and estuaries.
The challenge now is designing nature-based solutions on a larger scale
Upscaling small-scale pilot projects
Various sediment management measures have been tested already, but these are mostly small-scale pilot projects. “The challenge now is designing nature-based solutions on a larger scale", explains Hans Middelkoop, leader of the Water, Climate and Future Deltas hub. This requires an understanding of the connectivity of sediment pathways across the delta (Storyline: Pathways of delta development) and must be part of a more integrative, system-scale approach to sediment governance. Upscaling to larger delta areas will also mean that these strategies need to compete for space e.g. with population growth and urbanization, consider ecological and socio-economic aspects of the measures and ensure public acceptance of such large-scale interventions. Moreover, upscaling requires resources in terms of funding, technology and infrastructure which can be a challenge for many deltas.
Separate parts of the delta cannot be managed independently because they all impact each other through physical, chemical, biological and societal processes. This stresses the need for a holistic, or systemic, management strategy of deltas and their river catchment. Maarten Kleinhans explains, "This is especially true for so called delta ‘hotspots’, which are parts of the system that significantly influence the causes of and solutions to problems in other parts of the delta". Examples may be the river bifurcations, the tidal inlets and the estuaries. An important question for the future lies in locating these hotspots and assessing their influence on the entire delta system.
Strong international collaboration with upstream countries and transboundary governance are needed to secure sufficient sediment
Cooperative arrangements and transboundary governance
Delta system boundaries often do not overlap with administrative borders and institutional arrangements. "Strong international collaboration with upstream countries and transboundary governance are needed to secure sufficient sediment”, underlines Herman Kasper Gilissen, associate professor of environmental- and water law and governance. A major problem is that many transboundary river basins currently lack effective cooperative arrangements regarding water issues as a result of competing interests and political tension. Sediment-related arrangements are even more limited. In addition, existing transboundary regulations are often weak in terms of their design and enforcement instruments and conflict with the natural dynamic processes of rivers.