Nature-based solutions for flood protection in a systems perspective

The application of nature-based solutions for flood protection in low-lying coastal areas has recently received much attention. The solution is attractive because it may lower the costs for coastal defense, increase the effectiveness and resilience of defense measures, curb negative trends in biodiversity of globally important animal populations and contribute to carbon capture and storage.

Nature-based flood defense measures are based on relatively well-established biogeomorphological principles of self-organisation in coastal landscapes. Vegetated foreshores in particular contribute to wave damping, accumulation and stabilization of sediments maintaining a shallow bathymetry, capacity to adjust bed level to sea level rise and, in some conditions, provision of accommodation space for storm surges. Uncertainties in the contribution of ecosystems to flood safety arise from incomplete modeling capacity for sediment dynamics, inclusion of sub-grid phenomena, complex spatial structure, limited understanding of edge dynamics and sparse knowledge of ecosystem behavior under extreme loading. For application at regional scale, the most difficult problem is that nature-based solutions are needed most where natural processes are weakest, e.g. due to extreme encroachment of the coast.

In the broader context of use of nature-based solutions as measure to adapt to increased relative sea level rise, a crucial question is how these solutions will behave at longer term. Geological evidence shows that increased rates of sea level rise forces coastal wetlands to become narrower and to move faster inland. An important factor determining this behavior is the sediment mass balance, inhibiting the vertical rise of an ever-larger surface of marshes with increasing sea level. A second factor is the stability of the seaward edge under conditions of increased wave attack in deeper coastal profiles. These phenomena are observed in contemporary systems with high rates of subsidence.

Future-proof nature-based solutions for flood defense will have to resolve the issues related to the regional sediment mass balance and long-term stability of the coastal ecosystems. Whether the strategy is to hold the line of the coast or to withdraw, future coastlines will need sufficient sediment sources, sufficient sediment transport capacity and sufficient ecosystem resilience to provide extended services combining flood defense and biodiversity conservation. Major scientific challenges in the design and use of nature-based solutions are identified at this system scale.