Historic storms and the hidden value of coastal wetlands for nature-based flood defence

Zhu, Zhenchang; Vuik, Vincent; Visser, Paul J.; Soens, Tim; Wesenbeeck, Bregje K. van; Koppel, Johan van de; Jonkman, Sebastiaan N.; Temmerman, Stijn; Bouma, Tjeerd J.

doi:10.1038/s41893-020-0556-z

Cited by 128 publications

(120 citation statements)

References 39 publications

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“…Even when sediment supply is too low to fill basins entirely, the combination of vegetation helping to both fill the accommodation space while also reducing the tidal prism and together with sedimentation corridors (e.g. crevasses) along channels may be an attractive way to distribute sediments that contribute to land rise and enhance flood protection (Zhu et al 2020). When building artificial levees that are never or rarely flooded, most sediments are carried downstream via main channel, and do not contribute to land level rise.…”

Section: Considerations For Future Delta and Estuary Managementmentioning

confidence: 99%

Natural levee evolution in vegetated fluvial‐tidal environments

Albernaz

Roelofs

Pierik

et al. 2020

Earth Surf Processes Landf

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Natural levees are common features in river, delta and tidal landscapes. They are elevated near‐channel morphological features that determine the connection between channel and floodbasin, and consequently affect long‐term evolution up to delta‐scales. Despite their relevance in shaping fluvial‐tidal systems, research on levees is sparse and often limited to fluvial or non‐tidal case studies. There is also a general lack of understanding of the role of vegetation in shaping these geomorphic units, and how levee morphology and dimensions vary in the transition from fluvial to coastal environments, where tides are increasingly important. Our goal is to unravel the effects of fluvial‐tidal boundary conditions, sediment supply and vegetation on levee characteristics and floodbasin evolution. These conditions were systematically explored by 60 large‐scale idealized morphodynamic simulations in Delft3D which self‐developed levees over the course of one century. We compared our results to a global levee dataset compilation of natural levee dimensions. We found that levee height is determined by the maximum water level, provided sufficient levee building sediments are available. Discharge fluctuations increased levee width and triggered more levee breaches, i.e. crevasses, that effectively filled the fluvio‐tidal floodbasin. The presence of wood‐type (sparse) vegetation further increased the number of crevasses in comparison with the non‐vegetated scenarios. Conversely, reed‐type (dense) vegetation strongly dampened tidal amplitude and reduced the accommodation space and sedimentation further into the floodbasin, resulting in narrower levees, no crevasses and limited floodbasin accretion. However, dense vegetation reduced tidal forces which allowed levee growth further downstream. Ultimately, the levees merged with the coastal barrier, eliminating the floodbasin tides entirely. Our results elucidate the mechanisms by which levee and crevasse formation, and vegetation may fill fluvio‐tidal wetlands and affect estuary evolution. This brings new insights for geological reconstructions as well as for the future management of deltas and estuaries under sea‐level rise. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

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Section: Considerations For Future Delta and Estuary Managementmentioning

confidence: 99%

Natural levee evolution in vegetated fluvial‐tidal environments

Albernaz

Roelofs

Pierik

et al. 2020

Earth Surf Processes Landf

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“…Current predictions suggest that by 2100, annual coastal ooding will directly affect up to 5% of the world's population and cost up to 20% of global gross domestic product per year 4,20 . Although ood risks have traditionally been managed by building seawalls and other hardened defence structures [21][22][23] , the emerging paradigm of nature-based coastal protection holds that resilient, waveand surge-absorbing wetlands such as saltmarshes and mangroves should be integrated into coastal planning and management to more sustainably and effectively mitigate ood risk and impacts [24][25][26][27] . However, quantifying and valuing the contribution of ecosystems to ood mitigation is fraught with uncertainty due to multi-scale interactions between ecological features and hydrodynamic processes [28][29][30][31] .…”

Section: Introductionmentioning

confidence: 99%

Coastal wetlands mitigate storm flooding and associated costs in estuaries

Fairchild

Bennett

Smith

et al. 2021

Preprint

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As storm-driven coastal flooding increases under climate change, wetlands such as saltmarshes are held as a nature-based solution. Yet evidence supporting wetlands’ storm protection role in estuaries - where both waves and upstream surge drive coastal flooding - remains scarce. Here we address this gap using numerical hydrodynamic models within eight contextually diverse estuaries, simulating storms of varying intensity and coupling flood predictions to damage valuation. Saltmarshes reduced flooding across all studied estuaries and particularly for the largest – 100-year – storms, for which they mitigated average flood extents by 35% and damages by 37% ($8.4M). Across all storm scenarios, wetlands delivered mean annual damage savings of $2.7M per estuary, exceeding annualised values of better-studied wetland services such as carbon storage. Spatial decomposition of processes revealed flood mitigation arose from both localised wave attenuation and estuary-scale surge attenuation, with the latter process dominating: mean flood reductions were 17% in the sheltered top third of estuaries, compared to 8% near wave-exposed estuary mouths. Saltmarshes therefore play a generalised role in mitigating storm flooding and associated costs in estuaries via multi-scale processes. Ecosystem service modelling must integrate processes operating across scales or risk grossly underestimating the value of nature-based solutions to the growing threat of storm-driven coastal flooding.

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“…Vegetated foreshores, such as marshes and mangroves, are promoted globally for their capacities in reducing impacts of waves, winds and surges [1][2][3][4][5][6] . Besides along coastlines there is also potential for reducing wave heights and run-up in rivers and lakes by floodplain vegetation and riparian forests 7,8 .…”

Section: Introductionmentioning

confidence: 99%

Woods versus waves: Wave attenuation through non-uniform forests under extreme conditions

Wesenbeeck¹,

Wolters²,

Antolínez³

et al. 2021

Preprint

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Worldwide, communities are facing increasing flood risk, due to more frequent and intense hazards and rising exposure through more people living along coastlines and in flood plains. Nature-based Solutions (NbS), such as mangroves, and riparian forests, offer huge potential for adaptation and risk reduction. The capacity of trees and forests to attenuate waves and mitigate storm damages receives massive attention, especially after extreme storm events. However, application of forests in flood mitigation strategies remains limited to date, due to lack of real-scale measurements on the performance under extreme conditions. Experiments executed in a large-scale flume with a willow forest to dissipate waves show that trees are hardly damaged and strongly reduce wave and run-up heights, even when maximum wave heights are up to 2.5 meters. It was observed for the first time that the surface area of the tree canopy is most relevant for wave attenuation, but that the very flexible leaves hardly add to effectiveness. Overall, the study shows that forests can play a significant role in reducing wave heights and run-up under extreme conditions. Currently, this potential is hardly used but may result in considerable cost savings in levee designs.

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Historic storms and the hidden value of coastal wetlands for nature-based flood defence

Cited by 128 publications

References 39 publications

Natural levee evolution in vegetated fluvial‐tidal environments

Natural levee evolution in vegetated fluvial‐tidal environments

Coastal wetlands mitigate storm flooding and associated costs in estuaries

Woods versus waves: Wave attenuation through non-uniform forests under extreme conditions

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