Monitoring Impact of Salt-Marsh Vegetation Characteristics on Sedimentation: an Outlook for Nature-Based Flood Protection

B Martina Baaij,Jeroen Kooijman,Juul Limpens,Jantsje M Van Loon-Steensma,Richard J C Marijnissen

doi:10.1007/s13157-021-01467-w

Abstract

Salt marshes can protect coastlines against flooding by attenuating wave energy and enhancing shoreline stabilization. However, salt-marsh functioning is threatened by human influences and sea level rise. Although it is known that protection services are mediated by vegetation, little is known about the role of vegetation structure in salt-marsh accretion. We investigated the role of vegetation presence, vegetation type and structural vegetation characteristics in sedimentation and sediment grain size. We established 56 plots on a salt marsh on the Dutch Wadden island of Texel. Plots were divided over four vegetation types contrasting in vegetation structure and varied in elevation and distance to creeks. Vegetation presence was controlled by clipping in subplots. Within each plot, we measured seven vegetation characteristics, sedimentation and the sediment grain size distribution. Furthermore, we explored the effect of the natural variation in vegetation structure on wave attenuation with a simple model approach. For this, we developed vegetation scenarios based on the field measurements of stem height, diameter and density. We found that vegetation presence increased sedimentation on average by 42%. Sedimentation was highest in Salicornia vegetation and increased with stem height and branching level. Grain size also seemed to increase with branching level. Modelled wave attenuation was 7.5 times higher with natural vegetation compared to topography only, was strongest for Spartina vegetation and most sensitive to the natural variance in stem density. Our results can be used to improve predictions of salt-marsh accretion and the implementation of salt marshes in nature-based flood defences.

Highlights

Salt marshes provide important ecosystem services, such as protection against coastal erosion and flooding (Gedan et al 2009; Barbier et al 2011)
We investigated the effect of vegetation presence, vegetation type and natural variation in vegetation structure characteristics on sedimentation and sediment grain size on a salt marsh in the Netherlands
We found that modelled wave attenuation was stronger for the scenario with a zonation of the four vegetation types (60% for the first 100 m) compared to the average wave damping of the individual vegetation types weighted to their occurrence (51%), indicating the importance of including vegetation zonation in the model

Summary

Introduction

Salt marshes provide important ecosystem services, such as protection against coastal erosion and flooding (Gedan et al 2009; Barbier et al 2011). The expected sea level rise and increased storm activity highlights the role of salt marshes in coastal protection (Shepard et al 2011; Leonardi et al 2018). A meta-analysis by Shepard et al (2011) demonstrates positive contributions of salt-marsh vegetation to shoreline stabilization (i.e. sediment deposition, prevention of erosion and increase in elevation) and wave attenuation. Temmerman et al (2013) argue that coastal ecosystems, like salt marshes, are a preferable long-term flood protection measure over conventional hard infrastructure because of their natural adaptive capacity to sea level rise. The implementation of salt marshes in nature-based solutions requires an in-depth understanding of the role of vegetation in their adaptive capacity and flood protection function

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