Comment on esurf-2021-66

doi:10.5194/esurf-2021-66-rc2

Abstract

There is an increasing demand for creation and restoration of tidal marshes around the world, as they provide highly valued ecosystem services. Yet, tidal marshes are strongly vulnerable to factors such as sea level rise and declining sediment supply. How fast the restored ecosystem develops, how resilient it is to sea level rise, and how this can be steered by restoration design, are key questions that are typically challenging to assess. In this paper, we apply a biogeomorphic model to a planned tidal marsh restoration by dike breaching. Our modeling approach integrates tidal hydrodynamics, sediment transport and vegetation dynamics, accounting for relevant fine-scale flow-vegetation interactions (less than 1 m2) and their impact on vegetation and landform development at the landscape scale (several km2) and on the long term (several decades). Our model performance is positively evaluated against observations of vegetation and geomorphic development in adjacent tidal marshes. Model scenarios demonstrate that the restored tidal marsh can keep pace with realistic rates of sea level rise and that its resilience is more sensitive to the availability of suspended sediments than to the rate of sea level rise. We further demonstrate that restoration design options can steer marsh resilience, as it affects the rates and spatial patterns of biogeomorphic development. By varying the width of two dike breaches, which serve as tidal inlets to the restored marsh, we show that a larger difference in the width of the two inlets leads to more diversity in restored habitats. This study showcases that biogeomorphic modeling can support management choices in restoration design to optimize tidal marsh development towards sustainable restoration goals.

Highlights

Tidal marshes are among the most productive ecosystems on Earth (Barbier et al, 2011) providing invaluable services such as protection of coastal settlements against storms (Gedan et al, 2011; Zhu et al 2020), carbon sequestration
The success of restoration designs largely depends on the resulting rates of marsh vegetation development and sediment accretion, as they control the timescales at which target habitats, effective shoreline protection and carbon sequestration are reached
In the Southern basin, neither the presence of vegetation nor the speed of colonization seems to affect sediment accretion on the platforms (Fig. S1b, supplementary material), which suggests that the hydrodynamics is predominant in that part of the restored marsh

Summary

Introduction

Slow development (e.g., including bare mudflats) increases the risk of negative public perception in the first years, but may lead to 80 long-term persistence of high habitat diversity with different stages of succession All these examples illustrate the need for modeling tools that are based on state-of-the-art scientific knowledge, and that allow to predict how fast restored tidal marshes develop and how development rates can be steered by restoration design. One of the great challenges in numerical modeling of tidal marsh development is to combine large domains (order of 1 km or more), fine grid resolution (order of 1 m2 or less) and stochasticity in pioneer vegetation establishment, which are all essential to capture relevant scale-dependent biogeomorphic feedbacks, such as flow concentration between small-scale pioneer vegetation patches (order of m2 – van Wesenbeeck et al, 2008; Gourgue et al, 2021) and its impact on the landscape-scale formation of channel networks (order of km2 – Temmerman et al 2007; Schwarz et al 2018). We evaluate our model performance against data on vegetation and geomorphic development in adjacent tidal marshes

Methods

Results

Conclusion