An experimental study on the effect of tidal wetting on salt marsh resilience to sea level rise and wind waves

Introduction:

Salt marshes are vegetated surfaces located at the interface between the marine and terrestrial environment. They are among the most productive ecosystems on earth, and provide valuable ecosystem services (figure 1). However, salt marsh survival is threatened by wind waves and sea level rise and large salt marsh losses have been already documented worldwide; if these coastal environments continue declining, we risk losing all their valuable ecosystem services.

While there is a vast literature on salt marsh dynamics, the combined effect of storm surges and tides, and in particular the influence of wetting and drying on marsh properties are poorly understood even if they largely influence the ecology and morphology of these coastal environments.

Differently than other coastal environments, salt marshes are regularly flooded by tides. The time of flooding, as well as groundwater flow can cause the soil to shrink or swell depending on the water content of the substrate; In turn, this lead to perturbations in elevation influencing soil resistance, and vegetation species. The latter affect salt marsh resilience to external agents such as vertical accretion rates in response to sea level, as well as the resistance to the erosive action of wind waves.

Laboratory experiments are suitable tools to investigate salt marsh dynamics, as they allow testing real case scenarios under controlled conditions (e.g. specific wave height, specific water levels, or marsh height). The University of Liverpool houses a newly refurbished hydraulic laboratory equipped for the study of coastal areas (see also Methods). Field investigations will be used in combination with laboratory experiments for results comparison.

Project Summary:

Aims and objectives

This project aims at investigating factors controlling salt marsh dynamics and resilience to external agents by means of laboratory experiments, and field data. Research questions include but are not limited to:

- How the combined action of storm surges, tides, topographic features, and vegetation control wetting and drying processes along salt marshes?

- How wetting and drying processes influence salt marsh morphological and ecological features?

- Can the above processes affect salt marsh resilience to external agents (e.g. resistance to the erosive action of wind waves, and to sea level rise)?

Methods

The newly refurbished, state-of-the-art Hydraulic laboratory at Liverpool is equipped for the study of erosion processes in coastal areas. The hydraulic channels allow the independent or combined generation of wind waves and currents, and the recirculation of fine sediments. This allow reproducing a variety of scenarios representing storms of different intensity, storm surges, or projected sea level scenarios. Instrumentation for bed level and sediment transport monitoring at a high temporal and spatial resolution are available as well. Field experiments might be used in combination with laboratory investigations to test the accuracy of the obtained results.

Work plan

- Formulation of key questions based on literature review, available data, and existing knowledge.

- Familiarization with the laboratory equipment.

- Execution of laboratory experiments aimed at exploring the specified research questions.

- Data analysis, and results interpretation: Results will be critically analysed and discussed to provide useful indicators and parametrizations aimed at quantifying salt marsh morphological evolution and resilience to external agents

- Results dissemination: research outcomes will be disseminated trough presentations at international conferences, workshops, scientific papers and reports.