Salt Marsh Topography Reveals the Signature of Storm-Surge-Driven Sedimentation

Academic Background

Salt marshes are transitional zones located in low-lying, sheltered coastlines, situated between land and sea. They are among the most productive and fragile ecosystems on Earth, providing essential ecological services such as carbon sequestration, water quality improvement, biodiversity conservation, wildlife habitat provision, and shoreline protection. However, with rising sea levels and increasing human activities, salt marshes are facing the threat of disappearance. The vertical evolution of salt marshes primarily relies on sedimentation processes, which counteract the risk of inundation caused by sea-level rise through sediment accumulation. Traditionally, tidal flooding has been considered the main mechanism for salt marsh sedimentation, but storm surges and waves may also significantly influence sediment redistribution and deposition patterns. Therefore, studying the contribution of storm surges to salt marsh sedimentation and their impact on topography is crucial for understanding the evolutionary mechanisms of salt marshes and protecting these ecosystems.

Source of the Paper

This paper was co-authored by Davide Tognin, Andrea D’Alpaos, Massimiliano Ghinassi, and Luca Carniello, affiliated with the Department of Civil, Environmental, and Architectural Engineering and the Department of Geosciences at the University of Padova, Italy. The paper was published online on October 23, 2024, in the journal Geology, titled Marsh Topography Reveals the Signature of Storm-Surge-Driven Sedimentation. The research was funded by the NextGenerationEU project.

Research Process and Results

Research Process

1. Study Area and Data Collection
   The research was conducted in the Venice Lagoon, Italy, focusing on four study areas (SF, SE, CO, PA), each including channel-facing and tidal flat-facing salt marshes. Researchers collected sediment data using sediment traps from October 2018 to October 2021, either monthly or immediately after severe storm surge events. Organic matter content in the sediment was measured using the loss-on-ignition (LOI) method, and grain size was analyzed using laser diffraction.

2. Sediment Accumulation Analysis
   Researchers analyzed sediment accumulation during storm-dominated periods and fair-weather conditions. By comparing deposition patterns across different areas, they found that storm-dominated periods contributed an average of 70% to sediment accumulation, despite their shorter duration. The sediment supply during storm surges far exceeded that of regular tidal flooding.

3. Topographic Profile Analysis
   By comparing sedimentation patterns and topographic profiles, researchers found that channel-facing marshes exhibited a typical levee-shaped profile, with sediment accumulation rapidly decreasing away from the marsh edge. In contrast, tidal flat-facing marshes showed gentler profiles, with maximum sediment accumulation occurring further inland. This difference reflects the varying combinations of tide- and wave-driven depositional processes.

4. Global Comparative Analysis
   To validate the universality of the findings, researchers compared topographic profiles of salt marshes in five coastal systems worldwide with different tidal ranges and exposure conditions. The results showed that, regardless of tidal range or marsh maturity, channel-facing and tidal flat-facing marshes exhibited distinct topographic profiles, further supporting the link between marsh topography and sedimentation processes.

Key Findings

1. Contribution of Storm Surges to Sedimentation
   The study found that storm-dominated periods contributed 70%-90% to salt marsh sedimentation, particularly at the edges of channel-facing marshes. Elevated water levels and strong wind waves during storm surges significantly enhanced sediment resuspension and transport, leading to changes in sediment distribution at the marsh edge and interior.

2. Relationship Between Sedimentation Patterns and Topography
   Channel-facing marshes displayed levee-shaped profiles, with sediment accumulation rapidly decreasing away from the edge. In contrast, tidal flat-facing marshes showed gentler profiles, with maximum sediment accumulation occurring further inland. This difference reflects the varying combinations of tide- and wave-driven depositional processes.

3. Universality of Global Salt Marsh Topography
   The global comparative analysis revealed that, regardless of tidal range or marsh maturity, channel-facing and tidal flat-facing marshes exhibited distinct topographic profiles, further supporting the link between marsh topography and sedimentation processes.

Conclusions and Significance

The findings demonstrate that storm surges play a crucial role in salt marsh sedimentation, particularly in microtidal systems. Elevated water levels and strong wind waves during storm surges significantly enhance sediment resuspension and transport, leading to changes in sediment distribution at the marsh edge and interior. This challenges the traditional view that salt marsh sedimentation is primarily driven by tidal flooding. Salt marsh topographic profiles can serve as indicators of sedimentation processes, providing valuable information for salt marsh protection and restoration management policies.

Research Highlights

1. Key Discovery
   Storm surges contribute 70%-90% to salt marsh sedimentation, significantly higher than the traditionally assumed contribution of tidal flooding.

2. Methodological Innovation
   The study combined long-term sediment data collection with topographic profile analysis, revealing the relationship between sedimentation patterns and topography.

3. Global Universality
   Through global comparative analysis, the study validated the universal link between salt marsh topography and sedimentation processes.

4. Application Value
   The research provides a scientific basis for salt marsh protection and restoration management policies, particularly in the context of rising sea levels and increasing storm surge intensity.

Additional Valuable Information

The study also found that storm-surge-related sediments have higher inorganic content and coarser grain size, further emphasizing the long-term impact of storm surges on salt marsh topographic evolution. Additionally, the research explored the biomorphodynamic mechanism of salt marsh ecosystems, where vegetation growth and organic matter deposition feed back into the vertical evolution of salt marshes.

This study not only deepens our understanding of salt marsh sedimentation processes but also provides important scientific insights for the protection and management of salt marshes worldwide.