Wave attenuation by coastal heterospecific vegetation - modeling of synthetic plant meadows by Response Surface Methodology (RSM)

193-202Knowing the interactions between wave and aquatic vegetation is becoming increasingly important because of the phenomenon of plant-induced wave attenuation for the development of sustainable coastal management systems. Many of the wave-vegetation interaction studies focus on monotypic coastal plant meadows, while coastal plant meadows are typically heterospecific in nature, and the work on heterospecific plant meadows is still very limited. This research aims therefore to explain the heterospecific vegetation-wave interactions using a three-level four-factor surface response methodology (RSM) using controlled laboratory wave flume conditions. Heterospecific seagrass species, Cymodocea serrulata is physically simulated using synthetic plant mimics to establish a relationship between wave attenuation (E%) and four direct control factors, i.e. wave period (T), water depth (h), bed roughness factor (f) and plant density (N), using an empirical model. The model developed was evaluated using the methodology of variance analysis (ANOVA) and analyzed for the key and interaction effects of the parameters studied. The findings showed that both individually and in combination, all the parameters considered are significantly successful on E%. All model-based findings were compared with a new collection of experimental data, and validation tests were carried out

Salt marshes create more extensive channel networks than mangroves.

This is the final version. Available from Nature Research via the DOI in this record. Data availability: The data generated in this study have been deposited in the Zenodo database under accession code (https://doi.org/10.5281/zenodo.6331067).Coastal wetlands fulfil important functions for biodiversity conservation and coastal protection, which are inextricably linked to typical morphological features like tidal channels. Channel network configurations in turn are shaped by bio-geomorphological feedbacks between vegetation, hydrodynamics and sediment transport. This study investigates the impact of two starkly different recruitment strategies between mangroves (fast/homogenous) and salt marshes (slow/patchy) on channel network properties. We first compare channel networks found in salt marshes and mangroves around the world and then demonstrate how observed channel patterns can be explained by vegetation establishment strategies using controlled experimental conditions. We find that salt marshes are dissected by more extensive channel networks and have shorter over-marsh flow paths than mangrove systems, while their branching patterns remain similar. This finding is supported by our laboratory experiments, which reveal that different recruitment strategies of mangroves and salt marshes hamper or facilitate channel development, respectively. Insights of our study are crucial to understand wetland resilience with rising sea-levels especially under climate-driven ecotone shifts

The effect of sea‐level rise on estuary filling in scaled landscape experiments

When sea-level rise slowed down in the middle Holocene, fluvial and coastal sediments filled the newly created accommodation, whilst others remained largely unfilled because of limited sediment supply. In view of current and future rapid sea-level rise, the question arises how estuarine systems will adapt and whether the land-level rise may keep up. Besides geological data and conceptual models of large-scale and long-term estuary filling, little is known about the filling process during sea-level rise on the decadal-to-centennial time scale that is relevant for society. This study focusses on how sea-level rise affects the morphological and hydrodynamic development of filling estuaries. To this end, scaled laboratory experiments were conducted in a tilting flume facility that creates bidirectional tidal currents and develops entire estuaries. A net importing estuary with sand, mud and vegetation was formed that was subjected to linear sea-level rise. Findings show less of the imported sand was deposited landward following sea-level rise than in an experiment without sea-level rise. The bay-head delta and the flood-tidal delta retained nearly enough sediment to keep up with sea-level rise, whilst the tidal embayment in between drowned except for the highest vegetated bars. Sea-level rise also reduced vegetation survival and sprouting potential, as prolonged inundation increased mortality, negating the potential eco-engineering effect. This resulted in lower vegetation coverage with sea-level rise than under constant sea level. These findings suggest that sea-level rise may cause natural systems to drown even if nearly sufficient sediment is available to fill the newly created accommodation, particularly in areas further away from the fluvial and marine sediment sources. Finally, depending on the sea-level rise rate, the flood-tidal delta may show back-stepping like fluvial deltas, but in the reverse direction towards the sea

Experiment-supported modelling of salt marsh establishment

Recently, the use of salt marshes in front of hard structures is increasingly proposed as a more sustainable coastal protection measure. Yet, salt marsh restoration and creation is often hampered by the lack of a thorough understanding of initial vegetation establishment. Recent studies highlight the importance of bed level change for salt marsh development. In this study we continue the examination of the impact of bed level change on salt marsh development, focussing on the prediction of salt marsh establishment and the implications for coastal management. First, a test with Spartina anglica seedlings (Cordgrass) in a wave flume showed that long-term (seasonal) bed level change is more important for seedling survival than direct wave impact at the shoot. Therefore, we subsequently incorporated bed level change in the Windows of Opportunity (WoO) framework. Lastly, this revised WoO framework was applied to the design of the Marconi pioneer salt marsh (The Netherlands). Combining the WoO framework with a morphodynamic model (Delft3D) showed its potential for salt marsh design. The framework can be used to determine whether salt marsh establishment is possible, to find out which conditions are limiting establishment and to design engineering measures creating the conditions that facilitate salt marsh establishment

Algal-Induced Biogeomorphic Feedbacks Lay the Groundwork for Coastal Wetland Development

Ecosystem establishment under adverse geophysical conditions is often studied within the “windows of opportunity” framework, identifying disturbance-free periods (e.g., calm wave climate) where species can overcome establishment thresholds. However, the role of biogeophysical interactions in this framework is less well understood. The establishment of saltmarsh vegetation on tidal flats, for example, is limited by abiotic factors such as hydrodynamics, sediment stability and drainage. On tidal flats, raised sediment ridges colonized by algal mats (Vaucheria sp.) appear to accomodate high densities of plant seedlings. Such ridges were previously found to have higher sediment strength than substratum without algae. Here, we investigate whether these measurements can be explained by geophysical factors only, or that biological (Vaucheria-induced) processes influence tidal marsh establishment by forming stabilized bedforms. We performed two experiments under controlled mesocosm conditions, to test the hypotheses that (a) Vaucheria grows better on elevated topographic relief, that (b) the binding force of their algal filaments increases sediment strength, and that (c) Vaucheria consequently creates elevated topographic relief that further facilitates algal growth. Our experimental results confirm the existence of this algal-induced biogeomorphic feedback cycle. These findings imply that benthic algae like Vaucheria may contribute significantly to tidal marsh formation by creating elevated and stabilized substratum. This suggests biogeophysical feedbacks can “widen” the windows of opportunity for further ecosystem establishment. Our results could be useful for the design of managed realignment projects aimed at restoring the unique ecosystem services of coastal wetlands, such as habitat biodiversity, carbon sequestration potential and nature-based flood defense

Coupling nearshore and aeolian processes: XBeach and duna process-based models

A new dune profile model, Duna, is developed and coupled with the existing XBeach model, in which some key improvements allow a much better behaviour of the intertidal beach and the inclusion of structural erosion or accretion through a longshore transport gradient. The model is shown to represent typical behaviour of a beach-dune system in Praia de Faro, Portugal and to be able to simulate processes on a decadal timescale. The model captures a balance between longshore gradients and cross-shore processes in the surf zone, competing effects of moderate conditions and storms in the intertidal area and between build-up by storm waves and aeolian transport on the berm. Vegetation behaviour is shown to play a key role in the development of the shape of the foredunes. The relation between progradation or recession rate and foredune height as often reported in literature is reproduced and explained.FCT Investigator program [IF/01047/2014]Portuguese Science Foundation (FCT)Portuguese Foundation for Science and Technology [28949, UID/MAR/00350/2013]FEDER FundsEuropean Union (EU)info:eu-repo/semantics/publishedVersio

Do salt marshes survive sea level rise?: Modelling wave action, morphodynamics and vegetation dynamics

This paper aims to fundamentally assess the resilience of salt marsh-mudflat systems under sea level rise. We applied an open-source schematized 2D area model (Delft3D) that couples intertidal flow, wave-action, sediment transport, geomorphological development with a population dynamics approach including temporal and spatial growth of vegetation and bio-accumulation. Wave-action maintains a high sediment concentration on the mudflat while the tidal motion transports the sediments within the vegetated marsh areas during flood. The marsh-mudflat system attained dynamic equilibrium within 120 years. Sediment deposition and bio-accumulation within the marsh make the system initially resilient to sea level rise scenarios. However, after 50–60 years the marsh system starts to drown with vegetated-levees being the last surviving features. Biomass accumulation and sediment supply are critical determinants for the marsh drowning rate and survival. Our model methodology can be applied to assess the resilience of vegetated coast lines and combined engineering solutions for long-term sustainability

Characterization of vegetation patterns in a Venice lagoon saltmarsh from drone-based hyperspectral remote sensing

Coastal wetlands are unique and complex geomorphological systems that respond to a wide range of changing influences, and their responses remain poorly understood, emphasizing the need for and importance of this study. These ecosystems provide useful feedbacks to coastal systems, such as soil stabilization and coastal protection. They are very important carbon sinks. For carbon to be stored in the soils there must be biomass that is produced. This study focuses on the above ground biomass and the below ground biomass in the saltmarsh in order to evaluate the amount of organic matter that is stored in the soils. To obtain this, field campaigns were conducted to sample the above ground vegetation and core samples to analyse the amount of vegetation biomass and carbon stock in the soil. The marsh selected for this study is characterized by three different levels of elevation, high mid and low. We found that the middle marsh is the area that stores the highest amount of organic matter in the soil as compared to the lower and the higher marsh. In addition, we found that there is a linear positive correlation between the AGB and the BGB. Furthermore, the study concludes that it is possible to use vegetation indices retrieved from remote sensing to characterize the biomass. The NDVI (Normalized Difference Vegetation index) demonstrated to be a good proxy for the AGB only for low and mid-marsh vegetation species, while it saturates for high-marsh high-biomass vegetation. Studying the distribution of the NDVI ranges across the studied marsh, we found that it is mainly covered by dense vegetation, with AGB biomass larger than 400 g/m2.My deepest gratitude to the WACOMA administrative team for the help and support during these 2 academic years

Do salt marshes survive sea level rise? Modelling wave action, morphodynamics and vegetation dynamics

This paper aims to fundamentally assess the resilience of salt marsh-mudflat systems under sea level rise. We applied an open-source schematized 2D area model (Delft3D) that couples intertidal flow, wave-action, sediment transport, geomorphological development with a population dynamics approach including temporal and spatial growth of vegetation and bio-accumulation. Wave-action maintains a high sediment concentration on the mudflat while the tidal motion transports the sediments within the vegetated marsh areas during flood. The marsh-mudflat system attained dynamic equilibrium within 120 years. Sediment deposition and bio-accumulation within the marsh make the system initially resilient to sea level rise scenarios. However, after 50–60 years the marsh system starts to drown with vegetated-levees being the last surviving features. Biomass accumulation and sediment supply are critical determinants for the marsh drowning rate and survival. Our model methodology can be applied to assess the resilience of vegetated coast lines and combined engineering solutions for long-term sustainability