Littérature scientifique sur le sujet « Ecomorphodynamic »

An ecomorphodynamic model was developed to study how Avicennia marina mangroves influence channel network evolution in sandy tidal embayments. The model accounts for the effects of mangrove trees on tidal flow patterns and sediment dynamics. Mangrove growth is in turn controlled by hydrodynamic conditions. The presence of mangroves was found to enhance the initiation and branching of tidal channels, partly because the extra flow resistance in mangrove forests favours flow concentration, and thus sediment erosion in between vegetated areas. The enhanced branching of channels is also the result of a vegetation-induced increase in erosion threshold. On the other hand, this reduction in bed erodibility, together with the soil expansion driven by organic matter production, reduces the landward expansion of channels. The ongoing accretion in mangrove forests ultimately drives a reduction in tidal prism and an overall retreat of the channel network. During sea-level rise, mangroves can potentially enhance the ability of the soil surface to maintain an elevation within the upper portion of the intertidal zone, while hindering both the branching and headward erosion of the landward expanding channels. The modelling results presented here indicate the critical control exerted by ecogeomorphological interactions in driving landscape evolution.

Coastal dunes are often the first and primary form of defense against destructive surge and waves that accompany extreme storm events. Beach grasses are known to affect dune height, width, and stability, contributing to the dune’s ability to protect the hinterland from wave and flooding hazards (Hacker et al. 2012). However, the interaction and feedbacks between dune development and properties of beach grasses (e.g., species, density) is not fully understood. In particular, our knowledge of the ecomorphodynamic processes controlling the recovery of coastal dunes following storms and the long-term ability of dunes to adapt to changes in climate remains inadequate. The objective of this interdisciplinary research is to characterize the temporal and spatial variability of coastal foredune recovery following major storm events and the subsequent impact of this recovery on future vulnerability. The study region consists of three low-lying barrier islands within the Cape Lookout National Seashore (CALO) along the central coast of North Carolina. The 90 km stretch of coast exhibits spatial variability in dominant dune grass species, grass cover density, coast orientation, beach slope, and wave energy. Using physical and ecological field datasets and process-based numerical modeling, post-storm dune recovery is assessed following Hurricane Matthew (2016).

The elevation and extent of coastal marshes are dictated by the interplay between the rate of relative sea-level rise (RRSLR), surface accretion by inorganic sediment deposition, and organic soil production by plants. These accretion processes respond to changes in local and global forcings, such as sediment delivery to the coast, nutrient concentrations, and atmospheric CO2, but their relative importance for marsh resilience to increasing RRSLR remains unclear. In particular, marshes up-take atmospheric CO2 at high rates, thereby playing a major role in the global carbon cycle, but the morphologic expression of increasing atmospheric CO2 concentration, an imminent aspect of climate change, has not yet been isolated and quantified. Using the available observational literature and a spatially explicit ecomorphodynamic model, we explore marsh responses to increased atmospheric CO2, relative to changes in inorganic sediment availability and elevated nitrogen levels. We find that marsh vegetation response to foreseen elevated atmospheric CO2 is similar in magnitude to the response induced by a varying inorganic sediment concentration, and that it increases the threshold RRSLR initiating marsh submergence by up to 60% in the range of forcings explored. Furthermore, we find that marsh responses are inherently spatially dependent, and cannot be adequately captured through 0-dimensional representations of marsh dynamics. Our results imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climatic changes than previously thought.

This research investigated the feasibility of using the geological record to determine rates and drivers of morphological change in coastal lagoons. Substrate elevation in these environments is of primary importance for survival of wetland habitats, the effectiveness of drainage and flood mitigation functions delivered by those habitats, and the success of potential carbon sequestration programs. Investigating rates and trajectories of lagoon evolution will become more important given the effects of accelerating sea-level rise and human interventions, direct and indirect, on all coastal depositional environments. Elevation change on coastal lagoon shorelines is the net result of numerous sediment accretion and erosion cycles that are subject to considerable uncertainty. Numerous hydrological, biological, geological and anthropogenic processes interact over a range of timescales, and are subject to complex relationships and non-linear feedbacks. To successfully reproduce and predict long-term shoreline change with numerical models, the net effect of these processes must be captured and attributed to appropriate functions and parameter values. Shoreline processes are typically measured in-situ, and measurements would need to span several decades in order to reach an adequate level of confidence about the representativeness of the results. This is particularly true in regions subject to inter-decadal climate variability, such as the El Niño Southern Oscillation in southeast Australia. Even with a sufficiently long-term empirical dataset, the lasting effect of sediment accumulation for elevation change depends strongly on sub-surface processes (root production, decomposition, compaction and soil water content), which take place over still longer timescales and require sub-surface investigation. Reliance on the depositional history captured in the geological record would improve confidence in longer-term rates of morphological change. It would reduce the time and effort required from years (at least) of field measurements to a few months of laboratory work. The effectiveness of the geological record for model parameterisation and calibration, however, depends on the potential to infer drivers of elevation change as well as rates. For this research, soil samples up to 1.8 m depth were obtained in cross-shore core transects from prograded shorelines in three NSW coastal lagoons: Wooloweyah Lagoon near Yamba; Lake Innes near Port Macquarie; and Neranie Bay within Myall Lake. The three lagoons and the segments of shoreline sampled were selected to be as low-energy as possible by avoiding the effects of fluvial and tidal processes that could render intractable shoreline processes with already complex interactions. Each core sample was split and scanned for high-resolution optical images and down-core profiles of magnetic susceptibility, and geochemistry. These datasets enabled the identification and correlation of depositional units between cores and along cross-shore profiles, and thus high-level analysis of shorelines stratigraphy. From each site or transect at least one representative core was selected for detailed investigation, sub-sampled at 10 mm resolution and analysed for grain size, moisture content, density, organic content, and isotopic activity of 210Pb, 137Cs and 14C which provided the approximate timing of deposition for each sub-sample. Mass accumulation rates (g/cm2/yr) and vertical accretion rates (mm/yr) were calculated for correlation with physical sediment properties. At one site, Neranie Bay, this detailed level of analysis was performed for three cores, covering most of the cross-shore transect. Accretion rates calculated for approximately the last 100 years from 210Pb analysis averaged less than 2 mm/yr, consistent with figures reported for similar environments elsewhere in southeast Australia, and at the lower end of the spectrum for internationally reported rates. Preceding the timing of European settlement, accretion rates at the three sites were considerably lower. Recent rates of sediment mass accumulation mostly ranged from 0.02-0.2 g/cm2/yr, but this figure is rarely reported elsewhere and is therefore difficult to compare. Accretion and mass accumulation rates reduced rapidly down-core in the upper few centimetres of each sample, suggesting a significant role for organic matter decomposition for at least several decades following initial deposition. Changes in moisture content and bulk density were observed over similar depths. This research highlights the importance of analysing soil samples to sufficient depth and ensuring sub-surface processes have ceased to have significant impacts on down-core changes before making interpretations about trends over time. A controlling influence of organic content over vertical accretion (and therefore elevation change) was found for the three sites investigated. This control was independent of the inorganic sediment input, which was often higher (by mass) than the organic input. At Neranie Bay, cross-shore trends in organic content were evident. Organic matter input at the surface of the soil sample was greatest when the sample was taken from a higher elevation with less frequent inundation (i.e. short hydroperiod). The proportion of organic matter retained in the soil profile, however, was lowest where hydroperiod was shortest. On balance, organic matter makes the greatest contribution to elevation change when hydroperiod is longest. It could not be determined whether this was caused by higher rates of sub-surface decomposition with short hydroperiod, or high rates of below-ground productivity with long hydroperiod (or both). Either way the results are counter-intuitive and could not be determined without reliance on the geological record. The cross-shore trend that was established from this research is of vital importance. The relationship between hydroperiod and organically driven elevation change results in self-regulating, negative feedback and therefore greater resilience to increases in hydroperiod when the relationship is as reported here. When the reverse relationship is found, however, resilience to increased hydroperiod, and therefore sea level rise, would be compromised because inundation would continually decrease the ability of organic sedimentation to drive accretion, potentially resulting in habitat loss and exposing the shoreline to the risk of erosion. Previous studies suggest that this cross-shore relationship varies on a site-by-site basis. Determining the direction of the relationship with field measurements would take years and still be subject to much higher uncertainty than the methods employed here. This research has shown that the geological record is not only a feasible source of information about accretion rates and drivers, but also a preferable one. Provided further research can succeed in linking sub-surface retention of organic matter to contemporary primary production at the surface, the geological record will provide a more efficient and effective method of designing and calibrating much-needed predictive models to explore scenarios of shoreline development and wetland survival under changing conditions. Further research should also target a range of geologic and climatic settings to differentiate between drivers that can be generalised across all sites and those that vary on a site-by-site basis.