William Nardin

Dynamics of River Mouth Deposits

Bars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion.

Alongshore sediment bypassing as a control on river mouth morphodynamics

River mouths, shoreline locations where fluvial and coastal sediments are partitioned via erosion, trapping, and redistribution, are responsible for the ultimate sedimentary architecture of deltas and, because of their dynamic nature, also pose great management and engineering challenges. To investigate the interaction between fluvial and littoral processes at wave-dominated river mouths, we modeled their morphologic evolution using the coupled hydrodynamic and morphodynamic model Delft3D-SWAN. Model experiments replicate alongshore migration of river mouths, river mouth spit development, and eventual spit breaching, suggesting that these are emergent phenomena that can develop even under constant fluvial and wave conditions. Furthermore, we find that sediment bypassing of a river mouth develops though feedbacks between waves and river mouth morphology, resulting in either continuous bypassing pathways or episodic bar bypassing pathways. Model results demonstrate that waves refracting into the river mouth bar create a zone of low alongshore sediment transport updrift of the river mouth, which reduces sediment bypassing. Sediment bypassing, in turn, controls the river mouth migration rate and the size of the river mouth spit. As a result, an intermediate amount of river discharge maximizes river mouth migration. The fraction of alongshore sediment bypassing can be predicted from the balance between the jet and the wave momentum flux. Quantitative comparisons show a match between our modeled predictions of river mouth bypassing and migration rates observed in natural settings.

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

Abstract: Multiple stable states are established in coastal tidal wetlands (marshes, mangroves, deltas, seagrasses) by ecological, hydrological, and geomorphological feedbacks. Catastrophic shifts between states can be induced by gradual environmental change or by disturbance events. These feedbacks and outcomes are key to the sustainability and resilience of vegetated coastlines, especially as modulated by human activity, sea level rise, and climate change. Whereas multiple stable state theory has been invoked to model salt marsh responses to sediment supply and sea level change, there has been comparatively little empirical verification of the theory for salt marshes or other coastal wetlands. Especially lacking is long-term evidence documenting if or how stable states are established and maintained at ecosystem scales. Laboratory and field-plot studies are informative, but of necessarily limited spatial and temporal scope. For the purposes of long-term, coastal-scale monitoring, remote sensing is the best viable option. This review summarizes the above topics and highlights the emerging promise and challenges of using remote sensing-based analyses to validate coastal

FRESHWATER VEGETATION INFLUENCE ON SEDIMENT SPATIAL DISTRIBUTION IN RIVER DELTA DURING FLOOD

The morphology of a river delta stems from the interaction between flow and sediment transport. This morphodynamic interaction is potentially affected by freshwater marsh vegetation (e.g. Sagittaria spp. and Typha spp.) on the exposed surfaces of emergent deltaic islands. The vulnerability of these deltaic islands is a result of external forces like large storms, sea level rise, and trapping of sediment in reservoirs upstream. These factors can strongly determine the evolution of the deltaic system by influencing the coupling between vegetation dynamics and morphology. In the last few years, models have been developed to describe the dynamics of salt marsh geomorphology coupled with vegetation growth while the effect of the vegetation on a delta remains unexplored. The sediment flux through the system differs in salt marshes with respect to deltaic islands. During a flood in a river delta sediments are able to deposit on bars if the water flows over them, while in salt marsh the sediment is transported in and out through the channel edge flooding continuously the riparian marshes. Our results show a sediment spatial distribution in deltaic islands that depends on vegetation height and density. This dependence affects the slope of a river delta and its resiliency.