Sediment & Scour

Many large estuaries around the world are engineered to some degree for both flood protection and maintenance of shipping routes. Naturally, planforms of sandy estuaries have the tendency to converge landwards exponentially, where deviations from this shape provide alternating space and constrictions where tidal sandbars and channels form. In the case of embankment by bank protection and dikes, such channels can develop deep scour holes that endanger bank stability. Our objective is to study effects of channel and planform dynamics on scour depth variation near protected banks of estuaries in the tilting tidal flume the Metronome, through three experiments with completely fixed banks and a control run without fixed banks, allowing longer timeseries than available in nature. We observe repeatable patterns of topographic forcing and quasi-cyclic behaviour in channel position and scour depth. Notable variations within and between repeat experiments result from a degree of chaos in the system.

This work presents novel large-scale wave flume experiments (on a scale of 1:7) on the influence of a foredune in front of a secondary dune during storm surge situations. Working hypothesis is that the foredune acts as a sediment buffer, damping incoming waves and reducing overall erosion on the secondary dune. Experimental results show the significant influence of a foredune on the erosion behavior at a secondary dune as. Although investigated foredunes were commonly overwashed and eroded during each test, their remaining residual profiles still provide a protective effect as incoming waves break at greater distances from the secondary dune, causing reduced wave runup and erosion at the secondary dune.

Suspended sediment transport and retention within salt marshes is a key factor in their resilience against erosion associated with threats such as sea level rise and coastal squeeze. Salt marshes are vegetated intertidal wetlands found in temperate climate zones. Their presence contributes to flood protection, coastal flora and fauna, and carbon sequestration (Temmerman et al., 2013). Vegetation-induced resuspension of fine sediment helps to transport fine particles deeper into salt marshes. 
The interaction between waves, currents, and vegetation may resuspend sediment sooner than it would without vegetation. It has been shown in conditions with only currents (Liu et al., 2021; Tinoco & Coco, 2014) or only waves (Tinoco & Coco, 2018) that the threshold for resuspension is lower within vegetated meadows than without vegetation. However, the threshold has not yet been studied for combined wave-current conditions, which often exist in the marsh environment. 
Identifying this threshold will enable us to predict when in a tidal cycle sediment resuspension occurs and can be used to improve sediment transport model. We use flume experiments to methodically identify the threshold of sediment resuspension in artificial salt marsh meadows of three different area densities under combined wave-current flows.

The assessment of the coastal protection potential of dunes is far from being a streamlined, mature procedure, and it will demand more parameters than merely the dunes' height, dimensions or volume, but tentatively also their vegetation coverage. The role of stabilizing root components in the overall dune body matrix is currently not well understood, in particular, when highly dynamic processes such as wave attack is involved. The hypothesis of this novel work hence is that root systems of dune grass (i.e., A. arenaria) can be quantified for the modelling in experimental campaigns and for more reliable erosion results. To investigate this hypothesis novel physical experiments are currently conducted in the wave flume at the Leichtweiß-Institute for Hydraulic Engineering and Water Resources in Braunschweig (Germany), testing different root surrogate materials and quantities for the first time.