Effects of Shoal Margin Collapses on the Morphodynamics of a Sandy Estuary

Abstract

Shoal margin collapses of several million cubic meters have occurred in the Western Scheldt estuary, the Netherlands, on average five times a year over the last decades. While these collapses involve significant volumes of material, their effect on the channel‐shoal morphology is unknown. We hypothesize that collapses dynamicize the channel‐shoal interactions, which could impact the ecological functioning, flood safety, and navigation in the estuary. The objective is to investigate how locations, probability, type, and volume of shoal margin collapse affect the channel‐shoal dynamics. We implemented an empirically validated parameterization for shoal margin collapses and tested its effect on simulated estuary morphological development in a Delft3D schematization of the Western Scheldt. Three sets of scenarios were analyzed for near‐field and far‐field effects on flow pattern and channel‐shoal morphology: (1) an observed shoal margin collapse of 2014, (2) initial large collapses on 10 locations, and (3) continuous collapses predicted by our novel probabilistic model over a time span of decades. Results show that a single shoal margin collapse only affects the local dynamics in the longitudinal flow direction and dampen out within a year for typical volumes, whereas larger disturbances that reach the seaward or landward sill at tidal channel junctions grow. The direction of the strongest tidally averaged flow determined the redistribution of the collapsed sediment. We conclude that adding the process of shoal margin collapses increases the channel‐shoal interactions and that in intensively dredged estuaries shoal margins oversteepen, amplifying the number of collapses, but because of dredging the natural morphological response is interrupted.

Critical dependence of morphodynamic models of fluvial and tidal systems on empirical downslope sediment transport

“…Here we used a nested model of the NeVla-Delft3D schematization focusing on the Western Scheldt partly for reducing the computational time, which is also used by Van Dijk and others 56 . The model boundaries include the Western Scheldt from the mouth at Vlissingen to the Belgian border, in which the seaward boundary includes a water level fluctuation due to tides and the landward boundary a current.…”

“…Sediment fraction was uniform with a median grain size of 200 μm. The roughness field in the model is defined in Manning n and is variable over the model domain, which was 0.022 sÁm À1=3 for the eastern part, and 0.027 sÁm À1=3 for the western part 52,53,[56][57][58] . The bed consisted of erodible and non-erodible layers 59,60 , and therefore sediment thickness varies within the Western Scheldt model, which reduces the morphological changes but not the transverse bed slopes.…”

What Came First, Mud or Biostabilizers? Elucidating Interacting Effects in a Coupled Model of Mud, Saltmarsh, Microphytobenthos, and Estuarine Morphology

“…More details can be found in vanDijk et al (2019) andBraat et al (2019).10.1029/2019WR026945Water Resources Researchvegetation height h v (m), vegetation density n (m/m 2 ), and a bulk drag C D (-). Depending on the relative local water depth h (m), C is computed as with g is gravity (m/s 2 ), κ ¼ 0.41 (-) von-Kármán constant, and C b ¼ 25 ( ffiffiffiffi ffi m p =s), which is derived from the Manning of the vegetated bars of 0.028 and a water depth of 0.1 m. To compensate for higher local sediment transport induced by increased C, an additional flow resistance −λ/2*u 2 is included in the flow solver, where λ is defined as Finally, for each vegetation fraction present in each cell, both λ and C are weighted according to their relative coverage…”

Determining flow directions in river channel networks using planform morphology and topology

“…Multi-scale analyses of river valleys indicate that significant information about the local (link) scale is shared with the valley scale (Gutierrez and Abad, 2014;Vermeulen et al, 2016). VD attempts to impart flow direction information from the river valley scale to the link scale.…”