Modelling sand–mud morphodynamics in the Friesche Zeegat

Abstract

The objective of the study presented in this paper is to investigate the predictive capabilities of a process-based sand-mud model in a quantitative way. This recently developed sand-mud model bridges the gap between noncohesive sand models and cohesive mud models. It explicitly takes into account the interaction between these two sediment fractions and temporal and spatial bed composition changes in the sediment bed [Van Ledden (2002) 5:577-594, Van Ledden et al. (2004a) 24:1-11, Van Ledden et al. (2004b. The application of this model to idealized situations has demonstrated a good qualitative agreement between observed and computed bed levels and bed composition developments. However, in real-life situations, a realistic quantitative prediction of the magnitude and timescale of this response is important to assess the short-term and long-term impacts of human interventions and/or natural changes. For this purpose, the Friesche Zeegat in the Wadden Sea (the Netherlands) is used as a reference to hindcast the morphological response in the period 1970-1994. Due to the closure of the Lauwerszee in 1969, the tidal prism of this tidal basin was reduced by about 30%. Significant changes in the bed level and bed composition have occurred in the decades following the closure to adjust to the new hydrodynamic conditions. We modeled the long-term bed level and bed composition development in the Friesche Zeegat in the period 1970-1994 starting with the geometry of 1970 by using a research version of Delft3D, which incorporates the sand-mud formulations proposed by [Van Ledden (2002) 5:577-594].The computed total net deposition in the tidal basin in the period 1970-1994 agrees well with the observations, but the observed decrease of the import rate with time is not predicted. The model predicts net deposition in the deeper parts and at the intertidal area in the basin and net erosion in between, which resembles the observations qualitatively. Furthermore, the computed distribution of sand and mud in the basin of the Friesche Zeegat appears to be realistic. Analysis of the results shows that the absence of the decreasing import rate in the basin is caused by a poor quantitative prediction of the changes in the hypsometry of the basin. Because of this, the computed velocity asymmetry in the main channel tends toward flood dominance, whereas the observations indicate that the system is ebb-dominant in 1992. Although the sand-mud model needs to be further improved and verified, the results presented in this paper indicate that the model can be applied as a first step to estimate the effects of human interventions on the large-scale bed level and bed composition changes in tidal systems with sand and mud.

The settling dynamics of flocculating mud and sand mixtures: part 2—numerical modelling

“…Whilst this approach is often still adequate, the increasing need for a better understanding of potential changes in morphology and substrate has driven the use of combined mud-sand transport models. Examples of the use of these combined mud and sand models are Ockenden and Chesher (1994) in the Mersey Estuary, van Ledden et al (2006) in the Wadden Sea and Waeles et al (2007) in the Seine Estuary. Many of the more common sediment transport modelling suites (e.g.…”

The estimation of estuary dimensions using a simplified form model and the exogenous controls

“…flow dominated channel areas and wave dominated intertidal areas). This might, for example, require the inclusion of a mixed sediment description so that the critical bed shear stress is dependent on the mud content of the sediments, along the lines explored by van Ledden (; van Ledden et al , ).These descriptions of the sediments must then be related to the exogenous conditions based on the direction of change. Under conditions of sea level rise this will usually be depositional, driven by sediment supply from the marine and fluvial environments and represented by a suitable concentration.…”

“…flow dominated channel areas and wave dominated intertidal areas). This might, for example, require the inclusion of a mixed sediment description so that the critical bed shear stress is dependent on the mud content of the sediments, along the lines explored by van Ledden (2002;van Ledden et al, 2006).…”

Bed composition generation for morphodynamic modeling: case study of San Pablo Bay in California, USA

“…The model does not account for physical interactions between the fractions. For example, erosion of a sand fraction is not influenced by the presence of mud, although this may have considerable impact in reality (see, for example, Van Ledden et al 2006).…”