Stanislas Wartel

Modelling long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium

Abstract Existing numerical models simulating the vertical growth of tidal marshes have only, to a very limited degree, been validated using observed data. In this study, we describe a refined zero-dimensional time-stepping model, which is based on the mass balance approach of Krone [in: Coastal Sediments ’87, 1987, pp. 316–323], Allen [Mar. Geol. 95 (1990) 77–96] and French [Earth Surf. Process. Landforms 18 (1993) 63–81]. The model is applied and evaluated, using field data on suspended sediment and tidal regime as input and the historical growth of a specific minerogenic tidal marsh in the Scheldt estuary (Belgium) as independent data for model testing. First, the historical rise of the marsh surface during the past 55 years is reconstructed based on land use and vegetation cover changes, which are dated using aerial photographs and which are recognised in sediment cores. After marsh formation, the marsh surface builds up very quickly and asymptotically to an equilibrium level relative to the tidal frame. Second, temporal variations in suspended sediment concentration (SSC) were measured above the actual marsh surface during a 1-year period. These measurements show that the SSC, in the water that floods the marsh surface at the beginning of an inundation, increases linearly with maximum inundation height. The application of existing models, which assume a constant incoming SSC, leads to an underestimation of the observed historical growth and to biased predictions under scenarios of future sea-level rise. However, after incorporation of the relationship between SSC and inundation height, the observed vertical growth is successfully simulated. This leads to the conclusion that not only the decrease in tidal inundation, but also the decrease in SSC with decreasing marsh inundation height, is of great importance to fully explain and successfully simulate the long-term vertical morphodynamics of tidal marshes.

Seasonal variation of floc characteristics on tidal flats, the Scheldt estuary

The flocculation mechanism dominates the fate of suspended matter in the estuarine environment. By modifying the texture of suspended matter, flocculation is one of the principle factors determining the transport and deposition of suspended matter in estuaries. Surveys of the seasonal variation of dispersed particle and non-dispersed particle characteristics, organic matter content as well as suspended matter deposition in two contrasting intertidal environments, one freshwater and one brackish water, in the Scheldt estuary were undertaken at fortnightly intervals for a year. The study of non-dispersed particle, i.e. floc, is mainly focused on floc size, shape, and microstructure, properties presumed to be significant in the suspended matter transport processes in the estuary. In this study, floc size as well as floc sphericity correlate positively with the change of organic matter content and reveal that floc grows in a three-dimensional way with increasing organic matter. It is observed that relatively condensed, small and elongated flocs appear in winter and spring periods, while loose, large and spherical flocs occur during the summer. The study also reveals that suspended matter transported as dense flocs with size range of ca. 105–250 μm have a greater effect on its short-term deposition than loose flocs with size range of ca. 250–500 μm. As the measured suspended matter deposition is much higher in winter–spring than in summer, it is deduced here that highly compact and relatively dense flocs contribute to deposition during winter and spring periods resulting in a stable layer, while loosely formed flocs likely lead to an easier erodible layer during the summer. This study concludes that floc structure-related density is a more significant parameter than floc size in the suspended matter deposition processes.

Influence of harbour construction on mud accumulation in the Scheldt estuary

The bottom sediments of the turbidity maximum area of the Scheldt estuary were mapped in 1999 using echo sounding, sidescan sonar and grain-size analyses of bottom sediments. Four sediment types, sand, muddy sand, sandy mud and mud were recognised. Mud, with very little sand, occurs mainly in the access channels to the sluices giving access to the harbour docks of Antwerp. The sediments of the main channel have a sandier texture. One might conclude that the total mud stock in the middle estuary has increased, both between 1964–1986 and 1986–1999, but on the contrary the mud supply from the river, the mud stock in the river channel and the mud supply to the lower estuary have all decreased. The increase in the mud stock in the area as a whole was completely at the expense of mud deposition in the access channels to the sluice gates giving access to the harbour of Antwerp. The mud stock in the river channel decreased over the years because of a decreasing mud supply from the river. The mud stock in the river channel shows variations that are directly related to fluctuations in the river load. When the suspended matter decreased during a certain year the supply of silt and clay particles decreased correspondingly and the resuspension-deposition mechanism caused a relative increase of the sand fraction in the bottom sediments in that year.

ESTUARINE HYDRODYNAMICS AND ENERGY

The instantaneous and complex characters of an estuarine system are primarily subjected to the physical forces which are the driving power in an estuary. Based on the long-term field data in the Schelde estuary (Belgium, the Netherlands), this study is the first attempt of its kind to quantify estuarine energy derived from key physical forces, i.e., river flow, tidal activity and wave action, in an estuarine system. Tidal discharge, river discharge, flow velocity, tidal range and characteristics of geomorphology are the main variables considered in estuarine hydrodynamics analysis and energy calculation. The results show that the Schelde estuary is characterized by a specific total energy pattern dominated by tidal energy. The energy pattern determines the sediment transport and distribution especially the formation and locations of estuarine turbidity maxima (ETM) in the Schelde estuary.

Organic matter and dissolved inorganic nitrogen distributions in estuarine muddy deposits

Organic matter (OM) and dissolved inorganic nitrogen (DIN: nitrite, nitrate and ammonium) in the sediments as well as in the water column of two temperate estuaries, the Scheldt Estuary in Belgium and the Netherlands, and the Fraser Estuary in Canada, were investigated. Three representative stations, differing in salinity and representing areas of fast sedimentation, were selected in each estuary. Samples were taken during periods of high and low river discharge. The results show, in both estuaries, that the vertical distributions of OM and DIN in a sediment layer are affected by the instability, caused by episodic resuspension and re-deposition, of the uppermost sediment layer. The findings of this study suggest a hypothesis, next to biogeochemical processes, that the OM and DIN distributions in upper sediment layers are influenced by sedimentary processes in the estuarine environment. The same sedimentary processes even in different estuaries affect OM and DIN distributions in an equivalent way. Correspondingly, the similarity or difference in OM and DIN distribution to a certain extent reflects the sedimentary dynamics. River runoff and sediment resuspension and sedimentation have important impacts on sediment behaviour and thus regulate OM and DIN distributions and shape their vertical profiles in the sediments. As a reflection, the coupling of sediment resuspension followed by redeposition can be deduced from the vertical profile of DIN in the bottom sediments which, in turn, can provide a time-integrated periodic record of the most recent sedimentary history.