E&ES > Geography > Research projects > Sedimentation and morphodynamics of tidal marshes in the Scheldt estuary
Sedimentation and morphodynamics of tidal marshes in the Scheldt estuary: a field and numerical modelling study
| Researcher: | Dr. Stijn Temmerman |
| Promotor: | Prof. Dr. G. Govers, K.U.Leuven, Physical and Regional Geography Research Group |
| Co-promotor: | Prof. Dr. Patrick Meire, UIA, Ecosystem Management |
| Prof. Dr. Stanislas Wartel, KBIN, Unit of Sedimentology | |
| Funding: | IWT (specialisatiebursaal) |
Keywords:
| sediment transport, deposition, geomorphology, elevation change, tide, tidal marshes, salt marshes, numerical modelling, Schelde |
Research abstract:
|
Sedimentation on the surface of tidal marshes has an important effect on the functioning of the Scheldt estuary. In this research project, the spatial and temporal variability of tidal marsh sedimentation, and the factors that control these variations, are studied on different spatial and temporal scales. The field data are then used to construct and evaluate numerical models, which enable us to predict the morphological behaviour of tidal marshes when controlling factors are changing. |
Introduction
In the tidal marshes of the Scheldt estuary, deposition of suspended sediment takes place during tidal inundation (fig. 1). Tidal marsh sedimentation has an important effect on the functions of the estuary:
(1) it is one of the most important factors controlling the ecological functioning of the estuary, (2) it reduces the role of tidal marshes as inundation areas, which can protect the densely populated land from flooding, and (3) it can have a reducing effect on sedimentation in the stream channel, which is continuously dredged for intensive shipping. In spite of this importance, there are no data and numerical models that permit to estimate and predict sediment deposition and morphological changes in the tidal marshes of the Scheldt estuary.
Figure 1: A typical salt marsh near the mouth of the Scheldt estuary (Paulina marsh) just before and during tidal inundation at spring tide.
Objectives
The objectives of this research project are then:
- The execution of a measuring program on three tidal marshes in the Scheldt estuary, to get insight in the small-scale spatial and temporal variability of sedimentation and in the factors that control these variations.
- The collection of data on sediment accretion over the past century, in a relatively large number of tidal marshes along the estuary, to get insight in the long-term and large-scale sedimentological and morphological processes.
- The development and evaluation of numerical models, which can predict changes in sedimentation and morphology, when the controlling factors are changing.
Figure 2: The study area, with situation of the detailed study sites: Paulina (a salt marsh), Saeftinghe (a brackish marsh) and Notelaar (a freshwater marsh).
Small-scale sedimentation patterns
Field study.
Sedimentation patterns are studied at 3 study sites (of ca. 30 ha) (see fig. 2):
- a salt marsh (Paulinaschor)
- a brackish marsh (Saeftinghe, Kruispolderhaven)
- and a freshwater marsh (Notelaar).
On each study site sedimentation is measured using a spatial network of plastic sediment traps, which are replaced every 15 days after a spring-neap tidal cycle. Longer-term sedimentation rates are measured with a 1-year interval using feldspar marker horizons (fig. 4). The data show strong spatial variations, which are highly related to marsh topography. The distance to the nearest tidal creek or the stream channel of the Scheldt River plays an essential role in sediment delivery and deposition. Young (low lying) marshes have higher sedimentation rates than old (high) marshes, due to higher and more frequent tidal inundations (fig. 5).
Temporal variations in sediment delivery and deposition are studied in detail with 2 automatic sampling stations and sediment traps (fig. 4). These variations are especially affected by temporal variations in tidal inundation. The higher the inundation height and duration, the higher the suspended sediment concentration and deposition above the marsh surface. In addition, a strong seasonal pattern of tidal marsh sedimentation is observed.
Figure 3: Field sampling methods: (left) field station (automatic water level measurements and suspended sediment sampling with ISCO equipment), (right above) sediment trap, (left below) feldspar marker horizon after 1 inundation event.
Numerical modelling.
First, the extensive dataset of field measurements allow us to construct empirical models (by multiple regression techniques), which can predict spatio-temporal variations in short-term sedimentation, based on the major controlling parameters. Detailed information on tidal creek networks and surface elevation (by way of DEM's) are then used to generate fully 2-dimensional spatial sedimentation patterns.
Secondly, also physically-based models are used and evaluated for their possibility to simulate spatio-temporal patterns of tidal marsh sedimentation. The application of CFD-based hydrodynamic models (e.g. RMA-2), coupled with sediment transport models, is explored and evaluated against field measurements for the 3 study sites. Once validated, these models may be useful to simulate flooding and sedimentation patterns in other tidal marshes.
Figure 4: Aerial photograph of the Paulina marsh, with indication of the spatial sedimentation pattern for 1 spring-neap tidal cycle.
Large-scale sedimentation patterns
Field study.
At 14 marsh locations, scattered along the Scheldt estuary (see fig. 2), sediment accumulation rates during the past 100 years are estimated. In order to do this, undisturbed sediment cores are collected and dated using paleo-environmental techniques (see fig. 6) and radiometric techniques (unsupported 210Pb activity). Also the influence of sediment autocompaction and preservation of organic deposits on elevation changes of the marsh surface are investigated, using the same sediment cores.
Preliminary results show that young marshes are characterised by initially high accumulation rates (see fig. 7: light green balls). After an equilibrium level is attained, the accumulation rate is much lower and is then expected to be controlled by the increase of mean high water levels and by the suspended sediment characteristics, which vary considerably along the Scheldt estuary.
Figure 5: Aerial photographs of the Notelaar marsh, showing changes in land use (from agricultural use to development of natural marsh vegetation) and vegetation cover (from bare tidal mudflat over Scirpus sp. to Phragmites australis and finally Salix sp.). The levels, according to these land use and vegation cover changes, can be clearly observed in sediment cores, based on determination of plant detritus.
Numerical modelling.
First, long-term sediment accumulation is modelled following a 0-dimensional time-stepping modelling approach, based on long-term simulation of tidal marsh inundation and the solution of a simple mass balance equation over all these inundations. The mass balance equation describes the change in suspended sediment above a unit area of marsh surface between different time steps during a tidal inundation. Preliminary modelling results are in good agreement with observed field data (see fig. 7).
The possibility to simulate the long-term 2-dimensional morphological evolution of tidal marshes, is furhter explored by extending this modelling structure to 1 and 2 dimensions.
Figure 6. Observed (in dots and error bars) and predicted (lines) vertical rise of the young and old marsh of the Notelaar, using a constant C(0) (in broken lines) and C(0) as a function of H (in solid lines). MHWL = the yearly evolution of local mean high water level.
Finally
The integration of all field data and modelling structures will finally allow us to quantify and predict the behaviour of tidal marshes on different time and spatial scales. These data and models may then be used to support future management decisions in the Scheldt estuary, which take into account the possible effects of tidal marsh morphodynamics.
Links:
| KBIN, Unit of Sedimentology | http://www.natuurwetenschappen.be/sedimento/ |
| UIA, Ecosystem Management | http://bio-www.uia.ac.be/bio/ecobe/ |
| IWT | http://www.iwt.be/ |
Collaboration with:
| Prof. Dr. Patrick Meire Ecosystem Management University of Antwerp Universiteitsplein 1-c B-2610 Antwerp |
Prof. Dr. Stanislas Wartel Sedimentology department Royal Belgian Institute of Natural Sciences Vautierstraat 29 B-1000 Brussels |