luc lebbe , sarah jonckheere & alexander vandenbohede department of geology and soil...

Luc Lebbe, Sarah Jonckheere & Alexander VandenbohedeDepartment of Geology and Soil Science,

Ghent University, Gent, Belgium

Map of Belgium

Studied areais locatedbetweenZeebruggeandtheDutch-Belgianborder

The Zwin Estuary is the former waterway to Flemish medieval seaports of Sluis, Damme and Brugge (Bruges)

During the considered modeling period this estuary silted up and the modeled area changes

from an area around an estuary mouth over a tidal flat to a rather complex polder dune area.

- Applied software MOCDENS3D (Oude Essink, 1998) and VisualMOCDENS3D (Vandenbohede, 2007)

- Applied finite-difference grid: - 70 rows (width of all row = 50 m)- 88 columns (width of all columns = 50 m)- 12 layers (thickness of each layer = 2 m)- angle between north and row direction

is 75° turning to the east

Row direction is parallel to present day low water line

Phreatic aquifer is formed by sandy deposits.Heterogeneity is important.

Horizontal and vertical conductivities are deduced by the performance and the interpretation of three pumping tests (range Kh = 1.5 - 45 m/d, Kh/Kv = 25).

Hydrogeological data base (VMM) is used to generate the transmissivity matrix and the vertical leakance matrix.

+ Water conductive porosity is 0.38+ Longitudinal dispersivity is 0.2 m+ Transversal dispersivity for hor. flow is 0.02 m+ Transversal dispersivity for vert. flow is 0.01 m

+ Density dependent flow+ Density of salt water = 1020 Kg/m³+ Density of fresh water = 1000 Kg/m³

Boundary conditions on shore and banks of estuary inuppermost layer depend strongly on the surface elevation.

Therefore the reconstruction of the historical evolution of the surface elevation is very important.

Rech. dunes=280 mm/yRech. polders= 28mm/y

W. & E. boundary coincide with flow lines Impervious boundaries

N. & S. boundary are constant head boundaries

Based on work of Historian Johan Termote (2004) of the Tourist Development Agency ‘Westtoer

Digital Terrain Model (DTM or DEM) is deduced fromthe above mentioned documents

First document is painting of POURBUS (1571) of the region ‘Brugse Vrije’

Following are four old maps :Wilschut (1737-1740), Lammeire (1784),Van der Maelen (1846), De Mey (1880) and the present day map

Eight stress periods are considered with a duration of c. 60 y

+ To demonstrate the occurrence of inverse density distributions under the shores and the banks of estuaries

+ To demonstrate that the shapes of these distributionsare strongly depended of the slope of the estuary bankand of the heterogeneity of the phreatic aquifer

+ To demonstrate how these distributions evolve when entrapped under an area with a smallhydraulic gradient ( e.g. a tidal flat)

East ward movement of estuary with smaller draft for ships

Decrease slop left estuary bank ; increase slop right estuary bank .

East ward movement of estuary which is almost completely silted up

Erosion of eastern dunes

Eastern dunes fade away and the estuary is changed into a tidal flat

Enlargement of polders by the construction of the Saint Paul’s Dyke

Landward movement of high water line

Western dunes are eroded and shore has a rather large slope .

Tidal fluctuations on the shore and on the banks of the estuary result in inverse density distributions

Shapes of these inverse density distributions depend on slopeof shore or bank and on the heterogeneity of phreatic aquifer During silting up of estuaries these inverse distributions

became entrapped under the tidal flat area with a very slow GW flow (principally induced by density differences)

Results show very interesting evolutions of inverse density distributions which need more detailled modeling

These evolutions should be modelled more in detail in the future: – changes from one landscape into another (in sedimentologically well-founded events)- influence of discretization of space-time grid on evolutions

of inverse density distributions