Methods Used in a Mass-Balance Study of Holocene Sediment Accumulation on the...
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times> 1101 m. Thus the DEM 50 is unsuitable for studies in coastal lowland
areas.
The DEM 5 is based on a 12.5 x 12.5-m grid and contains much higher-quality
data; the elevation values have an accuracy < 10.51 m.A 3 D plot representing a 4
km 2 area of marshland at Krummhorn on the Ems estuary made from this kind
of digital data is shown in Fig. 3. The figure demonstrates the high vertical resolution of this model by dearly depicting a morphological depression and an artificial dwelling mound in the flat coastal marshland. Consequently, the DEM 5
is an excellent data base for calculation of the HON and UV volumes.
As far as possible, DEM 5 data were used in this research project for reconstructing the surface morphology of the coastal marshes and East Frisian barrier islands. However, DEM 5 covers only 10% of the study area; thus, the remaining land surface had to be reconstructed on the basis of other topographical data. Well locations were one source which contributed a number of scattered elevation values. The remaining areas were covered by manually digitizing contour
lines and point elevations on topographical map sheets. At least ten points per
km 2 were digitized from 1 :25000 maps to reconstruct the relief of the flat coastal
marshlands. The morphology of the dune landscape on the East Frisian Islands
required a much denser data set, which was obtained by digitizing approximately 250 points per km 2 from 1:5000 topographical map sheets.
The surface relief of subaquatic areas (foreshore zone of the barrier islands, tidal gullies, estuaries, and lakes) is the same as the sediment-water interface (Fig. 2).
There is a similar boundary in the intertidal zone. The surface of the sediment
wedge in both these areas was reconstructed on the basis of the digital coastal
model (DIGEKO; Schleider and Buziek 1991). Supplementary digital data sets
were obtained from the Dutch-German research project WADE (Wadden Sea morphological development due to an accelerated relative sea-level rise; Niemeyer et
al. 1995). The remaining gaps were filled by digitizing contour lines on 1:25000
coastal map sheets, which were published by the board of trustees for research in
coastal engineering. Thus, at least 10 elevation values per km 2 are available for reconstructions of the surface relief in subaquatic areas and in the intertidal zone.
4.3
The Base of the Accumulation Wedge
The base of the accumulation wedge (Fig. 2), which corresponds to the interface
between Holocene and Pleistocene deposits, was reconstructed on the basis of
borehole data. Over much of the coastal zone, this boundary is equivalent to the
surface of the original Pleistocene landscape. In other areas, especially in the seaward zone of the coastal lowlands, the original landscape underwent drastic
modification by tidal erosion during the Holocene transgression. The basal
boundary of the accumulation wedge can be reconstructed in two ways. One is by
manual construction of structure contour maps, the other is with the aid of computer programs and based directly on the depth of the Pleistocene-Holocene
boundary from borehole data.
365
times> 1101 m. Thus the DEM 50 is unsuitable for studies in coastal lowland
areas.
The DEM 5 is based on a 12.5 x 12.5-m grid and contains much higher-quality
data; the elevation values have an accuracy < 10.51 m.A 3 D plot representing a 4
km 2 area of marshland at Krummhorn on the Ems estuary made from this kind
of digital data is shown in Fig. 3. The figure demonstrates the high vertical resolution of this model by dearly depicting a morphological depression and an artificial dwelling mound in the flat coastal marshland. Consequently, the DEM 5
is an excellent data base for calculation of the HON and UV volumes.
As far as possible, DEM 5 data were used in this research project for reconstructing the surface morphology of the coastal marshes and East Frisian barrier islands. However, DEM 5 covers only 10% of the study area; thus, the remaining land surface had to be reconstructed on the basis of other topographical data. Well locations were one source which contributed a number of scattered elevation values. The remaining areas were covered by manually digitizing contour
lines and point elevations on topographical map sheets. At least ten points per
km 2 were digitized from 1 :25000 maps to reconstruct the relief of the flat coastal
marshlands. The morphology of the dune landscape on the East Frisian Islands
required a much denser data set, which was obtained by digitizing approximately 250 points per km 2 from 1:5000 topographical map sheets.
The surface relief of subaquatic areas (foreshore zone of the barrier islands, tidal gullies, estuaries, and lakes) is the same as the sediment-water interface (Fig. 2).
There is a similar boundary in the intertidal zone. The surface of the sediment
wedge in both these areas was reconstructed on the basis of the digital coastal
model (DIGEKO; Schleider and Buziek 1991). Supplementary digital data sets
were obtained from the Dutch-German research project WADE (Wadden Sea morphological development due to an accelerated relative sea-level rise; Niemeyer et
al. 1995). The remaining gaps were filled by digitizing contour lines on 1:25000
coastal map sheets, which were published by the board of trustees for research in
coastal engineering. Thus, at least 10 elevation values per km 2 are available for reconstructions of the surface relief in subaquatic areas and in the intertidal zone.
4.3
The Base of the Accumulation Wedge
The base of the accumulation wedge (Fig. 2), which corresponds to the interface
between Holocene and Pleistocene deposits, was reconstructed on the basis of
borehole data. Over much of the coastal zone, this boundary is equivalent to the
surface of the original Pleistocene landscape. In other areas, especially in the seaward zone of the coastal lowlands, the original landscape underwent drastic
modification by tidal erosion during the Holocene transgression. The basal
boundary of the accumulation wedge can be reconstructed in two ways. One is by
manual construction of structure contour maps, the other is with the aid of computer programs and based directly on the depth of the Pleistocene-Holocene
boundary from borehole data.