364
c. Hoselmann . H. Streif
wedge was reconstructed and displayed on 1:25000 structure contour maps.
Maps on the same scale showing the distribution of the clastic sediments and
peat layers present a picture of the internal makeup of the coastal Holocene. The
surface relief of the accumulation wedge in the subaquatic, eulitoral and terrestrial environments was derived from hydrographic soundings and topographic
levelling. This provided a good basis for determination of the volumes of the different units of the coastal Holocene.
4
Geometry of the Accumulation Wedge
For the mass-balance study, it was necessary to subdivide the sediment wedge
into a series of units, which are depicted in a schematic cross-section from the
North Sea to the Pleistocene hinterland (Fig. 2).
4.1
Units Used in the Calculation
Separate calculations were carried out to determine the volumes of Holocene
sediments above and below German zero datum NN (Normalnull), which is
nearly identical with mean sea level in this region. The sediment volumes above
and below NN (HON and HUN, respectively) are both mainly made up of marine to brackish deposits (HON mb and HUN mb), but also contain lacustrine to
semiterrestrial deposits (HON Is and HUN Is). The aeolian dune deposits (HON
al) on the East Frisian Islands, as well as anthropogenic structures (HON an)
such as dwelling mounds, dykes and dumps of dredged material, are limited to
positions above NN.
The volume of water bodies (WV) in the foreshore area, tidal gullies, estuaries, and freshwater lakes is of considerable importance for the mass-balance
study. Less important is the volume of morphological depressions (UV) in the
coastal marshes in which the land surface may lie 2 m below NN and which are
not filled with sediments or water. For the calculation of the total volume of the
Holocene deposits, the values ofWV and UV have to be subtracted from the sum
of the other units (HON+HUN).
4.2
The Surface of the Accumulation Wedge
In the terrestrial area (East Frisian Islands, salt marshes, and dyked coastal
marshland), the surface of the accumulation wedge is identical with the land
surface (Fig. 2) and can be derived from the topography or digital elevation
models. The ordnance survey of Lower Saxony currently offers two types of
data sets (Washausen 1992). The Digital Elevation Model DEM 50 is based on
a 50 x 50-m grid and is available for the whole of Lower Saxony. Each grid
node has one elevation above NN with an accuracy of a few meters and some-
c. Hoselmann . H. Streif
wedge was reconstructed and displayed on 1:25000 structure contour maps.
Maps on the same scale showing the distribution of the clastic sediments and
peat layers present a picture of the internal makeup of the coastal Holocene. The
surface relief of the accumulation wedge in the subaquatic, eulitoral and terrestrial environments was derived from hydrographic soundings and topographic
levelling. This provided a good basis for determination of the volumes of the different units of the coastal Holocene.
4
Geometry of the Accumulation Wedge
For the mass-balance study, it was necessary to subdivide the sediment wedge
into a series of units, which are depicted in a schematic cross-section from the
North Sea to the Pleistocene hinterland (Fig. 2).
4.1
Units Used in the Calculation
Separate calculations were carried out to determine the volumes of Holocene
sediments above and below German zero datum NN (Normalnull), which is
nearly identical with mean sea level in this region. The sediment volumes above
and below NN (HON and HUN, respectively) are both mainly made up of marine to brackish deposits (HON mb and HUN mb), but also contain lacustrine to
semiterrestrial deposits (HON Is and HUN Is). The aeolian dune deposits (HON
al) on the East Frisian Islands, as well as anthropogenic structures (HON an)
such as dwelling mounds, dykes and dumps of dredged material, are limited to
positions above NN.
The volume of water bodies (WV) in the foreshore area, tidal gullies, estuaries, and freshwater lakes is of considerable importance for the mass-balance
study. Less important is the volume of morphological depressions (UV) in the
coastal marshes in which the land surface may lie 2 m below NN and which are
not filled with sediments or water. For the calculation of the total volume of the
Holocene deposits, the values ofWV and UV have to be subtracted from the sum
of the other units (HON+HUN).
4.2
The Surface of the Accumulation Wedge
In the terrestrial area (East Frisian Islands, salt marshes, and dyked coastal
marshland), the surface of the accumulation wedge is identical with the land
surface (Fig. 2) and can be derived from the topography or digital elevation
models. The ordnance survey of Lower Saxony currently offers two types of
data sets (Washausen 1992). The Digital Elevation Model DEM 50 is based on
a 50 x 50-m grid and is available for the whole of Lower Saxony. Each grid
node has one elevation above NN with an accuracy of a few meters and some-