As described in §2, habitat quantification became an important parameter in research topic 2. As a first step in the quantification of habitats, a classification with 6 different classes was introduced:
- Subtidal deep (Sd): > 5 m below MLWL
- Subtidal moderately deep (Sm): 5 – 2 m below MLWL
- Subtidal shallow (Ss): 2 m below MLWL – MLWL
- Intertidal flat (If): MLWL – MHWL; slope < 2.5%
- Intertidal steep (Is): MLWL – MHWL; slope > 2.5%
- Marsh (M): > MHWL
This habitat classification was based on the work conducted by Bioconsult Schuchardt & Scholle for the TIDE project and by previous work by Brys et al. (2005).
Habitat mapping and quantification
A GIS algorithm was developed to map the different habitats based on above described classes. As a first step, MLWL and MHWL surfaces were created by interpolating (kriging) the water level values (see Figure 17) of the different water level stations (locations, see Figure 9, Figure 10, Figure 11 and Figure 12). By subtracting the MLWL surface with the topo-bathymetry grid¹, a subtidal grid file was created in which the 3 subtidal classes were defined. All pixel values below MLWL are thus defined as subtidal classes dependent on the depth, all values above MLWL were classified as ‘NoData’. By substracting the MHWL surface with the topo-bathymetry grid, a marsh grid was created in which the marsh class was defined. All pixel values above MHWL were defined as marsh, all values below MHWL were classified as ‘NoData’. In a next step, a slope grid (intertidal grid) was created based on the topo-bathymetry grid. Pixels values below 2.5% were hereby classified as intertidal flat, pixel values above 2.5% were classified as intertidal steep. By merging the marsh grid, the subtidal grid and the intertidal grid (in this order), and executing a clip with the dyke lines, a habitat grid was created for each estuary. Finally, a habitat map was created by converting the habitat grid to a polygon shapefile.
The habitats were quantified in GIS in two ways: (1) by their surface area, and (2) by their relative width compared to the local estuary width. Surface areas were calculated using the polygon shapefile. Quantification by surface areas is not only important for the hydro-geomorphology study, but also for the ecology and ecosystem service studies within work package 3. The relative width of each habitat was quantified by intersecting the habitat map with the cubage cross-sections (§3.2.1). In this way the cross-section lines were split into separate lines in which each segment represents the width of a habitat. By summing the lengths of the distinct habitat segments, the total habitat length along a cross-section was calculated. Dividing these values by the total cross-section length (*100) returns the habitat percentages along each cross-section. These percentages were then related to the tidal range gradient (§5.2).
¹ The topo-bathymetry grids described in §3.1.1 were used for the Elbe, Weser and Humber. For the Scheldt, a more recent topo-bathymetry grid was used (2007-2009), however with a coarser resolution (20 x 20).
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