2.3.1 Overview
SLAMM is written in Object Pascal and is open source software, distributed under a Common Public License. It is developed in Delphi 2007 and also requires the Delphi OpenGL libraries in order to be compiled. Its architecture is slightly unconventional in that the land cover classification is hard coded in accordance with the US National Wetland Inventory (NWI) scheme, which is not widely used elsewhere. Moreover, the forcing scenarios and various aspects of the sub-model parameterisations are also embedded in the source code rather than being read from external files. This means that source code alterations and re-compilation are required for application to sites outside North America. Accordingly, the first task undertaken in the present work is to modify the SLAMM source code to include a simplified land cover classification based on categories more suited to UK coastal and estuarine contexts. Also required, are a set of modified habitat transition rules and amended rules specifying their relation to the tidal frame. Support for UK-specific regional sea-level rise scenarios is also needed.
2.3.2 Implementation of a simplified habitat classification
SLAMM uses 25 wetland categories that follow the US National Wetland Inventory (NWI) but these are not readily transferable to the UK. Accordingly, this study is guided by a simple set of wetland categories defined by the INTERREG funded BRANCH project (BRANCH partnership, 2007) in the direction of promoting the use of spatial planning to help EU biodiversity to adapt to climate change. The “crosswalk” between these two classification systems is summarised in Table 2.4.
Table 2.4: Crosswalk between the UK and NWI wetland categories.
SLAMM code
NWI Classes UK Coastal Habitats
(BRANCH, 2007) Modified SLAMM Categories E stuary Mod el
1 Dev. dry land 10. Land 1. Dry Land
7 Trans.marsh 5.Transitional marsh 7. Trans.Marsh 20 Irr. Fl. marsh 4. Upper marsh 20. Upper Marsh
8 Reg.fl. marsh 3. Pioneer saltmarsh 8. Lower Marsh
11 Tidal flat 2.Mudflat 11. Tidal Flat
17 Estuarine water 1.Standing water 17. Est. subtidal
Coast
al
m
od
el 12 Ocean Beach Use of Leatherman
(2001) equation in GIS
12. Ocean beach
13 Ocean flat 13. Ocean flat
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2.3.3 Modification of habitat transition rules
Conversion of one wetland category to another occurs in response to either inundation or erosion. As described in section 2.2.3, erosion depends on the maximum fetch and the proximity of the wetland to the open ocean or the estuarine water, while the critical parameter that defines when a wetland category is inundated and therefore converted to another wetland category is its minimum elevation. The maximum elevation of each wetland category is only used when the Elevation Pre-Processor is utilised, in order to assign wetland elevation on the basis of wetland type, tide range and direction offshore, when LiDAR data are not available (Clough and Larson, 2010). All these transition rules are described on Table 2.5, and compared to the UK rules on Figure 2.6. In order to adapt the code to the UK, the decision tree is modified by assuming that the transitional marsh converts to upper marsh instead of lower marsh due to inundation, and the dryland to transitional marsh instead of estuarine beach when it is adjacent to the subtidal (Figure A-0.1 in Appendix).
Table 2.5: SLAMM decision tree (Clough et al., 2010).
Converting from Inundation - Converts to Erosion -Converts to
Dry Land Transitional Marsh
Ocean Beach (if adj to ocean water)
Ignored
Estuarine beach (if adj to water-erosion>heavy*)
Trans.Marsh Lower Marsh Tidal Flat
Upper Marsh Lower Marsh Tidal Flat
Lower Marsh Tidal Flat Tidal Flat
Tidal Flat/Beach Estuarine Subtidal Est. Subtidal
Oc. Flat/Beach Open Ocean Open Ocean
*heavy erosion when maximum fetch > 9km (see table 2.1)
Figure 2.6: SLAMM decision tree modification (grey arrows: inundation; red arrows: erosion).
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2.3.4 Adjustment of Habitat Elevation Ranges
In the original SLAMM code, the default elevation ranges of each wetland category are defined in the Elevation Inputs and Analysis Table as a function of the Salt Elevation, the Half Tide Unit (HTU) and the Mean Tide Level (MTL) (Table 2.6). The Salt elevation is that which is inundated by water once per month. This effectively defines the location where the dry land and the fresh water wetland begin. The Half Tide Unit (HTU) is defined from equation 2.13 (Clough and Larson, 2010), while the MTL is assumed to remain constant at zero. So, the HTU is equal to MHHW, which together with the MLLW are defined in the code from equations 2.14 and 2.15.
HTU = MHHW – MTL (2.13)
MHHW = GtideRange /2 (2.14) MLLW = MTL – GtideRange/2 (2.15) Table 2.6: SLAMM default elevation ranges.
SLAMM category No
Category Name Default Min. Elev. Default Max. Elev.
1 DryLand 1 Salt Elevation
7 Transitional Marsh 1 HTU 1 Salt Elevation
20 Upper Marsh 0.5 HTU 1 Salt Elevation
8 Lower Marsh 0 HTU 1.2 HTU
11 Tidal Flat -1 HTU MTL
17 Estuarine Subtidal 0 0
12 Ocean Beach -1 HTU 1 Salt Elevation
13 Ocean Flat -1 HTU MTL
19 Open Ocean 0 0
Although these could be useful for a secondary determination of tidal datum when primary data are not available (e.g. NOAA, 2000), they will likely vary from area to area. Thus, the modified code determines the position of each intertidal habitat according to their position in the tidal frame (Table 2.7; Figure 2.7), as used in the BRANCH project (after Chapman, 1960; Pye and French, 1993; Leggett and Dixon, 1994; Blott and Pye, 2004). Firstly, the tidal range parameters are added at the ‘Site
Parameter Table’ as required input values (see Figures A-0.2 to A-0.10 in Appendix)
and their values are linked to the default wetland elevation boundaries at the ‘Elevation
Input and Analysis Table’ (see Figures A-0.11 to A-0.16 in Appendix). Thus, in each
simulation the user defines the tidal range in the ‘Site Parameter Table’, and SLAMM automatically updates the elevation of each wetland category at the ‘Elevation Input and
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Table 2.7: UK default elevation ranges according to tidal ranges (BRANCH partnership, 2007).
SLAMM category No
Category Name Default
Min. Elev. Default Max. Elev. Estuarine Habitats 1 DryLand HAT
7 Transitional Marsh MHWS HAT
20 Upper Marsh MHW MHWS
8 Lower Marsh MHWN MHW
11 Tidal Flat LAT MHWN
17 Estuarine Subtidal LAT
Open Ocean Habitats
12 Ocean Beach LAT HAT
13 Ocean Flat LAT HAT
19 Open Ocean LAT
Figure 2.7: SLAMM decision tree modification, including tidal ranges (grey arrows: inundation; red arrows: erosion).
2.3.5 Addition of UK-specific sea level scenarios
The UKCP09 sea-level scenarios are incorporated into the modified SLAMM code (Table 2.8; Figure 2.8). These are based on the IPCC Fourth Assessment Report (IPCC, 2007), and assume that the rates of vertical land movement remain constant over the 21st century. Land movement uncertainty is neglected because this is likely to be small in comparison to the eustatic sea-level rise estimates (UKCP, 2009). These new scenario options were added to the SLAMM user interface (as illustrated in Figure 2.9). Accordingly, the mean sea-level trend is defined as required by the user parameter (1.7 mm yr-1 for the IPCC scenarios (IPCC, 2007), 1.4 mm yr-1 for the UK (Woodworth et al., 2009; Wahls et al., 2013)) (see Figures A-0.17 to A-0.26; update A-0.2 to A-0.4 and A-0.6 to A-0.10).
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Table 2.8: SLAMM inputs based on UKCP09 sea-level rise scenarios (mm).
UKCP09 sea-level rise scenarios (mm)
LONDON (SE) CARDIFF (SW) EDINBURGH (NE) BELFAST (NW)
H M L H M L H M L H M L
2025 137.5 116 98 137 115.5 98 91 69.5 52 94.5 73 66
2050 258 218 184 259 218 184 180 139 105 186 145 111
2075 402.5 337.5 284 402 336.5 284 303.5 225 171.5 298.5 233.5 180.5
2100 565 472 396 565 472 396 419 326 250 430 338 262
Figure 2.8: UKCP09 relative sea-level rise relative to 1990. Thick lines represent the central estimate values and thin lines the 5th and 95th percentile limits of the ranges of uncertainty.
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