In the present study, the modified SLAMM code was applied to Newtown, in order to undertake a sensitivity analysis of the various model parameters and external forcing involved. As noted previously, a key factor influencing the initial selection of the Newtown estuary was the availability of LiDAR data at no cost from the Channel Coastal Observatory (http://www.channelcoast.org/). At this stage in the project, LiDAR were not generally available for most of the estuaries in England and Wales. These data have a sampling interval of 1m and an indicative vertical accuracy of ±15 cm. LiDAR data combined with digitised bathymetry data in ARC-GIS 9.3 in order to create the input DEM for SLAMM (Figure 3.5). Given the quality of the LiDAR data, the elevation pre-processing option in SLAMM was not used. The DEM was used to derive the slope (Figure 3.6) and the land classification map based on wetland position in the tidal frame (Figure 3.7; Table 3.3), as described in Chapter 2. All input layers were resampled to a 5 m horizontal interval.
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Figure 3.6: Slope map of Newtown Estuary created using a GIS.
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Table 3.3: Tidal criteria for modelling vertical zonation of inter-tidal areas. Tidal levels are derived from the Admiralty tide tables (UK Hydrographic Office, 2000).
Coastal habitats and land classification
Criteria for defining habitat position based on elevation and tidal level
Tidal level Elevation (m)
E
stuary m
od
el
1. Dry land >HAT > 1.9
7. Transitional marsh MHWS-HAT 1.5 - 1.9
20. Upper marsh MHW-MHWS 1.15 - 1.5
8. Lower marsh MHWN-MHW 0.8 - 1.15
11. Tidal Flat LAT-MHWN (-2.6) - 0.8
17. Estuarine Subtidal <LAT < (-2.6)
13. Ocean Flat LAT – HAT (-2.6) - 1.9
19. Open Ocean <LAT < (-2.6)
Table 3.4 summarises the additional site-specific parameters used in the sensitivity analysis model runs. The historic sea-level trend includes the effect of vertical land movements and is estimated at 1.49 mm yr-1 using the closest tide gauge station at Portsmouth (almost 25 km to the northwest). The greater diurnal tide range at this site is calculated by the difference between MHWS and MLWS (3.4 m), while erosion and accretion parameters are obtained from the SMP2 (Isle of Wight SMP2, 2010) and the BRANCH project (BRANCH partnership, 2007).
Table 3.4: Site parameter table for Newtown estuary.
Parameter Newtown
DEM Date (YYYY) 2008
Direction Offshore[n,s,e,w] N
Historic Sea Level Trend (mm y-1) 1.49
GT Great Diurnal Tide Range (m) 3.4
Salt Elevation (m above MTL) 1.9
HAT (m) 1.9 MHWS (m) 1.5 MHW (m) 1.15 MHWN (m) 0.8 LAT (m) -2.6 Marsh Erosion (m y-1) 0.25
Tidal Flat Erosion (m y-1) 0.2
Lower Marsh Accr (mm y-1) 2 *
Upper Marsh Accr (mm y-1) 1.8 *
Beach Sedimentation Rate (mm y-1) 2
*Spatially varying accretion values for each wetland category. If the accretion model is to be used, these parameters are left blank.
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For the purpose of the sensitivity analysis, simulations were executed under the UKCP09 SE mean sea-level rise scenario using a time-step of 25 years (Simulation ‘N_UK’). Under this forcing scenario, the estuarine habitats are seemingly able to adapt to sea-level rise during the early time-steps, although changes in the lower marsh area are significant by the last time-step (Figure 3.8). In more detail, as presented in Table 3.5, sea-level drives enlargement of the estuarine subtidal by almost 30% by the year 2100. However, the area of the tidal flat increases from about 185 ha in year 2008 to about 208ha in year 2100, indicating that although part of it is inundated/eroded and therefore converted to estuarine subtidal, a larger part of it migrates upland to the marsh area. Consequently, the total area covered by marsh decreases by 2100. Most affected, though, is the lower marsh area with a decrease of 30%, because in most cases there is no space for upland migration. The transitional marsh remains quite stable by migrating upland to dryland, which is also inundated to ocean beach when it is adjacent to the ocean.
It is worth noting here that differences were not expected in the first time-step where SLAMM corrects the land classification based on the DEM, since the land classification is generated from the DEM based on the same conceptual model used in SLAMM. However, the resolution used for its creation can generate them. In this study, the land classification layer is generated by the original DEM file of 1m resolution, but they are both then resampled to 5 m, leading to small differences in the boundaries of some wetland categories between the two layers. SLAMM tries to correct these small differences at the first time-step by assuming inundation of the specific cells, ignoring the process of aggradation. Moreover, while inundation of a habitat within a specific cell is assumed, its transition to the correct one is based on empirical calculations which might result in bigger differences. Thus, it is wise to resample the DEM in the desired resolution first, and based on this generate the required land classification layer. In that case, the generated layers should have less differences and match he conceptual model used in SLAMM and therefore the current time-step could be skipped.
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Table 3.5: Impacts of sea-level rise at the Newton Estuary by the year 2100 (changes in ha).
Simulation name: "N_UK"
Date SLR Dry Land Trans. M. Upper M. Lower M. Tidal Flat Est. Subtidal Ocean Beach Ocean Flat Open Ocean
0 0 638.1 14.3 20.0 51.5 176.4 16.3 0.0 41.6 47.7 2008 0 628.7 19.5 15.8 49.2 185.0 17.3 0.8 41.2 48.4 2025 0.0563 627.7 14.5 15.9 51.7 187.6 17.8 0.4 40.8 49.5 2050 0.1583 625.7 14.4 14.1 49.6 192.5 18.6 0.0 39.7 51.2 2075 0.2778 623.0 14.6 13.4 46.5 197.1 20.2 0.0 37.7 53.4 2100 0.4123 619.5 14.8 13.5 36.8 208.2 21.6 0.0 35.4 56.2 0-2008 -9.4 5.3 -4.3 -2.3 8.6 1.0 0.8 -0.4 0.7 -8.3 -3.1 -7.3 -9.6 -1.0 -0.3 -0.4 -0.7 -1.1 2008-2025 -0.9 -5.1 0.1 2.5 2.6 0.5 -0.5 -0.3 1.0 -0.7 -5.8 -5.6 -3.2 -0.5 -0.7 -0.3 -1.0 -0.2 2025-2050 -2.0 0.0 -1.8 -2.07 4.93 0.80 -0.35 -1.17 1.73 -1.8 -1.8 -3.7 -5.7 -0.80 -0.6 -1.2 -1.73 -0.2 2050-2075 -2.7 0.2 -0.7 -3.2 4.6 1.5 0.0 -2.0 2.2 -2.4 -2.3 -3.0 -6.2 -1.5 -0.3 -2.0 -2.2 -0.3 2075-2100 -3.5 0.2 0.1 -9.7 11.0 1.4 0.0 -2.3 2.8 -3.0 -2.8 -2.7 -12.4 -1.4 -0.5 -2.3 -2.8 -0.5
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Figure 3.8: Habitat distribution in Newtown Estuary under the SE Mean UKCP09 sea- level rise scenario by the Year 2100 (Simulation ‘N_UK’).
The BRANCH project approach and SLAMM differ in that the former uses two different modelling approaches in order to assess the impacts of sea-level rise on an estuary; one for the open coast and one for the intertidal habitats. In contrast, SLAMM models the whole estuary at once. More specifically, in BRANCH the spits are modelled by using empirical relationships based on the assumption that the shoreline behaviour is included into the previous shoreline movements. However, the spits at the Newtown estuary have changed their shape and direction throughout the years, such that this approach tends to overestimate spit recession. On the other hand, SLAMM treats the open coastal flat in front of the spits exactly with the same way to the intertidal habitats within the estuary. Here, this tends to result in a more stable estuary mouth, and this approach therefore has its limitations.
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Within the estuary proper, previous modelling efforts, including BRANCH, largely evolve the progressive drowning of the existing topography under sea-level rise. This approach does not incorporate any mechanistic modelling of habitat transitions, as SLAMM does by incorporating a flexible decision tree and qualitative relationships. In addition, the process of erosion is totally ignored in most previous models, and although the accretion parameter is taken into account by the BRANCH project, it only applies to areas colonised by saltmarsh. SLAMM introduces more sophistication in that different accretion and erosion values can be applied for each wetland category. Moreover, SLAMM can take into consideration the spatiality of the accretion parameter within each habitat type by calculating it as a time-varying function of elevation, distance to channel (and salinity, when this sub-model is activated). Given this additional complexity, a sensitivity analysis is necessary in order to evaluate how important these parameters are and how they affect the prediction of sea-level rise impacts on the various parts of the estuarine intertidal.