As a follow-up to the simulations of estuarine habitat change, the modified SLAMM code was applied to the gravel and dune barrier and backbarrier saltmarsh complex of Blakeney, North Norfolk, eastern England. The aim here is to critically evaluate the ability of the modified model code to produce robust projections of habitat change in a system influenced more directly by the evolution of the open coast. In the case of Blakeney, this involves historic retreat of the outer barrier under the influence of storm- driven overwash (Clymo, 1964; Funnell et al., 2000)
As with the previous case studies, a key factor influencing the selection of this site was the availability of a composite LiDAR dataset; this was provided by the Environment Agency from surveys undertaken mostly between 2008 and 2010 at a 2 m resolution. These data were used as the base DEM and were integrated with subtidal bathymetry which was manually digitised based on UKHO bathymetry chart (chart no: 0108-0). The composite elevation dataset is resampled to 5 m resolution and visualised in Figure 5.1a. The derived land cover and slope layers used to define the model domain are presented in Figures 5.1b and 5.1c respectively.
Table 5.1 summarises the site-specific parameters used in the model runs. The historic sea-level trend is estimated at 2 mm yr-1 by interpolating the historic trend of the two closest tide gauge stations at Immingham and Lowestoft (PSMSL, 2012). The tidal reference levels for Blakeney, expressed in m OD, are obtained from the SMP2 project (SMP2-Appendix C, 2010). Due to missing data though, the highest and lowest astronomical levels are obtained by interpolating the relevant ones in Immingham and Cromer (Admiralty, 2000). Deposition rates have not been systematically measured at the backbarrier side of the island. Accordingly, the MARSH-0D model (French, 2006) is again used to compute the dependence of saltmarsh deposition rate on elevation; modelled sedimentation as a function of elevation was used to constrain the SLAMM saltmarsh accretion model parameters and a zero accretion rate assumed at the highest elevation of the marsh (Figure 5.2). In addition, the proximity to the creek network is
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known to exert a strong control on sedimentation in the marshes of North Norfolk (French and Spencer, 1993; French et al., 1995), and the distance to channel factor is also used in the SLAMM marsh accretion sub-models. In the absence of both data and a more mechanistic model of tidal flat processes, sedimentation in this environment is simply assumed to track sea-level rise.
Figure 5.1: SLAMM input layers for the Blakeney barrier-backbarrier complex; a) DEM, b) Land classification, c) Slope map.
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Table 5.1: SLAMM site parameter table for Blakeney.
Parameter Value
Date (YYYY) 2010
Direction Offshore[n,s,e,w] N
Historic Trend (mm yr-1) 2
Salt Elevation (m above MTL) 3
HAT (m) 3 MHWS (m) 2.6 MHW (m) 1.9 MHWN (m) 1.2 LAT (m) -2.8 Marsh Erosion (m yr-1) 0.1 T.Flat Erosion (m y-1) 0.1
Beach Sedimentation Rate / Tidal Flat Accretion (mm yr-1) 2
LOWER MARSH ACCRETION MODEL Max Accretion (mm yr-1) 24 Min Accretion (mm yr-1) 9.9 Coefficient a 0 Coefficient b 1 Coefficient c 3 DeffectMax (m) 100 Dmin (unitless) 0.1
UPPER MARSH ACCRETION MODEL Max Accretion (mm yr-1) 9.9 Min Accretion (mm yr-1) 0 Coefficient a 0 Coefficient b 1.4 Coefficient c 1 DeffectMax (m) 100 Dmin (unitless) 0.1
Figure 5.2: Modelled marsh accretion rates used to constrain the SLAMM marsh accretion sub-models for the Blakeney estuary.
The Blakeney barrier has retreated over recent historical timescale, in parallel with incremental westward extension of the spit through creation of sequential recurves
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under storm conditions (Funnell et al., 2000; Environment Agency, 2012). This behaviour is visualised in Figure 5.3 according to the coastal trend analysis undertaken by the Environment Agency (2012) for the period 1991 to 2011; a 2 m advance in the dune line is evident in the transect N2C1, in parallel with a foredune retreat of 20 m in 20 years in transect N2C2. To the east and along the spit length, a roll back of the shingle ridge is occurring in response to natural processes, on average rate of 0.6 m yr-1.
Figure 5.3: Coastal trend analysis for the Blakeney between 1991 and 2011, focusing on the a) westerly migration of the Blakeney Point system and b) the shoreline retreat along the barrier island (Environment Agency, 2012).
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Overwash is an important process driving the landward rollover of a barrier over the landward saltmarsh and tidal channel environments. SLAMM includes an overwash module, as mentioned in Chapter 2, that applies to barrier islands less than 500 m in width due to storms with a frequency of 25 years. During overwash, the barrier beach rolls back by 30 m, and overwashed sediment is carried over the crest of the barrier and deposited onto the adjacent marsh, converting it to undeveloped dryland and estuarine beach (Table 5.2; Figure 5.4). This behaviour is based on observations from the large sandy barrier beaches in the USA (Leatherman and Zaremba, 1986; Clough et al., 2010).
Table 5.2: SLAMM overwash decision tree (Clough et al., 2010).
Converting from to Default values
Ocean Beach Ocean 30 m
Dryland Ocean Beach 30 m
Transitional and upper marsh Undeveloped Dryland 50%
Lower marsh Estuarine Beach 50%
Estuarine Subtidal Estuarine Beach 60m
Figure 5.4: Overwash definition sketch within SLAMM
Accordingly, the overwash sub-model must be parameterised for the specific case study. An overwash frequency of 2 years is assumed, since the coast of North Norfolk experience extreme water levels almost every year (Figure 5.5). The maximum overwashed width is the width of the shingle ridge, estimated to be about 25 m, based on present aerial photos (Figure 5.6). The barrier roll over is estimated to be approximately 1.2 m in every overwash event, assuming that a retreat rate of 0.6 m yr-1 calculated on the coastal trend analysis undertaken by the Environment Agency (2012) is driven by the process of overwash (see Figure 5.3). Consequently, 1.2 m of sediment are overwashed in every event from the 25 m width shingle ridge and deposited on the marsh area at the back side of the ridge (‘marsh loss’: approximately 5%).
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Figure 5.5: Distribution of extreme water levels in North Norfolk (after SMP2, 2010).
Figure 5.6: Overwash sub-model parameterisation based on analysis of aerial imagery in Google Earth: 1) marsh loss, 2) maximum overwash width.
Three different scenarios are simulated under the UKCP09 SE sea-level rise scenario, as presented in Table 5.3, in order to investigate how this process affects the habitat distribution of Blakeney. The overwash sub-model is not incorporated in the first simulation (RUN_1), but a comparative evaluation in two subsequent simulations applies an overwash of 5% (RUN_2) and 100% (RUN_3).
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Table 5.3: SLAMM parameter table for the sensitivity analysis of the overwash model for Blakeney.
Overwash model parameters RUN_1 RUN_2 RUN_3
Freq. overwash (years)
NO OVERWASH
2 2
Max width overwash (m) 25 25
Beach to Ocean (m) 1.2 1.2
Dryland to beach (m) 1.2 1.2
Marsh loss overwash (%) 5 100