Modelling storm-induced beach/dune evolution: Sefton coast, Liverpool Bay, UK
Introduction
Coastal dune systems provide natural defence against erosion and flooding. They also provide an important natural habitat to local flora and fauna (Carter, 1988). Development and existence of coastal dunes are mainly controlled by cross-shore sediment transport delivering sediment to the upper beach and then Aeolian transport reshaping deposited sand (Harley and Ciavola, 2013). It is generally found that winter storms cause steep cross-shore profiles by dune erosion and offshore sediment transport while calm, mild summer condition system recovery results in a more gentle profile shape in most of the world's coastal systems (Callaghan et al., 2008). Severe storms in winter are responsible for non-recoverable erosion leading to dune breaching and then subsequent flooding of the hinterland areas.
There are four regimes of dune change during storm events depending on the water level and the upper limit (the 2% exceedance level, R2%) of wave run-up heights (Sallenger, 2000). They are as follows: 1) the swash regime — the dune system remains untouched, 2) the collision regime — wave bores collide with the dune face, 3) the overwash regime — a fraction of the waves overtop the dune crest and 4) the inundation regime — the dune is completely submerged. Episodic slumping of the dune face occurs during the collision regime (Vellinga, 1986, Erikson et al., 2007, Palmsten and Holman, 2012). The dune crest height can be rapidly reduced during the overwash and inundation regimes because sediment is transported both landwards and seawards from the dune (Donnelly et al., 2006).
Storm-induced dune erosion is one of the major concerns of coastal safety and sustainable development in the areas where frontal dune systems are present. In recent years, there is growing attention to investigate and understand the storm driven dune erosion processes in terms of numerical modelling approaches and statistical simulations (Callaghan et al., 2008, Lindemer et al., 2010, McCall et al., 2010, Ranasinghe et al., 2011, Williams et al., 2011, Harley and Ciavola, 2013, Pender and Karunarathna, 2013) due to possible changes in future storminess.
Numerical modelling approaches have been developed over the last years in order to predict more accurate and reliable dune evolution (Stive and Wind, 1986, Larson and Kraus, 1989, Roelvink and Stive, 1989, Bosboom et al., 2000, Larson et al., 2004, Roelvink et al., 2009). XBeach is one of the latest developments and an off-the-shelf model which is being continually improved by applications in different coastal environments around the world. This model has proven to be capable of predicting morphodynamic storm impacts of beach/dune systems in numerous case studies (Roelvink et al., 2009, Lindemer et al., 2010, McCall et al., 2010, Harley et al., 2011, Williams et al., 2011, Splinter and Palmsten, 2012, Harley and Ciavola, 2013). These studies motivated us to use and test the XBeach model to investigate storm driven beach/dune evolution in hyper-tidal conditions along the Sefton coast, Liverpool Bay, UK.
In previous research, different methods have been carried out in Liverpool Bay and specifically on the Sefton coast to hindcast and forecast wave climate, tidal-surge propagation and morphological evolution (Jones and Davies, 1998, Esteves et al., 2012, Brown et al., 2010a, Brown et al., 2010b, Brown et al., 2010c, Woodworth et al., 2007, Esteves et al., 2011, Wolf et al., 2011, Wolf and Woolf, 2006, Pye and Neal, 1994, Pye and Blott, 2008, Brown et al., 2012, Brown, 2010 and many others). Numerical models were mainly used to investigate the hydrodynamic characteristics (wave climate, tide, surges and their interactions leading to extreme events) under existing and future scenarios of sea level rise and climate change locally and also over the larger scale of the Irish Sea (Jones and Davies, 1998, Wolf and Woolf, 2006, Woodworth et al., 2007, Brown, 2010, Brown et al., 2010a, Brown et al., 2010b, Brown et al., 2010c, Wolf et al., 2011, Brown et al., 2012), to identify the importance of externally and locally generated conditions to Liverpool Bay. Although these results are not directly applicable to the Sefton coast, they provide potential offshore boundary conditions which can be used to model the local morphodynamics. Only a few studies discuss morphological evolution along the Sefton coast itself (Pye and Neal, 1994, Pye and Blott, 2008, Esteves et al., 2009, Halcrow, 2009, Esteves et al., 2011, Williams et al., 2011, Esteves et al., 2012) and they have mainly focused on historical data analysis implying the general patterns of morphological changes. Pye and Neal (1994) analysed the historical shoreline changes from 1845 to 1990 and found that middle reaches of the Sefton coast is eroding (~ 3 m/year) while northern and southern parts are accreting (~ 1 m/year). Decadal variation in dune erosion and accretion from 1958 to 2008 was investigated by Pye and Blott (2008) using a series of beach and dune surveys. This analysis shows that severe dune erosion occurs when storms generate positive surges on several successive tides. Esteves et al (2012) have quantified water level, significant wave height and dune erosion on the Sefton coast during several historical storm events and developed linear relationships among them in order to establish a threshold condition for dune erosion. In their study, dune erosion was estimated using one-dimensional (1D) profile data and they emphasized that inclusion of alongshore variation in the beach/dune morphology (i.e. 2D approach) is important to investigate dune evolution during stormy conditions. The MICORE project (Ciavola and Jimenez, 2011, Williams et al., 2011) has specifically focused on the storm driven dune erosion and potential hinterland flooding on the Sefton coast. They adopted the XBeach model (in 1D and 2D) imposing time-invariant wave boundary conditions (i.e. single wave condition) over a tidal cycle in a localised model domain for each tested scenario. These boundary forcings imply a conservative approach compared with the real-time storm-driven forcings and thus could lead to overestimation of morphodynamic changes of the beach/dune system.
The objective of the present study is to investigate the spatial variability of the exchange of sediment between dune face and beach during a storm, and to examine the alongshore variability of sediment dynamics in determining the evolution of the Sefton beach/dune system at engineering timescales. Such information is vital in taking effective and sustainable coastal management decisions.
There are a number of coastal management practices on the Sefton beach/dune system implemented by the Sefton Metropolitan Borough Council to deal with nature conservation and land management, shoreline management, coastal defence and flood risk, recreation, leisure and tourism (Houston, 2010, McAleavy, 2010). Success of these strategies depends on the understanding of how this complex beach/dune system interacts with coastal processes not only over the long-term, but also during storm conditions, with focus on the spatial and temporal variation of the resulting sediment fluxes and in turn the morphological changes. Application of numerical models is very efficient and effective in order to get such high resolution details of the beach/dune system. Previously, an event scale 1D early warning system for erosion has been developed for Formby Point (Souza et al., 2013). In this paper a 2D application of numerical models is used to identify the processes causing storm driven morphological change to support conceptual modelling based on beach monitoring that informs the local shoreline management plans. This research will therefore supplement the bi-annual beach surveys carried out by the Sefton Metropolitan Borough Council by providing detailed information of storm impacts at the individual event scale, in addition to the seasonal observations that capture the longer term beach and dune response.
In this study a nested modelling approach is used. A larger, coarse grid, 2D model domain is used to transform real-time offshore boundary forcings into the nearshore area. A high resolution, smaller domain, which represents the initial bed topography and in turn the resulting erosion and sedimentation patterns, is set up to investigate storm-induced dune evolution along the Sefton coast. Implementing real-time boundary forcing in the model allows more realistic storm induced interactions between the hydrodynamics and morphodynamic evolution.
This paper is structured as follows. 2 Study area — Sefton coast, 3 Storm event describe the study area and the selected storm event respectively. Section 4 describes the modelling approach used to obtain the results given in Section 5. A discussion of the overall findings is present in Section 6 while Section 7 provides conclusions.
Section snippets
Study area — Sefton coast
The Sefton coast is located between the Mersey estuary (to the south) and the Ribble estuary (to the north) in Liverpool Bay. It is an approximately 36 km long convex shape coastal stretch (Fig. 1a) (Pye and Blott, 2008, Brown et al., 2010a, Brown et al., 2010b, Plater and Grenville, 2010, Williams et al., 2011). The Sefton coastal system consists of natural beaches/dunes which have high recreational and nature conservational value, engineered beaches protected by seawalls, groynes and
Storm event
A storm event that occurred between 29 March 2009 and 01 April is modelled in this study. The selection of this event was purely based on the availability of pre-storm (Sefton Metropolitan Borough Council) and post-storm (Williams et al., 2011) beach profile measurements for model calibration. It should be noted that even though a significant number of profile measurements are available for the Sefton coast, the timing and frequency of surveys prevent accurate pre and post storm observations,
Model setup
A nested modelling approach is set up in order to optimize the computational time and accurately represent the nearshore topography (i.e. beach/dune system). Our study primarily applies the XBeach model (Roelvink et al., 2009) to investigate the storm impact on the beach/dune evolution while the SWAN (Booij et al, 1999) and Delft3D (Lesser et al., 2004) models are implemented to establish boundary forcings. The Delft3D model is used to develop spatial and temporal varying sea surface elevations
Water level (WL)
The total water elevation predicted by the Delft3D-FLOW module at the offshore boundary of the Formby domain (S1, S2 and S3, see Fig. 5a) is shown in Fig. 10. The mean water elevation at the offshore boundary of the Sefton domain (Bnd) and the observed tide (Tide) at the tide gauge (see TG in Fig. 1) are also included in this figure for comparison. In the Sefton model domain (i.e. cross-shore extent ~ 20 km), the boundary water elevation is almost identical to that of the other locations; S1, S2
Discussion
The LiDAR data (i.e. used to construct pre-storm bathymetry) and the observed post-storm profile data had different horizontal and vertical resolutions. This and some uncertainties regarding the accuracy of measurements may have caused some inaccuracies in the model predictions. This research, which is still continuing, is working alongside coastal managers, highlighting the observational needs for more detailed model validation; while understanding the model outputs required to advise regional
Conclusions
A numerical model study was carried out in order to hindcast the storm-induced dune evolution at the Sefton coast in the Liverpool Bay, UK, using a storm event that occurred during March–April 2010. A nested modelling approach was used by combining a coarser model domain to transform offshore hydrodynamics (i.e. tides, surge and waves) up to the nearshore area and a fine-grid model to investigate the morphological evolution. Predicted bed evolution was analysed and compared with measured
Acknowledgements
The work presented in this paper was carried out under the project ‘FloodMEMORY (Multi-Event Modelling Of Risk and recoverY)’ funded by the Engineering and Physical Sciences Research Council (EPSRC) under the grant number: EP/K013513/1. Prof. John Williams is greatly acknowledged for providing post-storm profile data, collected as part of the MICORE project (EU FP7 program Grant 202798). BODC, NTSLF and CEFAS (WaveNet) are acknowledged for providing tidal and wave data respectively. Sefton
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