2.10.- Barandilla vertical

Geomorphology and cliff type

Coastal morphological development is a physical expression of how energy is mitigated. Beach sediment will help to absorb, dissipate and protect coastal areas (Dawson et al., 2009). Whether the coast has sheltered areas, hard rock headlands or barriers, will influence its exposure to wave energy. Effects of future sea-level over the coast will largely depend on the characteristics of its coastal topography and sediments (Sharples, 2006; Dwarakish et al., 2009). Post-glacial processes have incorporated glacial sediments into beaches, dunes and estuaries in the study area. Even though

offshore sediment supply is currently limited, glacial sediments such clay and silts will help salt marshes and tidal areas to cope with SLR (Adam, 2002). Great emphasis has been placed on assembling seamless onshore-offshore geomorphological data in the near zone as well as sediment distribution for coastal vulnerability mapping assessments to projected sea-level rise (GEUS, 2013).

Deltas and low-lying plains are extremely sensitive to climate changes in water level downstream and by runoff upstream, exacerbating impacts such as erosion, flooding and anthropogenic degradation (IPCC, 2014). Sea-level rise and storminess will also alter the distribution and balance of sediment in lagoons and estuaries (Pilkey and Young, 2010). Wetlands and sea grass meadows will suffer coastal squeeze if there is no migration possibility. It is expected that sea-level rise might cause dry lands to be inundated and wetlands displaced from intertidal to sub tidal. In general, soft rock substrate is sensitive to erosion, whereas a sedimentary coast would be highly sensitive to both erosion and flooding (Sharples, 2006). Wave overtopping and sediment reworking on shingle and gravelly beaches and coastal man-made defences is also likely, particularly those without the possibility to migrate. However, wave energy changes will be accommodated on sandy beaches with enough sediment. Otherwise they will get excavated unless they retreat inland. Sediment accreted in embayed estuaries or marshes during storms can make them more resilient, depending on transport and sediment supply (Charlton & Orford, 2002; Pethick, 1984).

At small scales, in the short term, using cliff type is more accurate than using solid or drift-geology (McLaughlin and Cooper, 2010). In general, climate change will reduce the resilience of low cliffs as regards impacts, as once the cliffs have retreated, or been damaged, they will not recover their initial stability (Naylor et al., 2010). Cliffs retreat faster with sea-level rise, particularly those with high historical retreat trends (Trenhaile, 2010; Brooks and Spencer, 2014). Storminess and wave energy will also exacerbate erosion processes on both soft and hard cliffs (Naylor et al., 2010; Ashton et al., 2011). All this, combined with high tides and saturated ground from rainfall could quicken geomorphological processes over cliffs, and also increase rock falls (AGUBlogosphere, 2014). Hard cliffs were found to be more sensitive to low-frequency

strong events that previously thought (Hansom, 2001). However, compared with soft cliffs, the effects will be minimal (Dawson et al., 2009; Trenhaile, 2011).

The type of cliff and rock strength will determine the degree of erosion. The same factors that affect cliff instability also weaken the cliff, causing falls and slides (Lim et al., 2011). Consequently, in some places, retreat regularly comes in the form of sudden cliff failures and catastrophic events, indicating that erosion on those cliffs can occur fast (Lim et al., 2010a; Sistermans and Nieuwenhuis, 2013). For instance, the Holderness erosion hotspot in England currently erodes at 2-6m/yr, mainly during storms and surges. Sea-level effects combined with natural variability will maximise the impacts; for instance, high retreat on soft cliffs in East Anglia (UK) of ~10m/year has been linked to decadal North Atlantic Oscillation (NOA) energy fluctuation (Brooks and Spencer, 2014). Weakening in soft cliffs of south Dublin (Robinson, 2009) and resilience changes in other areas in Ireland have been identified (Jordan, 2016).

In the light of the above, first order mapping should be based on geomorphology and erodibility (Church et al., 2006; EUROSION, 2004; Harvey and Woodroffe 2008). Therefore, these two variables have been incorporated in a number of coastal and landslide vulnerability studies (Gornitz, 1990; Gornitz et al., 1994; Thieler et al., 2000; McGlauglin, 2001, 2002; Hampton & Griggs, 2004; McFadden et al,. 2007a; Hapke & Plant, 2010; McGlauglin and Cooper; 2010; Pendleton et al., 2010; Gutierrez et al., 2011; Ashton et al., 2011; BGS, 2012). For this research, these data were not available and needed to be created.

Coastal topography and slope

Coastal topography and slope is indicative of risk of inundation by flooding and shoreline retreat (Pilkey and David, 1987). Bathymetry and the subtidal substrate slope strongly influence the wave activity and coastline exposure in the near shore zone, and subsequently the physical response of sandy barriers to sea-level rise (Sharples, 2006). Thus, coastal elevation and slope are still valuable indicators for extrapolating future shoreline positions (Gutierrez et al., 2011).

Coastlines vulnerability will depend on sediment supply or migration ability. High slope gradients and man-made structures will accelerate squeeze by impeding

coastal adjustment (Pethick and Crooks, 2000). Sea-level rises and storms threaten systems that are backed by hard structures or cliffs and even dune systems that can migrate. If sea level rises quickly, systems will not have time for landward migration and barrier islands and wetlands might be seriously affected. This will lead to narrowing beaches and eroding dunes, and consequently, further exposing land to inundation before the next storm strikes (Plant et al., 2010). Low profile land immediately landwards of the mean high-water mark is very vulnerable. Shallow and wide-water inshore zones and continental shelf like this exists in the study area, backed by wide beach systems. These areas favour absorption of wave energy, minimising the impacts of wave-surge over the coast and thus protecting it (Carter, 1991; Carter and Woodroffe, 1994). However, gentler gradients will result in increased storm surge heights and waves and surges driven by winds, making the coast more vulnerable to erosion (ECOPRO, 1996).

Some argue that coastal damage is not proportional to the event’s energy. Sometimes high sea levels could be more damaging for coasts than isolated storm events (Betts et al., 2004). In general, low-lying areas might see a real threat from continuous sea-level rises while others will struggle from the combination of sea-level rise and storm surge events (McCarthy et al., 2001). The intertidal slopes provide information on wave energy dissipative gradients and potential storm surge heights while the hinterland zone provides information about semi-stable landforms. Hence good quality, continuous onshore-offshore high-resolution coastal topography data is indispensable for vulnerability and impact assessments (Gornitz, 1990; Gornitz et al., 1994; Thieler et al., 2000; McLaughlin, 2001; McFadden et al,. 2007a; Nageswara Rao et al., 2008; Wang et al., 2008; McLaughlin and Cooper. 2010; Pendleton et al., 2010).

Aspect

Topographic factors such as location and orientation of the coast are also responsible for the resistance of the coast towards impacts of sea-level rise (Sharples, 2006; Dwarakish et al., 2009). Aspect will affect the amount of energy spent at that particular location. Shoreline orientation relative to wave climate is a major factor in storm retreat-recovery interactions. However, this variable has only recently been included in some coastal studies (Harris et al., 2000; Mclaughlin, 2001; Mclaughlin et

al., 2002; Sharples, 2006; Ashton and Murray, 2006a, 2006b; Abuodha and Woodroffe, 2010b; Mclaughlin and Cooper, 2010; Brooks et al., 2016)

In the study area, some coastal segments are more exposed to the action of waves than others. Sheltering and orientation will govern whether the coast will be more exposed to wave significant heights and directions, which are in turn, indicative of storm wave direction approach. Consequently, it will determine whether the coast will change more or less rapidly with sea-level rise (ECOPRO, 2006; Sharples 2006). Consequently, exposure to high wave energy was included on this research.