1: An introduction to Slapton and SWISlapton Ley is a freshwater lake located behind a shinglebarrier extending from Torcross to Slapton Sands. It is 500m wide and reachesdepths of 4m (May, V.J. 2007). Slapton ley is vulnerable to saltwater intrusion (hereafterSWI) which involves the flow of seawater into freshwater environments,significantly disrupting the ecosystem. This occurs naturally in coastalaquifers across a groundwater gradient as saline water has a higher density(Boulton, 2005 cited in Austin et al.
2014). 2. Why is there an increased risk and What is theImportance of the Barrier?Projections of global sea level rise made in 2007 predict a60cm increase by 2100 (IPCC, 2007, cited in Nicholls and Cazenave: 1517),however more recent predictions (2013) show a rise up to 1m in worst caseatmospheric CO2 concentrations (Figure 1) (IPCC, 2013). Storm surges, energetic wave action and rising sea levelscause landward retreat of the barrier through rollover, subsequently increasingcrest height. This leads to planiform changes increasing the curvature andlength of the barrier. As Slapton is a closed sediment cell, the volume ofsediment per unit beach length decreases, reducing beach width (Pethick, 2001,cited in Chadwick et al., 2005:158).
Austin at el (2014) also found that theseevents can lead to “super-elevation” of the water table (PAGE).Saline water would enter the ley through the groundwaterpathway as opposed to breaching, however this is prevented by it’s physicalcharacteristics; in particular its’ 120m average width (Austin et al.,2014). Additionally, behind the bar water levels are 1 m above the spring tide,evidenced by a net seaward hydraulic gradient. This acts as natural capital forthe maintenance of freshwater habitats.
3. How is the barrier currently managed?The Slapton SMP (Halcrow, 2011) outlines that ManagedRealignment will be adopted in the short term along with localised realignmentof the A379. In the medium term emphasis will be on developing adaptivemeasures for the inevitable closure of the A379 and an assessment will becarried out to judge whether defences should be strengthened in their currentposition or realigned inland; basing the decision on economic viability.Finally in the long term, assuming the A379 will be abandoned, No Active Interventionwill be adopted. The barrier will be allowed to breach as part of its naturalevolution. 4. What Are the impacts of SWI?SWI would create an intertidal environment, displacingeutrophic-enriched freshwater, carr and reedbeds. This reduces breeding ofspecies e.
g. reed bunting whilst the resulting saline lagoon; featuring saltmarshes and coastal grazing would support shelduck. However, when modellingclimate change scenarios it becomes apparent that, due to the higher leys’larger catchment and therefore higher deposition rate, it would remainunaffected by breach in the lower ley due to the natural formation of a dam(Morris and Denbigh, 2006). 5.
Conclusion- How could the barrier be managed ina changing climate?Recognition that current management plans accommodate staticconditions means an integrated approach may be more effective in a changingclimate (Zanuttigh, 2011). Therefore a proposal of reprofiling in the shortterm and No Active Intervention in the long term could prove effective forSlapton Ley. Reprofiling by beach nourishment would counteract the barrier’sincreasing crest height due to rollover and also mitigate impacts of the closedsediment cell (Chadwick et al., 2005).
By increasing its’ width, theintegrity of the barrier could be maintained. This would be applicable in theshort term to allow adaptation procedures, for example development of inlandroutes to reduce dependency on the A379, to be implemented. The increasedresilience these strategies could create would enable adoption of the SMP’srecommendation of No Active Intervention in the long term (Halcrow, 2011). Thiswould allow SWI to occur.However, SWI need not been seen as negative. The Slapton LeyLand Management Committee believe the resulting intertidal environment presentsa more diverse habitat (Morris and Denbigh, 2006), and the creation of saltmarshes acts as natural capital.
Salt marshes form a natural coastal defencethereby increasing the resilience of the saline lagoon by dissipating incidentwave energy. This leads to increased sedimentation and could replicateprocesses in the higher ley, raising the bed of the lagoon to keep pace withrising sea levels (Morris et al, 2002, cited in Zanuttigh, 2011:856).One approach that could be adopted in response to the lossof freshwater habitats is to recreate them in the Gara and Start Valleys. Thiscould replace a ? of lost habitats. However by incurring costs of £542,000, TheSlapton Ley Management Committee suggested they would oppose it as thecost-benefit analysis does not appear positive (Morris and Denbigh, 2006).Figure 1.
Ranges for the projection of global mean sea levelrise. The blue scenario predicts atmospheric CO2 concentrations of 421ppmand the red shows concentrations of 936ppm (IPCC, 2013).