1: An introduction to Slapton and SWI
Slapton Ley is a freshwater lake located behind a shingle
barrier extending from Torcross to Slapton Sands. It is 500m wide and reaches
depths of 4m (May, V.J. 2007).
Slapton ley is vulnerable to saltwater intrusion (hereafter
SWI) which involves the flow of seawater into freshwater environments,
significantly disrupting the ecosystem. This occurs naturally in coastal
aquifers 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 the
Importance of the Barrier?
Projections of global sea level rise made in 2007 predict a
60cm increase by 2100 (IPCC, 2007, cited in Nicholls and Cazenave: 1517),
however more recent predictions (2013) show a rise up to 1m in worst case
atmospheric CO2 concentrations (Figure 1) (IPCC, 2013).
Storm surges, energetic wave action and rising sea levels
cause landward retreat of the barrier through rollover, subsequently increasing
crest height. This leads to planiform changes increasing the curvature and
length of the barrier. As Slapton is a closed sediment cell, the volume of
sediment per unit beach length decreases, reducing beach width (Pethick, 2001,
cited in Chadwick et al., 2005:158). Austin at el (2014) also found that these
events can lead to “super-elevation” of the water table (PAGE).
Saline water would enter the ley through the groundwater
pathway as opposed to breaching, however this is prevented by it’s physical
characteristics; 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 for
the maintenance of freshwater habitats.
3. How is the barrier currently managed?
The Slapton SMP (Halcrow, 2011) outlines that Managed
Realignment will be adopted in the short term along with localised realignment
of the A379. In the medium term emphasis will be on developing adaptive
measures for the inevitable closure of the A379 and an assessment will be
carried out to judge whether defences should be strengthened in their current
position or realigned inland; basing the decision on economic viability.
Finally in the long term, assuming the A379 will be abandoned, No Active Intervention
will be adopted. The barrier will be allowed to breach as part of its natural
4. What Are the impacts of SWI?
SWI would create an intertidal environment, displacing
eutrophic-enriched freshwater, carr and reedbeds. This reduces breeding of
species e.g. reed bunting whilst the resulting saline lagoon; featuring salt
marshes and coastal grazing would support shelduck. However, when modelling
climate change scenarios it becomes apparent that, due to the higher leys’
larger catchment and therefore higher deposition rate, it would remain
unaffected 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 in
a changing climate?
Recognition that current management plans accommodate static
conditions means an integrated approach may be more effective in a changing
climate (Zanuttigh, 2011). Therefore a proposal of reprofiling in the short
term and No Active Intervention in the long term could prove effective for
Slapton Ley. Reprofiling by beach nourishment would counteract the barrier’s
increasing crest height due to rollover and also mitigate impacts of the closed
sediment cell (Chadwick et al., 2005). By increasing its’ width, the
integrity of the barrier could be maintained. This would be applicable in the
short term to allow adaptation procedures, for example development of inland
routes to reduce dependency on the A379, to be implemented. The increased
resilience these strategies could create would enable adoption of the SMP’s
recommendation of No Active Intervention in the long term (Halcrow, 2011). This
would allow SWI to occur.
However, SWI need not been seen as negative. The Slapton Ley
Land Management Committee believe the resulting intertidal environment presents
a more diverse habitat (Morris and Denbigh, 2006), and the creation of salt
marshes acts as natural capital. Salt marshes form a natural coastal defence
thereby increasing the resilience of the saline lagoon by dissipating incident
wave energy. This leads to increased sedimentation and could replicate
processes in the higher ley, raising the bed of the lagoon to keep pace with
rising sea levels (Morris et al, 2002, cited in Zanuttigh, 2011:856).
One approach that could be adopted in response to the loss
of freshwater habitats is to recreate them in the Gara and Start Valleys. This
could replace a ? of lost habitats. However by incurring costs of £542,000, The
Slapton Ley Management Committee suggested they would oppose it as the
cost-benefit analysis does not appear positive (Morris and Denbigh, 2006).
Figure 1. Ranges for the projection of global mean sea level
rise. The blue scenario predicts atmospheric CO2 concentrations of 421ppm
and the red shows concentrations of 936ppm (IPCC, 2013).