Community Driven Coastal Management An Analysis of the Implementation of a Coastal Defence Bund in South Uist, Outer Hebrides, Scotland Elizabeth Young, David Muir, Sue Dawson, & Alistair Dawson 1. Context
firstname.lastname@example.org 4. ICZM Response
Climate change, extreme weather events, and sea-level rise are already considered serious threats to the low-lying machair coastline of the island of South Uist. In January 2005 a severe storm caused widespread damage along the western coast of South Uist, including damaging a primary school, depositing debris across roads and agricultural land, and causing coastal retreat of up to 10 m within 48 hours. In the aftermath of the storm local people put pressure on authorities to erect coastal defences.
This research investigates the sensitivity of a sandy headland called Cille Pheadair to flooding during storms. Additionally, the ICZM process which led to the erection of defensive bund at this site is analysed.
Figure 1. Aerial imagery and map of Cille Pheadair (adapted from Thorsen et al., 2010)
3. Flood Sensitivity
Figure 2. Water level at highest astronomical tide (blue surface) superimposed on DTM of Cille Pheadair.
Figure 4. Aerial photography of Cille Pheadair with present day position of mean high water at ordinary spring tide (dashed line), and 1878 position of mean high water at ordinary spring tide (solid line).
Figure 3. Cross-shore profile of the lowest part of the headland showing water levels for i) high tide, ii) high tide + 0.5 m surge, iii) high tide + 1 m surge, iv) high tide + 2 m surge.
The sensitivity of the headland to flooding and over-wash during storms is highlighted by several lines of investigation. Fig. 2. shows the extremely low-lying topography , with large areas of the site beneath the high tide water level. Fig. 3. shows the elevation of storm surges relative to the coastal crest height. Fig. 4 shows that the coastline has retreated up to 30 m at this site since 1878.
A coastal defence bund made of crushed rock was erected to ‘hold’ the existing coastline. This was against the advice of coastal experts who felt this approach would be unsustainable environmentally, and would not address the larger issues of reduced sediment budget and sea-level rise. The ICZM approach adopted was selected as the community “would not wear the loss of land” associated with more environmentally suitable management options. Storms have damaged the bund, but no maintenance has been carried out since its construction in 2011.
Figure 5. Photographs of the defence bund at Cille Pheadair showing cuspate erosion of the defence, revealing the crushed rock and shingle base. Damaged sand traps are shown in the lower left photo.
Cille Pheadair is socially vulnerable and geomorphologically sensitive to coastal change. Extreme storms represent the biggest threat to this site. A lack of integrative instruments in the ICZM process has prevented adaptation to coastal change—instead the community has taken a mitigative approach. While community involvement has led to some positive outcomes (e.g. cessation of beach sand extraction, education), socio-economic conditions make sustainable ICZM options (e.g. managed retreat) unacceptable to local people. This has led to the installation of an environmentally insensitive defence which disrupts normal machair processes.
Thorsen, M et al. (2010). Soil. Use. Manag.. 26 (2), 149-157.
7. Acknowledgments Aerial imagery and LiDAR data © Scottish Natural Heritage on behalf of the Western Isles Data Partnership. Stewart Angus (Scottish Natural Heritage); The British Society for Geomorphology; Comhairle nan Eilean Siar.