Machair Soil and Landform Sensitivity to Erosion at Cille Pheadair, South Uist, Outer Hebrides Elizabeth J. Young1,2 Sue Dawson1 & Blair M. Mckenzie2 1 University of Dundee, 2The James Hutton Institute
Introduction The machair is a low-lying coastal grassland system found only in parts of northern and western Scotland, and western Ireland. The machair provides a habitat for many rare and endangered species of flora and fauna, has unique geomorphology, and a strong cultural significance. Much of the machair is managed in a form of low intensity agriculture known as crofting, which is a major source of employment in the island of South Uist.
Figure 3b. Profile A-A’ shown with water levels for: i) 2012 highest tide (1.86 m O.D., Balivanich tide table, 2012), ii) highest tide combined with a 0.5 m storm surge; iii) highest tide combined with a 1 m storm surge; iv) highest tide combined with a 2 m storm surge. Chart datum-ordnance datum conversion of 2.59 m (Lochmaddy)4 used. Vertical exaggeration x8.
A combination of low-lying topography, limited vegetation cover, an exposed location, and a sediment deficit have made the machair susceptible to marine and aeolian erosion. The vulnerability of the machair system to erosion is evident in reports of widespread sand blow causing disruption to agriculture1, and coastal retreat of 5 -10 m in response to severe storms2. There is concern among local people and policy makers that climate change and ongoing sea-level rise may increase the vulnerability of the machair to erosion.
Methods
Results
Assessment of spatial and temporal variations in coastal change: Quantification of short-term change through repeat RTK-dGPS surveys of the coastal zone (Fig. 2). Quantification of medium-term change using a 2005 LiDAR dataset (© SNH on behalf of Western Isles Data Partnership). Quantification of long-term change using historic maps and aerial photographs. Analysis of historic meteorological and oceanic data.
Figure 3a shows a digital terrain model and map of the Cille Pheadair area derived from 2005 LiDAR data. The transparent blue surface indicates the level of the highest astronomical predicted between 2005 and 2025 and is at a level of 5.52 m CD. It is evident that large areas of agricultural land are below this level, and potentially vulnerable to flooding if the machair front is breached or over washed during storms.
Assessment of spatial variation in soil sensitivity to aeolian erosion: Wind abrasion resistance measured by rotary sieving soil samples after the method of Tisdall et al., 20123, and soil separated into erodible, potentially erodible, and non-erodible fractions (Fig. 2). Characterisation of soil properties which may be linked to abrasion resistance using: particle size analysis; Fourier-transform infrared analysis; stable isotope analysis. Measurement of water-drop penetration time and water repellency.
This project aims to investigate spatial and temporal variations in the sensitivity of soils and coastal landforms to erosion at three sites in South Uist (Fig. 1) to enhance understanding of the drivers of environmental change in the machair landscape. In this poster, results from one of the sites, Cille Pheadair, are presented. Cille Pheadair is characterised by a soft headland, and lacks a protective dune cordon along the central ~150 m of the headland. Additionally, cultivated plots extend to within < 10 m of the dune crest or machair front at this site. These factors make this area particularly susceptible to marine erosion and aeolian erosion.
Figure 2. Rotary sieve (mesh size 850 µm), used to simulate wind abrasion. 40 g of soil is placed in the drum of the sieve, which rotates at 7 rpm for 300 s. Soil passing through the sieve is collected on the balance.
Figure 1. Map of Scotland with inset of Benbecula, South Uist and Barra, showing the location of Cille Pheadair. Acknowledgements With thanks to Stewart Angus of Scottish Natural Heritage for provision of the 2005 LiDAR dataset.
References 1
Angus, S. 2001. The Outer Hebrides: Moor and Machair. White Horse Press. Angus, S., Rennie, A. 2008. An Ataireachd Ard: The great sea surge. SNH commissioned report. 3 Tisdall, J. et al., 2012. Soil. Biol. Biochem. 50:134-141. 4 Dawson, A. et al. 2007. Assessment of the rates and causes of change in Scotland’s beaches and dunes—Phase 2. SNH commissioned report. 2
Figure 3b shows a graph of the profile A-B, with water levels for different tidal and storm surge conditions. The crest is only ~50 cm higher than expected water levels for a severe storm combined with high, but not extreme tidal conditions. ~ 15 m inland from the crest elevation is similar to the highest water levels shown.
Figure 4 shows the results from rotary sieving samples collected along profile C-D. The proportion of soil that is erodible decreases with distance from the coast, and the proportion of soil that is non-erodible decreases with distance from the coast.
Figure 4. Graph showing % of bulk soil samples classed as erodible (soil passing through sieve between 0-40 s), potentially erodible (soil passing through sieve between 41-300 s), and non-erodible (soil remaining on sieve after 300 s) along the length of profile C-D.
Conclusions Crest height and width at Cille Pheadair vary considerably along the coastline. The low-lying centre of the headland is particularly vulnerable to flooding due to over wash and breaching during storms. Coastal retreat of ~15-20 m at the centre of the headland would place crest height at the same level as severe storm surges under high tidal conditions.
Figure 3a. LiDAR derived digital terrain model (DTM) and topographic map of Cille Pheadair showing low-lying topography. View is to the south. The blue surface is at the level of the highest astronomical tide predicted between 2005 and 2025 (5.52 m CD prediction for 19th September 2024, Stornoway4). Note that high tide levels on the western coast of South Uist are typically 60-80 cm lower than for Stornoway Vertical exaggeration x5. Chart datum-ordnance datum conversion value of -2.71 m used (Stornoway).
Funding for March 2012 fieldwork provided by:
There is a clear trend of increasing soil wind abrasion resistance with distance inland. Samples from the first 100 m of the transect were predominantly erodible, while samples from the last 100 m of the transect were predominantly non-erodible. Erosion of the machair front is a major concern at Cille Pheadair. The slightly raised machair front protects crofting land from flooding and erosion. Marine erosion of the machair front, and erosion of topsoil from the back dune are different processes, but both may contribute to crest narrowing and lowering, increasing vulnerability to storms.