Post-emplacement coastal dune blowout development and re-colonization

Katherine C. Bolles Dr. Helene Burningham, Supervisor 2012

This research dissertation is submitted for the MSc Environmental Mapping at University College London.

UCL DEPARTMENT OF GEOGRAPHY

DEPARTMENT OF GEOGRAPHY

M.Sc. in Environmental Mapping

Please complete the following declaration and hand this form in with your M.Sc. Research Project.

I, ...... Katherine C. Bolles......

hereby declare :

(a) that this M.Sc. Project is my own original work and that all source material used is acknowledged therein;

(b) that it has been prepared specially for the MSc in Environmental Mapping of University College London;

(c) that it does not contain any material previously submitted to the Examiners of this or any other University, or any material previously submitted for any other examination.

Signed : ......

Date : ...... 3 September, 2012......

Abstract

Coastal dunes are a noteworthy habitat, comprising 20% of the world’s coastlines. Blowouts are a highly dynamic part of these systems, contributing to the overall stability of such environments. As such, they are a significant area of study for coastal dune dynamics, yet they are not well understood. This study examines blowout development and re-colonisation once initiated in a dune system in northwest Ireland in order to understand the ecogeomorphic influences at work. An analysis of aerial imagery from 1951 to 2009 was undertaken for the Magheramore dune system, located in County Donegal, examining changing blowout morphologies and vegetation coverage. Fieldwork employing a differential Global Positioning System (dGPS) was done to capture high-resolution data on the geomorphology of 30 blowout samples. For each blowout, a 1 x 1 meter vegetation quadrate was assessed for coverage and species on the north, south, east and west slopes. To assess regional wind climate, wind data from Met Éireann was analysed for storminess and wind regime. Analysis of aerial imagery and dGPS data was carried out in ArcGIS 9.3 and 10, as well as visualisations of morphological and vegetation data. A variety of multivariate statistics was carried out on both morphology and wind data in MATLAB, and vegetation data was analysed in TWINSPAN and CANOCO for Windows 4.5. A cycle of ecogeomorphic feedbacks is shown to control blowout evolution, mainly relating to the orientation of the blowout and the length of its dominant axis. The wind climate commands blowout initiation and growth, though the ultimate shape is a function of physical parameters. Blowout morphology in turn controls vegetation re-colonisation, including species and coverage, while vegetation determines how much a blowout can develop, while constraint by wind strength. This cycle is shown to be consistent across all blowouts, allowing for placement within a model of blowout evolution. This enables wider inferences to be made about the dune system as a whole, and a mode to understand coastal morphodynamics facing climate change.

In total, this dissertation is 11,351 words.

i Acknowledgements

Many thanks are given to Dr. Helene Burningham for her excellent guidance and support with this project. I could not have asked for a better supervisor. Thanks are also due to my coursemates on the MSc Environmental Mapping and MSc Remote Sensing programmes, for the feedback and banter. The UNIX lab will miss us all.

Thanks to the National Parks & Wildlife Service for providing accommodation for the duration of fieldwork.

ii Table of Contents

1. Introduction ...... 1 2. Literature Review ...... 4 2.1. Coastal dune morphodynamics ...... 4 2.2. Studies of blowouts ...... 6 2.2.1 Blowout wind dynamics ...... 9 2.2.2 Vegetation growth and controls in coastal dunes and blowouts ...... 10 3. Methods ...... 12 3.1 Study site ...... 12 3.2 Data acquisition ...... 14 3.2.1 Wind data ...... 14 3.2.2 Aerial imagery ...... 14 3.2.3 Differential Global Positioning System (dGPS) data ...... 15 3.2.4 Vegetation data ...... 16 3.3 Data analysis ...... 17 3.3.1 Analysis of wind data ...... 17 3.3.2 Geomorphological analysis ...... 17 3.3.3 Ecological analysis ...... 19 4. Results ...... 20 4.1 Climactic variables and trends ...... 20 4.2 Geomorphology of blowouts ...... 22 4.2.1 Aerial imagery ...... 22 4.2.2 dGPS data ...... 26 4.3 Ecogeomorphological interactions ...... 32 5. Discussion ...... 40 6. Conclusions ...... 44 7. Appendix 1 ...... 45 8. Auto-critique ...... 46 9. References ...... 48

iii Table of Figures

Figure 1. Schematic of dune evolution (Ritchie 2008)………………………………………………………………………………5 Figure 2. Hysteresis curve related to wind power and vegetation cover (Tsoar 2005)……………………………..5 Figure 3. Diagrams of typical saucer- and trough-shaped blowouts (Hesp 2002). Typical windflow patterns are indicated by arrows………………………………………………………………………………………………..8 Figure 4. Location of the Magheramore dune system, Co. Donegal, NW Ireland……………………………………12 Figure 5. Dune ridge changed observed in aerial imagery from 1951, 1977, 1995 and 2000. From 2000, the ridge is largely stable. A recession rate of approximately 11.3m/year along the estuarine margin can be inferred……………………………………………………………………………………………………………..13 Figure 6. Representation of measurements taken for a (A) saucer- or cup-shaped blowout, where transects are along ordinal directions (in blue) and rim (in black), and for (B) a ‘mega-blowout’ where the dominant axis (red) is measured. Feature outlines (yellow) were also measured when present.…………………………………………………………………………………………………………………………………….16 Figure 7. Digital elevation model (DEM) of the Magheramore dune system..………………………………………..18 Figure 8. Wind roses for hourly wind data from Malin Head and Belmullet meteorological stations from 1956 to 2011…………………………………………………………………………………………………………………………….21 Figure 9. Geomorphological interpretation of aerial photographs for the period 1951-2009. A comparison of dGPS data to satellite imagery of the 30 sub-sampled blowouts was carried out for 2012...... 23 Figure 10. Summary of association between blowout age and selected morphological variables………….27 Figure 11. Summary of association between blowout location and selected morphological variables.….27 Figure 12. Summary of associations between blowout activity grade and selected morphological variables…………………………………………………………………………………………………………………………………...28 Figure 13. Summary of associations between blowout orientation and selected morphological variables……………………………………………………………………………………………………………………………………28 Figure 14. Initial statistical analysis. Principal component analysis (A) shows both the first and second principal components. Within cluster differences (B) establishes the number of clusters in the data. …………………………………………………………………………………………………………………………………………30 Figure 15. Results of the cluster analysis. Blowouts were grouped into 5 clusters (A) and morphological attributes compared: (B) dominant axis to circularity, (C) dominant axis to secondary axis, (D) volume to circularity and (E) surface area to circularity…………………………………………………………….31 Figure 16. Frequency of bare sand and species occurrence on west-, east-, north-, and south-facing blowout slopes. ………………………………………………………………………………………………………………………..32 Figure 17. Dendrogram of the 14 vegetation clusters distinguished for sampled blowouts. There is a gradation left to right, from mature vegetation (e.g. R. pimponellifolia) to no vegetation, characterized by bare sand. Samples are named as B# (number of blowout sample), direction slope is facing and location (A indicates sea-facing blowouts, Y represents estuary-facing blowouts). ………………………………………………………………………………………………………………………………..34 Figure 18. Spatial distribution of the 14 vegetation clusters demarcated in the dendrogram (Fig. 17). Blowouts are identified by sample number……………………………………………………………………………….35 Figure 19. Species (A) and environmental variables (B) principal component analyses……………...……...….35 Figure 20. Samples classified by key environmental variables: (A) location, (B) shape (axis ratio), (C) activity grade and (D) average slope (degrees) of the blowout wall and (E) aspect (degrees North), as related to species. ……………………………………………………………………………………………………37 Figure 21. Samples classified by indicator species derived from TWINSPAN analysis: (A) bare sand, (B) A. arenaria, (C) A. vulneraria, (D) F. rubra, (E) L. cornaculatis, and (F) moss, as related to environmental variables. ………………………………………………………………………………………………………….38 Figure 22. Vegetation cover observed in aerial imagery for blowout sample one (A), a cup blowout initially identified in 1977, and sample two (B), a trough blowout identified in 1951, over the period 1995 to 2009. ………………………………………………………………………………………………………………..41

iv Table of Tables

Table 1. The historical aerial photographs and satellite imagery used in this study……………………………....15 Table 2. Scales used to assess vegetation coverage in quadrats (Braun-Blanquet scale from Kent and Coker 1992). Mid-points were later substituted for statistical analysis………………………………….….16 Table 3. Activity grade assigned to each blowout for its overall level of activation……………………….……….17 Table 4. Storminess measured at (A) Malin Head and (B) Belmullet for the period of 1956-2009. To qualify as a storm event, winds must have blown at 22 knots or higher for a duration of 5 hours………………………………………………………………………………………………………………………………...... 20 Table 5. Blowout presence and development visible since 1951…………………………………………...………….…..22 Table 6. Evolution of sub-sampled blowout shape between 1951 and 2009. Blowouts with an axis ratio of <1.8 are considered saucer- or cup-shaped; those over 1.8 are trough-shaped…...……...…...... 24 Table 7. Morphological attributes of sampled blowouts………………………………………………………………………..25 Table 8. Eigenvalues and variance from the detrended component analysis and principal component analysis performed on species and environmental data in CANOCO………………………………….……..36

v 1. Introduction

Coastal dunes have long been a prominent area of research interest, and for good reason. Such environments provide extensive protection to many of the world’s shorelines and are an important factor in regulating coastal groundwater (Carter 1991). Sensitive to a wide range of environmental factors, coastal dunes present the opportunity to test and observe interactions between climate, land use and geomorphic processes on a relatively short timescale (Levin 2011). Driven by the aforementioned elements, dunes typically evolve from embryonic to mobile to semi-fixed to fixed stages (Liverpool Hope University 2012). Jones et al. (2004) note that over the past three to four decades, UK sand dunes have shifted from mobile dunes to ones with somewhat continuous vegetation cover interrupted by blowouts. A blowout is defined as a saucer-, cup- or trough-shaped depression or hollow formed by wind erosion on a pre-existing sand deposit (Hesp and Hyde 1996, Hesp 2002) whose boundaries are sharply delineated by depositional ridges or erosional scarps (Gares and Nordstrom 1995). Their morphology can be highly variable (Hesp and Hyde 1996, Hesp 2002), resulting in a wide spectrum of shapes and sizes. Saucer blowouts are typically semi-circular in shape, cup blowouts being deeper versions, and trough blowouts are more elongated, marked by deeper deflation basins and steeper erosional walls (Gares and Nordstrom 1995, Hesp 2002). Blowouts are very common to dune landscapes (Hesp and Hyde 1996) and can be initiated in a variety of ways. Hesp (2002) lists seven primary causes of blowout formation: wave erosion, topographic acceleration of airflow over the dune crest, climate change, vegetation variation, water erosion, high velocity wind erosion and human activities. Blowout recovery relies mostly on the same factors, depending generally on vegetation growth and sediment deposition in the throat of the blowout (Gares 1992). Change within dunes is typically attributed to a competition between vegetation and sand transport (Murray et al. 2008). Blowout development is akin this this, primarily forced by vegetation and wind patterns (Gares 1992, Hesp and Hyde 1996, Hesp 2002). Dissimilar to a dune system however, large changes can take place within blowouts over short periods of time (Gares 1992). Blowouts are the most dynamic aspects of coastal dune systems (Gares 1992) and as such are an important field of study in understanding coastal morphodynamics. Blowouts are highly relevant to vegetation-aeolian interactions and topography of dune environments, as they alter the pre-existing topography and consequently the near-surface boundary layer airflow (Hugenholtz and Wolfe 2006). Further to this, blowout development is a significant

factor in dune behaviour and subsequently the long-term stability of the system as a whole. Development of blowouts is intimately related to the climate and climate change, which control wind regime, which in turn determines sediment supply and availability (Hugenholtz and Wolfe 2006). This prominent role in the transport of sediment means blowouts are a large factor in the sediment budget of the dune system as architects of sediment transport (Gares and Nordstrom 1995); this consequently influences ecogeomorpholgical change within blowouts and the dune field in which they appear. Thus, as highly variable parts of dune dynamics, blowouts and their morphological evolution need to be understood in order to understand and manage coastal systems effectively. Until relatively recently, studies on blowouts focused predominantly on fully evolved samples. Emphasis now lies with the evolutionary process of blowout morphology, but there still exists vast gaps in the knowledge of what drives this evolution. Little is known about the role of various factors such as topographic position, wind regime and directional variability, and vegetation cover and species in determining blowout type and morphological evolution (Hesp 2002). Without considering the relationships between physical and biological processes, the morphological evolution of some landscapes cannot be understood (Murray et al. 2008). This issue forms the crux of this study. Over the past 1000 years there have been episodes of sand mobilization, mostly concentrated in the period of 1500 to 1900, which spans the Little Ice Age (Clarke and Rendell 2009). However, during the last 100 years or so, many northwest European dune systems have tended toward stability (Jones and Sowerby 2010). Very little research has been undertaken on blowout development and contrasting blowout behaviour within one system. This study focuses on the Magheramore dunes in northwest Ireland, County Donegal, where the presence of blowouts can be documented since the 1950s. Using a series of historical aerial photographs and satellite imagery, differential Global Positioning System unit data and vegetation surveys this study aims to uncover the driving forces behind blowout development in a coastal dune system and determine any geomorphic constraints on habitat development. External controls, such as storminess and atmospheric nitrogen deposition, exert significant control on development, but local variation cannot be explained by these regional trends. Therefore, the main objective of this work is to explore this variation to test the following two hypotheses:

2 1. Ecogeomorphic factors control blowout morphological evolution and vice versa. The associated suggestion of topographical heterogeneity will translate into geomorphological and ecological heterogeneity. 2. Blowouts and blowout evolutionary processes can be used as indicators to the state of the system in which they develop, so conclusions about the Magheramore dunes can be made based on its blowouts. Through this, this study aims to fill the knowledge gaps and increase the understanding of biodiversity and its development in response to system disturbance. Further, to understand how climate change will impact the geomorphological processes prevailing in a dune landscape it is essential to understand blowout formation and important parameters (Jungerius et al. 1991). This work hopes to provide a foundation from which this is possible.

* * * * *

3 2. Literature Review

Blowouts first appear in literature in 1898 (Hesp and Hyde 1996, Hesp 2002), but did not become a focus area for research until much later. Until recently, most literature discussed the morphology and evolution of coastal dunes as a whole and, while not specific to blowouts, lend important context and evidence for controls on blowout evolution.

2.1 Coastal dune morphodynamics

Dunes are a unique system that can fluctuate between mobility and stability. The mobility indices of dunes are related to a variety of factors including: wind, precipitation, temperature and evaporation (Tsoar 2005, Levin 2011). Coastal dune fields evolve and adapt to these climactic variables through complex ecogeomorphic interactions, which can form recognisable patterns if conditions continue across a long time period (Nield and Baas 2008). Figure 1 provides a schematic of dune development, though there is no simple, single cycle. Likewise, there is no agreement which of the prime or secondary factors is the most important control within this landform process. Sand dunes are clearly established as dynamic, sensitive settings in which it is possible to observe environmental and geomorphic interactions. Levin (2011) suggests storminess plays a large role in dune development, noting that periods of increases storminess have been related to dune accretion in Western Europe. In areas will more than 50 mm/year of rainfall, wind stands out as the most important physical factor in regulating dune mobility (Levin 2011). Tsoar (2005) also hypothesizes that wind potential, in particular the drift potential, is the variable of most significance. Dunes are hysteric in nature (Tsoar 2005, Levin 2011), meaning that dunes can vegetate when wind power is suitably low. Operating on the base of the hysteresis curve, Tsaor (2005) determines three states of sand dunes: free of vegetation and active, partially vegetated and active and fully vegetated and fixed. Following his theory of drift potential1, Tsoar postulates that vegetation, symbiotically with a decrease in wind power below 1000 DP, will start covering dunes. When DP falls below 200, vegetation cover can reach the maximum the system is able to support. Simply, change in wind power will cause a change in vegetation cover along hysteresis lines of stability (see Figure 2, Tsoar 2005).

! 1 ! !! !! !" = ! = ! where U is the wind velocity, U is the threshold wind velocity, t is the degree of !"" t windiness and q is calculated separately for each wind direction above Ut (Tsoar 2005).

4 Figure 1. Schematic of dune evolution (Ritchie Figure 2. Hysteresis curve related to wind power 2008). and vegetation cover (Tsoar 2005). Nield and Baas (2008) also emphasize the importance of wind, but give weight to vegetation cover itself as a controlling factor. Vegetation can determine if a dune system will become stabilised or activated given that its rate of colonisation is determined by changes in climate, nutrient level and sediment conditions (Nield and Baas 2008). The authors suggest that vegetation and the ‘ecogeomorphic feedback’ are the prevailing influence on dune behaviour. If this is the case, coastal dune dynamics relate strongly to disturbance and the system’s response. If wind and vegetation remain consistent over a long period of time, dunes will self-organised into distinct patterns. Ritchie (2008) focuses on sand movement as it relates to grain size and wind velocity. This connects to vegetation, as different vegetation layers are of different ‘roughness’ (Ritchie 2008). Vegetation can therefore affect the movement of sediments. In turn, vegetation patterns are closely linked with water availability, so the water table could in fact be regarded as the fundamental base of dune evolution (Ritchie 2008). At the same time, blowout patterns typically align with the direction of maximum exposure of wind, so wind pattern compounds morphological development as well (Ritchie 2008). It can be said, though others find a dominant controlling factor, the evolution of coastal sand dunes is affected by an array of pressures. Jackson and Cooper (2011) highlight the role of climate in morphological change, including variance in rainfall, temperature and wind regime attributed to the changing climate. Local wind specifically plays a key role, acting as the transport mechanism for sediment movement; yet, vegetation is the main component of dune construction, which also

5 controls the movement of sand (Jackson and Cooper 2011). The ability of sand to move ultimately determines the stability and shape of the dune system. In summation, the local wind regime and vegetation (coverage and species) are the most important variables controlling dune morphodynamics. Local and regional factors both play a significant role in landform developments, blowouts being a prime example of local factors forcing dune morphology (Nield and Baas 2008).

2.2 Studies of blowouts

One of the definitive texts on dunes is that of Melton (1940), which established a classification system for sand dunes, including a description of blowouts. Melton (1940) postulates that if vegetation is killed and the climate is more arid or ground-water surface is lowered, wind will remove sand from the spot until conditions other than the growth of vegetation halt the erosion. This forms the basin of a blowout. This is limited by depth to the water table or resistant layer (such as bedrock) and/or the initial size of the area of bare sand (Melton 1940). Similar to those studies on dune systems, Melton (1940) describes climactic factors as the main control on blowout development. While this text provides an early mention of blowouts, detailed studies are not found in the literature until later. The first suggestion of a blowout evolutionary model came from Jungerius et al. (1981). In this study, the authors propose all blowouts begin when sand is moved by wind in such a way that the width and depth are in nearly constant ratio. Sand is taken up the windward side of the blowout, growing the length against the dominant wind. As the blowout deepens over time, the more difficult it becomes for sand to exit the blowout, gradually halting further development. Findings indicate blowouts can reach a critical size in length, width and depth, at which point removal of sand is no longer possible (Bate and Ferguson 1996, Hugenholtz and Wolfe 2006). With increasing length, the erosivity power of the wind lessens until erosion finally stops, completing the blowout’s evolution. Jungerius et al. (1981) state any departures from this model are caused by local variation in surrounding vegetation. Following this, Jungerius and van der Meulen (1989) examine system dynamics as they pertain to blowout development. Similar to the previous study, it was found that blowouts begin to form in places where resistant surface soil has been removed, exposing the subsoil sand. Deflation patches appear in such area in upper dune slope positions, sparking blowout evolution. Over time, wind creates erosion features within blowouts. The fluctuating number

6 of blowouts relates to the wind regime from year to year. Most importantly, the study establishes stabilized blowouts can be reactivated. Gares and Nordstrom (1995) explore this idea of reactivation further in their work, articulating a cyclic model for landform evolution. Blowouts are a result of a process between wind speeds and direction, topography and vegetation cover. These depressions are spawned by the deflation of unconsolidated dune sediments by aeolian processes. Once formed, blowouts evolve through distinct morphological stages over the course of one to two decades beginning with notching, which lasts for about one year. This is followed by an incipient blowout stage, for a duration of two to five years; this can be rapid as vegetation is destroyed and is an inverse function of foredune width. Over the next five to ten years, the full blowout forms through widening and deepening, the new morphology altering the wind profile. Closure begins over the next ten to twenty years, though this process can be truncated. Blowout evolution is forced by the same physical factors as the wider dune system (Jungerius et al. 1991, Hugenholtz and Wolfe 2006), closely related to climactic conditions that control the transport capacity of wind, sediment supply and sediment availability. Development is distinctly related to wind velocity and direction, though storms have little effect (Jungerius et al. 1991). More specifically, blowouts are formed by the most frequent winds above threshold velocities between 6.25 and 12.5 m/s (Jungerius et al. 1991). The size of a blowout is grossly related to its orientation to dominant wind patterns of the region (Gares and Nordstrom 1995). If there are no changes in land use, management, vegetation and the water table, wind can be used to predict blowout behaviour. Less frequency of wind at or above threshold velocities will lead to the stabilization; conversely, higher frequency of such winds will alter blowout dimensions (Jungerius et al. 1991). If these winds became stronger, it could change the whole system, especially if the wind also shifted direction (Jungerius et al. 1991). While this is an interesting theory of evolution, it has already been demonstrated that the aforementioned variables are intertwined in coastal systems, thus future studies of blowouts need to incorporate them as a collective of influences. The primary morphologic features of a blowout are an erosional throat, steep lateral walls adjacent to the throat and large depositional fans at the landward end of the blowout (Gares and Nordstrom 1995). Blowouts tend to develop where off-shore wind speeds, sediment transport and deflation reach their maxima. Once blowouts are initiated, wind speeds within the blowout depend on the wind direction and orientation of the blowout axis (Gares and Nordstrom 1995). Hesp and Hyde (1996) attribute the spatial variability in blowout

7 morphologies to a variety of factors not limited to: where in the dunes they develop, size of the initial constriction, height and width of the dune, degree and type of vegetation cover, and wind regime. Blowout development continues until the deflation basin is eroded to a base level, such as the water table, calcrete surface or armoured surface (such as shell) (Melton 1940, Hesp and Hyde 1996). Sand transport typically reaches its peak along unvegetated, avalanche slopes, and often a flow ‘escape’ is found at the top of the lateral walls. Within the blowout, wind speeds can be significantly higher than remotely sensed wind speeds (Hesp and Hyde 1996, Hesp 2002, Hugenholtz and Wolfe 2009); furthermore, wind dynamics within the blowout can experience topographic steering such that the flow will be turned against the approach wind direction. Trough blowouts in particular display higher velocities in deflation basins and can continue to develop and/or migrate downwind for significant distances. Figure 3 shows the difference in shape between blowout types, topographic features and common wind flows.

Figure 3. Diagrams of typical saucer- and trough-shaped blowouts (Hesp 2002). Typical windflow patterns are indicated by arrows.

Hesp (2002) elaborated on these points further, and in exploring blowout geomorphology found that blowouts can become too wide for effective creation of jets and lessen sand transport. Vegetation density, much like blowout development, varies both spatially and temporally and this further contributes to blowout initiation. For instance, where upper slopes are partially to fully vegetation, exposed slope sediment is removed and slumping occurs, causing the wall to recede and increasing flow complexity within the blowout. The evolution of blowouts and their orientation may be influences by degree of variation in regional wind strength and direction. The pattern of blowout evolution depends on wind speeds, dominant wind direction, vegetation types and re-vegetation processes, magnitude and occurrence of storms and beach status.

8 There is wide speculation in the literature as to which parameters are of most importance. Soil nutrition is cited as a contributor to blowout development (Bate and Ferguson 1996, Zhang and Baas 2012), initiation a result of unfertile soil conditions for vegetation growth. González-Villanueva et al. (2011) found that the formation and migration of blowouts is a decadal scale process, governed by a combination of topography, winds and wave erosion and changes in vegetation cover. Related to climate, in colder months aspect wields an important control on sediment availability (Hugenholtz and Wolfe 2006); this is most pronounced in larger blowouts and may contribute to seasonal asymmetry. Gares (1992) finds that vegetation is the primary influence on blowout change. He suggests that blowout recovery is dependent on vegetation growth and that movement is dictated by vegetation cover in addition to sediment size and topographic characteristics. His study even proposes that differences in elevation between blowouts can be attributed to variations in vegetation cover through key areas of the blowout: the throat, floor and rim. Most studies like Gares (2002) choose either wind or vegetation and explore only that particular aspect of blowout development.

2.2.1 Blowout wind dynamics

It is widely acknowledged that wind is the major driving force behind blowout development (Jungerius et al. 1991, Gares and Nordstrom 1995, Bate and Ferguson 1996, Hesp and Hyde 1996, Hesp 2002, Tsoar 2005, Walker et al. 2006, Walker et al. 2009, Hugenholtz and Wolfe 2009, and Lynch et al. 2010 among others). As previously discussed, regional winds often determine blowout formation and orientation, and therefore development. Southwesterly winds in particular have greater potential for eroding the deflation basin and westerly winds for transporting sand out of blowouts (Hugenholz and Wolfe 2006). However, the significance of local winds and internal form-flow interactions can rival that of the prevailing wind regime (Hesp and Hyde 1996, Hugenholtz and Wolfe 2009, Walker et al. 2009). Foredune topography and morphology of the blowout itself can cause topographic steering, causing wind within the blowout to turn back on itself (Hesp and Hyde 1996, Walker et al. 2006). Hugenholtx and Wolfe (2009) performed an in-depth study of form-flow interaction in a saucer blowout and found for both saucer and trough blowouts there is a strong link between blowout morphology and the pattern of airflow. For both shapes, the degree of steering and acceleration is determined by blowout orientation and long axis of the blowout.

9 Size is another important morphological trait that constrains form-flow feedbacks with positive or negative effects, the latter leading to vegetation and stabilisation. Additionally the ‘critical size’ of a blowout will often lead to modifications to the airflow pattern and likely stabilisation. The most common form-flow patterns begin with airflow passing over the windward rim of the blowout. Near the deflation basin, airflow accelerates up the opposing side, where it can be 50% faster than its original approach speed. There is a persistent interchange between oblique and axis-parallel wind erosion, causing relatively consistent morphometric changes, though oblique winds are apt to be more effective in deepening of the blowout. Overall, understanding these interactions enables understanding of reactivation processes on stabilised and semi-stabilised dunes.

2.2.2 Vegetation growth and controls in coastal dunes and blowouts

Separately, many studies can be found on vegetation growth within a dune system. Like wind, vegetation type and coverage is a regularly recognized component of ecogeomorphic processes (Oosting and Billings 1942, Kachi and Hirose 1983, Box 1996, Jones et al. 2004, Jones et al. 2008, Judd et al. 2008, Jones and Sowerby 2010, Baas 2007, Nield and Baas 2008, Corenblit et al. 2011, Zhang and Baas 2012, among others). Often what dominates in blowouts can indicate the state of the dune system; a dominance of pioneer or succession species can denote a mobile or advanced stabilised dune system (Zhang and Baas 2012). Vegetation is effected by a number of variables. There is a strong link with climate for vegetation establishment (Nield and Baas 2008, Jones and Sowerby 2010), more often attributed to nitrogen deposition (Jones et al. 2004, Jones et al. 2008), temperature (Jones et al. 2008) and nutrient status of the sand (Kachi and Hirose 1983, Nield and Baas 2008). In turn, vegetation cover and species are grossly related to dune development and subsequently blowouts. Plant cover affects sediment transport, a large factor in sand stabilisation and rehabilitation (Jones et al. 2008); only 15-20% of vegetation coverage is needed to inhibit wind-blown sand transport (Zhang and Baas 2012). Increased surface stability prepares the sediment for the next range of species, typically belonging to functional vegetation types, from pioneer to succession species (Box 1996, Zhang and Baas 2012). This results in specific geomorphic landforms, or potentially the lack of, such as blowouts. Ultimately, Baas (2007) is the first to incorporate vegetation dynamics into models of morphological evolution, using ‘vegetation effectiveness’ as a way to simulate species effects on sediment transport, and

10 therefore the ability of a blowout to develop. This adds another set of feedback links between spatial and temporal variability.

* * * * * The overriding theme in studies related to blowout morphometry is that evolution is controlled by wind, topography and vegetation cover; yet, there is a distinct gap in the literature about the ecogeomorpholgical relationship between those recognized forces. Most research has focused on the morphology, sediment transport and airflow in coastal blowouts (Hugenholtz and Wolfe 2006) and often of only a handful of samples. Hesp (2002) exposes many gaps in blowout research where little is known about: role of topographic position, wind regime, wind direction variability and vegetation cover and species in determining blowout type and evolution, and vegetation dynamic processes and change in blowout dune environments. There is a poor understanding of multiple controlling factors operating within a range of timescales; studies only speculate into what drives fundamental change in coastal dune fields (Jackson and Cooper 2011). Often blowout evolution studies are inhibited by the lack of field data on wind characteristics and changes in blowout morphology (Gares and Nordstrom 1995). Furthermore, Hugenholtz and Wolfe (2006) point out there have been an immense amount of studies on dunes, but the same attention has not been given to blowouts. Thus, this project attempts to synthesize blowout evolution accounting for all major driving forces highlighted in previous studies.

* * * * *

11 3. Methods

Regional climatic variables were derived from hourly wind data obtained from two weather observing stations in northwest Ireland. Locally, aerial imagery, differential GPS and vegetation surveys were performed on a sub-sample of 30 blowouts scattered throughout the Magheramore dune system. The acquired spatial data was interpolated in a GIS. Vegetation data was analysed through statistical packages and also visualised within a GIS. Finally, the structure of the blowouts was contrasted with morphological changes obtained from analysis of the aerial imagery from known dates in both a GIS and through statistics carried out in MATLAB.

3.1 Study site

Figure 4. Location of the Magheramore dune system, Co. Donegal, NW Ireland Dune fields along the Irish shoreline began to form around 5000 years BP, when a slight fall in sea-level allowed sediment to be moved onshore (see Figure 4, Carter and Wilson 1993, Barrettt-Mold and Burningham, 2010, Jackson and Cooper 2011). Since 4000 BP, a morphological shift has occurred in many of these systems, as the sediment budget came strongly negative (Barrettt-Mold and Burningham 2010). Currently, as a whole, Ireland had over 140km2 of dune fields around its coastline (Jackson and Cooper 2011). As previously

12 mentioned, during the last 1000 years, evidence shows a ‘spatially variable, episodic sand mobilization’ (Clarke and Rendell 2009). Northwest Ireland is one of the most exposed and windiest parts of Europe (Carter and Wilson 1993). The highest average annual windspeed (8m s-1) is recorded at Malin Head, north Donegal (Carter and Wilson 1993, Barrett-Mold and Burningham, 2010), indicative of a high- energy wind climate. Northwest Ireland comprises County Donegal and small parts of other counties; much of central and west Co. Donegal is an ice-eroded surface (Carter and Wilson 1993). Knight and Burningham (2011) note that several studies indicate that Northwest Ireland coastal sand accumulations are sensitive to external forcing from changes in wind and wave climate, storminess and sediment supply. Such changes can be seen in documentary records of sand drift in Ireland between 1600 and 1800 AD and a strong interannual variability in storminess between 1876 and 1996 (Clarke and Rendell 2009).

Figure 5. Dune ridge changed observed in aerial imagery from 1951, 1977, 1995 and 2000. From 2000, the ridge is largely stable. A recession rate of approximately 11.3m/year along the estuarine margin can be inferred. The Magheramore dune system, the focus of this study, sits at the northern edge of the Loughros Beg estuary mouth, encompassing a 1.5km2 area of embryonic, shifting and fixed dunes (Barrett-Mold and Burningham 2011). The system is bordered to the southwest by a pelite bedrock headland and to the west by Tramore beach with a steep dune face (up to 15m high) on the estuarine-margin (Barrett-Mold and Burningham 2010). It falls within the northeast to southwest trending Dalradian metasediments of the Precambrian age and the surface rocks include quartzites, pelites and granites (Carter and Wilson 1993). Since 1907, the ebb-tidal channel of the estuary has shifted northwards to its current position (Barrett-Mold and Burningham 2010). This is confirmed by observed dune ridge change over the past six decades (Figure 5). This shift of about 650m resulted in 500m of dune recession and release of the equivalent amount of sand into the system (Barrett-Mold and Burningham 2010).

13 However, a rapid reduction in bare surface area can be observed from 1995 to 2005, indicating increased stability (Jackson and Cooper 2011). Throughout the dunes rabbits and cattle grazing are present. Blowouts feature strongly, with over 40 depressions being visible in satellite imagery and persist in a variety of shapes, sizes, depths and vegetated states. They are present in all ages of dunes from embryonic to fixed, some containing dune slack and/or ponds, while others remain active. Multiple blowouts are eroded down to the water table, resulting in dune ponds in the deflation basin. Others have eroded down to a shell layer associated with middens, left from prehistoric human occupation (Knight and Burningham 2011) or to bedrock.

3.2 Data acquisition

3.2.1 Wind data

Wind data was procured from the Malin Head (Co. Donegal, 55°22’20’’N, 7°20’20’’W) and Belmullet weather observing stations (Co. Mayo, 54°13’40’’N, 10°0’25’’W)2 from 1956 to 2011. Both data sets were used for corroboration since local climactic data was unavailable. This does open analysis to uncertainties in interpretation, as the trends in regional winds might not adequately reflect local winds at the site (Hugenholtz and Wolfe 2006). However, this does not diminish the relevance of correlations between morphology and climate variables since the main purpose of the study is to explore such regional controls on development. Further, as stipulated by Gares and Nordstrom (1995) blowout size is related to their orientation to dominant wind patterns; thus, if those two traits can be obtained from data, it is possible to uncover relevant relationships in the climactic feedbacks.

3.2.2 Aerial imagery

Using remotely sensed data is a common method for studying coastal dune systems (see Jungerius and van der Meulen 1989, González-Villanueva et al. 2011, Jackson and Cooper 2011, Levin 2011, Zhang and Baas 2012 among others). Recent historical (decadal-scale) change was examined using aerial imagery. Aerial photographs of Magheramore from Ordnance Survey Ireland were available 1951, 1977 and 1995; imagery was also available for the years 2000, 2005 and captured from Google Earth for the year 2009. Images from 2000

2 Met Eirann…

14 and 2005 were obtained already georeferenced.3 Using a geographic information system (GIS)4, the remaining images were georeferenced against a digitized map of the area from 1907, the 2000 and 2005 images and dGPS data. A minimum of 20 ground control points (GCPs) were used for each image, resulting in an average Root Mean Square Error of 4.8m (see Table 1). Images from 1951 and 1995 were then mosaicked together to ease analysis.

Table 1. The historical aerial photographs and satellite imagery used in this study. Imagery type Number Year Approximate Number of GCPs Median RMS of photos scale error (m) Historical aerial photograph 3 1951 1:27,500 34 4.91 Historical aerial photograph 1 1977 1:56,400 20 7.84 Ortho photograph 5 1995 1:25,200 29 1.13 Ortho photograph 1 2000 1:79,250 -- -- Ortho photograph 1 2005 1:75,000 -- -- Satellite photograph 1 2009 1:23,650 22 4.83

3.2.3 Differential Global Positioning System (dGPS) data

Thirty blowouts chosen at random were surveyed with a dGPS, a Leica 1200 base station and Leica G315 rover, to collect high-resolution data on topography. Data was collected using the Irish National Grid. For measurements of blowouts facing the estuary, the base station was positioned at McGlinchey (52°54’24’’N, 3°58’43’’W) so as to decrease likelihood of losing the real-time correction. Similarly for blowouts present nearer to the sea- facing side of the site, the base station was placed at Tramore (50°12’38’’N, -5°14’58’’W). Data was post-processed in Leica desktop software to correct points not receiving a real-time correction. Points outside of an acceptable quality range (>1m) were rejected; the remaining data points have a mean quality measure of 2.3cm. Upon identification of a sample, measurements were collected in one of two ways. For saucer- or cup-shaped blowouts and smaller trough-shaped blowouts, five measurements were taken (Figure 6a): the rim and four transects along ordinal directions (i.e. north to south, east to west, northeast to southwest and northwest to southeast). ‘Mega-blowouts’, the exceptionally large trough-shaped blowouts observed in this system, were mapped by measuring the rim, the main axis and six to eight transects perpendicular to the main axis in order to capture more detail (Figure 6b). The outlines of features were also measured, for example active features in partially stable blowouts or ponds in the basin of samples eroded down to the water table.

3 Credit to Helene Burningham. 4 Versions 9.3 and 10 of ArcGIS were used to carry out analyses requiring a GIS.

15 Figure 6. Representation of measurements taken for a (A) saucer- or cup-shaped blowout, where transects are along ordinal directions (in blue) and rim (in black), and for (B) a ‘mega-blowout’ where the dominant axis (red) is measured with parallel transects. Feature outlines (yellow) were also measured when present.

3.2.4 Vegetation data

For each of the 30 blowouts surveyed, four 1 x 1m vegetation quadrats, located on the north, south, east and west slope faces, were used to record species present and their percent cover. The slope face was first assessed from a distance to determine overall vegetation coverage and quadrat location selected that best represented the blowout wall as a whole. 5 In the field the DAFOR (Dominant, Abundant, Frequent, Occasional, Rare) scale was used to assess the species coverage, including bare sand, in the quadrats, and later substituted for midpoint values from the Braun-Blanquet scale (Table 1). Special features outside of the quadrats were noted such as orchids present in the basin of blowouts, or in the case of one sample, a tree. Also an activity grade, similar to Tsoar’s (2005) classification system for dunes, was assigned to each blowout; criteria was based on the coverage of exposed sand and vegetation and vegetation type present in the blowout.

Table 2. Scales used to assess vegetation coverage in quadrats (Braun-Blanquet scale from Kent and Coker 1992). Mid-points were later substituted for statistical analysis. Value Braun-Blanquet (% cover) Mid-point (%) D 76-100 88 A 51-75 63 F 26-50 38 O 6-25 15.5 R 1-5 3 -- >1 0

5 Gares (1992) utilized a similar method in his study, making a visual estimation of vegetation density and assigning a sample point to one of five density categories.

16 Table 3. Activity grade assigned to each blowout for its overall level of activation. Value Level of activity 1 Inactive 2 Mostly inactive 3 Equally inactive and active 4 Mostly active 5 Active

3.3 Data analysis

3.3.1 Analysis of wind data

From the Malin Head and Belmullet data sets wind lineations and wind strength were calculated for every year from 1956 to 2011. With this, wind rose data was collected and the number of hours of wind above the threshold velocities calculated for each direction. Data was divided into periods mirroring the available aerial images (e.g. 1956-1976). Next, storminess was determined by calculating the characteristics of a wind speed event, defined by a threshold speed maintained for a threshold duration, in this case 22 knots over 5 hours. Storm events for analogous timescales were tabulated, along with mean wind speed and average wind direction.

3.3.2 Geomorphological analysis

Morphology data was first collected from the dGPS measurements and digitized features from aerial imagery. From the imagery, both the dune ridge and all visible blowouts were digitized in a GIS. From this an array of blowout morphological traits could be gathered: location, surface area, perimeter, blowout length and width and shape (ratio of length to width). Additionally this allowed for the identification of the first appearance of the 30 sub- sampled blowouts as well as the percentage of area developed and re-colonised from year to year. This made possible a visualisation of temporal blowout evolution and gathering of features for statistical analysis of samples. In a GIS, individual data for each sample was added, with the rim transect separated from the rest of the data points. Natural neighbour interpolation was run on the rim data and all blowout points for each sample and data extracted with a polygon of the specific blowout. The resultant layer of all data was subtracted from the rim layer. This calculation was exported as an ASCII file and volume of sediment removed from each blowout calculated in Matlab by summing the grid data and multiplying by the cell area. Orientation of the samples was determined using the dGPS data. In a GIS, the length and width were measured to determine circularity and therefore blowout type. Then, the data

17 points were visualised using elevation to identify the throat and depositional lobe of the blowout. The dominant axis of the trough blowouts indicated the orientation of the blowout, the main transport direction verified by the location of the throat and lobe. For saucer-shaped blowouts, orientation was based on the elevation data at the rim to determine the main opening and depositional lobe and axis aligned accordingly. Further, the length, width, highest and lowest elevation, maximum depth and location were collected for each sub-sampled blowout. Using the interpolated blowout data and broader topography layer,6 a digital elevation model (DEM) was created (Figure 7). From this, aspect and slope could be calculated. To collect environmental variables for vegetation data, zonal statistics were run to find the mean slope of each north-, south-, east- and west-facing wall of the sampled blowouts.

Figure 7. Digital elevation model (DEM) of the Magheramore dune system.

Matlab scripts were used to perform a variety of statistical analyses on the morphological traits. Data was first explored through boxplots for comparison of size and shape characteristics between blowouts of different topographic locations, ages, orientations and grades. Then a cluster analysis was carried out using an Euclid approach. Most variance could be explained by four clusters, however the optimum number of clusters from this method was determined to be five. In selecting a linkage option, it was found that regardless of the method, the same blowouts were clustered together so average linkage was selected,

6 Provided by Helene Buringham. The layer is a composite of contours from the 1907 Ordnance Survey map, a map from 1995 and various dGPS data.

18 utilizing its robust calculations of distance. Then the clusters were explored through visualizations of relationships between various morphological characteristics. Finally, a principal component analysis (PCA) was carried out to determine the dominant controlling factor.

3.3.3 Ecological analysis

Vegetation data was first run through a two-way indicator species analysis (TWINSPAN). Canonical correspondence analysis (using CANOCO for Windows 4.5) was then used to explore ordination of samples. First a detrended correspondence analysis (DCA) was run on the 120 samples. Based on this statistical test, nearly 50 percent of the variance can be explained in the vegetation species. As the gradient is relatively short (see Table 6), a PCA was selected then to explore samples as they relate to species and environmental variables. Additionally, in order to understand the ecogeomorphic feedbacks of vegetation on blowout development, vegetation samples were then visualized in a GIS.

* * * * *

19 4. Results

4.1 Climactic variables and trends

Table 4 displays storminess for the region for the period of 1956 to 2009, the period reflecting that of the historical analysis. Between 1956 and 1976, on average there were 508 and 341 storms per five years registered at Malin Head and Belmullet respectively. From 1977 to 1994, there was a slight increase in storm events to 537 and 358 per five years. This trend continued through 1999, when the number of storms began to decrease, the lowest rates being found between 2005 and 2009. Similarly, there was a minor decrease in the mean wind speed over the decades, demonstrating a slight change in climate. This overall decrease in the number of storm events and slight decrease in strength mirrors the trend toward stability in the historical analysis from aerial photographs (see section 4.2.1).

Table 4. Storminess measured at (A) Malin Head and (B) Belmullet for the period of 1956-2009. To qualify as a storm event, winds must have blown at 22 knots or higher for a duration of 5 hours. A. Malin Head Storm Events Mean Wind Speed (m/s) Average Wind Direction (degN) 1956-1976 2034 26.7 204.8 1977-1994 2167 26.9 205.0 1995-1999 570 26.3 201.2 2000-2004 484 26.0 208.0 2005-2009 432 26.2 209.1 B. Belmullet Storm Events Mean Wind Speed (m/s) Average Wind Direction (degN) 1956-1976 1367 26.1 213.9 1977-1994 1219 26.2 214.4 1995-1999 368 25.9 213.4 2000-2004 290 25.6 210.6 2005-2009 260 25.9 217.4

Storms, however, do not impact blowout development as much as more frequent winds at a lower velocity (Jungerius et al. 1991). Over time, the wind climate has become milder, with less time spent at stronger velocities. As can be observed in Figure 8, the most consistently occurring wind speeds are those between 7 and 16 meters per second, which coincide with established threshold velocities for blowout development (Jungerius et al. 1981, Jungerius et al. 1991). For the period from 1956 to 1976, the average median wind speed was m 14 meters per second. From 1977 to 1994, the median speed was 15.6 /s, represented a change in the wind regime similar to storminess. Years between 1995 and 2004 experiences a m m median wind speed of 14.8 /s, decreasing to 14.0 and further to 13.0 /s between 2005-2008 and 2009-2011 respectively. The predominant direction is southwesterly and westerly, though

20

Figure 8. Wind roses for hourly wind data from Malin Head and Belmullet meteorological stations from 1956 to 2011.

21 southerly winds are not insignificant. More infrequently, wind blows from a northwesterly and northerly direction, however these winds tend to be at threshold velocities, rather than strong, storm-level winds. 4.2 Geomorphology of blowouts

4.2.1 Aerial imagery

Figure 9 shows the evolution of blowouts from 1951 to 2009 based on aerial imagery. Change was noted between 2009 and 2012 for the sub-sample of blowouts measured in fieldwork. Prior to 1951, there was a record period of storminess leading to a decimation of vegetation in the dune system, apparent in the aerial photographs from the time. By tracking the visible surface area, it is possible to qualify the development and re-colonisation of existing and new blowouts. It is apparent in the aerial imagery that individual blowouts follow comparable trends over time. Thus, with the percentage of area in both categories, the state of the dune system as a whole can be classified. Table 5. Blowout presence and development visible since 1951. Year Number of Total surface area of Area developed Area re-colonised Classification blowouts blowouts (m2) (%) (%) 1951 35 227,225 ------1977 40 398,507 68 39 Mobile 1995 68 264,575 38 59 Vegetating 2000 66 300,049 31 22 Mobile 2005 65 218,235 16 39 Vegetating 2009 57 232,562 20 15 Steady Sub- samples: 2009 29 140,451 -- -- 2012 30 132,155 8 13 Steady

As shown in Table 5, from 1951 to 1977, though only 5 new blowouts were initiated, those already present expanded over 150,000m2 in area. This represents a 68% growth in area developed; when compared to the 39% of blowout area re-vegetated, it is evident the dune system was experiencing a period of mobility. This trend reversed between 1977 and 1995, with only 38% of new blowout area but 59% of previous area re-colonised by vegetation. During this time, 28 new blowouts were initialised, but overall surface area decreased. Most likely, this is related to the increased storminess during the same timescale (Table 4). The strong winds would clear vegetation and erode sand, allowing frequent threshold winds, m blowing at a higher median wind speed (15.6 /s) during this period, to develop the blowout. This would not have much effect on established blowouts (Jungerius et al. 1981), however, allowing for re-colonisation. The dune system continued to shift between mobile and

22 Figure 9. Geomorphological interpretation of aerial photographs for the period 1951-2009. A comparison of dGPS data to satellite imagery of the 30 sub-sampled blowouts was carried out for 2012.

23 - or cup

-

shaped. sampled blowout shape between 1951 and 2009. Blowouts with an axis ratio of <1.8 are considered saucer - -

Table 6. Evolution of sub shaped; those over 1.8 are trough

24 le 7. Morphological attributes of sampled blowouts. Tab

25 vegetating states until recent years, apparent in Figure 9, from around 2005, when equilibrium is noticeable between area developed and re-colonised, though the percentage of area developed has nearly consistently decreased year to year. Bate and Ferguson (1996) wrote that blowouts greater than 30 meters in length are rare. Historical analysis of the Magheramore system shows blowouts with an average length of approximately 108m during any given year. Some of these blowouts are identified as ‘mega-blowouts,’ a term coined to represent those large samples with a dominant axis length of more than 100m. These blowouts are nearly all trough-shaped and can be identified in 1951 photographs. Shape of each of the 30 sub-sampled blowouts was delineated from its first appearance in aerial imagery, shown in Table 5. With few exceptions, the blowouts kept the same ratio over the course of development, consistent with the model from Jungerius et al. (1981). Those trough-shaped blowouts, samples 2, 8, 10, 11, 19 and 26 (with 21 and 30 on the cusp) are mainly oriented in a southwesterly direction (26 is southerly and 30 is westerly) as shown in Table 6, aligning with the most frequent wind directions. The remaining 22 samples are cup-shaped and represent an array of orientations, though southwesterly is still most common.

4.2.2 dGPS data

Morphological attributes gleaned from the dGPS data are represented in Table 7. The majority of the blowouts within the Magheramore dunes were established by 1995 and this is reflected in the 30 samples, 26 of which were visible in the 1995 aerial photograph. Age does not appear to affect blowout morphology significantly, with the exceptions of shape (circularity) and maximum depth, which coincides with lowest elevation (Figure 10). Those blowouts present in 1951 are dominantly oriented along a southwest to northeast axis. While the median value is the same for those initiated by 1977, the range in orientation is greater. By 1995 most blowouts were associated with southwesterly and westerly orientations. Following this, there are too few new blowouts initiated to draw meaningful conclusions about trends in orientation. Figure 10 also illustrates that the older a blowout is, the more likely it is to become a trough blowout, the axis ratio increasing with age. Similarly, maximum depth and volume of sediment removed increase with age as wind has more time to erode blowouts down to the water table and/or other immovable layer; likewise, the lowest elevations are found in older blowouts. Highest elevation does not appear to correlate with

26 Figure 10. Summary of association between blowout age and selected morphological variables.

Figure 11. Summary of association between blowout location and selected morphological variables.

27 Figure 12. Summary of associations between blowout activity grade and selected morphological variables.

Figure 13. Summary of associations between blowout orientation and selected morphological variables.

28 age, however. Though the widest range of activity is observed in blowouts dated to 1995, level of activity is fairly evenly distributed across blowouts of all ages. Location within the dune system, on either the estuary-facing or sea-facing side of the headland, is suspected to influence blowout development. However, as Figure 11 shows, this is not really the case. Blowouts in either location are oriented in an assortment of directions, though those on the sea-side pick up more of the westerly winds. On the estuary side, the exaggerated trough blowouts, ‘mega-blowouts,’ are more likely occur, though trough blowouts do occur in sea-facing locations. Other morphological traits, including maximum depth, volume, and lowest and highest elevations, are not specific to either topography, though sea-facing blowouts are marginally more active than their estuary-facing counterparts. Given this, Figure 12 explores associations between level of activity and morphological traits. For each activity grade, the median orientation is relatively consistent, aligning with prevailing wind directions. For those more active blowouts, there is a wider range of orientations, indicating the influence of local winds, outside of the regional trends. Those blowouts completely active and completely inactive both tend toward cup-shaped, while those only partially active or inactive range wider in shape. Referring to Table 5, it can be seen that those blowouts with a grade of five, while varying in age and orientation, have an axis ratio of 1.5 or less. The same description applies to blowouts with a grade of one. All trough blowouts possess some level of activity. For blowout development this demonstrates that there is a gradient in shape over activity, over which blowouts tend to be most active and conversely most stable when circular in shape. Those blowouts that are equally active and inactive tend toward trough-shaped, indicating that trough blowouts are more likely to become reactivated after undergoing re-colonisation. Furthermore, a blowout tends to be more active the deeper it is. This again can be associated with shape, as trough blowouts are traditionally deeper than cup-shaped depressions, and internal wind dynamics exert a significant force. While volume and lowest elevation do not exhibit a strong relationship with activity, those blowouts with higher elevation tend to be more active. This is likely related to regional winds, as such climactic forces would have better access to sediments at higher elevations. Orientation influences wind effects on blowout development both in shape and internal wind speeds (Gares and Nordstrom 1995). Its influence on specific blowout morphologies can only be observed in those aligned with most frequent wind directions (Figure 13), as those oriented otherwise are too infrequent for meaningful conclusions. If a

29 blowout is aligned with dominant wind lineations, it is more likely to be active than those that are not, but it is still able to transition to inactivity. Trough blowouts are predominantly oriented to the main, most frequent southwesterly and westerly winds, demonstrating that orientation is influenced by the degree of variation in regional wind speed and direction (Hesp 2002), as the Magheramore system experiences fairly consistent wind regimes. Orientation appears to affect other attributes beyond circularity and level of activity in a similar manner, with the dominant winds forcing most of the development.

Figure 14. Initial statistical analysis. Principal component analysis (A) shows both the first and second principal components. Within cluster differences (B) establishes the number of clusters in the data.

The principal component analysis of morphological data (Figure 14a) shows that orientation accounts for most of the variability in the data and length of the dominant axis has the next highest variance uncorrelated with orientation.7 Cluster analysis offers another aspect of these associations. With the principal components in mind, Figure 14b establishes the number of clusters within the data, with most variance explained in two clusters, but five covering nearly all. Figure 15a visualises the spatial distribution of the blowout clusters and allows for characterisation of common morphologic traits. Cluster 1, trough blowout sample 26, is unique to the system; it is oriented in a southerly direction, its dominant axis forcing sand from south to north, the only such orientation in observed in the data. Cluster 2, samples 8 and 10, are similar in that they are also trough blowouts qualifying as ‘mega-blowouts.’ They, along with blowouts in Cluster 3 (samples 2, 24 and 25) are the most developed in the system. As can be seen in Figure15b, these three clusters tend toward longer dominant axis length,

7 This is consistent with findings by Hugenholtz and Wolfe (2009), who found the same characteristics to be significant for wind dynamics.

30 A

B C

D E

Figure 15. Results of the cluster analysis. Blowouts were grouped into 5 clusters (A) and morphological attributes compared: (B) dominant axis to circularity, (C) dominant axis to secondary axis, (D) volume to circularity and (E) surface area to circularity.

31 though vary over circularity. Clusters 4 and 5 group closely together, with shorter dominant axes and a more circular shape, though Cluster 4 tends towards slightly longer axis length. This gradation in size and shape is verified in the correlation between dominant and secondary axes lengths, from Cluster 1 measurements shrinking decreasing through to those of Cluster 5, observed in Figure 15c. Similar trends can be observed in Figure 15d and e: the smaller the blowout in volume and area, the more strongly associated with a smaller axis ratio (i.e. cup- shaped).

4.3 Ecogeomorphological interactions

Figure 16. Frequency of bare sand and species occurrence on west-, east-, north-, and south-facing blowout slopes.

Twenty-nine species of vegetation were identified within blowout quadrats, ranging from colonising species such as Ammophilia arenaria to mature dune species like Ranunculus

32 and mosses.8 Exposed sand, associated with blowout activity, is far more common on the south- and west-facing blowout slopes (Figure 16). Those species commonly associated mobile dunes are found at almost equal rate on all slope faces, while species recognised as mature dune type are more likely to be found on north- and east-facing slopes. This can partly be attributed to conditions required by individual species for growth: colonising species are able to grow in nutritionally poor soil, but mature species often required moist soil, which is more likely found on slopes not facing dominant winds. Samples were grouped into 14 clusters in TWINSPAN analysis (Figure 17), and the progression from one cluster to the next reflects the gradation between mature dune and mobile dune vegetation. Positive indicator species are typically indicative of active or mobile dunes, such as bare sand and A. arenaria. Negative indicator species suggest semi-fixed or fixed dune varieties, such as Anthyllis vulneraria, Festuca rubra, Lotus corniculatus, Polygala vulgaris and moss. When visualised in a GIS (Figure 18), it is observed that clusters tend to group together; blowouts of a higher grade (see Table 5) tend to hold vegetation clusters associated with bare sand. Exceptions to this pattern are seen in equally active and inactive blowouts where colonising to mature vegetation is observed, such as ‘mega-blowout’ samples 8 and 10. As the DCA resulted in a gradient of less than four (see Table 6), a principal component analysis was selected for further exploration. Principal component analysis reveals relationships between species and environmental variables (Table 6, Figure 19).9 For species, the primary component is the presence of bare sand, akin to the TWINSPAN indicator species. When bare sand is not present, variance can be explained by ecological succession. A. arenaria indicates a blowout at the beginning of this process, whereas moss is more telling for a blowout that has completely stabilised. Less predominantly, A. vulneraria, L. corniculatus and Leontodon hispidus present in samples show a blowout moving towards inactivity, before species such as Silene maritime, Pteridium aqualinum, and Holcus lanatus colonise. Environmental variables related to blowout morphology also influence the species able to grow. The activity grade explains most of the variance between samples, though the average slope, aspect and highest elevation are also recognised as forces behind re-colonisation.

8 Species associated with pioneering and succession were corroborated with previous vegetation studies of Magheramore and similar dune sites (Barrett-Mold and Burningham 2010, Zhang and Baas 2012). 9 See Appendix for vegetation codes.

33

B09SY B09NY B09EY B09WY B14SY B21SA B23WA B27SA

R. B12SY B14EY B17SY B21NA B23SA

Y Y B05SY B10E B15S B16SY B21WA B24EA B29SA B29WA

rom mature vegetation (e.g. B02SY B17WY B19WA B25WA B27WA B27NA B27EA

B02EY B06SY B08NY B10NY B16NY B16EY B17EY B18SY B24WA B29NA B30WA

blowout sample), direction slope is facing and location (A B01WY B03SY B04SY B04WY B08SY B11SY B13WY B15NY B16WY B18WA B22WA B23NA B25SA

B01SY B11WY B13SY B13EY B14WY B14NY B15WY B19SA B21EA B22NA B22EA B23EA B24NA B25NA

B03WY B12WY B19NA B22SA

B02NY B05WY B07SY B07WY B07NY B07EY B08EY B10SY B17NY B18NA B24SA B30SA B30NA B30EA

facing blowouts). - B02WY B03NY B19EA

B03EY B05EY B06WY B06EY B08WY B11EY B12NY B12EY B25EA B26SA B26WA B26EA

B01NY B01EY B04NY B04EY B05NY B10WY B20WA B20NA B28SA

) to no vegetation, characterized by bare sand. Samples are named as B# (number of B06NY B11NY B13NY B15EY B18EA B20SA B20EA B26NA facing blowouts, Y represents estuary -

B28WA B28NA B28EA Figure 17. Dendrogram of the 14 vegetation clusters distinguished for sampled blowouts. There is a gradation left to right, f pimponellifolia indicates sea

34 Figure 18. Spatial distribution of the 14 vegetation clusters demarcated in the dendrogram (Fig. 17). Blowouts are identified by sample number.

A

B

Figure 19. Species (A) and environmental variables (B) principal component analyses.

35 Table 8. Eigenvalues and variance from the detrended component analysis and principal component analysis performed on species and environmental data in Canoco. DCA 1 2 3 4 Eigenvalue 0.49 0.25 0.16 0.13 Gradient 3.21 1.96 1.9 % Variance 18.2 27.5 33.7 38.6 PCA (species) Eigenvalue 0.25 0.03 0.02 0.01 Species/environmental variable correlations 0.77 0.46 0.4 0.36 % Variance (species) 25.3 28.2 30.2 31.4 % Variance (spec/EV relation) 76.8 85.4 91.5 95.1

When the relationship between species and samples is explored more in-depth, it appears that, other than exposed sand, species do not strongly affect blowout morphologies. Like the morphological features, vegetation is not effected by location in relation to the headland (Figure 20a), thus type of vegetation does not determine where blowout will develop. Likewise, highest elevation does not show a strong connection with species present in samples. Aspect, represented in Figure 20b, does experience species controls in certain directions: south-facing slopes are somewhat more clustered around indicators of development (bare sand, A. arenaria), while east-facing slopes are chiefly clustered around mature dune species, implying east-facing slopes are more likely to be stabilised, tapering blowout development. This is supported by previous results depicted in Figure 16. Blowout grade (Figure 20c) displays a progression from mobile to fixed species as the level of activity decreases. Those samples found in blowouts with a grade of 4 or 5 are more strongly related to bare sand and colonising species, whereas samples from inactive blowouts cluster around the more diverse, established species. Average slope of blowout walls does show a small influence of species (Figure 20d), as on the steepest slopes samples generally cluster around bare sand and A. arenaria is sometimes found, demonstrating that without the stabilising force of other species, sediment will continue to be moved from blowout walls. Further analysis of samples as related to environmental variables shows the influence of morphology on ecological growth. Utilising the indicator species derived from TWINSPAN analysis, samples show strong correlations with their surrounding morphologies. There is a distinct gradation in the amount of bare sand present in relation to morphological characteristics (Figure 21a). Where exposed sand is dominant, samples found in the higher- numbered clusters are concentrated around the first principal component. Figures 21b-f show a progression along indicator species (a succession of functional plant types), and reflect the second principal component axis in the spcies PCA, from A. arenaria to moss. While the lower

36

C egrees) of the ) average slope (d

E B

A

D Figure 20. Samples classified by key environmental variables: (A) location, (B) shape (axis ratio), (C) activity grade and (D blowout wall and (E) aspect (degrees North), as related to species.

37

L. (E)

F C F. rubra, (D)

A. vulneraria, (C)

A. arenaria,

E B

D A and (F) moss, as related to environmental variables.

rnaculatis, Figure 21. Samples classified by indicator species derived from TWINSPAN analysis: (A) bare sand, (B) co

38 percentages of cover do not strongly correlate with any variables, when a species becomes abundant or dominant it is less aligned to activity, slope or aspect, and more affiliated with orientation and circularity. This trend is consistent along this ecological succession, and becomes more distinct at each stage.

* * * * *

39 5. Discussion

Every blowout possesses easily identifiable environmental variables. Vegetation grows in conditions specific to species and cluster around distinct environmental variables. Thus, for re-colonisation to occur, a blowout (or part of a blowout), must obtain a certain shape, possibly related to the maximum sediment transport for its topographic location. So, what forces shape a blowout? Mainly the orientation of the dominant axis to the prevailing winds, as Hesp (2002) explains orientation may be influenced by degree of variation in regional wind strength and direction; though, this does not explain all variation. It is hypothesized that the shape of the original opening in vegetation cover that initialized the blowout determines, in part, the morphological development of the blowout. This cannot be confirmed in this study owing to the lack of frequent aerial imagery. Further, as a blowout grows, so does the importance of internal wind dynamics, which is also not possible to test from the data presented here. However, there is much to be inferred from the analysis and statistical results gathered in this study. Gares (1992) states that blowout size is grossly related to its orientation to dominant winds. While this study confirms that orientation is indeed a primary factor in development, average-to-large sized blowouts still develop while forced by less dominant winds. Both trough- and cup-shaped blowouts observed in this study are forced by a wide swath of wind lineations. Most likely, as established in the boxplots, shape is related to age, i.e. the amount of time a blowout develops before becoming stabilised. It appears that trough-blowouts are more likely to become reactivated and continue to expand, as they are more strongly associated with the dominant winds in the system. The longer a blowout is active, the more the wind forces its growth both laterally and vertically, causing mega- blowouts to form. Those not developing in to a trough-shape may become stabilised before this occurs. One possibility for this is the fact cup-shaped blowouts can also represent less dominant winds and local wind regime, sometimes aligning with the most infrequent wind directions, such as sample five. Differences in blowout shape are not effected by location, both cup and trough and ‘mega-blowouts’ appearing throughout the dune system. Likewise, vegetation species appear with equal likelihood in blowouts of either shape, no species being strongly associated with either. The one exception is exposed sand and A. arenaria, which is more frequently observed in those blowouts on the cusp between shapes. Typically these blowouts are still active and

40 developing. So, the main difference in shape is related to wind climate, with trough blowouts exclusively oriented to dominant wind directions. Cup-shaped blowouts experience forcing from several different wind lineations, and not always the prevailing direction. It is of note that those in-between shapes are oriented to dominant winds, indicating the potential to continue to develop into a trough blowout. Activity is specific to individual blowouts; there is no singular clustering of activity in the dune system. The most active blowouts are oriented in the widest range of directions. This indicates that local wind dynamics are very significant to blowout re-colonisation. However, activity is also related to the gradation in vegetation itself, an apparent form of ecological succession. Bare sand is more common on south- and west-facing slopes, while A. arenaria, a colonising plant, is most frequently observed on east- and north-facing slopes. Further, more plants associated with more mature dunes, such as F. rubra and T. farfara, when present, are mainly observed on north-facing slopes. This indicates that re-colonisation occurs when A. arenaria can begin to grow those slopes facing away from the dominant winds. Therefore, blowouts oriented away from these winds will be more likely to achieve vegetating to stable states. This is reliant on orientation and length of the dominant axis, as observed in the principal component analysis, but also to shape and higher elevation, attributed to the distance from the water table (and therefore not ideal for a range of vegetation).

Figure 22. Vegetation cover observed in aerial imagery for blowout sample one (A), a cup blowout initially identified in 1977, and sample two (B), a trough blowout identified in 1951, over the period 1995 to 2009.

41 Gares and Nordstrom (1995) determined that blowouts are relatively stable after one to two decades. This study finds this to be consistent within the evolution of blowouts in the Magheramore dunes. From assessment of aerial photographs and in the field, it is possibly to identify where in the model of blowout succession individual samples reside. The common model of blowout development (notch, hollow, incipient, large, to re-vegetated and stabilised and possibly reactivated) can be identified in this method of study, though this study finds it is not possible to predict and overall timeframe for each stage. Figure 22 shows that post- emplacement, closure and re-vegetation will occur in both cup and trough blowouts after about two decades, but this is not consistent over the entire system. Once pioneer species take root, colonisation is fairly rapid, the majority of the blowout being colonised in as little as five years. Biodiversity within blowouts is not as rich as the entire dune system, however. Only 29 species were identified within blowout quadrats, compared to 85 species found in the vegetation study in Magheramore from Barrett-Mold and Burningham (2010), implying that mature species requiring greater stability do not have a chance to take root. Thus, the capacity to reactivate limits biodiversity in blowout environments. This in part confirms the first hypothesis of this study, in that geomorphic features will constrain ecological development. Trends in vegetation and wind dynamics found in this analysis also correspond with previous findings. Jones et al. (2004) found that over the last 30 to 40 years, dunes in the United Kingdom have shifted from mobile to fairly continuous vegetation cover. This applies to this Irish dune system, with vegetation coverage increasing and development of new blowouts decreasing. Winds are chiefly southwesterly and westerly in direction, which is seen to drive blowout development. Morphologies pick up on these prevailing winds, but also threshold velocity winds from less predominant lineations. Blowouts driven by the latter are empirically cup-shaped as trough blowouts are only seen to form when aligned to dominant winds. Cup blowouts affiliated with ascendant winds have the potential to develop into trough blowouts given time, as shown by the maximum depth and shape of aged trough blowouts. The recurring theme in blowout development is that orientation and length of the primary axis force geomorphological evolution. Gradation in blowout morphologies occurs along these lines, mostly related to size and shape, which, as already demonstrated, are linked with activity. A shift to re-colonisation occurs when wind sediment carrying capacity is reached, but this point appears to be individual to each blowout, possibly due to topographic

42 effects (ex. proximity to other blowouts, initial constriction, elevation above the water table, etc.). Consequently ecological development is forced by blowout morphologies. Aspect and slope exert significant control on vegetation growth. Some species will only grow in certain aspects, and often, steeper slopes are less vegetated and typically found downwind, making implantation difficult. A gradient in vegetation is observed in both TWINSPAN and CANOCO analyses, clusters coinciding with environmental variables associated with blowout development. Furthermore, abundance and dominance is effected by orientation and axis length; the ability of a plant to achieve high levels of coverage is intimately related to the exposure to wind allowed by blowout morphology. Finally, vegetation and re-colonisation is linked back to wind along a hysteresis curve, where weaker winds mean more vegetation cover (Tsoar 2005), as observed in this study. Overall, both hypotheses tested in this study prove to be true for blowout evolution. Blowouts are developed by wind and driven by their morphologies. Re-colonisation occurs when these same morphological features allow for pioneering species to take root and coincide with a decrease in wind. Therefore it is reasonable to conclude that morphology does control ecology in a blowout environment. The reverse is also true, though ecology does not constrain how a blowout develops, just how much; however, topographic heterogeneity does not have a large effect in general. Blowouts are indicative to the state of the dune system as a whole. In the case of Magheramore, by studying blowouts, it can be inferred that the dunes are tending toward fixed, mature dunes though this process is decades away from stabilisation.

* * * * *

43

6. Conclusions

Blowout morphometry is driven by wind, which is in turn controlled by vegetation, which is in turn forced by morphology. It is this cyclical feedback between ecogeomorphic variables that dominants blowout development and re-colonisation. If data on blowout orientation, dominant axis length, axis ratio, slope and aspect is available, preferably over a decadal timescale, it is possible to determine if stabilising and/or more mature plants can grow via analysis of these morphological traits. If further vegetation species data is available, the blowout can be placed in an evolutionary model. Trends in blowouts will reflect trends in the dune system, as can be seen in studies of Magheramore as a whole (Barrett-Mold and Burningham 2010, Jackson and Cooper 2011). For a larger study site this would be most useful, as blowouts are more manageable samples to collect data on. Blowouts are indicators to greater coastal dune dynamics. This is substantiated by this study; the ability to draw such conclusions is owed to the field data from the 30 blowouts surveyed, over half of the blowouts currently in the Magheramore system.10 In regards to Magheramore, dunes display characteristics of mobile, semi-fixed and fixed dunes (Barrett- Mold and Burningham 2010, Knight and Burningham 2011). The blowouts within the system share this spectrum of ecological and morphological succession. This suggests that blowouts reflect the climate, including wind regime, storminess and potentially nitrogen deposition, found to be affecting dunes. Furthermore, blowouts are seen to be sensitive to local winds, allowing insight into forces overshadowed by regional trends. While blowouts can look very different, their development and subsequent re- colonisation is very similar. These geomorphic features indeed do play a large role in dune dynamics, but are predictable in their development. If more refined timescales of evolution can be established, blowouts can provide an invaluable insight into coastal systems and the more subtle effects of climate change.

* * * * *

10 The lack of field data on blowout morphology is a critique made by Gares in Nordstrom (1995) of many studies of blowouts.

44 7. Appendix 1: Vegetation codes

Code Scientific Name Common Name BARE BARE SAND BC Ranunculus BUTTERCUP BFT Lotus corniculatus BIRD'S-FOOT-TREFOIL, COMMON BHP Plantago coronopus BUCK'S-HORN PLANTAIN BK Pteridium aquilinum BRACKEN BR Rosa pimponellifolia BURNET ROSE CEN Centaurium erythraea CENTAURY, COMMON CF Tussilago farfara COLT'S FOOT CME Cerastium holosteoides MOUSE-EAR, COMMON CMW Polygala vulgaris MILKWORT, COMMON CV Trifolium CLOVER DD Taraxacum officinale DANDELION DY Bellis perennis DAISY GS Veronica chamaedrys GERMANDER SPEEDWELL JUN Juniperus communis JUNIPER KV Anthyllis vulneraria KIDNEY VETCH LBS Galium verum LADY'S BEDSTRAW MEH Pilosella officinarum MOUSE-EAR HAWKWEED MM Ammophila arenaria MARRAM GRASS MOSS MOSS RF Festuca rubra FESCUE, RED RH Leontodon hispidus ROUGH-HAWKBIT RWP Plantago lanceolata RIBWORT PLANTAIN SC Silene maritima SEA CAMPION SH Prunella vulgarid SELFHEAL SS Rumex acetosella SHEEP'S SORREL WA Angelica sylvestris WILD ANGELICA WS Carex sylvatica WOOD SEDGE WT Thymus drucei WILD THYME YF Holcus lanatus YORKSHIRE FOG

45 8. Auto-Critique

Originally I was attracted to this project for the fieldwork. After discussing potential projects with various supervisors, the diversity and breadth of this project appealed to me the most. In my future career, I wish to explore spatial relationships in ecological settings, so the ecogeomorpholgical aspect, and associated skills I would gain from studying it, lead for me to choose this topic. I have discovered a new passion in geomorphology and hope to pursue it further. The strengths of this dissertation primarily lay in the fieldwork. There is a dearth of detailed morphological data in studies of blowouts. Studies that do employ fieldwork typically only compare two samples. This project examines 30 samples over a large temporal and spatial scale. This allowed for compelling conclusions to be drawn about the dune system as a whole, and provided some insight into gaps in the literature. Furthermore, novel methods were developed over the course of fieldwork. There is no precedent found by the author for the mapping of blowout features, so a method was developed. Similarly, blowout activity was not specifically qualified in previous studies, something that is offered in this dissertation. The scope of the project is considered to be another strength. By examining wind, morphology and vegetation I was able to synthesise previous studies into a broader, cohesive picture of blowout development. However, this breadth is also a weakness as analysis was limited by the diversity in topics. While I attempted to give each part enough attention, there was not enough time to really go deeply into each area. Additionally, I must admit that my lack of background in the subject area is a weakness to the project, as there are likely some ecological or geomorphological facets to the study I missed or glossed over unwittingly. A more in-depth background knowledge outside of the literature review would have aided this. Regarding work already done, if I were to do it differently, I would have completed the historical analysis prior to fieldwork and deliberately selected blowouts to sample to ensure I was collected a balanced representation of the study site. As it turned out, I did this inadvertently, but next time I wouldn’t leave it up to chance. This historical analysis would ideally be in more detail, such as tracking vegetation coverage within each blowout. I did add this at the end for two samples, but it would be nice to establish that data for all the samples. Another method for mapping of blowouts was to create a contour by circling the blowout until reaching the deflation basin; for time’s sake this was not employed, but I believe that would have captured even more detail of blowout morphology.

46 Given more time, other areas I would have liked to examine are sediment variables, such as age, size statistics, pH, etc. for each blowout. I would collect local wind data and measure internal blowout winds. Finally, if not at this site, a different site with more frequent aerial imagery, and over a longer timespan, would provide a better opportunity for historical analyses.

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50