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Migration of parabolic dunes at Aberffraw, Anglesey, north Wales
Geomorphology 59 (2004) 165 – 174 www.elsevier.com/locate/geomorph
Migration of parabolic dunes at Aberffraw, Anglesey, north Wales S.D. Bailey, C.S. Bristow * School of Earth Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK Accepted 16 July 2003
Abstract Aberffraw is a 1-km-wide and 3-km-long transgressive dunefield that extends inland along a northeast – southwest-trending valley from a southwest-facing beach, Traeth Mawr. The prevailing wind is from the southwest, and both the parabolic dunes and the valley within which they lie are sub-parallel to the prevailing wind. The dunefield at Aberffraw includes two foredune ridges and three rows of active compound parabolic dunes. At the landward end is a lake, Llyn Coron, which has been formed by dunes migrating up the valley and damming the river, Afon Ffraw. Between the parabolic dunes are gently sloping interdune areas with a close cropped vegetation. The parabolic dunes at Aberffraw have been migrating inland across the interdune areas. Rates of parabolic dune migration are derived from three sets of aerial photographs taken in 1940, 1982 and 1993. The aerial photographs have been scanned and manipulated in ArcView GIS software. Registration of the aerial photograph to an Ordnance Survey (OS) map was performed using ground control points (GCPs), common fixed features that are identifiable on both the aerial photographs and the baseline map. Attempts to correct for the inherent distortions of aerial photography were made during registration. Standardising the projection of the photographs to a common baseline allows meaningful spatial analysis, and the dune ridges, trailing edges and areas of bare sand were mapped from each photograph as a series of overlays. Rates of dune migration are calculated from the spatial distance between linear trend lines, parallel to the dune crests and perpendicular to the dune migration orientation, applied to sections of dune ridges for 1940 and 1993. Trend lines were only fitted to sections where continuity of dune form was maintained over the given period. The method provides an improved representation of the actual migration rate as it incorporates the whole of the parabolic dune form, and the whole of the compound dune ridge form into the calculation. It effectively measures the centre point or line of a dune or dune ridge as opposed to the variable positions and orientations of the dune crest noses, which represent maximum migration, rather than the mean. Rates of parabolic dune migration range from a minimum of 0 m year 1 to a maximum of 3.6 m year 1, with an average migration rate of 1 m year 1. D 2003 Published by Elsevier B.V. Keywords: Aerial photographs; Coastal dunes; Migration rates; Parabolic dunes; Wales
1. Introduction Records of dune migration often use aerial photographs to record successive changes in dune location * Corresponding author. Fax: +44-20-7383-0008. E-mail address: [email protected] (C.S. Bristow). 0169-555X/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/j.geomorph.2003.09.013
(Pye, 1982; Vance and Wolfe, 1996). Most studies of dune migration have involved barchan dunes where the dune form is relatively conservative and the morphology is relatively simple (e.g., Stokes et al., 1999). Simple dunes consist of individual dune forms that are spatially separate from their neighbours (McKee, 1979).
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For parabolic dunes, migration of the apex of the dune, its nose, which is readily identifiable, can be used to measure rates of migration. However, it is possible for the morphology to change to such an extent that it is not possible to identify the same morphological feature between two sets of aerial photographs. Blowouts along a compound parabolic dune ridge can lead to an inversion where a blowout in a trailing dune limb can migrate faster than the nose of the dune. Furthermore, dune morphology is frequently either complex (dunes that consist of two or more different types of simple dunes that have coalesced or superimposed) or compound (dunes consisting of two or more of the same type that have coalesced or are superimposed) (McKee, 1979). The compound and complex nature of many dunes raises additional problems for the quantification of dune migration. The aims of this paper are to measure the migration rates and orientation of the complex compound parabolic dune ridges at Aberffraw from georeferenced aerial photographs dated 1940, 1982 and 1993, in a method that encompasses the movement of the whole of the dune form rather than just the leading edge of the apex.
2. Aberffraw dunefield Aberffraw is a 1-km-wide and 3-km-long transgressive dunefield on the southwest coast of Anglesey, an island off the coast of Wales (Fig. 1). This site has been selected for study because of its geomorphology, which includes good examples of foredunes and migrating parabolic dunes (Fig. 2). In addition, the extent of the dunefield can be readily defined and the potential sand inputs and wind regime are well constrained. The seaward limits of the dunefield are defined by a beach, Traeth Mawr. The beach faces southwest, towards the Atlantic Ocean, and is almost perfectly swash aligned. Either side of the beach are low headlands composed of Precambrian schist, which extend inland as ridges on either side of the dunefield. The landward edge of the dunefield is defined by a freshwater lake, Llyn Coron, which was created by dune damming of a small river, the Afon Ffraw, which runs along the northeast side of the dunefield. The prevailing wind is from the southwest, and both the parabolic dunes and the valley within which they lie are subparallel to the prevailing wind. Thus, the dunefield is topographically constrained and sediment input is
Fig. 1. Locality map for Aberffraw, showing the position of the dunefield on the Isle of Anglesey.
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Fig. 2. 1940 Aerial photograph with interpretation, highlighting the crests of the dunes and dune ridges labelled A to F.
restricted to a bay head beach, which is swash aligned. The wind regime for Aberffraw is evaluated using data representative of the southwest coast of Anglesey (Anderson, 1994), recorded at the Met. Office meteorological station at RAF Valley, approximately 10 km northwest of Aberffraw. Mean
monthly wind speeds range from 11.2 to 15.6 knots (5.7 – 8 m s 1), with the strongest winds occurring during the winter months of October to March. The annual wind rose (Fig. 3A) constructed from the period 1961 –1990 shows a predominance of south – southwesterly winds, with maximum wind speeds in the 28 –33 knots category. Conversion of the wind
Fig. 3. (A) Annual wind rose representative of the southwest coast of Anglesey, constructed from the period 1961 – 1990 (Anderson, 1994). (B) Sand rose for Aberffraw, constructed from the annual wind rose using the method of Fryberger (1979).
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data to a sand rose (Fig. 3B) using the Fryberger (1979) method gives a resultant drift direction (RDD) of 40j northeast, and a resultant drift potential (RDP) of 1657 vector units or 116 m3 m-w 1 yr 1 (cf. Fryberger et al., 1984). These are an expression of the net trend of sand drift and the net sand transport potential of which the wind regime is capable. The modern dunefield at Aberffraw contains two foredune ridges (termed A and B) and three rows of active parabolic dunes (termed C, D and E). The remains of a fourth parabolic dune ridge, F, are visible by Llyn Coron on the 1940 aerial photograph (Fig. 2) but are no longer apparent on subsequent photographs. The parabolic dune ridges are compound in nature, composed of chains of parabolic dunes that range up to 12 m in height at the crest, with a depression of 0.5– 1 m depth marking the erosive scour between the trailing arms. Between the parabolic dunes are gently sloping interdune areas with a close cropped vegetation of grass and herbs. The interdune vegetation changes from dune slacks and semi-fixed dune grassland near the coast to fixed dune grassland with gorse and heather further inland. The dune crests, trailing limbs and leeside slopes are usually vegetated with marram grass, whilst the windward slopes are often bare sand. Sand deflated from the interlimb areas and the windward slopes of the dunes is blown inland over the dune crest and deposited on the lee slope of the dune and on a broad apron that extends downwind. The interdune areas are generally dry, but interlimb corridors have deflated down to the watertable, and some of these areas are occasionally subject to flooding during the winter.
one another. To provide a meaningful comparison between aerial photographic plates, it is necessary to standardise the projection of each plate to a common baseline. This includes scaling and orientating each in alignment with the set, and attempting to apply a degree of correction to the inherent distortions of aerial photography introduced through sensor, panoramic and perspective distortions (Jensen, 1996). The registration of the aerial photographs to the known projection of an Ordnance Survey (OS) map of Aberffraw allows spatial operations such as measuring distance and orientation relating to aeolian features, and the measurement of temporal changes, in this case dune movement, against a fixed projection. The georeferencing and distortion corrections were applied using the geographical information systems software ArcView v3.2a. The aerial photographs were scanned to convert them into digital format for entry into ArcView. A digital copy of the Ordnance Survey Landline 1:2500 map for Aberffraw was used as the known grid projection to which the aerial photographs were registered. Registration of the aerial photographs to the OS map was performed using ground control points (GCPs), fixed locality features common to the aerial photographs and the basemap (e.g., road intersections, corners, buildings). The ImageWeb extension for ArcView generates a pair of x and y coordinate polynomial equations that describe the deviation in two dimensions of the photograph from the map projection, based on the GCPs. The ImageWeb extension performs the transformation using a leastsquares fit for the coefficients of a given polynomial using the GCPs and applies it to each individual pixel of the image. The brightness of each pixel is adjusted using a nearest-neighbour analysis.
3. Methods 3.2. Mapping aeolian features 3.1. Geometric correction of aerial photographs Aerial photographs provide a good record of dune morphology, and historic aerial photographs, which in this case extend back to 1940, can provide a good record of dune migration. However, aerial photographs are often taken at different orientation and different magnification and they contain a number of errors that impair their direct correlation with
Once transformed, the aerial photographs and the OS basemap become directly comparable in spatial terms, allowing features from the photographs to be mapped onto the OS map as a series of overlays. Using ArcView, polylines (short lines joined by points) were used to trace the dune ridges, the trailing edges of the dune ridges and the extent of bare unvegetated sand for each photograph. The georefer-
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Fig. 4. Aerial photographic interpretations for 1982 and 1993.
enced aerial photograph for 1940 is presented alongside the interpretation in Fig. 2. The interpretations for the 1982 and 1993 aerial photographs are pre-
sented in Fig. 4. The foredune ridge A is not included in this analysis, and parabolic dune ridge B, situated directly behind the foredune ridge A, was omitted
Fig. 5. Changes in the extent of bare sand and vegetation cover from 1940, 1982 and 1993. There has been a reduction from 28% in 1940 to 5% and 6% in 1982 and 1993, respectively.
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from the migration analysis due to the difficulty in accurately identifying the dune crests along this ridge.
4. Measurements 4.1. Changes in vegetation cover The areas of bare sand were mapped in ArcView as a series of polygons in order to produce quantifiable measurements of change (Fig. 5). The aerial photographs show clear reduction in the area of bare, unvegetated sand between 1940 and 1982 – 1993, indicating a substantial increase in vegetation cover-
age. In 1940, there was a significant quantity of unvegetated bare sand, 920,000 m2, representing approximately 28% of the total dunefield area, 3,339,000 m2. By 1982, this has shrunk to 123,000 m2 or 4% of the dunefield area, rising slightly to 5% in 1993. Much of the bare sand in 1940 is associated with the foredune ridges A and B, the immediate interdune area behind them, and ridges C, D and E. By 1982– 1993, the lee slipfaces of the dune ridges were extensively vegetated with marram grass, with only parts of the stoss slipfaces of the trailing edges remaining bare sand in dune ridges C, D and E. Most of the interdune became vegetated, with the areas of bare sand remaining in 1983– 1993 resulting from northeast –
Fig. 6. Comparison of dune ridge positions in 1940 and 1993 derived from interpretation of georeferenced aerial photographs.
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southwest-trending blowouts, reflected in the area patterns, particularly between dune ridges B and C at the seaward end of the dunefield. The small remaining patches of bare sand associated with the interdune areas result from recreational use, in particular, the motorcycle tracks and footpaths (documented in Liddle and Grieg-Smith, 1975a,b) visible on the 1982 aerial photograph, and from the activity of rabbits. The effect of the latter are particularly noticeable from observations during fieldwork, with collapsed rabbit burrows a considerable source of bare sand, often initiating small blowouts and encouraging sand activity. 4.2. Dune migration rates The movement rates of dune ridges are commonly derived from distance measurements between their positions at two points in time. Traditionally, the measurement is made from crest to crest of the dune ridge, and in parabolic dunes, the measurement is made in the direction of travel along the centre axis through the nose of the dune, perpendicular to the trend of the dune ridge. Maximum and minimum rates of migration were measured in this fashion, from the apex of nose to nose, from the 1940 – 1993 comparison of dune ridge positions (Fig. 6), and are summarised in Table 1. Measurements were restricted to the dune sections used in the linear fit method (see Fig. 7) to remain comparable. The maximum rate of migration in the dunefield is represented by a parabolic dune in dune ridge C, which has moved a distance of 190 m Table 1 Minimum, maximum and mean migration rates for dunes in ridges C, D, and E Migration rates and orientations from nose-to-nose method Dune ridge
C D1 D2 E Mean
Dune migration rates
Dune nose migration direction
Minimum (m year 1)
Maximum (m year 1)
Mean (m year
0.7 0 0.9 1.3 –
3.6 2.0 1.6 1.3 –
1.8 0.9 1.2 1.3 1.3
Mean (j) 1
) 33 32 14 25 29
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between 1940 and 1993, migration rate of 3.6 m year 1. The lowest rate of migration, 0 m year 1, occurs at the western end of dune ridge D, where there has been no forward movement of the dune crest at the apex of the nose. The average distance travelled by parabolic dunes in the dunefield between 1940 and 1993 is 71 m, an average migration rate of 1.3 m year 1 (n = 17). Measuring the migration rates from crest to crest distances at the forward most point of each parabolic dune (the apex or nose) suffers a number of limitations. Foremost, it is not representative of the whole of the dune structure, be that an individual parabolic dune or a compound chain of parabolic dunes forming a dune ridge. The crest to crest measurement at the nose of the dune represents the maximum distance travelled for this part of the dune, it does not take into account the rest of the dune, particularly the trailing arms, which have often moved to a much lesser degree. This is apparent in the 1940 – 1993 comparison of aerial photographs (Fig. 6), where the distance migrated by a dune is considerably greater at the nose than at the flanks and the arms. This is also reflected in the compound dune ridges (e.g., C and D), where the dunes towards the centre of the ridges have moved a greater distance than at the ends of the ridges. To compensate for this effect, the overall mean migration rate for each dune ridge and the dunefield was calculated by fitting a best-fit linear trend line to the data points (Fig. 7). This trend line represents the mean centre point between the foremost point of the dune ridge at the nose and the rearmost trailing edge at the tips of the arms. In order to perform this calculation, the polylines mapping the dune ridges in ArcView were converted to a point data set using a conversion script (poly2point). This provided a series of points that describe the dune ridge positions in terms of x and y coordinates consistent with the map scale and projection. A spreadsheet was used to fit a linear trend line to each of the dune ridges. Trend lines were only applied to sections of dune ridge where continuity of form was maintained over the given period. Significant deviation in dune form, e.g., loss of individual parabolas through erosion, or the creation of new parabolas through blowout division, severely skews the trend lines, and so were omitted from the analysis. In the case of dune ridge
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Fig. 7. A plot of the dune ridge sections, after conversion to point coordinates, used in the migration rate calculations. Linear best-fit trend lines represent the mean centre position and orientation of the dune ridges. The linear trend lines are parallel to the dune crests and perpendicular to the transport direction. The foredune ridge A is moving seawards, whilst the parabolic dune ridges C, D and E are moving landwards.
D, this has necessitated splitting the ridge into two sections (D1 and D2) for the purpose of the analysis. The mean distance travelled was derived from the distance between the centre point of each trend line for a given dune ridge and is summarised in Table 2. Using the linear fit method, the minimum migration rate of a dune ridge in the dunefield is represented by dune ridge D2, which has moved 30 m between 1940 and 1993, a mean migration rate of 0.6 m year 1. The maximum rate of migration is represented by dune ridge C, which has moved 95 m over the same period, a migration rate of 1.8 m year 1. The average dune ridge migration for the whole of the
dunefield is 1 m year 1, equating to a distance of 53 m travelled between 1940 and 1993. This rate is 0.3 m year 1 slower than the mean calculated by measurement of migration rates at the dune noses, indicating that with the linear fit method the dune ridges would have travelled on average 15.9 m less between 1940 and 1993. 4.3. Migration orientation Using the crest to crest method of measurement at the nose of the dunes provides an array of orientations, with individual parabolic dune migration orientations ranging from 6jN to 59jNE, mean dune
S.D. Bailey, C.S. Bristow / Geomorphology 59 (2004) 165–174 Table 2 Dune ridge migration rates and direction, and change in orientation as determined from the linear fit method Migration rates and orientations from linear fit method Dune ridge
Dune ridge orientation 1940
1993
1940 – 1993 Change
C D1 D2 E Mean
42j 33j 14j 352j –
30j 25j 14j 8j –
8j 8j 0j + 16j –
Dune ridge migration rates (m year 1)
Dune ridge migration direction (m year 1)
1.8 0.8 0.6 0.9 1
36j 28j 14j 0j 23j
ridge orientations ranging from 14jNE to 33jNE and an overall mean orientation of 29jNE. For the linear fit method, the direction of migration was derived by measuring the orientation perpendicular to the trend line resulting in a mean orientation of 23jNE. Unlike the crest to crest method, the linear fit method also highlights the change in the orientation of dune ridges between 1940 and 1993 for dune ridges C and D1. This is most apparent in Fig. 7, where the direction of migration as evidenced from the divergence of the trend lines between 1940 and 1993 indicates rotation towards the west by up to 8j. This appears to be a product of increased dune migration rates towards the eastern end of these two dune ridge sections.
5. Discussion The mean migration rates for the dune ridges within the dunefield were calculated as 1.3 and 1 m year 1, as determined by the crest to crest method and the linear fit method, respectively. The latter is probably a closer representation of the actual migration rate as it incorporates the whole of the parabolic dune form, and the whole of the compound dune ridge form into the calculation. It effectively measures the centre point or line of a dune or dune ridge as opposed to the highly variable positions and orientations of the dune crest noses, which represent maximum migration, rather than the mean. Over the 53-year period of study, the 0.3 m year 1 difference between the migration rate of the whole dune ridge, calculated from the linear fit method, and that measured from the apex of the dunes, equates to 14.9 m of dune migration.
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These rates of migration are low in comparison with the neighbouring parabolic dunefield of Newborough Warren, where Ranwell (1958) determined migration rates of 1.5– 6.7 m year 1 over a 3-year period, based on staked measurements at various locations of the dune profiles. The low rates at Aberffraw may be a reflection of the long-term (53 years) analysis period and the increase in the vegetated state of the dunefield over this time. Over a similar period at Newborough Warren, Rhine et al. (2001) record 75% of the total dune area as mobile dunes in 1950, with military activity at the site contributing to destabilisation, compared to just 28% at Aberffraw in 1940. Although Newborough Warren was largely fixed by 1991, with only 6% classed as mobile by Rhine et al. (2001), it appears that it was considerably more active than Aberffraw in the 1940s, and this may be reflected in the differing migration rates of the parabolic dunes at the two sites. Other rates of parabolic dune migration range from 0.75 to 4.75 m year 1 for dunes in the Great Sand Hills region of Saskatchewan (Wolfe and Lemmen, 1999), who based their measurements on the slipface of the dunes over a 2-year period; and 2.5 – 4 m year 1 with an average of 2.2 m year 1 for parabolic dunes in the Seward sand hills of Saskatchewan (David et al., 1999), based on an optical dating chronology. Pye (1982) determined maximum average rates of 4.8 m year 1 from aerial photographs for coastal parabolic dunes at Cape Bedford, North Queensland, with other movement averaging less than 2 m year 1. The two mean directions of migration as calculated by the crest to crest method and the linear fit method are of a similar order of magnitude, being 29j and 23j, respectively. These orientations are, however, somewhat divergent from the calculated resultant drift direction of 40j, derived from wind data for the region. It is apparent from the trend lines in Fig. 7 that, whilst the foredune A, retains an orientation of 47j, close to that of the resultant drift direction, the other three dune ridges, particularly C and D1 exhibit rotation towards the west. This is also reflected in the change in orientation of dune ridges C and D1 between 1940 and 1993. A possible cause for this may be the Afon Ffraw that bounds the western margin of the dunefield, leading to a higher water table, with wetter conditions and the associated re-
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duction in sand mobility towards the west, and a drier margin with increased sand mobility towards the east. Dune ridge E, which consists of a large parabolic dune, has rotated east, but this may be the result of deflation on the western limb skewing the trend line.
6. Conclusions Comparison between different sets of aerial photographs has been achieved by georeferencing the photographs to a basemap of the location, and mapping the aeolian features using GIS software. In order to quantify the migration rate of compound parabolic dune ridges where the rates of migration vary along the dune ridge, we have applied a linear trend line to the dune crest. The linear fit allows an average rate of migration to be derived for a compound dune ridge. The same method could be applied to other complex and compound dune forms. Quantifying rates of dune migration is an important step towards determining actual mass sediment transport rates (e.g., Stokes et al., 1999; Bristow and Lancaster, this volume).
Acknowledgements The digital basemaps are Crown Copyright Ordnance Survey, an EDINA Digimap/JISC supplied service. Will Sandison of CCW is thanked for his support and cooperation at Aberffraw, and the Bodorgan Estate for permission to work at Aberffraw. Simon Bailey is in receipt of NERC studentship GT04/97/189/ES.
References Anderson, G., 1994. The climate of Anglesey with particular reference to Newborough Warren, Cors Erddreiniog and Cors Bodeilio. Unpublished report for The Countryside Council for Wales. The Meteorological Office, 1 – 27. Bristow, C.S., Lancaster, N., this volume. Movement of a small slipfaceless dome dune in Namibia. Geomorphology.
David, P.P., Wolfe, S.A., Huntley, D.J., Lemmen, D.S., 1999. Activity cycle of parabolic dunes based on morphology and chronology from Seward sand hills, Saskatchewan. In: Lemmen, D.S., Vance, R.E. (Eds.), Holocene Climate and Environmental Change in the Palliser Triangle: A Geoscientific Context for Evaluating the Impacts of Climate Change on the Southern Canadian Prairies. Geological Survey of Canada, Bulletin, vol. 534, pp. 223 – 238. Fryberger, S.G., 1979. Dune forms and wind regime. In: McKee, E.D. (Ed.), A Study of Global Sand Seas. U.S. Geological Survey Professional Paper, vol. 1052, pp. 37 – 169. Fryberger, S.G., Al-Sari, A.M., Clisham, T.J., Rizvi, S.A.E., AlHina, K.G., 1984. Wind sedimentation in the Jafurah sand sea, Saudi Arabia. Sedimentology 21, 413 – 431. Jensen, J.R., 1996. Introductory Image Processing: A Remote Sensing Perspective. Prentice Hall, New Jersey, pp. 127 – 137. Liddle, M.J., Grieg-Smith, P., 1975a. A survey of tracks and paths in a sand dune ecosystem: 1. Soils. Journal of Applied Ecology 12, 893 – 908. Liddle, M.J., Grieg-Smith, P., 1975b. A survey of tracks and paths in a sand dune ecosystem: 2. Vegetation. Journal of Applied Ecology 12, 909 – 930. McKee, E.D., 1979. A study of global sand seas. U.S. Geological Survey Professional paper, 1052. Pye, K., 1982. Morphological development of coastal dunes in a humid tropical environment. Cape Bedford and Cape Flattery, North Queensland. Geogr. Ann. 64 A (3 – 4), 213 – 227. Ranwell, D., 1958. Movement of vegetated sand dunes at Newborough Warren, Anglesey. Journal of Ecology 46, 83 – 100. Rhine, P.M., Blackstock, T.H., Hardy, H.S., Jones, R.E., Sandison, W., 2001. The evolution of Newborough Warren dune system with particular reference to the past four decades. In: Houston, J.A., Edmondson, J.A., Rooney, P.J. (Eds.), Coastal Dune Management: Shared Experience of European Conservation Practice. Liverpool University Press, Liverpool, pp. 345 – 380. Stokes, S., Goudie, A.S., Ballard, J., Gifford, C., Samieh, S., Embabi, N., El-Rashidi, O.A., 1999. Accurate dune displacement and morphometric data using kinematic GPS. Zeitschrift fu¨r Geomorphologie. Supplementband 116, 195 – 214. Vance, R.E., Wolfe, S.A., 1996. Geological indicators of water resources in semi-arid environments: southwestern interior of Canada. In: Berger, A.R., William, J.I. (Eds.), Geoindicators: Assessing Rapid Environmental Changes in Earth Systems. Balkema/Rotterdam/Brookfield, pp. 251-263. Wolfe, S.A., Lemmen, D.S., 1999. Monitoring of dune activity in the Great Sand Hills region, Saskatchewan. In: Lemmen, D.S., Vance, R.E. (Eds.), Holocene Climate and Environmental Change in the Palliser Triangle: A Geoscientific Context for Evaluating the Impacts of Climate Change on the Southern Canadian Prairies. Geological Survey of Canada, Bulletin, vol. 534, pp. 199 – 219.
Migration of parabolic dunes at Aberffraw, Anglesey, north Wales S.D. Bailey, C.S. Bristow * School of Earth Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK Accepted 16 July 2003
Abstract Aberffraw is a 1-km-wide and 3-km-long transgressive dunefield that extends inland along a northeast – southwest-trending valley from a southwest-facing beach, Traeth Mawr. The prevailing wind is from the southwest, and both the parabolic dunes and the valley within which they lie are sub-parallel to the prevailing wind. The dunefield at Aberffraw includes two foredune ridges and three rows of active compound parabolic dunes. At the landward end is a lake, Llyn Coron, which has been formed by dunes migrating up the valley and damming the river, Afon Ffraw. Between the parabolic dunes are gently sloping interdune areas with a close cropped vegetation. The parabolic dunes at Aberffraw have been migrating inland across the interdune areas. Rates of parabolic dune migration are derived from three sets of aerial photographs taken in 1940, 1982 and 1993. The aerial photographs have been scanned and manipulated in ArcView GIS software. Registration of the aerial photograph to an Ordnance Survey (OS) map was performed using ground control points (GCPs), common fixed features that are identifiable on both the aerial photographs and the baseline map. Attempts to correct for the inherent distortions of aerial photography were made during registration. Standardising the projection of the photographs to a common baseline allows meaningful spatial analysis, and the dune ridges, trailing edges and areas of bare sand were mapped from each photograph as a series of overlays. Rates of dune migration are calculated from the spatial distance between linear trend lines, parallel to the dune crests and perpendicular to the dune migration orientation, applied to sections of dune ridges for 1940 and 1993. Trend lines were only fitted to sections where continuity of dune form was maintained over the given period. The method provides an improved representation of the actual migration rate as it incorporates the whole of the parabolic dune form, and the whole of the compound dune ridge form into the calculation. It effectively measures the centre point or line of a dune or dune ridge as opposed to the variable positions and orientations of the dune crest noses, which represent maximum migration, rather than the mean. Rates of parabolic dune migration range from a minimum of 0 m year 1 to a maximum of 3.6 m year 1, with an average migration rate of 1 m year 1. D 2003 Published by Elsevier B.V. Keywords: Aerial photographs; Coastal dunes; Migration rates; Parabolic dunes; Wales
1. Introduction Records of dune migration often use aerial photographs to record successive changes in dune location * Corresponding author. Fax: +44-20-7383-0008. E-mail address: [email protected] (C.S. Bristow). 0169-555X/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/j.geomorph.2003.09.013
(Pye, 1982; Vance and Wolfe, 1996). Most studies of dune migration have involved barchan dunes where the dune form is relatively conservative and the morphology is relatively simple (e.g., Stokes et al., 1999). Simple dunes consist of individual dune forms that are spatially separate from their neighbours (McKee, 1979).
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For parabolic dunes, migration of the apex of the dune, its nose, which is readily identifiable, can be used to measure rates of migration. However, it is possible for the morphology to change to such an extent that it is not possible to identify the same morphological feature between two sets of aerial photographs. Blowouts along a compound parabolic dune ridge can lead to an inversion where a blowout in a trailing dune limb can migrate faster than the nose of the dune. Furthermore, dune morphology is frequently either complex (dunes that consist of two or more different types of simple dunes that have coalesced or superimposed) or compound (dunes consisting of two or more of the same type that have coalesced or are superimposed) (McKee, 1979). The compound and complex nature of many dunes raises additional problems for the quantification of dune migration. The aims of this paper are to measure the migration rates and orientation of the complex compound parabolic dune ridges at Aberffraw from georeferenced aerial photographs dated 1940, 1982 and 1993, in a method that encompasses the movement of the whole of the dune form rather than just the leading edge of the apex.
2. Aberffraw dunefield Aberffraw is a 1-km-wide and 3-km-long transgressive dunefield on the southwest coast of Anglesey, an island off the coast of Wales (Fig. 1). This site has been selected for study because of its geomorphology, which includes good examples of foredunes and migrating parabolic dunes (Fig. 2). In addition, the extent of the dunefield can be readily defined and the potential sand inputs and wind regime are well constrained. The seaward limits of the dunefield are defined by a beach, Traeth Mawr. The beach faces southwest, towards the Atlantic Ocean, and is almost perfectly swash aligned. Either side of the beach are low headlands composed of Precambrian schist, which extend inland as ridges on either side of the dunefield. The landward edge of the dunefield is defined by a freshwater lake, Llyn Coron, which was created by dune damming of a small river, the Afon Ffraw, which runs along the northeast side of the dunefield. The prevailing wind is from the southwest, and both the parabolic dunes and the valley within which they lie are subparallel to the prevailing wind. Thus, the dunefield is topographically constrained and sediment input is
Fig. 1. Locality map for Aberffraw, showing the position of the dunefield on the Isle of Anglesey.
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Fig. 2. 1940 Aerial photograph with interpretation, highlighting the crests of the dunes and dune ridges labelled A to F.
restricted to a bay head beach, which is swash aligned. The wind regime for Aberffraw is evaluated using data representative of the southwest coast of Anglesey (Anderson, 1994), recorded at the Met. Office meteorological station at RAF Valley, approximately 10 km northwest of Aberffraw. Mean
monthly wind speeds range from 11.2 to 15.6 knots (5.7 – 8 m s 1), with the strongest winds occurring during the winter months of October to March. The annual wind rose (Fig. 3A) constructed from the period 1961 –1990 shows a predominance of south – southwesterly winds, with maximum wind speeds in the 28 –33 knots category. Conversion of the wind
Fig. 3. (A) Annual wind rose representative of the southwest coast of Anglesey, constructed from the period 1961 – 1990 (Anderson, 1994). (B) Sand rose for Aberffraw, constructed from the annual wind rose using the method of Fryberger (1979).
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data to a sand rose (Fig. 3B) using the Fryberger (1979) method gives a resultant drift direction (RDD) of 40j northeast, and a resultant drift potential (RDP) of 1657 vector units or 116 m3 m-w 1 yr 1 (cf. Fryberger et al., 1984). These are an expression of the net trend of sand drift and the net sand transport potential of which the wind regime is capable. The modern dunefield at Aberffraw contains two foredune ridges (termed A and B) and three rows of active parabolic dunes (termed C, D and E). The remains of a fourth parabolic dune ridge, F, are visible by Llyn Coron on the 1940 aerial photograph (Fig. 2) but are no longer apparent on subsequent photographs. The parabolic dune ridges are compound in nature, composed of chains of parabolic dunes that range up to 12 m in height at the crest, with a depression of 0.5– 1 m depth marking the erosive scour between the trailing arms. Between the parabolic dunes are gently sloping interdune areas with a close cropped vegetation of grass and herbs. The interdune vegetation changes from dune slacks and semi-fixed dune grassland near the coast to fixed dune grassland with gorse and heather further inland. The dune crests, trailing limbs and leeside slopes are usually vegetated with marram grass, whilst the windward slopes are often bare sand. Sand deflated from the interlimb areas and the windward slopes of the dunes is blown inland over the dune crest and deposited on the lee slope of the dune and on a broad apron that extends downwind. The interdune areas are generally dry, but interlimb corridors have deflated down to the watertable, and some of these areas are occasionally subject to flooding during the winter.
one another. To provide a meaningful comparison between aerial photographic plates, it is necessary to standardise the projection of each plate to a common baseline. This includes scaling and orientating each in alignment with the set, and attempting to apply a degree of correction to the inherent distortions of aerial photography introduced through sensor, panoramic and perspective distortions (Jensen, 1996). The registration of the aerial photographs to the known projection of an Ordnance Survey (OS) map of Aberffraw allows spatial operations such as measuring distance and orientation relating to aeolian features, and the measurement of temporal changes, in this case dune movement, against a fixed projection. The georeferencing and distortion corrections were applied using the geographical information systems software ArcView v3.2a. The aerial photographs were scanned to convert them into digital format for entry into ArcView. A digital copy of the Ordnance Survey Landline 1:2500 map for Aberffraw was used as the known grid projection to which the aerial photographs were registered. Registration of the aerial photographs to the OS map was performed using ground control points (GCPs), fixed locality features common to the aerial photographs and the basemap (e.g., road intersections, corners, buildings). The ImageWeb extension for ArcView generates a pair of x and y coordinate polynomial equations that describe the deviation in two dimensions of the photograph from the map projection, based on the GCPs. The ImageWeb extension performs the transformation using a leastsquares fit for the coefficients of a given polynomial using the GCPs and applies it to each individual pixel of the image. The brightness of each pixel is adjusted using a nearest-neighbour analysis.
3. Methods 3.2. Mapping aeolian features 3.1. Geometric correction of aerial photographs Aerial photographs provide a good record of dune morphology, and historic aerial photographs, which in this case extend back to 1940, can provide a good record of dune migration. However, aerial photographs are often taken at different orientation and different magnification and they contain a number of errors that impair their direct correlation with
Once transformed, the aerial photographs and the OS basemap become directly comparable in spatial terms, allowing features from the photographs to be mapped onto the OS map as a series of overlays. Using ArcView, polylines (short lines joined by points) were used to trace the dune ridges, the trailing edges of the dune ridges and the extent of bare unvegetated sand for each photograph. The georefer-
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Fig. 4. Aerial photographic interpretations for 1982 and 1993.
enced aerial photograph for 1940 is presented alongside the interpretation in Fig. 2. The interpretations for the 1982 and 1993 aerial photographs are pre-
sented in Fig. 4. The foredune ridge A is not included in this analysis, and parabolic dune ridge B, situated directly behind the foredune ridge A, was omitted
Fig. 5. Changes in the extent of bare sand and vegetation cover from 1940, 1982 and 1993. There has been a reduction from 28% in 1940 to 5% and 6% in 1982 and 1993, respectively.
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from the migration analysis due to the difficulty in accurately identifying the dune crests along this ridge.
4. Measurements 4.1. Changes in vegetation cover The areas of bare sand were mapped in ArcView as a series of polygons in order to produce quantifiable measurements of change (Fig. 5). The aerial photographs show clear reduction in the area of bare, unvegetated sand between 1940 and 1982 – 1993, indicating a substantial increase in vegetation cover-
age. In 1940, there was a significant quantity of unvegetated bare sand, 920,000 m2, representing approximately 28% of the total dunefield area, 3,339,000 m2. By 1982, this has shrunk to 123,000 m2 or 4% of the dunefield area, rising slightly to 5% in 1993. Much of the bare sand in 1940 is associated with the foredune ridges A and B, the immediate interdune area behind them, and ridges C, D and E. By 1982– 1993, the lee slipfaces of the dune ridges were extensively vegetated with marram grass, with only parts of the stoss slipfaces of the trailing edges remaining bare sand in dune ridges C, D and E. Most of the interdune became vegetated, with the areas of bare sand remaining in 1983– 1993 resulting from northeast –
Fig. 6. Comparison of dune ridge positions in 1940 and 1993 derived from interpretation of georeferenced aerial photographs.
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southwest-trending blowouts, reflected in the area patterns, particularly between dune ridges B and C at the seaward end of the dunefield. The small remaining patches of bare sand associated with the interdune areas result from recreational use, in particular, the motorcycle tracks and footpaths (documented in Liddle and Grieg-Smith, 1975a,b) visible on the 1982 aerial photograph, and from the activity of rabbits. The effect of the latter are particularly noticeable from observations during fieldwork, with collapsed rabbit burrows a considerable source of bare sand, often initiating small blowouts and encouraging sand activity. 4.2. Dune migration rates The movement rates of dune ridges are commonly derived from distance measurements between their positions at two points in time. Traditionally, the measurement is made from crest to crest of the dune ridge, and in parabolic dunes, the measurement is made in the direction of travel along the centre axis through the nose of the dune, perpendicular to the trend of the dune ridge. Maximum and minimum rates of migration were measured in this fashion, from the apex of nose to nose, from the 1940 – 1993 comparison of dune ridge positions (Fig. 6), and are summarised in Table 1. Measurements were restricted to the dune sections used in the linear fit method (see Fig. 7) to remain comparable. The maximum rate of migration in the dunefield is represented by a parabolic dune in dune ridge C, which has moved a distance of 190 m Table 1 Minimum, maximum and mean migration rates for dunes in ridges C, D, and E Migration rates and orientations from nose-to-nose method Dune ridge
C D1 D2 E Mean
Dune migration rates
Dune nose migration direction
Minimum (m year 1)
Maximum (m year 1)
Mean (m year
0.7 0 0.9 1.3 –
3.6 2.0 1.6 1.3 –
1.8 0.9 1.2 1.3 1.3
Mean (j) 1
) 33 32 14 25 29
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between 1940 and 1993, migration rate of 3.6 m year 1. The lowest rate of migration, 0 m year 1, occurs at the western end of dune ridge D, where there has been no forward movement of the dune crest at the apex of the nose. The average distance travelled by parabolic dunes in the dunefield between 1940 and 1993 is 71 m, an average migration rate of 1.3 m year 1 (n = 17). Measuring the migration rates from crest to crest distances at the forward most point of each parabolic dune (the apex or nose) suffers a number of limitations. Foremost, it is not representative of the whole of the dune structure, be that an individual parabolic dune or a compound chain of parabolic dunes forming a dune ridge. The crest to crest measurement at the nose of the dune represents the maximum distance travelled for this part of the dune, it does not take into account the rest of the dune, particularly the trailing arms, which have often moved to a much lesser degree. This is apparent in the 1940 – 1993 comparison of aerial photographs (Fig. 6), where the distance migrated by a dune is considerably greater at the nose than at the flanks and the arms. This is also reflected in the compound dune ridges (e.g., C and D), where the dunes towards the centre of the ridges have moved a greater distance than at the ends of the ridges. To compensate for this effect, the overall mean migration rate for each dune ridge and the dunefield was calculated by fitting a best-fit linear trend line to the data points (Fig. 7). This trend line represents the mean centre point between the foremost point of the dune ridge at the nose and the rearmost trailing edge at the tips of the arms. In order to perform this calculation, the polylines mapping the dune ridges in ArcView were converted to a point data set using a conversion script (poly2point). This provided a series of points that describe the dune ridge positions in terms of x and y coordinates consistent with the map scale and projection. A spreadsheet was used to fit a linear trend line to each of the dune ridges. Trend lines were only applied to sections of dune ridge where continuity of form was maintained over the given period. Significant deviation in dune form, e.g., loss of individual parabolas through erosion, or the creation of new parabolas through blowout division, severely skews the trend lines, and so were omitted from the analysis. In the case of dune ridge
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Fig. 7. A plot of the dune ridge sections, after conversion to point coordinates, used in the migration rate calculations. Linear best-fit trend lines represent the mean centre position and orientation of the dune ridges. The linear trend lines are parallel to the dune crests and perpendicular to the transport direction. The foredune ridge A is moving seawards, whilst the parabolic dune ridges C, D and E are moving landwards.
D, this has necessitated splitting the ridge into two sections (D1 and D2) for the purpose of the analysis. The mean distance travelled was derived from the distance between the centre point of each trend line for a given dune ridge and is summarised in Table 2. Using the linear fit method, the minimum migration rate of a dune ridge in the dunefield is represented by dune ridge D2, which has moved 30 m between 1940 and 1993, a mean migration rate of 0.6 m year 1. The maximum rate of migration is represented by dune ridge C, which has moved 95 m over the same period, a migration rate of 1.8 m year 1. The average dune ridge migration for the whole of the
dunefield is 1 m year 1, equating to a distance of 53 m travelled between 1940 and 1993. This rate is 0.3 m year 1 slower than the mean calculated by measurement of migration rates at the dune noses, indicating that with the linear fit method the dune ridges would have travelled on average 15.9 m less between 1940 and 1993. 4.3. Migration orientation Using the crest to crest method of measurement at the nose of the dunes provides an array of orientations, with individual parabolic dune migration orientations ranging from 6jN to 59jNE, mean dune
S.D. Bailey, C.S. Bristow / Geomorphology 59 (2004) 165–174 Table 2 Dune ridge migration rates and direction, and change in orientation as determined from the linear fit method Migration rates and orientations from linear fit method Dune ridge
Dune ridge orientation 1940
1993
1940 – 1993 Change
C D1 D2 E Mean
42j 33j 14j 352j –
30j 25j 14j 8j –
8j 8j 0j + 16j –
Dune ridge migration rates (m year 1)
Dune ridge migration direction (m year 1)
1.8 0.8 0.6 0.9 1
36j 28j 14j 0j 23j
ridge orientations ranging from 14jNE to 33jNE and an overall mean orientation of 29jNE. For the linear fit method, the direction of migration was derived by measuring the orientation perpendicular to the trend line resulting in a mean orientation of 23jNE. Unlike the crest to crest method, the linear fit method also highlights the change in the orientation of dune ridges between 1940 and 1993 for dune ridges C and D1. This is most apparent in Fig. 7, where the direction of migration as evidenced from the divergence of the trend lines between 1940 and 1993 indicates rotation towards the west by up to 8j. This appears to be a product of increased dune migration rates towards the eastern end of these two dune ridge sections.
5. Discussion The mean migration rates for the dune ridges within the dunefield were calculated as 1.3 and 1 m year 1, as determined by the crest to crest method and the linear fit method, respectively. The latter is probably a closer representation of the actual migration rate as it incorporates the whole of the parabolic dune form, and the whole of the compound dune ridge form into the calculation. It effectively measures the centre point or line of a dune or dune ridge as opposed to the highly variable positions and orientations of the dune crest noses, which represent maximum migration, rather than the mean. Over the 53-year period of study, the 0.3 m year 1 difference between the migration rate of the whole dune ridge, calculated from the linear fit method, and that measured from the apex of the dunes, equates to 14.9 m of dune migration.
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These rates of migration are low in comparison with the neighbouring parabolic dunefield of Newborough Warren, where Ranwell (1958) determined migration rates of 1.5– 6.7 m year 1 over a 3-year period, based on staked measurements at various locations of the dune profiles. The low rates at Aberffraw may be a reflection of the long-term (53 years) analysis period and the increase in the vegetated state of the dunefield over this time. Over a similar period at Newborough Warren, Rhine et al. (2001) record 75% of the total dune area as mobile dunes in 1950, with military activity at the site contributing to destabilisation, compared to just 28% at Aberffraw in 1940. Although Newborough Warren was largely fixed by 1991, with only 6% classed as mobile by Rhine et al. (2001), it appears that it was considerably more active than Aberffraw in the 1940s, and this may be reflected in the differing migration rates of the parabolic dunes at the two sites. Other rates of parabolic dune migration range from 0.75 to 4.75 m year 1 for dunes in the Great Sand Hills region of Saskatchewan (Wolfe and Lemmen, 1999), who based their measurements on the slipface of the dunes over a 2-year period; and 2.5 – 4 m year 1 with an average of 2.2 m year 1 for parabolic dunes in the Seward sand hills of Saskatchewan (David et al., 1999), based on an optical dating chronology. Pye (1982) determined maximum average rates of 4.8 m year 1 from aerial photographs for coastal parabolic dunes at Cape Bedford, North Queensland, with other movement averaging less than 2 m year 1. The two mean directions of migration as calculated by the crest to crest method and the linear fit method are of a similar order of magnitude, being 29j and 23j, respectively. These orientations are, however, somewhat divergent from the calculated resultant drift direction of 40j, derived from wind data for the region. It is apparent from the trend lines in Fig. 7 that, whilst the foredune A, retains an orientation of 47j, close to that of the resultant drift direction, the other three dune ridges, particularly C and D1 exhibit rotation towards the west. This is also reflected in the change in orientation of dune ridges C and D1 between 1940 and 1993. A possible cause for this may be the Afon Ffraw that bounds the western margin of the dunefield, leading to a higher water table, with wetter conditions and the associated re-
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duction in sand mobility towards the west, and a drier margin with increased sand mobility towards the east. Dune ridge E, which consists of a large parabolic dune, has rotated east, but this may be the result of deflation on the western limb skewing the trend line.
6. Conclusions Comparison between different sets of aerial photographs has been achieved by georeferencing the photographs to a basemap of the location, and mapping the aeolian features using GIS software. In order to quantify the migration rate of compound parabolic dune ridges where the rates of migration vary along the dune ridge, we have applied a linear trend line to the dune crest. The linear fit allows an average rate of migration to be derived for a compound dune ridge. The same method could be applied to other complex and compound dune forms. Quantifying rates of dune migration is an important step towards determining actual mass sediment transport rates (e.g., Stokes et al., 1999; Bristow and Lancaster, this volume).
Acknowledgements The digital basemaps are Crown Copyright Ordnance Survey, an EDINA Digimap/JISC supplied service. Will Sandison of CCW is thanked for his support and cooperation at Aberffraw, and the Bodorgan Estate for permission to work at Aberffraw. Simon Bailey is in receipt of NERC studentship GT04/97/189/ES.
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