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Wave spray-induced sand transport and deposition during a coastal storm, Magilligan Point, Northern Ireland
Marine Geology 161 Ž1999. 377–383 www.elsevier.nlrlocatermargeo
Short communication
Wave spray-induced sand transport and deposition during a coastal storm, Magilligan Point, Northern Ireland J.A.G. Cooper ) , D.W.T. Jackson School of EnÕironmental Studies, UniÕersity of Ulster, Cromore Road, Coleraine, Londonderry, Northern Ireland, BT52 1SA, UK Received 7 December 1998; accepted 1 April 1999
Abstract Observations during a coastal storm at Magilligan Point, Northern Ireland reveal characteristic surficial deposits onshore produced by landward transport of sand within spray generated by strong winds and breaking waves. Conditions necessary for formation of such deposits include an adequate sediment supply, strong onshore waves and winds, onshore waves of short period and a low coastal scarp. The process, which may be locally important, is unlikely to be of great significance in contemporary sediment budget considerations and indicative figures suggest net landward transport rates of about 0.5 m3rmetre of shorelinerhour during optimal conditions. Its importance as a source of cliff-top sediment over longer time periods may be greater. q 1999 Elsevier Science B.V. All rights reserved. Keywords: sediments; wave spray; deposition
1. Introduction Transport of sediments on sandy coasts during storms has traditionally been difficult to assess and quantify due to the intensity of processes operative and the infrequency of storm occurrence. While the nature of coastal response to storm-related sediment dynamics varies spatially, the main modes of sediment transport include landward transport by barrier overwash, aeolian action, enhanced tidal inlet efficiency and a net landward transport vector within low gradient waves. Seaward transport has been documented through storm-return surge and nearshore profile adjustments ŽVellinga, 1982., while alongshore transport during storms often exceeds nonstorm rates by orders of magnitude. Quantifica)
Corresponding author. E-mail: [email protected]
tion of sediment movement under these transport modes enables sediment budgets to be computed at a variety of temporal and spatial scales. Recently, Morton et al. Ž1995. have drawn attention to the fact that high magnitude events may exceed morphodynamic thresholds and be the dominant process affecting mesoscale sediment budgets. This paper deals with sediment transport by wave spray associated with coastal storm conditions. Given the difficulties in measurement of transport processes under storm conditions, it is not surprising that this process appears to have been overlooked in the literature, although transport of foam and the production of related foam impressions on beach sediments during strong onshore winds has been noted ŽAllen, 1967.. This paper documents the occurrence of the wave-spray mode of transport, quantifies the volume of sediment transported and provides insights into
0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 9 . 0 0 0 4 4 - 4
378
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
the spatial controls on its occurrence. The observations were made at the apex of a cuspate foreland ŽMagilligan Point. in Northern Ireland ŽCarter and Wilson, 1990. under storm conditions which produced locally generated waves on a semi-enclosed coastal embayment. The pattern of wave energy dispersal and the topography of the foreland apex produced distinctive distributions of wave-spray intensity and associated sediment transport. Carter Ž1980. previously described longshore variations in nearshore wave patterns in this location and Carter and Stone Ž1989. documented processes of coastal dune erosion during storms in the same location. The objectives in this paper are: Ži. to document the nature of spray-related deposits during a coastal storm, Žii. to examine the spatial and temporal controls on spray-related deposition, and Žiii. to assess the role of this process in sediment budgets.
water levels in the lough reached a peak at 10:00 am at a water level that coincided with the base of an eroding dune scarp which has been retreating progressively since at least 1995. The incidence of spray-induced transport during this storm was first noted by the presence of a thin film of sand which accumulated on the seaward face of a vehicle parked approximately 5 m from the high water mark during high tide. Closer examination of the adjacent dune and road surface during the storm at high tide, revealed this to be a widespread phenomenon around the study area, but with distinctive spatial variability. In general, the deposit took the form of scattered patches of sand which adhered to stems of the Ammophila arenaria vegetation, forming a distinctive pattern. These patches were irregular in outline and up to 1 m in diameter, but were
2. Methods During a storm on November 9th 1998, Magilligan Point was subject to wave attack from locally generated winds blowing from the SE quadrant across the maximum Ž22 km. fetch of Lough Foyle. During a 2-h period centred on high tide, observations were made of the storm impacts on a 2-km stretch of coastline around Magilligan Point. Measurements were made using a ranging rod and sediment samples were collected along a shore-normal transect parallel to the dominant storm wind direction. Sediment textural analysis was carried out in a settling tube Žfall column..
3. Results Wind velocities measured at Coleraine Ž15 km distant. averaged 16.5 m sy1 over the 2-h period of observations with a maximum recorded gust of 27.8 m sy1 . Mean wind direction was from the SW along the longest fetch afforded by Lough Foyle Ž22 km.. Wave heights generated during the storm at Magilligan Point were estimated to be in the region of 1.5 m with a period of 3–4 s. The waves were short crested and exhibited discontinuous offshore breaking. Still
Fig. 1. Study area and distribution of wave spray-deposited sediments. The patterns of wave approach and refraction are shown schematically ŽA.. Note the distribution of spray-related deposits in relation to onshore winds and waves. The affected area lies at the downwind extremity of Lough Foyle during SW storms ŽB..
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
379
Fig. 2. Schematic cross-shore distribution of spray-related deposits. The process of deposition involves entrainment in breaking waves and upward transport. These sediment-laden pockets of water are then blown onshore as spray and deposited on the dune surface.
more typically about 30 cm across. The occurrence of direct aeolian transport was precluded during this
period, due to the high water levels which left no exposed sand downwind. Spray-induced transport
Fig. 3. Patches of spray-transported sand on the dune surface about 4 m from the eroding scarp. Note the surface cover. Visible section of scale bar is 0.5 m long.
380
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
Fig. 4. Distribution of spray deposits about 18 m from scarp edge. Note the thinner deposit and reduced surface cover. Scale bar is 1 m long.
was further confirmed by the deposition of a veneer of sand on a road fronted by a 2-m high stone revetment.
The distribution of spray-transported sediment varied systematically around Magilligan Point ŽFig. 1.. It was most extensive on the margin facing SW
Fig. 5. Detail of spray deposit which comprises sand adhering to stems of A. arenaria. Scale is 12 cm long.
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
Fig. 6. Cross-shore distribution of spray deposit textural characteristics.
into Lough Foyle, where storm waves impacted directly on the shoreline and were backed by strong winds from the same direction. During the rising tide, these waves typically dissipated through breaking, generating longshore sediment transport and swash processes; however, when water levels reached the base of an eroding dune, swash processes were almost eliminated and instead, the breaking waves generated landward-directed splash and spray as they impacted on the base of the scarp. The cross-shore distribution of spray-transported sediment varied from a 3-cm thick depositional layer within 2 m of the dune scarp which covered virtually
381
the entire surface ŽFig. 2.. Landward, this gave way to a progressively reduced thickness of sand ŽFigs. 3 and 4. which formed a distinctive veneer on underlying A. arenaria stems ŽFig. 5.. When developed, this deposit covered between 0 and 80% of the surface area of the sand dune with an estimated overall average coverage of about 20%. The typical landward extent of this deposit was approximately 20 m from the dune scarp where the deposit thinned to a few sand grains. Investigation of the sediment texture on a shore-normal transect revealed no consistent variation landward of the scarp crest ŽFig. 6.. Transported sand was dominated by the fine sand category, had a mean grain size of between 0.15 and 0.17 mm, was well sorted and was slightly positively skewed. The only marked textural variation noted on the profile was a near lack of skewness in the sample closest to the eroding scarp where the deposit was at its thickest and greatest lateral extent. A marked spatial distribution of the spray deposit was evident. In areas facing the dominant wave and wind direction, the deposit was developed where the scarp height was less than 2 m. Where the scarp exceeded this dimension, splash deposits were present on the upper part of the scarp face ŽFig. 7.
Fig. 7. Spray deposits adhering to eroding dune scarp. This situation characterises scarps in excess of 2 m high under the wave and wind conditions observed. The sand has also adhered to exposed root systems of A. arenaria.
382
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
where they adhered to eroded roots of the dune vegetation. Further, although the estuary waves were refracted around the point of the foreland and approached the shoreline from the NW ŽFig. 1., no such spray deposits were recorded there as the wind was directed offshore in this area and prevented onshore sediment transport by spray. Observations during the rising tide indicated that splash and spray deposition only occurred when the tide was at its highest and lasted for approximately 60 min. Thereafter, although wind speeds did not diminish, the focus of wave energy dissipation shifted to the exposed intertidal beach rather than the dune scarp and limited splash and associated spray processes, preventing them from extending onto the upper edge of the scarp. Thus the volume of sediment deposited by the storm could be quantified both volumetrically and temporally. An estimate based on deposition along a 1-m wide transect with a 3-cm thick deposit within 1 m of the scarp edge and an average 20% coverage of the remaining area with an average deposit thickness of 0.5 cm indicates a deposited volume of 0.5 m3 per metre of affected shoreline. Of this, 0.3 m3 is deposited within the first metre of the scarp edge and the remainder is spread over a distance of about 19 m. The length of shoreline affected in the study area was about 100 m and thus, about 50 m3 of sand was deposited within a 1-h period.
4. Discussion Observations during storm conditions coincident with high tide revealed the occurrence of sand deposition landward of an eroding dune scarp by processes related to wave spray. The mechanism for generation of these deposits requires entrainment of sand in nearshore breaking waves and their ejection into the onshore wind field within pockets of spray. These spray packages are blown onshore with the contained sediment grains and deposited in an irregular pattern related to the strength of the wind and the height of the scarp over which they are transported. The depositional process is augmented close to the scarp edge by occasional wave overtopping which produces a laterally continuous and thicker deposit ŽFig. 2..
The spatial distribution of the spray-related deposits bears a clear relationship to both scarp elevation and the juxtaposition of wave and wind approach directions. The absence of spray deposits on the dune surface where the scarp elevation exceeded 2 m indicates this to be a limiting factor for waves of the type described. Since spray is generated as the wave impacts on the scarp face, the landward trajectory appears to require the scarp crest to be less than 0.5 m higher than the wave crest. Further, although refracted waves approached the northern shore of Magilligan Point with similar dimensions to those approaching the southwest-facing shore, and the scarp in the eroding dune was - 2 m in elevation, no spray deposits were recorded since wind was directed offshore, thus preventing onshore spray transport. The spray deposits show little textural variability in a cross-shore direction, although a marked decrease in skewness at the lip of the dune scarp is suggestive of splash and possible grainflow processes akin to small-scale overwashing within 1 m of the scarp. Further from the scarp edge, the good sorting and positive skewness of the sediments and fine sand grain size vary little suggesting deposition from a single depositional process. It is envisaged that the spray packages, laden with sand entrained in the surf zone follow a trajectory during which little further sorting of the sediment takes place and the spray and its contained grains are transported intact to the ground where they adhere to the stems of existing vegetation. The patchy distribution of concentrations of sediment suggests such a package-like transport mode. The volumes of sediment transported by this mode are not large and are likely only to occur under quite specific conditions of coincident onshore winds and waves at a water level that reaches the base of a scarp which is only slightly higher than the incident wave height. Thus the process is likely to be greatly restricted in time and space. Under intense storms, however, it may persist for several hours. The indicative sediment volumes transported during the field observations are likely to be representative of most conditions since the incident waves were of short period and thus the calculated sediment transport rate of about 0.5 m3rmetre of shorelinerhour may be broadly indicative of sediment transport rates during
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
such events. Except under prolonged storms, or periods of intense storminess, this process is unlikely to be a major feature in sediment budget calculations. During known historical periods of more intense storminess, such a process may have been volumetrically more important. Under storm conditions, normal aeolian modes of sediment transport to coastal dunes are likely to be impeded by high water levels which reduce available source area and by increased surface wetness of sands. The dominant geomorphological process during such storms is coastal erosion driven by both alongshore and offshore sediment transport. Carter Ž1988. cites figures of sediment erosion from dunes of up to 50 000 m3rkm of coast in a number of hours during storms. This is two orders of magnitude greater than the rates of spray-induced deposition reported here and serves to place the current observations in perspective. Under appropriate morphological conditions, washover may be more characteristic of storm impact ŽPenland et al., 1980.. Washover has been recognised as a means by which sediment is transported landward and stored as a future sediment source under conditions of barrier recession ŽLeatherman, 1979.. The mechanism reported in this paper acts in a similar manner to washover in terms of landward transport of sediment during storms. Although it is volumetrically much less important, spray-related deposits may, under certain circumstances, be more widely distributed than overwash deposits which tend to be spatially focused. The nature of the resulting deposit ensures that it has a low preservation potential as it is likely to be rapidly dispersed by wind when the storm abates and the accumulated sediment dries. If preservation was to be envisaged, it would be only through rapid burial by continued deposition by spray or rapid inundation by aeolian deposition. The mechanism may, however, be important in delivering sediment to cliff-top locations for subsequent incorporation into cliff-top dunes ŽJennings, 1967. in high energy
383
coastal settings. At sufficiently long time scales, appreciable volumes of sediment could be delivered to such locations.
Acknowledgements This research was undertaken in the course of investigations associated with European Commission Project ENV4-CT97-0488 Storminess and Environmentally Sensitive Atlantic Coastal Areas of the European Union. Thanks are expressed to Declan Lawlor for the grain size analyses, Mark Millar and Killian McDaid for drafting the diagrams and Nigel McDowell for photographic reproduction.
References Allen, J.R.L., 1967. A beach structure due to wind-driven foam. Journal of Sedimentary Petrology 37, 691–692. Carter, R.W.G., 1980. Longshore variations in nearshore wave processes at Magilligan Point, Northern Ireland. Earth Surface Processes and Landforms 5, 81–89. Carter, R.W.G., 1988. Coastal Environments. Academic Press, London. Carter, R.W.G., Stone, G.W., 1989. Mechanisms associated with the erosion of sand dune cliffs, Magilligan, Northern Ireland. Earth Surface Processes and Landforms 14, 1–10. Carter, R.W.G., Wilson, P. 1990. The geomorphological, ecological and pedological development of coastal foredunes at Magilligan point, Northern Ireland. In: Nordstrom, K.F., Psuty, N.P., Carter, R.W.G. ŽEds.., Coastal Dunes: Form and Process. Wiley, Chichester, pp. 129–157. Jennings, J.N., 1967. Cliff-top dunes. Australian Geographical Studies 5, 40–49. Leatherman, S.P., 1979. Beach and dune interaction during storm conditions. Quarterly Journal of Engineering Geology 12, 281–290. Morton, R.A., Gibeaut, J.C., Paine, J.G., 1995. Meso-scale transfer of sand during and after storms: implications for prediction of shoreline movement. Marine Geology 126, 161–179. Penland, S., Nummedal, D., Schramm, W.E. 1980. Hurricane impact at Dauphin Island, AL. Coastal Zone’ 80, ASCErHollywood, FL. Vellinga, P., 1982. Beach and Dune Erosion During Storm Surges. Delft Hydraulics Laboratory Publication No. 276.
Short communication
Wave spray-induced sand transport and deposition during a coastal storm, Magilligan Point, Northern Ireland J.A.G. Cooper ) , D.W.T. Jackson School of EnÕironmental Studies, UniÕersity of Ulster, Cromore Road, Coleraine, Londonderry, Northern Ireland, BT52 1SA, UK Received 7 December 1998; accepted 1 April 1999
Abstract Observations during a coastal storm at Magilligan Point, Northern Ireland reveal characteristic surficial deposits onshore produced by landward transport of sand within spray generated by strong winds and breaking waves. Conditions necessary for formation of such deposits include an adequate sediment supply, strong onshore waves and winds, onshore waves of short period and a low coastal scarp. The process, which may be locally important, is unlikely to be of great significance in contemporary sediment budget considerations and indicative figures suggest net landward transport rates of about 0.5 m3rmetre of shorelinerhour during optimal conditions. Its importance as a source of cliff-top sediment over longer time periods may be greater. q 1999 Elsevier Science B.V. All rights reserved. Keywords: sediments; wave spray; deposition
1. Introduction Transport of sediments on sandy coasts during storms has traditionally been difficult to assess and quantify due to the intensity of processes operative and the infrequency of storm occurrence. While the nature of coastal response to storm-related sediment dynamics varies spatially, the main modes of sediment transport include landward transport by barrier overwash, aeolian action, enhanced tidal inlet efficiency and a net landward transport vector within low gradient waves. Seaward transport has been documented through storm-return surge and nearshore profile adjustments ŽVellinga, 1982., while alongshore transport during storms often exceeds nonstorm rates by orders of magnitude. Quantifica)
Corresponding author. E-mail: [email protected]
tion of sediment movement under these transport modes enables sediment budgets to be computed at a variety of temporal and spatial scales. Recently, Morton et al. Ž1995. have drawn attention to the fact that high magnitude events may exceed morphodynamic thresholds and be the dominant process affecting mesoscale sediment budgets. This paper deals with sediment transport by wave spray associated with coastal storm conditions. Given the difficulties in measurement of transport processes under storm conditions, it is not surprising that this process appears to have been overlooked in the literature, although transport of foam and the production of related foam impressions on beach sediments during strong onshore winds has been noted ŽAllen, 1967.. This paper documents the occurrence of the wave-spray mode of transport, quantifies the volume of sediment transported and provides insights into
0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 9 . 0 0 0 4 4 - 4
378
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
the spatial controls on its occurrence. The observations were made at the apex of a cuspate foreland ŽMagilligan Point. in Northern Ireland ŽCarter and Wilson, 1990. under storm conditions which produced locally generated waves on a semi-enclosed coastal embayment. The pattern of wave energy dispersal and the topography of the foreland apex produced distinctive distributions of wave-spray intensity and associated sediment transport. Carter Ž1980. previously described longshore variations in nearshore wave patterns in this location and Carter and Stone Ž1989. documented processes of coastal dune erosion during storms in the same location. The objectives in this paper are: Ži. to document the nature of spray-related deposits during a coastal storm, Žii. to examine the spatial and temporal controls on spray-related deposition, and Žiii. to assess the role of this process in sediment budgets.
water levels in the lough reached a peak at 10:00 am at a water level that coincided with the base of an eroding dune scarp which has been retreating progressively since at least 1995. The incidence of spray-induced transport during this storm was first noted by the presence of a thin film of sand which accumulated on the seaward face of a vehicle parked approximately 5 m from the high water mark during high tide. Closer examination of the adjacent dune and road surface during the storm at high tide, revealed this to be a widespread phenomenon around the study area, but with distinctive spatial variability. In general, the deposit took the form of scattered patches of sand which adhered to stems of the Ammophila arenaria vegetation, forming a distinctive pattern. These patches were irregular in outline and up to 1 m in diameter, but were
2. Methods During a storm on November 9th 1998, Magilligan Point was subject to wave attack from locally generated winds blowing from the SE quadrant across the maximum Ž22 km. fetch of Lough Foyle. During a 2-h period centred on high tide, observations were made of the storm impacts on a 2-km stretch of coastline around Magilligan Point. Measurements were made using a ranging rod and sediment samples were collected along a shore-normal transect parallel to the dominant storm wind direction. Sediment textural analysis was carried out in a settling tube Žfall column..
3. Results Wind velocities measured at Coleraine Ž15 km distant. averaged 16.5 m sy1 over the 2-h period of observations with a maximum recorded gust of 27.8 m sy1 . Mean wind direction was from the SW along the longest fetch afforded by Lough Foyle Ž22 km.. Wave heights generated during the storm at Magilligan Point were estimated to be in the region of 1.5 m with a period of 3–4 s. The waves were short crested and exhibited discontinuous offshore breaking. Still
Fig. 1. Study area and distribution of wave spray-deposited sediments. The patterns of wave approach and refraction are shown schematically ŽA.. Note the distribution of spray-related deposits in relation to onshore winds and waves. The affected area lies at the downwind extremity of Lough Foyle during SW storms ŽB..
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
379
Fig. 2. Schematic cross-shore distribution of spray-related deposits. The process of deposition involves entrainment in breaking waves and upward transport. These sediment-laden pockets of water are then blown onshore as spray and deposited on the dune surface.
more typically about 30 cm across. The occurrence of direct aeolian transport was precluded during this
period, due to the high water levels which left no exposed sand downwind. Spray-induced transport
Fig. 3. Patches of spray-transported sand on the dune surface about 4 m from the eroding scarp. Note the surface cover. Visible section of scale bar is 0.5 m long.
380
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
Fig. 4. Distribution of spray deposits about 18 m from scarp edge. Note the thinner deposit and reduced surface cover. Scale bar is 1 m long.
was further confirmed by the deposition of a veneer of sand on a road fronted by a 2-m high stone revetment.
The distribution of spray-transported sediment varied systematically around Magilligan Point ŽFig. 1.. It was most extensive on the margin facing SW
Fig. 5. Detail of spray deposit which comprises sand adhering to stems of A. arenaria. Scale is 12 cm long.
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
Fig. 6. Cross-shore distribution of spray deposit textural characteristics.
into Lough Foyle, where storm waves impacted directly on the shoreline and were backed by strong winds from the same direction. During the rising tide, these waves typically dissipated through breaking, generating longshore sediment transport and swash processes; however, when water levels reached the base of an eroding dune, swash processes were almost eliminated and instead, the breaking waves generated landward-directed splash and spray as they impacted on the base of the scarp. The cross-shore distribution of spray-transported sediment varied from a 3-cm thick depositional layer within 2 m of the dune scarp which covered virtually
381
the entire surface ŽFig. 2.. Landward, this gave way to a progressively reduced thickness of sand ŽFigs. 3 and 4. which formed a distinctive veneer on underlying A. arenaria stems ŽFig. 5.. When developed, this deposit covered between 0 and 80% of the surface area of the sand dune with an estimated overall average coverage of about 20%. The typical landward extent of this deposit was approximately 20 m from the dune scarp where the deposit thinned to a few sand grains. Investigation of the sediment texture on a shore-normal transect revealed no consistent variation landward of the scarp crest ŽFig. 6.. Transported sand was dominated by the fine sand category, had a mean grain size of between 0.15 and 0.17 mm, was well sorted and was slightly positively skewed. The only marked textural variation noted on the profile was a near lack of skewness in the sample closest to the eroding scarp where the deposit was at its thickest and greatest lateral extent. A marked spatial distribution of the spray deposit was evident. In areas facing the dominant wave and wind direction, the deposit was developed where the scarp height was less than 2 m. Where the scarp exceeded this dimension, splash deposits were present on the upper part of the scarp face ŽFig. 7.
Fig. 7. Spray deposits adhering to eroding dune scarp. This situation characterises scarps in excess of 2 m high under the wave and wind conditions observed. The sand has also adhered to exposed root systems of A. arenaria.
382
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
where they adhered to eroded roots of the dune vegetation. Further, although the estuary waves were refracted around the point of the foreland and approached the shoreline from the NW ŽFig. 1., no such spray deposits were recorded there as the wind was directed offshore in this area and prevented onshore sediment transport by spray. Observations during the rising tide indicated that splash and spray deposition only occurred when the tide was at its highest and lasted for approximately 60 min. Thereafter, although wind speeds did not diminish, the focus of wave energy dissipation shifted to the exposed intertidal beach rather than the dune scarp and limited splash and associated spray processes, preventing them from extending onto the upper edge of the scarp. Thus the volume of sediment deposited by the storm could be quantified both volumetrically and temporally. An estimate based on deposition along a 1-m wide transect with a 3-cm thick deposit within 1 m of the scarp edge and an average 20% coverage of the remaining area with an average deposit thickness of 0.5 cm indicates a deposited volume of 0.5 m3 per metre of affected shoreline. Of this, 0.3 m3 is deposited within the first metre of the scarp edge and the remainder is spread over a distance of about 19 m. The length of shoreline affected in the study area was about 100 m and thus, about 50 m3 of sand was deposited within a 1-h period.
4. Discussion Observations during storm conditions coincident with high tide revealed the occurrence of sand deposition landward of an eroding dune scarp by processes related to wave spray. The mechanism for generation of these deposits requires entrainment of sand in nearshore breaking waves and their ejection into the onshore wind field within pockets of spray. These spray packages are blown onshore with the contained sediment grains and deposited in an irregular pattern related to the strength of the wind and the height of the scarp over which they are transported. The depositional process is augmented close to the scarp edge by occasional wave overtopping which produces a laterally continuous and thicker deposit ŽFig. 2..
The spatial distribution of the spray-related deposits bears a clear relationship to both scarp elevation and the juxtaposition of wave and wind approach directions. The absence of spray deposits on the dune surface where the scarp elevation exceeded 2 m indicates this to be a limiting factor for waves of the type described. Since spray is generated as the wave impacts on the scarp face, the landward trajectory appears to require the scarp crest to be less than 0.5 m higher than the wave crest. Further, although refracted waves approached the northern shore of Magilligan Point with similar dimensions to those approaching the southwest-facing shore, and the scarp in the eroding dune was - 2 m in elevation, no spray deposits were recorded since wind was directed offshore, thus preventing onshore spray transport. The spray deposits show little textural variability in a cross-shore direction, although a marked decrease in skewness at the lip of the dune scarp is suggestive of splash and possible grainflow processes akin to small-scale overwashing within 1 m of the scarp. Further from the scarp edge, the good sorting and positive skewness of the sediments and fine sand grain size vary little suggesting deposition from a single depositional process. It is envisaged that the spray packages, laden with sand entrained in the surf zone follow a trajectory during which little further sorting of the sediment takes place and the spray and its contained grains are transported intact to the ground where they adhere to the stems of existing vegetation. The patchy distribution of concentrations of sediment suggests such a package-like transport mode. The volumes of sediment transported by this mode are not large and are likely only to occur under quite specific conditions of coincident onshore winds and waves at a water level that reaches the base of a scarp which is only slightly higher than the incident wave height. Thus the process is likely to be greatly restricted in time and space. Under intense storms, however, it may persist for several hours. The indicative sediment volumes transported during the field observations are likely to be representative of most conditions since the incident waves were of short period and thus the calculated sediment transport rate of about 0.5 m3rmetre of shorelinerhour may be broadly indicative of sediment transport rates during
J.A.G. Cooper, D.W.T. Jacksonr Marine Geology 161 (1999) 377–383
such events. Except under prolonged storms, or periods of intense storminess, this process is unlikely to be a major feature in sediment budget calculations. During known historical periods of more intense storminess, such a process may have been volumetrically more important. Under storm conditions, normal aeolian modes of sediment transport to coastal dunes are likely to be impeded by high water levels which reduce available source area and by increased surface wetness of sands. The dominant geomorphological process during such storms is coastal erosion driven by both alongshore and offshore sediment transport. Carter Ž1988. cites figures of sediment erosion from dunes of up to 50 000 m3rkm of coast in a number of hours during storms. This is two orders of magnitude greater than the rates of spray-induced deposition reported here and serves to place the current observations in perspective. Under appropriate morphological conditions, washover may be more characteristic of storm impact ŽPenland et al., 1980.. Washover has been recognised as a means by which sediment is transported landward and stored as a future sediment source under conditions of barrier recession ŽLeatherman, 1979.. The mechanism reported in this paper acts in a similar manner to washover in terms of landward transport of sediment during storms. Although it is volumetrically much less important, spray-related deposits may, under certain circumstances, be more widely distributed than overwash deposits which tend to be spatially focused. The nature of the resulting deposit ensures that it has a low preservation potential as it is likely to be rapidly dispersed by wind when the storm abates and the accumulated sediment dries. If preservation was to be envisaged, it would be only through rapid burial by continued deposition by spray or rapid inundation by aeolian deposition. The mechanism may, however, be important in delivering sediment to cliff-top locations for subsequent incorporation into cliff-top dunes ŽJennings, 1967. in high energy
383
coastal settings. At sufficiently long time scales, appreciable volumes of sediment could be delivered to such locations.
Acknowledgements This research was undertaken in the course of investigations associated with European Commission Project ENV4-CT97-0488 Storminess and Environmentally Sensitive Atlantic Coastal Areas of the European Union. Thanks are expressed to Declan Lawlor for the grain size analyses, Mark Millar and Killian McDaid for drafting the diagrams and Nigel McDowell for photographic reproduction.
References Allen, J.R.L., 1967. A beach structure due to wind-driven foam. Journal of Sedimentary Petrology 37, 691–692. Carter, R.W.G., 1980. Longshore variations in nearshore wave processes at Magilligan Point, Northern Ireland. Earth Surface Processes and Landforms 5, 81–89. Carter, R.W.G., 1988. Coastal Environments. Academic Press, London. Carter, R.W.G., Stone, G.W., 1989. Mechanisms associated with the erosion of sand dune cliffs, Magilligan, Northern Ireland. Earth Surface Processes and Landforms 14, 1–10. Carter, R.W.G., Wilson, P. 1990. The geomorphological, ecological and pedological development of coastal foredunes at Magilligan point, Northern Ireland. In: Nordstrom, K.F., Psuty, N.P., Carter, R.W.G. ŽEds.., Coastal Dunes: Form and Process. Wiley, Chichester, pp. 129–157. Jennings, J.N., 1967. Cliff-top dunes. Australian Geographical Studies 5, 40–49. Leatherman, S.P., 1979. Beach and dune interaction during storm conditions. Quarterly Journal of Engineering Geology 12, 281–290. Morton, R.A., Gibeaut, J.C., Paine, J.G., 1995. Meso-scale transfer of sand during and after storms: implications for prediction of shoreline movement. Marine Geology 126, 161–179. Penland, S., Nummedal, D., Schramm, W.E. 1980. Hurricane impact at Dauphin Island, AL. Coastal Zone’ 80, ASCErHollywood, FL. Vellinga, P., 1982. Beach and Dune Erosion During Storm Surges. Delft Hydraulics Laboratory Publication No. 276.