internal structure of mixed-sand-and-gravel beach deposits revealed using ground-penetrating radar

Internal structure of mixed-sand-and-gravel beach depositsrevealed using ground-penetrating radar

ADRIAN NEAL*, NIGEL I. PONTEE� , KEN PYE� and JULIE RICHARDS�1

*School of Applied Sciences, University of Wolverhampton, Wulfruna Street, Wolverhampton WV1 1SB,UK (E-mail: [email protected])�ABP Marine Environmental Research Ltd, Pathfinder House, Maritime Way, Southampton SO14 3AE,UK�Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

ABSTRACT

Mixed-sand-and-gravel beaches are a distinctive type of coarse-clastic beach.

Ground-penetrating radar (GPR) and photographic records of previous

excavations are used to investigate the stratigraphy and internal sedimentary

structure of mixed-beach deposits at Aldeburgh in Suffolk, south-east England.

The principles of radar stratigraphy are used to describe and interpret migrated

radar reflection profiles obtained from the study site. The application of radar

stratigraphy allows the delineation of both bounding surfaces (radar surfaces)

and the intervening beds or bed sets (radar facies). The deposits of the main

backshore berm ridge consist of seaward-dipping bounding surfaces that are

gently onlapped by seaward-dipping bed sets. Good correspondence is

observed between a sequence of beach profiles, which record development of

the berm ridge on the backshore, and the berm ridge’s internal structure. The

beach-profile data also indicate that backshore berm ridges at Aldeburgh owe

their origin to discrete depositional episodes related to storm-wave activity.

Beach-ridge plain deposits at the study site consist of a complex, progradational

sequence of foreshore, berm-ridge, overtop and overwash deposits. Relict berm-

ridge deposits, separated by seaward-dipping bounding surfaces, form the main

depositional element beneath the beach-ridge plain. However, the beach ridges

themselves are formed predominantly of vertically stacked overtop/overwash

units, which lie above the berm-ridge deposits. Consequently, beach-ridge

development in this progradational, mixed-beach setting must have occurred

when conditions favoured overtopping and overwashing of the upper

beachface. Interannual to decadal variations in wave climate, antecedent

beach morphology, shoreline progradation rate and sea level are identified as

the likely controlling factors in the development of such suitable conditions.

Keywords Beach ridge, berm ridge, ground-penetrating radar (GPR), overtop,overwash, south-east England, Suffolk.

INTRODUCTION

Coarse-clastic beaches are composed of varyingmixtures of sand and gravel (Carter & Orford,1984). They can be subdivided into three maintypes, based on the relative proportions and

distribution of sand and gravel across the fore-shore and backshore: (a) pure-gravel types; (b)those with an upper foreshore and backshore ofgravel and a lower foreshore of sand; and (c)mixed-sand-and-gravel types (mixed beaches),where no clear spatial division exists betweenthe sand and gravel components (Pye, 2001).

Investigations into the internal sedimentarystructure of coarse-clastic beaches can provideinsights into their medium- to long-term process

1Present address: The Environment Agency, KingfisherHouse, Goldhay Way, Orton Goldhay, PeterboroughPE2 5ZR, UK.

Sedimentology (2002) 49, 789–804

� 2002 International Association of Sedimentologists 789

dynamics, complementing studies based onshort-term monitoring of process–form relation-ships. However, such studies have been limitedin number, primarily because of an over-ridingresearch interest in sand beaches and the diffi-culty in obtaining relevant information throughtraditional sampling methods such as trenchingand coring. Notable exceptions, based on thestudy of both recent and ancient deposits, includethe work of Bluck (1967, 1999), Hey (1967),Jennings & Coventry (1973), Maejima (1982,1983), Orford & Carter (1982), Dupre (1984),Massari & Parea (1988), Nielsen et al. (1988), Hart& Plint (1989), Postma & Nemec (1990), Mathers &Zalasiewicz (1996) and Bluck et al. (2001).

The ongoing development of ground-penetrat-ing radar (GPR) technology since the 1950s hasallowed the non-invasive acquisition of data onthe stratigraphy and internal sedimentary struc-ture of unconsolidated sedimentary deposits(Reynolds, 1997). The technique is particularlywell suited to the investigation of deposits dom-inated by low-conductivity sands and gravels. Ithas been successfully applied to the study of suchdeposits in a variety of coastal settings, mostcommonly in North America (for example Leath-erman, 1987; Jol et al., 1996; Meyers et al., 1996;van Heteren et al., 1996, 1998; van Heteren & vande Plassche, 1997; Smith et al., 1999), but also inEurope and elsewhere (for example Baker, 1991;van Overmeeren, 1994, 1998; Harari, 1996; Bri-stow et al., 2000; Neal & Roberts, 2000, 2001; Nealet al., 2002). However, studies relating princi-pally to the stratigraphy and internal sedimentarystructure of coarse-clastic beach deposits havebeen restricted to those of Neal et al. (2001) fromthe Isle of Man and Solway Firth, UK. At thesestudy sites, Neal et al. (2001) were able to useGPR to infer the presence of all three of the largesedimentary growth forms associated with gravelbeaches (Bluck, 1999), namely regressive barrierbars, regressive (prograding) gravel sheets andtransgressive gravel sheets.

Although one of the principal types of coarse-clastic beach, mixed-sand-and-gravel beacheshave received surprisingly little attention. Therehave been few studies of their morphodynamicsor sedimentology (see review articles by Kirk,1980; Mason & Coates, 2001). This paper presentsthe results of GPR investigations into the internalsedimentary structure of a mixed-sand-and-gravelbeach at Aldeburgh, Suffolk, south-east England.According to Otvos (2000), a morphodynamicdistinction is made between �berm ridges�, whichare part of the active beachface, and �beach

ridges�, which are relict features no longer signi-ficantly affected by wave activity. Shallow (1–2 m), high-frequency GPR surveys from a bermridge on the upper beachface at Aldeburgh arepresented and compared directly with data fromshort-term (months to years) topographic surveys.Results from the survey of the berm ridge are thenused to help interpret the detailed medium- tolong-term progradational history of a small, relict,beach-ridge plain, based on a series of moreextensive GPR surveys and photographic recordsof previous beach excavations.

STUDY SITE

Aldeburgh lies on the Suffolk coast in south-eastEngland (Fig. 1a). The coast is characterized by acontinuous series of narrow beaches and barrierscomposed predominantly of mixed sand andgravel (Fig. 1b). The beach and barrier depositsenclose areas of low-lying marsh, or front cliffscut into Pleistocene glacial till or Plio-PleistoceneCrag deposits. The shoreline at Aldeburgh hasrecently shown a net progradation, which hasbeen ongoing for at least the last 100 years(Babtie-Dobbie Ltd, 1991). This has led to thedevelopment of a small beach-ridge plain(Fig. 1c), which has infilled a narrow coastalembayment north of the town.

The study site experiences semi-diurnal tides,with a mean spring tidal range of 2Æ3 m. Averagewave heights range from 0Æ4 to 0Æ5 m, and theaverage annual wave period is 6 s (Fortnum &Hardcastle, 1979). Storm waves with returnperiods of 1 in 100 years can reach heights of2–4 m along the Suffolk coast (Anglian Water,1988). Positive storm surges up to 1 m occurseveral times a year, with the predicted 1 in50 years storm surge component typically 1Æ5 m(HR Wallingford, 1989).

The active beach is characterized by a back-shore composed of berm ridges formed underhigh-wave-energy conditions. The upper fore-shore typically displays a berm ridge at thehigh-water mark (HWM), which is connected bya planar, seaward-dipping lower foreshore to aplunge step at the low-water mark (LWM). Over-all, beach sediments have a coarser sand fractionand finer gravel fraction than is typical for coarse-clastic beaches (Pontee, 1995). Landward of theHWM, the backshore is characterized by anabundance of gravel, in a similar manner to othercoarse-clastic beaches (Carter & Orford, 1984).However, the area between HWM and LWM is

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� 2002 International Association of Sedimentologists, Sedimentology, 49, 789–804

distinctive, being composed of mixed sand andgravel that shows a high degree of both spatialand temporal grain-size variation (Pontee, 1995).In contrast to other coarse-clastic beaches (e.g.Bluck, 1967, 1999; Carter & Orford, 1984), shapesorting is largely absent.

The backshore berms and beach ridges arecomposed almost entirely of gravel (Fig. 2). Lackof cohesion in the gravels prevents free-standingfaces from forming during excavations. As a

result, it is impossible to discern their internalstructure visually (Fig. 2). Consequently, GPRsurveying provides the only means of obtainingdetailed information regarding the stratigraphyand internal structure of the backshore deposits.

The presence of significant amounts of sand inthe deposits below HWM provides cohesion tothe sediments and allows the development offree-standing faces when trenched. The internalstructure of the deposits can be seen in these faces(Fig. 2). Shallow pits at the study site and inter-pretation of photographic records of previousexcavations reveal that the foreshore deposits arecharacterized by alternating, often mixed, centi-metre- to decimetre-thick layers of sand-, granule-and pebble-sized material that impart awell-defined stratification. Under suitable condi-tions, such sediments would be ideal for radarreflection profiling. However, on the active fore-shore, rapid signal attenuation occurs, leading toa very limited depth of penetration (<1 m), andprimary reflections are obscured by �ringing�(noise comprising multiple reflection events).These effects are likely to be caused by thepresence of saline pore waters within the sedi-ments. Consequently, the active foreshore isunsuitable for GPR deployment.

GROUND-PENETRATING RADARSURVEYS

GPR surveying is based on the transmission,reflection and reception of electromagneticwaves in the MHz frequency range. The prin-ciples behind the technique and the practical-ities of data collection are described thoroughlyin the literature (Conyers & Goodman, 1997;Reynolds, 1997). Reflections occur in the sub-surface as a result of the varying electricalproperties of earth materials. Changes in sedi-ment composition, water content/type andgrain shape, orientation and packing can causesignificant changes in electrical properties(Davis & Annan, 1989; Baker, 1991; van Dam& Schlager, 2000). Consequently, subsurfacefeatures such as sedimentary structures, litho-logical changes and the water table can allgenerate primary reflections.

GPR surveys were performed in May 1995 usinga Geophysical Survey Systems Inc. SIR-2 systemand in January 2000 using a Sensors and SoftwarePulseEKKO PE1000 system. The GPR surveyswere partly performed along transect lines usedpreviously as part of a 2-year (March 1993 to May

Fig. 1. (a) Location of the study site on the Suffolkcoast in south-east England. (b) The surficial geologyfor the section of coastline around Aldeburgh, based onBritish Geological Survey sheets 191 and 208 (1:50000series, solid and drift editions). (c) Location and con-text of the ground-penetrating radar surveys at Alde-burgh (transect lines A-L1 to A-L4).

Internal structure of beaches 791


1995) beachface morphology monitoring pro-gramme (Pontee, 1995).

Using the SIR-2 system, initial trials indicatedthat a 500-MHz antenna provided the bestcompromise between resolution and depth ofpenetration (the �range-resolution trade-off� ofDavis & Annan, 1989). The single transmitting–receiving antenna was moved along survey linesat a constant, slow speed, while data were recor-ded continuously at 32 traces s)1 over a 100-nstime window. Individual traces were stacked fourtimes. Horizontal position was recorded by man-ual marking every 1 m. Two transects were per-formed normal to the coast (A-L1 and A-L2), andone intersecting transect (A-L3) was performedparallel to the coast (Fig. 1).

In January 2000, a second series of GPR surveyswas performed using the PE1000 GPR system.Surveys were repeated along transect lines A-L1,A-L2 and A-L3. In addition, a further transect wasperformed parallel to the coast (A-L4, Fig. 1).During these surveys, the best range-resolutiontrade-off was achieved when the GPR system was

used with 450-MHz antennae. Data were collec-ted using connected, but separate, transmittingand receiving antennae, which were moved alongtransects in step-mode. Preliminary trials indica-ted that individual traces should be stacked eighttimes, and that a time window of 100 ns and step-size of 0Æ05 m were most appropriate for optimalimaging.

Common mid-point (CMP) surveys were carriedout in order to estimate the velocities of radarwaves in the subsurface. To calculate the correctvelocities from CMP surveys, the reflections usedmust be horizontal. Consequently, CMP surveyswere performed on transects parallel to theshoreline, as these were dominated by horizontalor subhorizontal reflections (Fig. 3). Velocitieswere calculated as in seismic reflection studies,using the principles of �normal moveout� (Robin-son & Coruh, 1988). Velocities ranged from 0Æ130to 0Æ132 m ns)1, with an average of 0Æ131 m ns)1.The average velocity was used to convert two-waytravel time to individual reflections into depth(Annan & Davis, 1976). Comparison of reflection

Fig. 2. Trench 1, excavated on the beach at Aldeburgh on 16 November 1987, 950 m south of the location of the GPRtransects. Note the lack of vertical free-standing faces in the upper part of the trench as a result of the gravel-dominated nature of the backshore sediments. As a consequence, no internal structure is discernible in thesedeposits. In contrast, the lower part of the trench is composed of beds formed of sand and/or gravel. As a result of thepresence of the sand, vertical free-standing faces form, and the seaward-dipping nature of the foreshore beds can bevisualized. The engineer in the trench provides an approximate vertical scale. The timber piles of the groyne arespaced at �2Æ2-m intervals. Photograph provided courtesy of the Babtie Group Ltd, Glasgow.

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depths, on radar profiles collected along the sametransect lines, indicated that an average subsur-face velocity of 0Æ131 m ns)1 was also a reason-able estimate for the 1995 survey.

Data collected with the SIR-2 system wereprocessed and plotted using radan for Windowsversion 3Æ1 software (Geophysical Survey Sys-tems Inc., 1996). A six point linear gain andvertical high-pass triangular filter, with a 110-MHz lower cut-off, were applied. Profiles weretopographically corrected relative to OrdnanceDatum (OD) and plotted in greyscale colour-amplitude format. The return-centre frequencyfor primary reflections was estimated to be245 MHz. In combination with an average sub-surface radar wave velocity of 0Æ131 m ns)1, thisresults in a vertical resolution of 0Æ13 m.

Data collected with the PE1000 system wereprocessed using version 4Æ2 of the system soft-ware. Data were filtered using dewow, whichremoves low-frequency noise generated by thelarge transmit pulse of the GPR system. Subse-quently, all radar reflection profiles were migra-ted. In seismic reflection, which is broadlyanalogous to GPR, migration is routinely appliedin order to return dipping reflections to their truehorizontal and vertical position, remove struc-tural distortions associated with undulating ref-lectors and remove diffraction patterns generatedby point and strongly curved reflectors (Robinson& Coruh, 1988). However, despite these obviousadvantages, migration is not routinely applied toGPR data (Neal & Roberts, 2001). The migrationprogram used in this study employs the fre-

quency-wavenumber approach of Stolt (1978) andassumes a single, constant velocity. Conse-quently, all profiles were migrated with a velocityof 0Æ131 m ns)1.

The return-centre frequency for primary reflec-tions collected with the 450-MHz antennae wasestimated to be between 380 and 405 MHz. Incombination with an average subsurface radarwave velocity of 0Æ131 m ns)1, this results in anaverage vertical resolution of 0Æ08 m.

Topographic corrections were applied to themigrated reflection profiles, again using elevationdata relative to OD. The profiles were plottedwith the appropriate near-surface velocity, anautomatic gain control (AGC) gain with a maxi-mum limiting value of 250 and no vertical orhorizontal averaging.

RADAR REFLECTION PROFILES

Radar reflection profiles collected along transectsparallel to the crests of both berm and beachridges show predominantly horizontal or subhor-izontal reflection configurations (Fig. 3). As aresult, migrated radar profiles normal to suchtransects will contain reflections that are veryclose to, or actually correspond to, the true dip ofthe reflector that generated them. Consequently,only reflection profiles generated on shore-normaltransects will be presented in the followingsection.

The reflection profiles were interpreted usingthe principles of radar stratigraphy (Beres &

Fig. 3. Radar reflection profile obtained from 10 to 34 m along transect line A-L4. The reflection profile has beenplotted with a 0Æ1-m step-size. The intersection with transect line A-L1 and the location of common mid-point profile1 (CMP1) are indicated on the horizontal axis.



Haeni, 1991; Jol & Smith, 1991), a techniquebased on seismic stratigraphy (Mitchum et al.,1977). Traditionally, in radar stratigraphy, sys-tematic reflection terminations have been used todelineate �radar sequence boundaries�, whichthen define genetically related packages of reflec-tions termed �radar sequences�. However, owingto some confusion over the use of the term�sequence�, Neal et al. (2002) have suggested thatthis terminology be modified, such that system-atic reflection terminations are used to identify�radar surfaces�, which then define �radar pack-ages�. Within this framework, it is then possible toidentify various �radar facies�. These are sets ofreflections with distinctive configurations, conti-nuity, frequency, amplitude, velocity characteris-tics and external form. It should be noted that theuppermost part of the profiles cannot be inter-preted in this way because of �noise� associatedwith the air and ground waves. The air andground waves are the first two waves recorded onthe reflection profiles (see Neal & Roberts, 2000)and obscure approximately the upper 0Æ6 m of the1995 profiles and the upper 0Æ4 m of the 2000profiles.

Radar surfaces are interpreted geologically asrepresenting non-depositional or erosional hia-tuses in a sedimentary sequence (Gawthorpeet al., 1993). In deposits formed entirely of sandand gravel, they effectively mark the positions ofbounding surfaces. The intervening radar faciesare then formed from reflections generated by thestratification within the sediments.

Berm ridges

Only the backshore berm ridges well above thelevel of MHW spring tides (1Æ2 m OD) weresuitable for GPR surveying, on account of rapidsignal attenuation at lower beach elevations. TheMay 1995 radar reflection profile from the sea-ward portion of A-L1 (Fig. 1) shows a large, singleberm ridge on the backshore (A-BR1, Fig. 4a).Radar stratigraphic interpretation of the profileindicates the presence of two radar packages(Fig. 4b). These are defined by two seaward-dipping (�7–8�), slightly sigmoidal radar surfa-ces: A-AE and A-AF. The reflections of radarfacies A-31 are also gently sigmoidal and arelargely concordant with A-AE and A-AF, exceptat their extreme landward margin, where low-angle onlap onto A-AE and erosional truncationby A-AF takes place. Radar facies A-32 displayssubhorizontal and gently sigmoidal reflectionsthat onlap A-AF at very low angles.

The reflection profile collected from the samesection of transect A-L1 in January 2000 showstwo berm ridges (Fig. 4c), the most landward ofwhich is largely an eroded remnant of A-BR1.Delineation of its radar stratigraphy (Fig. 4d)reveals that the two radar packages associatedwith A-BR1 in the 1995 survey can be identifiedagain. However, the higher resolution of thissurvey, in combination with the slightly raisedelevation of the berm’s upper surface, allows amore detailed picture of the reflection configu-rations to be obtained. First, reflections associ-ated with the upper part of A-32 (present, butobscured by the air and ground waves in 1995)have a more distinct seaward dip of around 6�.Secondly, there is an obvious seaward trunca-tion of radar surface A-AF and radar faciesA-32, compared with the 1995 survey. In addi-tion, a third radar package can be identifiedassociated with a second, more seaward bermridge (A-BR2, Fig. 4c and d). The base of thisradar package is defined by radar surface A-AG,which dips seaward at �10� and erosionallytruncates underlying radar facies A-31. A-AG isoverlain by radar facies A-33, which displaysconcordant reflections at its base. However, withincreasing elevation, they obtain gentle land-ward (�3�) and moderately steep seaward(�19�) dips, which mimic the external ridgemorphology.

Fig. 4. (a) Radar reflection profile obtained from 52Æ6 to72 m along transect line A-L1 in May 1995. The profilecrosses a single berm ridge (A-BR1) on the backshore.(b) Interpretation of the radar stratigraphy of A-BR1,defining two radar surfaces (A-AE and A-AF) and tworadar facies (A-31 and A-32). Note that interpretationbecomes more difficult below A-AE because of thepresence of �ringing� multiples that interfere with, andthen replace, primary reflections in a seaward direc-tion. (c) Radar reflection profile obtained from 52Æ6 to72 m along transect line A-L1 in January 2000. Theprofile is plotted with a 0Æ05-m step-size and crossestwo berm ridges. One berm ridge is the eroded remnantof that surveyed in 1995 (A-BR1), and the second is anew ridge (A-BR2) formed at a lower elevation on thebackshore. (d) Interpretation of the radar stratigraphyof A-BR1 and A-BR2. The two radar surfaces (A-AE andA-AF) and two radar facies (A-31 and A-32) are againassociated with A-BR1, and one new radar surface (A-AG) and one new radar facies (A-33) are identified withA-BR2. Note that the primary reflections below A-AEare more clearly defined than those in the May 1995profile. This results from the higher resolution of theJanuary 2000 profile and significantly reduced �ringing�.The primary reflections below A-AE are interpretedfully in Fig. 7.

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Geological interpretation of the radar stratigra-phy (Fig. 4), combined with a time series of beachprofile data collected between 1993 and 2000(Figs 5 and 6), indicates that two of the threeradar packages identified can be related to speci-fic events during the development of berm ridge

A-BR1. Given a vertical resolution limit of 0Æ13 mfor the GPR profile and vertical surveying errorsof less than 0Æ1 m for the beach profile data(Pontee, 1995), bounding surface A-AE correlateswell with a March 1993 beach profile (Fig. 5a).The fact that A-AE erosionally truncates a series



of stratigraphically older primary reflections(Fig. 4a and c) suggests that this bounding surfaceand the corresponding beach profile were gener-ated through erosion of the entire backshore.

The seaward-dipping stratification character-ized by radar facies A-31 (Fig. 4) formed primarily

between March 1993 and September 1993, duringa period of vertical accretion across the wholebackshore (Fig. 5a). The bounding surface delin-eated by radar surface A-AF appears to be relatedto a series of stable backshore profiles (54Æ6–69 m,Fig. 5b) developed between September 1993 and

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December 1994. This suggests that A-AF owes itsorigin to a significant period of non-depositionor minor sediment reworking on the backshore.However, during the same period and immedi-

ately to seaward, a large berm ridge was pro-gressively being emplaced (Fig. 6a). This wassubsequently reworked landward betweenDecember 1994 and January 1995 to formA-BR1, with further minor erosion and depositionacross the berm top between January and May1995 (Figs 5c and 6b). The formation of A-BR1has been related by Pontee (1995) to knownstorm-surge events that affected the Suffolk coastin January 1995.

The emplacement of A-BR1 resulted in thedevelopment of the stratification portrayed byradar facies A-32 (Fig. 4). In the 1995 GPR survey,the internal structure largely mirrors the externalform of the ridge. Subhorizontal to gently sea-ward-dipping strata lie beneath the extensive,gently seaward-dipping berm top, whereas mod-erately seaward-dipping, sigmoidal-shaped bedsare found beneath the berm-ridge crest andseaward-dipping berm slope. In the 2000 GPRsurvey, the moderately seaward-dipping strata arelargely absent as a result of intervening erosion ofthe seaward portion of the berm (Fig. 5d). Theprogressive upward and landward growth of the

Fig. 5. Radar reflection profile obtained for berm ridgeA-BR1 on transect line A-L1 in May 1995, with super-imposed topographic profile data from surveys in (a)March and September 1993 (radar surfaces A-AE andA-AF are also shown), (b) September 1993 and March,September and December 1994, and (c) December 1994,and January and May 1995. (d) Radar reflection profileobtained for berm ridges A-BR1 and A-BR2 on transectline A-L1 in January 2000, with superimposed topo-graphic profile data from surveys in May 1995 andJanuary 2000. Note the correspondence between radarsurface A-AE and the topographic profile from March1993 and similar correspondence between radar surfaceA-AF and the four profiles obtained between Septem-ber 1993 and December 1994. Also note the significantvertical accretion from 55 to 71 m between December1994 and January 1995 and the subsequent erosionof the seaward (ESE) portion of A-BR1, followed byemplacement of A-BR2 between May 1995 and January2000.

Fig. 6. Beach profile data collectedfrom 52Æ6 to 110 m along the line oftransect A-L1 between (a) March1993 and December 1994, and (b)December 1994 and January 2000.



berm ridge during emplacement is illustrated bythe onlapping relationship between the mostlandward beds of A-32 and the underlyingbounding surface A-AF (Fig. 4). These strata arebelieved to have formed because of overtopping/overwashing of the berm crest and consequentsediment deposition on the berm top. Overtop-ping and overwashing are complementary pro-cesses resulting from wave/swash action (Orford& Carter, 1982). Overtopping occurs when swashcarries water and sediment just over a berm-ridgecrest or beach crest, resulting in vertical aggrada-tion at the crest. Overwashing occurs when thelandward excursion of the sediment-laden swashis greater, resulting in horizontal accretion land-ward of the berm-ridge crest or beach crest.

The bounding surface represented by radarsurface A-AG (Fig. 4d) cannot be related to spe-cific events on the backshore/upper foreshorebecause of a lack of profile monitoring betweenMay 1995 and January 2000. However, A-AG doeserosionally truncate the strata of radar facies A-31and is overlain by A-33, which characterizes bermridge A-BR2. This suggests that A-AG is anerosional surface formed before the emplacementof A-BR2. The progressive upward steepening ofbeds associated with A-33, dipping in bothlandward and seaward directions and mirroringthe external form of the berm ridge, suggests thatA-BR2 has subsequently grown largely in situthrough overtopping.

Aldeburgh beach-ridge plain

The complete January 2000 GPR survey of tran-sect A-L1 (Fig. 1) crossed two beach ridges (A-BHR1 and A-BHR2), in addition to the two bermridges (A-BR1 and A-BR2) already discussed fromits seaward end (Fig. 7a). Owing to the inherentlimitations to radar-wave penetration at the studysite, detailed imaging was limited to a maximumdepth of �4 m. As a consequence, relevant datawere only obtained from deposits formed pre-dominantly on the backshore. Radar stratigraphicinterpretation of the reflection profile generatedshows a complex arrangement of at least 33 radarpackages (Fig. 7b). Seaward-dipping (typically�12�) radar surfaces dominate and commonlyunderlie both the swales and the lower parts ofthe beach ridges. These surfaces are typicallyslightly concave upwards (e.g. A-G, A-J, Fig. 7b)or sigmoidal in form (e.g. A-W, A-X, Fig. 7b). Theconcave-up surfaces may show an oblique-tan-gential relationship, but this is difficult to discernbecause of insufficient radar wave penetration.

Where seaward-dipping radar surfaces domin-ate, the reflections of the intervening radar faciesare predominantly seaward-dipping and concor-dant with the underlying radar surface at lowerelevations. However, with increasing elevation,the reflections acquire progressively lower dips(e.g. A-7, Fig. 7b) and may become subhorizontalto gently landward dipping (up to �7�), onlap-ping the underlying radar surface (e.g. A-9, A-24,Fig. 7b). These radar facies are typically erosion-ally truncated by the overlying radar surface (e.g.A-10, A-23, Fig. 7b).

Directly beneath the upper part of the beach-ridge crests, the radar packages display a morecomplex, stacked arrangement. Radar surface geo-metries include both gently landward-dipping,concave-upward forms (e.g. A-AC, A-AD, Fig. 7b)and strongly convex-upward forms (e.g. A-S, A-T,Fig. 7b). Radar surface truncation is also common(e.g. A-AD truncating A-Z and A-AC, Fig. 7b).

In addition to differences in radar surfacecharacteristics beneath the beach-ridge crests,the radar facies are also somewhat different innature. For example, radar facies A-18 (beneathA-BHR1, Fig. 7b) is a lens-shaped unit withreflections that are landward dipping on itslandward side (up to �8�) and seaward dippingon its seaward side (up to �12�). Dip angles in theupper part of A-18 approach those of the overly-ing radar surface (A-S). Lying directly above andto landward is radar facies A-19, which is dom-inated by landward-dipping reflections (up to�8�). Lying above both these is radar facies A-20,the base of which is defined by radar surface A-T.A-T erosionally truncates the seaward side ofA-18 and radar surface A-S. A-20 displays land-ward-dipping reflections (�6�), which gentlydownlap onto A-T beneath the crest and land-ward-dipping slope of A-BHR1, and displaysseaward-dipping reflections (�12�) beneath thebeach ridges’ seaward-dipping slope.

Beneath the crest of A-BHR2, the verticallystacked radar packages observed have a slightly

Fig. 7. (a) Radar reflection profile obtained from 0 to72 m along transect line A-L1 in January 2000. Theprofile is plotted with a 0Æ1-m step-size and crosses twobeach ridges (A-BHR1 and A-BHR2) and two bermridges (A-BR1 and A-BR2). The intersection points withtransects A-L3 and A-L4 are indicated. More detailedGPR survey results for the two berm ridges are pre-sented in Fig. 4c and d. (b) Line-drawing interpretationof the radar stratigraphy for reflection profile A-L1,showing a complex sequence of radar surfaces (A-A toA-AG) and radar facies (A-1 to A-33).

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different character (Fig. 7b). Both radar faciesA-29 and A-30 are dominated by laterally con-tinuous, gently landward-dipping reflections (upto 11�) and are underlain by gently landward-dipping, concave-up surfaces (A-AC and A-ADrespectively). Radar facies A-30 lies above anddirectly to landward of A-29, with its underlyingradar surface truncating various radar packages.

Geological interpretation of the radar stratigra-phy suggests that the sedimentary sequence isdominated by prograding berm-ridge deposits.Numerous seaward-dipping bounding surfaces(¼radar surfaces) separate, and often erosionallytruncate, units with a distinctive style of stratifi-cation (¼radar facies). The strata are often sea-ward dipping and concordant with theunderlying bounding surfaces at lower elevations,but acquire progressively lower dips, oftenbecoming subhorizontal or gently landward dip-ping, at higher elevations. Cross-strata at higherelevations typically onlap the bounding surfacesthat underlie them. Such internal sedimentarystructure is directly comparable with that of thecontemporary berm ridges at Aldeburgh anddeposits described in the literature (Maejima,1982; Massari & Parea, 1988; Bluck, 1999). Insome instances, the entire berm ridge appears tohave been preserved (e.g. A-7, Fig. 7b) but, typ-ically, the berms’ upper surface has been erosion-ally truncated by the overlying bounding surface.

Beneath the beach-ridge crests, the sedimentarysequence is more complex. At low elevations,berm-ridge deposits dominate but, at higher eleva-tions, the bounding surfaces and cross-strataacquire different characteristics. The sequencebeneath the crest of A-BHR1 consists of boundingsurfaces and cross-strata with both distinct sea-ward and landward dips. The cross-strata repre-sented by radar facies A-18 (Fig. 7b) are interpretedas representing deposits formed as a result ofovertopping (Orford & Carter, 1982). Overlyingthese deposits are two further sets of deposits(represented by radar facies A-19 and A-20), whichhave distinct sets of landward-dipping cross-strata. These are interpreted as either beach-crestovertop or minor, beach-crest overwash deposits.

The sedimentary sequence beneath A-BHR2shows similar characteristics to that beneathA-BHR1. At lower elevations, deposits that haveresulted from typical berm-ridge seaward progra-dation are evident. These are overlain at higherelevations by a stacked sequence of overtop/overwash deposits resulting from swash excur-sions onto, and beyond, the beach crest. Theovertop/overwash deposits display cross-strata

with a distinct landward dip (e.g. A-29, A-30,Fig. 7b) and, in this instance, are underlain bybounding surfaces that are concave up and trun-cate underlying deposits (e.g. A-AD, Fig. 7b).

As already noted, GPR profiling was unsuc-cessful on the present-day foreshore because ofhigh signal attenuation. In addition, relict fore-shore deposits preserved beneath the beach-ridgeplain were not imaged in detail on account of thefinite nature of radar wave penetration. Shallowpits on the present-day foreshore provided somelimited information regarding the internal struc-ture of the deposits (see earlier section describingthe study site). However, more extensive trench-ing, both on the active foreshore and on thebeach-ridge plain, was not permissible because ofenvironmental and aesthetic considerations. Con-sequently, primary data regarding the internalstructure of the foreshore deposits were limited,and secondary sources had to be used in order togain further insight.

Photographic records of previous excavations atAldeburgh during coast protection works (Babtie-Dobbie Ltd, 1991) suggest that the foreshoredeposits are generally dominated by seaward-dipping sedimentary units (trench 1, 950 m southof the GPR profiles, Fig. 2). On the limitednumber of occasions where more detailed recordsare available (e.g. trench 2, 650 m south of theGPR profiles, Fig. 8), it is evident that the depos-its are characterized by a series of seaward-dipping bounding surfaces that separate beds ofsand and/or gravel, with the importance of sand-sized material often increasing down dip. Thisgeneral sedimentary sequence was confirmed inthe vicinity of GPR transect A-L1 by a series ofshallow pits excavated on the foreshore.

Interpretation of the photographic records fromtrench 2 shows that the base of the visiblesedimentary sequence is marked by a well-devel-oped bounding surface (Bs1, Fig. 8). OverlyingBs1 is a packet of downlapping, gently landward-dipping cross-beds, which are, in turn, overlainby an erosional bounding surface (Bs2, Fig. 8).Bs2 is overlain by a series of gently seaward-dipping beds, separated by minor, erosional,gently concave-up, bounding surfaces (Bs3, Bs4and Bs5, Fig. 8). Bs3, Bs4 and Bs5 are erosionallytruncated by the overlying bounding surface Bs6.Bs6 defines the base of a thin, laterally discon-tinuous, seaward-dipping sedimentary unit, thetop of which is marked by the planar boundingsurface Bs7. Bs7 is, in turn, overlain by a thickpacket of downlapping, moderately seaward-dip-ping, gently concave-up beds.



The gently landward-dipping cross-strata lyingabove bounding surface Bs1 are interpreted asberm-ridge deposits, by analogy with those des-cribed from the GPR transects and those in theliterature (Maejima, 1982; Massari & Parea, 1988;Bluck, 1999). These deposits are likely to haveformed on the uppermost foreshore, which ischaracterized by a HWM berm ridge. The berm-ridge deposits are overlain by erosional boundingsurface Bs2, which separates them from a series ofyounger deposits interpreted as having formed byseaward progradation of the foreshore. Seawardprogradation appears to have been punctuated bya series of minor erosional episodes, leading tothe formation of bounding surfaces Bs3, Bs4 andBs5. A major erosional event then followed,resulting in the formation of bounding surfaceBs6, which cross-cuts the progradational fore-shore deposits. After a period of minor reworkingand erosion on the foreshore, seaward prograda-tion occurred again, with the consequent depos-ition of the gently concave-up beds overlyingbounding surface Bs7.

DISCUSSION

Based on the detailed interpretation of high-resolution GPR reflection profiles, it has beendemonstrated that the contemporary backshoreberm ridges at Aldeburgh show a relativelysimple internal structure. Seaward-dippingbounding surfaces separate sedimentary unitsresulting from a limited number of discretedepositional episodes on the backshore. Such astructure is comparable to that described for other

modern coarse-clastic berm ridges (Maejima,1982; Bluck, 1999). Beach profile monitoringindicates that the construction of berms on thebackshore occurs through landward reworking ofberm ridges developed lower on the beachface(Pontee, 1995) and/or landward reworking of theseaward portion of the berm ridge itself. Pontee(1995) indicate that backshore berm building andlandward migration can be linked to knownstorm-surge events on the coast. Massari & Parea(1988) have suggested that such accretion on theupper beachface of coarse-clastic beaches may berelated to the activity of low-steepness �construct-ive� waves. These can occur as part of storm-decayspectra or heavy swell related to distant storms.In contrast, the bounding surfaces that underlieberm deposits are ascribed to upper-beachfaceerosion resulting from steep �destructive� wavesassociated with the storm itself (Massari & Parea,1988).

The data presented in this study indicate thatthe overall structure of the Aldeburgh beach-ridgeplain most closely resembles the progradationalcoarse-clastic beach sequences described by Mas-sari & Parea (1988), Bluck (1999) and Bluck et al.(2001). Bluck (1999) and Bluck et al. (2001)emphasized the importance of seaward-dippingcusp and berm bedding in the upper part ofregressive (prograding) gravel sheets from LowerPleistocene and Holocene beach sequences inSouth Wales, Scotland and Namibia. Massari &Parea (1988) also described similar berm beddingand washover deposits from the uppermost partof Italian Messinian and Pleistocene prograda-tional sequences. However, backshore depositsare likely to make up a greater proportion of the

Fig. 8. Trench 2, excavated on thebeach at Aldeburgh on 17 November1987, 650 m south of the location ofthe GPR transects. The exposedforeshore deposits show a complexarrangement of seaward-dippingbeds and bounding surfaces. Rele-vant bounding surfaces have beenhighlighted (Bs1 to Bs7) to aid dis-cussion in the text. The timber pilesof the groyne are spaced at �2Æ2-mintervals. Photograph providedcourtesy of the Babtie Group Ltd,Glasgow.

800 A. Neal et al.


full progradational sequence at Aldeburgh com-pared with these other coarse-clastic beachdeposits. This is because of the narrow verticalextent of the foreshore, caused by the low tidalrange on the open Suffolk coast, and the con-siderable elevation of the beach-ridge plaindeposits above MHW.

There has been considerable ongoing debateregarding the origin of both sandy and coarse-clastic beach ridges (see reviews by Tanner, 1995;Taylor & Stone, 1996; Otvos, 2000). The beachridges at Aldeburgh are clearly associated withthe lateral and vertical stacking of multiple berm-ridge, overtop and overwash deposits, separatedby often erosional bounding surfaces. Massari &Parea (1988) indicated that �destructive� wavesassociated with storm events can result in bothplanation of the upper beachface (as alreadynoted with respect to the formation of boundingsurfaces in modern berm-ridge deposits) andwashover, which passes over the beach-ridgecrest and deposits material as a thin washoversheet. However, as crestal overtopping and over-washing form part of a natural continuum (Orford& Carter, 1982), it might be anticipated that suchwaves could also result in overtop deposits.Consequently, all the deposits that form the beachridges can be related to storms (beach-crestwashover/overtop deposits, bounding-surface for-mation) or immediate post-storm wave activity(berm-ridge deposits). As a result, they wouldappear to have a similar origin to other coarse-clastic, �storm� beach ridges (Taylor & Stone, 1996;Otvos, 2000). This is probably a reflection of thegravel-rich nature of the sediments that make upthe backshore deposits of these mixed beaches.

Temporal variation in storm-wave climate, withrespect to both individual storms and their decay,and longer term variations in the frequency andintensity of storm events, is likely to have playedan important role in determining the evolution ofthe beach-ridge plain and the nature and timingof beach-ridge formation. However, it is highlyunlikely that variations in storminess have beenthe sole control on beach-ridge plain develop-ment.

In the short term, beach morphology and maxi-mum crestal elevation have been shown to influ-ence the nature of backshore/upper-foreshoreprocesses significantly during any given stormevent (Orford & Carter, 1982, 1985). Examinationof the seaward-dipping bounding surfaces identi-fied at Aldeburgh (Fig. 7), which effectivelydefine the morphology of the backshore beforeemplacement of the overlying sediments, indi-

cates significant changes in the form and crestalelevation of the upper beachface during shorelineprogradation. Vertical aggradation of beach ridgesresulting from overtopping/minor overwashinghas been shown to increase resistance to furtherovertopping and overwashing (Orford & Carter,1982). Consequently, storm events that mightresult in overtopping/overwashing when incipi-ent beach-ridge crest elevations are lower mayonly result in storm-wave planation on the sea-ward side of the beach ridge when crest eleva-tions are higher. In addition, Orford & Carter(1985) demonstrated that changes in upper-beachface morphology can significantly modifystorm-wave behaviour, resulting in either erosionor, conversely, high-level deposition duringotherwise similar storm events.

In the medium to long term, sediment supplywill determine the progradation rate of theshoreline. Analysis of changes in the position ofMHW mark, based on Ordnance Survey maps(Babtie-Dobbie Ltd, 1991), indicates that theslight embayment that the Aldeburgh beach-ridgeplain occupies has been infilling with sedimentfor at least the last 120 years. Beach ridgesA-BHR1 and A-BHR2 appear to have formedduring a period of relatively rapid shorelineprogradation, between approximately 1925 andthe present day. This followed a period of relativeshoreline stability and even slight erosionbetween 1880 and 1925, during which timebeach-ridge development did not take place. Thisresponse appears to be opposite to that whichoccurs in sandy beach-ridge systems, where highsediment supply results in rapid progradationand limited beach-ridge development, and lowersediment supply results in slower progradationand more pronounced beach-ridge development(Taylor & Stone, 1996). Consequently, drawing asimple analogy with beach-ridge systems domi-nated by sand-sized sediment appears to bemisleading. However, Bluck et al. (2001) haveindicated a link between progradation rate andthe style of gravel-beach deposition on the Nam-ibian coast and, although its role on the Suffolkcoast is unclear, it clearly warrants further inves-tigation.

Over the last 70 years, average rates of meansea-level rise in south-east England have beenbetween 1 and 3 mm year)1 (Woodworth et al.,1991; Shennan & Woodworth, 1992). Detailedspatial analyses for the east coast of Englandsuggest that, over the same time period, extremesea levels have risen at around 1Æ3 mm year)1

(Dixon & Tawn, 1995). However, Orford et al.



(1995) have demonstrated that subdecadal varia-tions in sea level can also be significant and canhelp to control the potential for overwash andovertopping associated with coarse–clastic barri-ers. Orford et al. (1995) indicated that theoccurrence of beach-crest overtopping and over-washing is also governed by the annual surgepotential (related to storminess) and antecedentbarrier morphology. These latter two factors havealready been identified as potentially importantin this study.

It is evident from the preceding discussion thatthe nature of beach-ridge plain development atAldeburgh has been controlled by complex inter-actions between short- and medium-term changesin wave climate, beach morphology and crestalelevation, shoreline progradation rate and sealevel. Perhaps most importantly, when thesefactors have combined to raise the potential forovertopping and overwashing, this has clearlyfavoured beach-ridge development.

CONCLUSIONS

This study of mixed-sand-and-gravel beaches onthe Suffolk coast reinforces the distinction madeby Otvos (2000) between berm ridges and beachridges. They can be differentiated not only interms of their current dynamic status (active bermridges vs. relict beach ridges), but also in terms oftheir stratigraphy and internal sedimentary struc-ture, as derived in this study by deploying GPR.The typical berm ridges found on the backshore ofthe beach display a simple, but characteristic,internal structure, consisting of one or two sea-ward-dipping bounding surfaces gently onlappedby sets of seaward-dipping beds. This structure iscomparable to that displayed by backshore bermridges on other coarse-clastic beaches. In addi-tion, the internal structure of the berm ridges canbe successfully related to their known morpholo-gical development, in this case obtained frombeach profile monitoring. The monitoring dataalso indicate that formation is related to specific,storm-induced, depositional episodes, whichcause reworking and landward migration of bermridges from lower on the beachface.

In contrast to the berm ridges, the beach-ridgeplain is clearly made up of multiple, laterally andvertically stacked, backshore and foreshoredeposits, which were formed during seawardprogradation of the shoreline. Berm-ridge depo-sits form only one component of the sedimentarysequence, but typically underlie the whole of the

beach-ridge plain and are separated by seaward-dipping bounding surfaces. Where overtop andoverwash deposits are vertically stacked on top ofthe berm-ridge deposits, a distinct beach-ridgetopography is evident. Consequently, the beachridges must have formed during periods whenpotential for overtop and overwash of the beachcrest was enhanced. Such potential is likely tohave been controlled by complex interactionsbetween interannual to decadal changes in waveclimate, antecedent beach morphology, shorelineprogradation rate and sea level.

ACKNOWLEDGEMENTS

Martin Fenn provided GPR field assistance.Numerous friends and colleagues aided in beachprofile monitoring at Aldeburgh. Peter Fenningand Andy Brown of Earth Science Systems Ltdand the staff of Allied Associates Geophysicalprovided additional logistical and technical sup-port. Roy Stoddard and John Davies of SuffolkCoastal District Council, and Alison Collins ofEnglish Nature provided permission to carry outthe research. Barry Sanders and Roy Stoddard ofSuffolk Coastal District Council, and Keith Rid-dell and Chris Powell of Babtie Group Ltd,Glasgow, helped to track down the photographicrecords of the 1987 excavations on Aldeburghbeach. Kay Lancaster prepared the diagrams. Theresearch was supported by Cambridge Environ-mental Research Consultants Ltd, by the BillBishop Memorial Fund and by the School ofApplied Sciences, University of Wolverhampton.Brian Bluck and a second, anonymous, refereeprovided reviews of a previous version of thismanuscript that led to substantial improvements.

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