Sediment transport in the Bristol Channel: a review

Michael Collins

COLLINS, M. 1987. Sediment transport in the Bristol Channel: a review. Proc. Geol. Ass.98(4), 367-83. Investigations into the physical oceanographic characteristics of the BristolChannel and Severn Estuary are reviewed; these include the general pattern of watermovement and the influence of high freshwater discharges on the salinity distribution. Variousapproaches to the problem of defining sediment transport paths in the macrotidal estuarinesystem, for fine grained and coarse grained material, are summarised. Transport patterns areexamined, for example, through: the distribution of surficial sediments and their associatedbedforms; seabed drifter studies; the analysis of tidal current and wave data, separately and incombination, because of their influence on bedload transport paths; satellite imagery, withinthe visible part of the spectrum; and on the basis of output from depth-averaged numericalmodels. Comparison between the interpretations shows that there are discrepancies andcontradictions in the derived sediment transport paths. In particular, it is difficult to explainthe up-channel movement of fine grained material against a background of inferred offshoremovement, shown by the orientation of sandwaves and the direction of movement of thesurface waters. An early model to overcome this contradiction incorporated 'two-waydifferential transport' throughout the water column. Localised contradictions in the transportpaths for sand and 'fine sediment' occur in the inner part of the Bristol Channel.

The most recent conjectural models for sediment movement incorporate an ebb-dominatedzone along the central axis of the Channel, together with flood-dominated coastal zones. Thislateral variation in transport paths, across the Channel, constrasts with the earlier propositionfor vertical differentiation; it is suggested by seabed drifter recovery patterns and confirmed bythe output from one of the numerical models. Transport towards the flood-dominated zones atthe boundaries is enhanced under the superimposed influence of waves, which is demonstratedthrough the use of sediment transport formulae and sand tracer experiments. Although theannual contribution of the rivers to the overall sediment budget is shown to be small, the timeof retention of waters and sediment within the system could result in fluviatile sediment inputsbeing retained for tens of years.

In an attempt to extrapolate observations of transport processes into the ancient record, apreliminary facies model for the macrotidal Bristol Channel is presented. The model shows theavailability of sand increasing in an onshore direction; progressing in this direction, the faciespass from scoured bedrock and lag deposits, through cross bedded megaripple foresets and thetopset planar bedding of intertidal flats, to Holocene fill and supratidal deposits.

Department of Earth Sciences, University of Wales, Singleton Park, Swansea, SA28PP andUniversity of CambridgePresent address: Department of Oceanography, University of Southampton

1. INTRODUCTION

Sedimentation processes in estuaries vary according tothe supply and transport of fine grained and coarsegrained material and their response to the physics ofwater movement. Within restricted waters, such asestuaries, the available potential energy is reduceddue in part to conversion to kinetic energy associatedwith currents, and also through progressive frictionaland turbulent losses (Shaw, 1980).

The source of sediments to some estuarine systemshas been ascribed to a terrestrial supply, although thisis most certainly not always the case. A recent reviewof sediment exchanges across the coastal margins ofnorthwest Europe (Kirby, 1987) has emphasised theneed to understand whether a particular margin is asink or a source for sediment. For example, in theNorth Sea, the coasts of Holderness and East Angliaare important input points, whereas the estuaries of

the Wadden Sea and perhaps the Wash are sinks. Themarginal estuaries of the eastern Irish Sea are,similarly, sinks for fine sediment derived fromseaward, and not sources. Importantly, estuariesknown not to be sinks are those already filled bysediment, from both seaward and landward directions.Such estuaries are more likely to carry sediment to sea(Kirby, 1987). The Seine Estuary exemplifies thisbalance, where man has changed the natural role ofthe estuary from a sink to a source of fluvial sedimentsto the adjacent shelf (Avoine, 1987). When extendedto the other macrotidal estuaries along the south coastof the English Channel, this concept has demonstratedthat estuaries at the eastern end of the system are'filled'; those towards the west are 'unfilled' (Avone &Larsonneur, 1987). Hence, the dynamics andbehaviour of sediments within estuarine systems, suchas the Bristol Channel, require understanding. Withthe possible construction of the Severn (tidal)

367

368 MICHAEL COLLINS

TAB LE 1. Sediment transport investigations in the Bristol Channel

Investigator(s) Date

Stride 1963Belderson & Stride 1966Kenyon & Stride 1970Hamilton 1973Murray & Hawkins 1977Culver & Banner 1978Pingree & Griffiths 1979Barrie 1980Murray et al. 1980Culver 1980

Hamilton et al. 1980Owen 1980Davies 1980Collins et al. 1979Mantz & Wakeling 1981Uncles 1982Harris 1982Evans 1982Parker & Kirby 1982Collins 1983Collins & Ferentinos 1984Murray & Hawkins 1976Robinson 1978Heathershaw & Hammond 1980Heathershaw 1981Harris & Collins 1984

Harris & Collins 1985Harris, Ashley & Collins 1986

Murray 1987Harris 1987

Type of evidence

Data on bedforms and sediment distributionBedform asymmetries and sediment typeBedform asymmetriesWater circulation from temperature and salinity propertiesForaminifera from borehole samplesForaminiferal assemblagesSand transport paths predicted from numerical modelHeavy mineral analysisBedform asymmetries and sediment typeDifferential transport paths from indirect sedimentological and physical

oceanographic evidenceNear-bed current speeds and sediment typesFinite difference numerical modelBedform asymmetries and sediment typeSediment distribution, type and current dataSediment and current dataNumerical model and current meter dataBedform distributionSediment distributionSummary of other studiesAnalysis of satellite imagerySeabed drifter investigationsThe distribution of foraminiferal speciesNumerical modellingPrediction using sediment transport formulae on the basis of field observationsAs Heathershaw & HammondRepeated side-scan survey investigation, under varying hydrographical

conditionsReview of various physical oceanographic and sedimentological investigationsInterpretation of SEASAT imagery and comparison with sea-truth data from the

sea-bedBiogenic indicators of suspended transportSynthesis of information available from various macrotidal estuarine systems,

including the presentation of a facies model

Barrage, there is even more need to understand theexisting patterns of water and sediment movementover the region.

Various investigators have proposed patterns ofwater movement and sediment pathways in the BristolChannel based upon the interpretation of watertemperature/salinity data, observations of currents,analysis of satellite imagery, sedimentological andmicropalaeontological data, and the use of numericalmodels (see Table 1). This paper attempts tosynthesise these various approaches to the problemand to provide an up-to-date assessment of availabledata from a wide range of publications.

2. GEOLOGICAL AND PHYSICAL SETTING

The Bristol Channel consists of a series ofembayments and sections of cliffed coastline sur-

rounding a major submarine valley system thatconnects the estuary of the River Severn to the CelticSea. The inner part of the Channel forms the seawardextension of the Severn Estuary and is bordered byextensive intertidal flats, with a series of linearsandbanks located offshore. Such sandbanks alsooccur adjacent to coastal discontinuities along thenorthern coastline (Fig. 1).

The location of the embayments is related to theonshore geology with, in many cases, the headlandsbeing formed of resistant Carboniferous limestoneoutcrops. Offshore, the water depth varies from 50 to60 m at the mouth of the Channel, in the vicinity ofLundy Island, to 10 to 20 m in the inner part of theChannel. Based on the interpretation of seismicprofiles and gravity cores, the floor of the InnerBristol Channel/Severn Estuary has been shown toconsist of Carboniferous to Lower Jurassic limestones,

SEDIMENT TRANSPORT IN THE BRISTOL CHANNEL 369

IMinehead

Bluff

OUTER BRIS TOl CHANNEL

• I ,300

INNER BRISTOL CHANNEL S E V E R ~S T UARY

Fig. 1. Bathmetry of the Bristol Channel/Severn Estuary system (in metres), based upon U.K. Admiralty Chart 1179. Thelocations of embayments and sandbanks referred to in the text are shown. The division of the system in various Channelsegments and the Estuary is based upon the scheme adapted by IMER (Plymouth, U.K.) .

mudstones and siltstones, together with glacial till,valley infill, and surficial sediments (Evans, 1982; seealso Banner, Brooks & Williams, 1971; Evans &Thompson, 1979). Bedrock is exposed mainly to thewest and southwest of the area , following the mainaxis of the Channel , with the surficial sedimentspresent as gravel, sand and mud deposits, andoffshore sandbanks (Fig. 2). The coarse grainedoffshore deposits exist in the form of flow-parallelstructures (linear sandbanks and sand ribbons) andflow-transverse features, such as megaripples andsandwaves (see BGS, 1986: Harris, 1987). Thethickness of the surficial sediment cover is in excess of20 m in places, but is generally less than 10m (Evans,1982, fig. 3).

The general pattern of water movement in theChannel is controlled by tidal action, wind drift, andthe influence of the Earth's rotation (the Corioliseffect). Superimposed upon the semi-diurnal tidaldisplacement is a residual water circulation patterncreated by the intrusion of relatively high salinity andtransparency waters along the southern coastline,from the North Atlantic (for comparison, the mainbody of water in the Irish Sea has its origins in the

North Atlantic Drift (Lee, 1960» . Such movementcontrasts with that of more turbid and less salinewaters out along the northern coastline (Banner &Collins, 1975), which is enhanced at times of highfreshwater discharge into the Channel (Fig. 3). Thenet effect of these opposing directions of movement isto create anticlockwise residual water transport at themouth of the Channel, which has been confirmed bythe collection and analysis of conventional physicaloceanographic data (Hamilton, 1973; Abdullah,Dunlop & Gardner, 1973). This pattern is consideredto be enhanced at times of severe southwesterlywinds, as predicted from the output of a numericalmodel which incorporated the influence of wind stress(Pingree & Griffiths, 1980).

The net drift of water , enhanced by the Corioliseffect, is obscured by the superimposition ofephemeral wave- and wind-drift currents from variousdirections but , above all, by the oscillating tidalcurrents. The currents and tidal amplitudes areamong the largest in the world; they are caused by theinfluence of up-Channel reductions in width and depthon the tidal wave progressing from the Celtic Sea.Tidal ranges at Swansea, around 4°W (Fig. 1), are

370

l:W

o o

MICHAEL COLLINS

20 m

3

Sandbank=Gravel '~.~ · ~8 f'('Sand ...;.....;

Mud ::=:~:

Fig. 2. Distribution of surficial sediments in the Bristol Channel (from Harri s, 1987, with permission of Elsevier SciencePublisher s B.V .; see also Harris , 19114; and BGS , 19116).

8.6 m on springs and 4.1 m on neaps. The dominanttidal constituent is the lunar semi-diurnal (M2)

component, which accounts for some 72% of theoverall tidal amplitude at Swansea (Wilding & Collins,1980). The mean spring tidal ranges vary from 7.0 mat the mouth of the Channel to 11.0 mat Avonmouth.Measurements of tidal phase and amplitude atselected coastal stations along the central part of thenorthern coastline infer that there is a standing tidalwave over this section of the Channel (Wilding &Collins, 1980), caused possibly by reflection ofa progressive wave from offshore. The progressivetidal wave moves in along the southern coastline andextends across the complete width of the Channel ,where it narrows (Heaps, 1968; see also Taylor , 1921).Additionally , tides in the Bristol Channel have beenmodelled numerically by, amongst others, Bennett(1975), Robinson (1978), Fong & Heaps (1978),Owen & Heaps (1979), Heaps (1982), and Uncles(1982). These models produce a supplementary database for understanding water and sediment movementin the Channel. In particular, they are being usednnw to predict changes that might take place in the

event of the Severn (tidal power) Barrage beingconstructed (Miles, 1982; Odd , 1982).

In the same way that the tidal wave is enhanced inits passage up-Channel, storm surges undergo thesame modification (Heaps, 1967); these surges canresult in sea levels being above those predicted on thebasis of tides alone, although the regional setting ofthe Channel is such that they would not be expectednormally to coincide with HW spring tides . At thehead of the Estuary, on spring tides, a 'tidal bore'develops .

Although the tidal currents move water particlessome considerable distance (10 to 22 km) during eachof the ebb and flood phases of a tidal cycle (Shaw,1980), the net displacement of an individual particle isrelatively small (IMER, 1974). Such characteristics ofmovement create high retention times for freshwaterrunoff, throughout the various compartments of theBristol Channel , which is important when consideringthe movement and deposition of fine grainedsedimentary material within the complete estuarinesystem .

Some reference has been made already to the

SEDIMENT TRANSPORT IN THE BRISTOL C H A N N E L 371

Fig. 3. Surface salinity data relating to IMER Cruise 2/78 (February, 1978) at a time of high freshwater discharge. Surfacesalinity corrected to low water at Swansea . Sampling points denoted by squares (from Stephens, 19R6; with permission ofPergamon Journals Ltd) .

ephemeral nature of the mechanisms controlling watermovement in the Channel, including the presence ofwind-driven waves. Some areas of the system areparticularly susceptible to wave influence, such as thenorthern coastline in the vicinity of Carmarthen andSwansea Bays (Fig. 1). These regions have anunrestricted fetch of some 6000 km out into theAtlantic Ocean. Wave data for these embayments,and other parts of the Channel, are available from thepermanently anchored Lightships (Helwick andScarweather) at the western ends of the respectivesandbanks (for location, see Fig. 1), local CoastguardStations (e.g. see, Shackley & Collins, 1984), or fromspecific pressure-sensor or wave rider buoydeployments.

Data collected using a wave-rider buoy, deployedand maintained by lOS (Taunton) at the ScarweatherLight Vessel over a 12 month period, showed waves tovary from 2 to 16 sec in period and up to 5 m in height(Fortnum & Hardcastle, 1979; see fig. 2 of Pattiaratchi& Collins, 1984). At Port Talbot Harbour, on theeastern coastline of Swansea Bay , occasional waveheights of 7 m have been recorded , and heights of upto 5 to 6 m are not uncommon in times of storms(Jackson & Norman, 1980). Up-Channel, within theSevern Estuary itself, 50-year significant wave heightsof around 4 m have been predicted on the basis ofmeasurements at selected sites, as part of the

pre-feasibility study for the proposed Severn EstuaryTidal Barrage; comparable wave height maxima are ofthe order of 5 m (Shuttler, 1982).

The dominant physical processes affecting watermovement in the Channel, tides and wind-generatedwaves, create currents that interact with thebathymetry and available sedimentary material. Thetidal currents are essentially rectilinear and directedalong the main axis of the Channel , with maximumsurface water speeds of the order of 1.5 and 0.9 m S-l

in the Inner and Outer Channel, respectively (StationsL and A, Admiralty Chart 1179). This system ismodified within the embayments of the northerncoastline, leading to the development of tidal currenteddies and their associated sandwave fields (Harris &Collins, 1984) and linear sandbanks (Ferentinos &Collins, 1979, 1980). These eddies have beenpredicted throughout the Channel, on the basis of adepth-averaged tidal model of the Estuary (Owen,1980). In other areas of the Channel , the distributionsof bed shear stress , predicted from numericalmodelling approaches , have been shown to correlatewell with the exposure of bedrock and/or the presenceof seabed sediments (e.g. see Collins, Banner, Tyler,Wakefield & James, 1980, fig. 14.1) . The sedimentarysubstrate, in turn, is related to the distribution ofsublittoral macrofaunal communities in the BristolChannel (Warwick & Davies, 1977). Hence, the

372 MICHAEL COLLINS

benthic macrofauna show some direct association withtidally-induced shear stress (Warwick & Uncles ,1980).

Currents induced by wave action in the Channel ,either in response to local winds or created by swellswith their areas of generation out in the AtlanticOcean, have been shown also to be an importantmechanism in determining the distribution of recentsediments or the dimensions of flow-parallel bed­forms . Investigations into variations in sublittoralsediments and their associated macro-infauna inSwansea Bay have used the presence of storm-inducedwaves in a model to explain the presence , in asublittoral deposit, of allochthonous bivalves of aninshore muddy-sand Abra community together withan offshore Spisula community. The latter weretransported shorewards, under storm conditions, froma sandwave zone located to the southwest (Shackley &Collins , 1984). Substrate response has been shownelsewhere to provide an extremely important mechan­ism for the onshore transport and supply of sand tothe area (Pattiaratchi & Collins, 1984, 1985; see alsodiscussion below on sediment transport paths) . Waveaction in the Channel is considered also to be thelimiting factor in the development of the heights ofthe various sandbanks present , with the maximumheight being limited by breaking waves (Britton &Britton . 1980). Progressing in an easterly, up­Channel direction , it has been demonstrated thatthe linear sandbanks here (Scarweather, Nash andCulver) are exposed to a narrower sector of approachof long period destructive wave action than, forexample , the Helwick Sands (Fig. 1). On the mostwave-exposed of the sandbanks, faunal diversity isreduced greatly; the benthic ecology of the bank ismodified, through wave action, by destabilising thesubstratum and winnowing out organic matter (Tyler& Shackley, 1980). The sedimentological characteris­tics of one of the Inner Channel sandbanks has beendiscussed in detail by Davies (1980) .

Combinations of wave- and tidally-induced currentsare considered below within the context of long-termand short-term patterns of sediment movement withinthe Channel/estuarine system.

3. SEDIMENT TRANSPORT PATHS

Various authors have attempted to synthesiseelsewhere the present state of knowledge ofsedimentation processes in the Bristol Channel(Collins , 1983; Dyer, 1984; BGS , 1986: Harris , 1987),or have reviewed changes in overall environmentalconditions that may occur in the event of theconstruction of a tidal (power) barrage (Shaw , 1980;Mettam , 1982). Certainly , the supply of fine grained(muddy) and coarse grained (sandy) sedimentarymaterial to the system and their transport paths withinit are complex and not clearly understood , although

various investigations have been undertaken over thepast 20 years (Table 1).

(a) The Severn EstuaryAt the upper end of the system, within the SevernEstuary, a schematic sand circulation pattern has beenproduced (Fig. 4), on the basis of the interpretation ofa wide range of sedimentological, geochemical, andphysical oceanographical observations. Even so ,there are some contradictions in the derived directionsof transport within a regional context. From theavailable evidence it 'appears that sand . .. moveseastward into the Severn Estuary. Along this routethe various banks form local recirculation cells ...some sand may return westward along the southernsand zone, but there is no firm evidence for this'(Parker & Kirby, 1982).

The movement of fine grained sediment over thesame area of the estuary is complicated by thepresence of high concentrations of suspended materialin the water column, which form 'fluid mud' layersnear the bed and are represented as a 'suspendedsediment front' in the surface and underlying waters.The suspended load over the greater part of the watercolumn ranges between 0.25 to > 10g r 1; its vertica ldistribution is controlled by the tidal range and phase(neap-spring). At maximum flow on spring tides, thesuspended load is mixed throughout the watercolumn. Near slack water on springs and throughoutthe neap tidal cycles , stratification occurs withnear-bed concentration levels well in excess of 10 g 1-1(Kirby & Parker, 1982). Although mechanisms forthe formation and maintenance of the front are notyet known, it can be seen occasionally at the seasurface, in calm weather, as "a sharp interfaceseparating ' blue' and 'brown' water" (Kirby & Parker,1982). The major long term source of fine grainedsediment in the area is the extensive and settled muddeposit in Bridgwater Bay. Sediment from thisdeposit is considered to move along the Estuary ,principally on the English side, to form the turbidwater mass (Fig. 5). In addition to Bridgwater Bay,sediment sinks arc the deep water area oppositeNewport and the peripheral estuaries and accretingsalt marshes. Other fluid mud layers have beenidentified in the dredged approach Channel toSwansea Docks (Banner, 1980). Similarly , a finegrained depositional area, with flaser bedding withinthe deposit indicating variability in (tidal and wave)energy input, has been identified in eastern SwanseaBay (Collins , Ferentinos & Banner, 1979). The se twoareas may constitute the major zones of net finegrained sediment deposition within the outer part ofthe Bristol Channel system (see Fig. 2).

(b) The Bristol Channel (see Table 1)Attempts at understanding the sediment dynamics ofthe Bristol Channel commenced in the mid-1960s with

SEDIMENT TRANSPORT IN THE BRISTOL C H A N N E L 373

15 '

Cardiff

/ ' ~ ,/......... _- .....,;'

15I

101

krn

5m

10m

15m

5

..?.: :.. ::........ . "..

. . .. .; :: :. :.;~::.~

oI

......

..:.:... ..:

.::: .....

30 3·W

Fig. 4. Schematised patterns of sand circulation in the Inner Bristol Channel and Severn Estuary, based upon data fromvarious source s (from Parker and Kirby, 1982).

the investigat ions of Belderson & Stride (1966) andKenyon & Stride (1970), following on from the earlierwork of Stride (1963). The se studies relied upon theanalysis of side-scan sonar data collected, in part atleast , from NERC research vessels entering andleaving the research base at Barry. Initially, a largefield of sandwaves was identified in the Out erChannel; these were oriented asymmetrically towardsthe west , inferr ing the transport of coarse grainedsediment in that direction. A correspondingly smallergroup of easterly-oriented sandwaves was present inthe inner part of the Channel. These opposingtransport directions were described by Belderson &Stride (1966) as a 'bed-load parting zone' . From thiszone (Harris & Collins, 1984, fig. Sa), sediment istransported as bed load towards the east into theSevern Estuary and towards the west into the CelticSea; these patterns of movement originate from azone of divergent transport paths. These directions

are confirmed by output from MzIM4 tidal constituentanalyses from numerical models (Pingree & Griffiths ,1979). It should be noted that the sandwaves arerestri cted to the deep er water offshore and channelledareas and, in some cases, have been identified recentlyin satellite (SEASAT) synthetic aperture radarimages. Based on comparative side-scan sonarsurveys (in 1977 and 1983) and the satellite dat a(1979), the sandwaves are considered to vary from'rel ative stabilit y' to migrating at rates of up to100 m/yr. Bedforms > 11m in relief and with crestlinespacings > 500 m, which were visible to SEASAT,were present in water depths of 45 m and with surfacecurrent speeds of 70 em s" I (Harris , Ashley & Collins,1986). Bed-load transport rates and directions, basedon the analysis of self-recording current meter dataand the applicat ion of tran sport formulae for(unidirectional) tidal flow, have provided confirmationof the paths inferred from the sandwaves. Offshore

374 MICHAEL COLLINS

oI

5I

10I

15!

.:::'..

km

5m

tOm.. .. ..... ... . 15m

..:.., .~ . :, ' ,

Suspended 15'sedimentfront

30 ' 3·W

Fig. 5. SchemaIised circulation of fine sediment in the Inner Bristol Channel and Severn Estuary, based upon data fromvarious sources (from Parker and Kirby, 1982).

from the Gower Peninsula (Fig. 1), the derivedtransport directions were towards the west, with ratesof 2 tonnes/m width (of seabed)/day (Heathershaw &Hammond, 1980; Heathershaw, 1981). In this case,two different approaches to understanding theproblem of sediment transport have providedcomparable results; these have demonstrated themovement of coarse grained sediment as bed-load outof the estuarine system, towards the Celtic Sea.

In contrast to the correspondence between tran­sport paths described above, foraminiferal evidencefrom the Channel is indicative of movement in theopposite direction. Very small planktonic foraminif­era tests (c.l40 /lm) have been found in the SevernEstuary, although the area does not support livingindividuals ; these are thought to have been trans­ported in suspension from the Celtic Sea (Murray &Hawkins, 1976). Furthermore, sediments of theintertidal zone of the Severn Estuary and BristolChannel have been found to contain marineallochthonous foraminiferal assemblages (Culver &Banner, 1979). Biogenic debri s in the muds and silts

of Llanrhidian Marsh, to the north of the GowerPeninsula, for example, consists of echinoid ossicles,with marine benthic and planktonic foraminifera; thisindicates that the bulk, at least, of the fine sedimentsoriginated offshore (Banner & Collins, 1975). Amodel to explain these apparent contradictions intransport paths was proposed by Culver (1980, fig. 1).The model (Fig. 6) represents the bed-load partingzone and the sandwaves as part of 'tidal-currentdriven bed-load transport system', with movement ina predominantly offshore direction. Fine grainedsediment, particularly the biogenic component, isconsidered to move in the opposite (onshore)direction as 'suspended-load transport , (by a)wind-driven current' . Interestingly, contradictionsbetween transport directions inferred from biogenicindicators and those from bed-load movement , are notrestricted to the Bristol Channel. The distribution ofnannoplankton and foraminifera is indicative , oftransport from the shelf into the estuaries of theEnglish Channel , with the tests being thrown intosuspension by tidal and wave energy . It is suggested

SEDIMENT T R A NSPO RT IN THE BRISTOL CHA NNE L 375

km

20 40I I

oI

SOUTH WALES

--- ---.................. •................ '-----~--- - -=-- .........----

- ......--- .....a----- -r------.....'"-~~

IRISH SEA

-- -/' "-/ \

/ \Area

I \\ of mud J\ /\ /

"- /'

Approximate southern 11m i t ofpresent day thick surficial sed imentTidal current drivenbed-load transportWind driven currentsuspended-load transport

Bed-load parting

CELTIC SEA

IRELAND

--Fig. 6. Present-day sediment transport regime in the Bristol Channel and Severn Estuary (modified from Culver, 1980, withpermission of Elsevier Science Publishers B. V.)

that transport is most active during the winter andparticularly during severe storms at times of springtides (Murray, 1987). Bed-load transport of sandgrade material in the English Channel is also knownto be from east to west (Lee & Ramster quoted inMurray, 1987).

Additional information on the landward transfer ofsediment in the Bristol Channel, in this case withinthe fine sand size range , is provided by analysescarried out on sea-bed sediment s and the near-beddistribution of tidal currents. Interaction between thefluid flow and the loosely consolidated sediments isconsidered to have led to selective sorting ofsediments, leading to the migration up the estuary ,from the Celtic Sea continental shelf, of increasinglyfiner sediment (Hamilton, Somerville & Stanford,1980). Within this context, the sedimentary regime ofthe Estuary is considered to be part of a larger systemrelated to the Celtic Sea. The inverse relationship

between current speed and grain size , which iscontrary to normal expectation, results from suchfactors as tidal asymmetry and residual flows of thecompetent currents (Hamilton et al., 1980). Flow andgrain size parameters for three well sorted sands fromthe shelf and Estuary are listed in Table 2, showingthe presence of finer grained sediments in the higherenergy environment of the Estuary.

Without recourse to any additional dat a sets, theCulver 'two-layered' conceptual model provides anadequate explanation for differences between bed andsuspended load transport paths in the Bristol Channel.On the basis of the analysis of LANDSAT satelliteimagery within the visible part of the spectrum, usingsuspended sediment in the surface waters as a passivetracer of sediment and water movement , analtern ative interpretation became possible (Collins,1982). Using sate llite imagery representative of awide range of tidal and meteorological conditions

376 MICHAEL COLLINS

TABLE 2. Flow and grain size parameters of sediment samples from the Severn Estuary and continentalshelf of the Celtic Sea (adapted from Hamilton , 1982)t

82134 Outer shelfCeltic Sea

82246 Mid-shelfbank , CelticSea .

SE8420 SheperdineSand , UpperSevernEstuary

Water Sand Size,Depth Md Sorting

(m) (mm) a

176 0.23 0.27

82 0.27 0.25

BoundaryShear Shear

Velocity StressV,max 1'0 maxmm/s (N/m 2

)

21.5 1.02

49.8 2.54

SampleRef· Location

Intertidal 0.15 0.25

Max. currentspeed atheight Z

in cm(mm/s)

305(at Un)

411(at U100)

3320(at U 100) 169.7 29.37

t Flow data were obtained from a velocity gradient rig, operating within the bottom 1.8 m of the watercolumn.

(Collins , 1983, table 1), the boundaries of high,medium, and low concentration waters were iden­tified. From the orientation of tongues of waterassociated with these boundaries, movement of waterand suspended material has been inferred. Super­imposition of a number of interpretations of individualimages led to the preparation of a summary diagram(Fig. 7), in which directions of movement inferredfrom the satellite imagery are compared with:observed and computed residual currents in thesurface waters , based upon self-recording currentmeter data and numerical model outputs, respectively;and the release and recovery positions of surface(Woodhead) drifters and drift cards .

Survey data and satellite imagery indicate nettransport of sediment and waters in an offshoredirection , with the movement being stronger along thenorthern coastline; this is consistent with the general(residual) pattern of water movement in the Channel,enhanced by the Coriolis effect (Collins, 1983).Movement to seaward is accompanied by sometransfer towards the southwest, into Barnstaple Bay,which is confirmed by the orientation of the surfacetidal streams (Admiralty, 1973) and with bothobserved and computed (Eulerian) residual currents(Uncles , 1982). This pattern of residual watermovement in the central/outer Channel is confirmedby the surface drifter recovery patterns from Station X(Fig. 7). Forty-five per cent of those released at thestation were recovered in Barnstaple Bay within thefirst 75 days (North (MAFF), pers . comm.).

Superimposed upon this pattern is the influence ofwind drift, which can be identified in the satelliteimagery by producing diffuse boundaries to the watercontaining different levels of material in suspension .Furthermore, drift cards released at Station Y (Fig. 7)

moved initially in the direction of the tidal currents;subsequent recovery was dominated by wind direc­tion . Under northwe sterly winds, for example, driftcards have been found to reach the Porlock/Mineheadarea within 2 to 6 days (Borthwick & Collins, 1976).Hence, wind drift appears to cause movement which isoblique to the tidally-dominated residual currents,acting as a secondary mechanism for the cross­Channel dispersion of fine grained material in thesurface waters (Collins, 1983). Wind/wave effects canalso remove material from the upper surfaces, orcrestlines of the linear sandbanks, making it availablefor subsequent transfer by the tidal currents(Pattriaratchi , Hammond & Collins, 1986); as statedpreviously , this acts as a mechanism to control theheight of the sandbanks.

The net seaward movement of fine grained materialin suspension in the surface waters of the Channel , inthe presence of wind drift, provides a mechanism ofsupply to mud deposits offshore (Stride, 1963), but issomewhat contradictory to the model proposed byCulver (1980). Indeed, each of the mechanisms oftransfer described so far relates to the seawardtransfer of fine and coarse grained sediments. Inorder to explain the landward transfer of foraminiferaand the presence of minerogenic material in theSevern Estuary, considered to have originated formthe Celtic Sea, movement must also take place in anupstream direction . The results of a seabed drifterinvestigation, together with the analysis of detailedside-scan sonar records and the identification ofwave-induced transport paths, provide such amechanism.

Woodhead seabed drifters were released from 11stations in the Channel between 1976 and 1977 (fordetails see Collins & Ferentinos, 1984); the recovery

SEDIMENT TRANSPORT IN THE BRISTOL CHANNEL 377

L!'lr--­e-,o­~o

BARNSTAPLEBAY

~co c~

• I

51 30

• I

51 15

Fig. 7. Summary of inferred residual surface water and suspended sediment movement in the Bristol Channel , based on datafrom a variety of sources. +- Transport directions , based upon satellite imagery and survey data (<l-- - Possible Directions) ;

- '-'-' Supended sediment 'front' , identified from tatellite imagery; I(M/D7/751 Upper Boundary of zone of High

Turbidity 124/04/751 as above , but Lower Boundary ; 0~ Observed (0) and computed (C) surface residual currents,based upon density and nonlinear tidal effects (Uncles 1982); +- Schematized residual surface water circulation (from Parkerand Kirby, 1982); ® Release position for surface (Woodhead) drifters , with 48% recovery in Barnstaple Bay (within hatchedarea of coastline) (Norton (MAFF) , personal communication) ; 0 Release position for drift cards, some of which wererecovered from Porlock/Minehead (from stippled area of coastline) .

levels were between 51% and 68%. Drifter transport Bristol Channel, originating from density gradients,paths, inferred on the basis of the patterns of recovery non-linear advection, and continuity (Uncles, 1982).and times elapsed between recovery and release, Furthermore, the residual currents are consistent withrequired careful interpretation. Such drifters are used currents derived from the pattern of movement of thenormally in regions where water movement can be drifters , assuming that the drifters move at ap­more simply understood. In this particular case, due proximately half the speed of the surrounding waterconsideration needed to be given to the large tidal mass (based upon laboratory investigations; Collins &excursion of water particles and the susceptibility of Barrie, 1979). Also shown on the model (Fig. 8) isthe drifters to be influenced by wave action . The some tidal current and wave-induced transfer from theconjectural near-bottom drift pattern derived for the central to the northern coastal region . Wave-inducedBristol Channel (Fig . 8) takes into account the fact transfer would be enhanced particularly under thethat peaks in drifter recovery were often separated by superimposed influence of southwesterly swell wavesperiods of between 200 and 300 days. (see below). The forcing mechanism for the frictional

The suggested near-bed water circulation pattern (tidal) residuals is related to the relative amplituderepresents seaward transfer along the mid-Channel and phase differences between the main semidiurnallongitudinal axis and landward transport along the (M 2) and first , shallow water, harmonic (M.) tidalcoastal zones (Fig. 8). These directions of movement constituents (see Collins & Ferentinos , p. 39).are consistent with predicted residual currents for the Analysis of some 3000 km of detailed side-scan

378 MICHAEL COLLINS

Fig. 8. Conjectural near-bottom drift pattern in the Bristol Channel. The shaded area represents upstream coastal boundarytransfer (based upon Collins & Ferentinos , 1984).

sonar data from the Bristol Channel has demonstratedthe presence of megaripples, sandwaves , and sandribbons on the seabed (Harris & Collins, 1985).Although the megaripple asymmetries have beenshown to reverse between ebb and flood phases of thetidal cycle, the sandwaves retain their asymmetricalorientation even under extreme hydrodynamic condi­tions (see also Harris & Collins, 1984). Using thesandwaves as indicators of net bed-load transportdirections in the Bristol Channel, therefore, theextensive data set has shown that there are differencesbetween the offshore mid-Channel zone and thoseadjacent to the coastline. The offshore pattern ofmovement is consistent with that predicted by Kenyon& Stride (1970) as part of their 'bed-load partingzone' . In contrast, up-Channel transfer of coarsegrained sediment is inferred along the areas of theseabed adjacent to the northern and southerncoastlines (Harris & Collins, 1985, fig. 6). Thesedirections of movement are consistent with thoseshown from the recovery patterns of Woodheaddrifters (Fig. 8) and with the 'selective entrainment'mechanism for the tidally-induced landwardmovement of fine grained sand from the Celtic Sea to

the Sherperdine Sands (see Table 2) in the SevernEstuary (Hamilton et al. , 1980). Supporting evidencefor movement of landward movement along thesouthern coastline is provided by heavy minerals thathave their origin in the rivers of Barnstaple Bay(Barrie, 1980). Whereas the sandwaves mayrepresent the up-Channel movement of coarse grained(sandy) material, the seabed drifter recovery patternsmight be considered to represent fine sand/silt movingas bed/saltation load along the coastal zones .

This model for lateral inhomogeneity in net,residual transport paths (Fig. 8) differs from thatproposed by Culver (1980) (Fig. 6), which assumesvertical two-way differentiation throughout the watercolumn . Nevertheless, it can be accommodatedwithin the pattern of movement of the surface waters(Fig. 7) and could provide: a feeder mechanism forsediment to the 'bed-load parting zone '; and anexplanation for the long-term landward movement ofbiogenic and clastic particles within the large scaleestuarine system.

The enhanced transfer of sedimentary materialtowards the northern coastline, in response tosouthwesterly waves and shown on the model (Fig. 8),

SEDIMENT TRANSPORT IN THE BRISTOL CHANNEL 379

is exemplified by investigations concerned specificallywith wave current interaction (see Pattiaratchi &Collins, 1987). The results of these studies aresummarised in Fig. 9 and demonstrate how theresultant (R) bed-load transport vector can changeunder the superimposed action of waves (w) upontidally-induced transport (1;, = tidal current).

In the first of the studies (Fig. 9a), self-recordingcurrent meters were deployed and 1 tonne offluorescent sand (400~m mean diameter) tracer wasreleased in water depths of between 20 and 40 moffshore from the Gower Peninsula (Pattiaratchi &Collins, 1984, 1985). Under spring tidal currentsalone, the directions of transport shown by the tracerand computed from the current meter data weretowards the west. Under the superimposed influenceof waves (7.5 s period/1.5 m height (average);12.5 s/2.3 m (oceanic); and 12.5 s/3.2 m (stormy)), thepredicted transport vectors were reversed. Thelandward direction of movement was confirmed bythat of the centroid of the tracer, particularly duringconditions of high wave energy and low (neap) tidalcurrents (Pattiaratchi & Collins, 1984, fig. 6).

In another study, carried out just to the north of theScarweather Sands (Fig. 1), radioactively labelledmaterial was deposited and monitored subsequently.

I aJCOASTLINE

//////////////////////

This research was undertaken by the Institute ofOceanogr:phic Sciences (lOS, Tautnon) and utilisedScandium to treat the sediments (170~m mean grainsize) and movement was traced over a 12 monthperiod using a scintillation counter (Heathershaw &Carr, 1977). The results again demonstrated somedifferences between transport paths under theinfluence of waves and those under tidal currentsalone . In this particular case , southwesterly wavesprovided the mechanism to move material across thetidal current direction, which ran in a northwesterly­southeasterly direction (Fig. 9).

These mechanisms for the transfer of sedimentsfrom the central Channel ebb-dominated zonetowards the northern and eastern coastline, help toexplain the onshore movement of sand from thesublittoral embayment to the modern supra-littoralsand dunes. This transfer has continued at least sincethe Early Bronze Age (c. 3900B.P.) and is supportedby historical evidence of inundation of the area bysand during the 12th, 13th, and 15th centuries. A16th century visitor, for example, wrote of ruins'almost choked and devoured with the sands that theSevern Sea there casteth up' (Collins & Banner, 1980;see also North, 1964). In this sense, the conjecturalmodel for sediment movement in the main part of theBristol Channel is consistent with the availablehistorical and geological evidence for the transfer ofsand from offshore to onshore.

4. OTHER CONSIDERATIONS

I bJ

Fig. 9. Examples of wave (w) and tidal current (Tc )

interaction from the Swansea Bay area of the BristolChannel, with the resultant transport vector (R) shown: (a)offshore from the Gower Peninsula ; and (b) adjacent to theeastern coastline of the embayment.

Te 4

(a) Sediment budget

Although much of the evidence available is indicativeof a marine source of sediment supply, it is interestingto consider the potential contribution of the rivers tothe amount of material contained within the system(see also Parker & Kirby, 1982).

Using a regression equation relating annualsuspended sediment load (y) to catchment area (x),y = 12.9x\.zl derived from other catchment investiga­tions in the British Isles (Collins, 1981), the annualpredicted load for all rivers discharging into theBristol Channel was 1.63 x 106 tonnes . Of thisamount, the Wye, Avon and Severn contribute1.25 x 106 tonnes , or 77% of the total. Contributionsto the various embayments are : Carmarthern-D.lO x106 tonnes ; Swansea-D.28 x 106 tonnes ; Bridfwater­0.29 x 106 tonnes; and Barnstaple-D.lO x 10 tonnes.Comparison between these predictions and thosemade by other investigators (e .g. see Brookes, 1974)is reviewed in Collins (1983) and are shown to be ofthe same order of magnitude.

For the Bristol Channel as a whole , attempts havebeen made to quantify the satellite imagery in termsof suspended sediment concentration (SSC, in mg/l)and to make allowance for vertical density gradients

380 MICHAEL COLLINS

within the water column. Using these criteria , it hasbeen estimated that around 9 x 106 tonnes are presentthroughout the system on MHWN tides and 13.4 x 106

tonnes on springs (Collins, 1983). These estimatescan be compared with: 5 x 106 tonnes of mobile finesediment which is considered to be 'potentiallyavailable for resuspension, especially during stormconditions. . . for the area between Swansea Bay tojust above Avonmouth' (Hamilton et al., 1980,p. 142); between 10 and 30 x 106 tonnes of mobile finesediment being available in the estuary section fromthe shoals, just upstream of Avonmouth, to 3°30'W(see Fig. 2) (Parker & Kirby, 1982); and, on the basisof geophysical surveys, a conservative estimate ofsettled mud in Bridgwater Bay of 270 x 106 tonnes forthe sublittoral area. 7.5 km2 of the overall 30 km' ofthe latter area is thought to be erosional (Parker &Kirby, 1982). Rates of erosion of the surroundingcliffs, to provide comparable data, are less easilydefined but are considered to be generally low.

On the basis of the sediment loads outlined above,the mobile population of fine grained material present

Scoured bedrockand lag depos its

in the Channel is equivalent to either 3 to 4 years ofannual river sediment supply (Collins, 1983) or 20years of river input. The settled mud population inBridgwater Bay alone is equivalent to of the order of200 years of river input (Parker & Kirby, 1982).

(b) Fades reconstruction

Although the understanding of modern sedimenttransport processes within the Bristol Channel/SevernEstuary system is important in itself and is an essentialprerequisite for the construction of any tidal barragescheme , other geologists will be interested in thisparticular estuarine sequence in terms of a faciesmodel. Concerning estuarine sequences in general,Frey & Howard (1986) make the following statement:'Although much has been written about estuaries andestuarine sediments, very few facies models have beenproposed to date and considerable ambiguity remainsin coastal classification schemes... an 'estuarinesequence' consists of complex intertidal to subtidal,mostly Channel form facies dominated to some extent

Suprat idal deposits

~~~~~In t e r t i d a l flats

top set planarbedding

Bioturbated,Megaripple foreset s

Cross-bedding ofmegaripple foresets?

-or­Unstr a t ifi ed?

Fig. 10. Block diagram showing the lateral distribution of bedform facies in the Bristol Channel from the offshore sedimentstarved areas characterised by sand ribbon s, grading into headland-associat ed linear sand banks ; these culminate in intertidalflat deposits , backed by subaerial dune systems (from Harris, 1987, with permission of Elsevier Science Publishers B.V.).

SEDIMENT TRANSPORT IN THE BRISTOL CHANNEL 381

by tidal effects.. . .' These investigators then proceedto describe mesotidal estuarine sequences.

For the macrotidal Bristol Channel , Harris (1987)has attempted to reconstruct a facies model (Fig. 10),which represents the lateral distribution of bedformfacies in the Channel. Sand is scarce in the centre ofthe Channel (Fig. 2) and sand ribbons, grading intoelongate sandwave trains and an extensive sandwavefield, are separated by bedrock exposures. Inshore,where sand is more plentiful, headland-associatedsandbanks have formed ; farther inshore, intertidalflats backed by aeolian dunes are deposited (Harris,1987). Within the inshore deposits, the superimposedinfluence of waves can be detected by the presence ofsand horizons containing Spisula shells. The surfaceof these sand layers is often rippled , with symmetricalbedforms (Shackley & Collins, 1984, fig. 8). Theinshore sediments form thickly laminated deposits ofmud and sand (predominantly sand) or thinlylaminated (predominantly mud) deposits.

5. CONCLUDING REMARKS

(a) On the basis of the assessment of varioussedimentological and physical oceanographic data setsfrom the Bristol Channel , some showing correspon­dence between transport paths and others contradic­tions, a transport model for the Channel has beenpresented. In contrast to earlier models, such as thatof Culver (1980) which assumes vertical two-waydifferentiation, the latest attempt to explain move­ment (Collins & Ferentinos, 1984) proposes lateralinhomogeneity in direction. Upstream movementtakes place along the coastal boundary zones of thenorthern and southern coastline, enhanced by wave

action : the central, longitudinal, axis of the Channel isebb-dominated. A similar model of lateral variationin transport paths was proposed by Robinson (1978)to explain the seaward origin of mud deposits in theinner Humber Estuary. The model may also findsome application in the macrotidal Bay of Fundy(Middleton, pers. comm .)(b) The annual input of sediments from riversconstitutes only a proportion of the amount of materialpresent at anyone instant within the large scaleestuarine system. Various estimates of the sedimentbudget suggest that it would require between 3 and 20years of river input to be equivalent to the mobile finegrained sediment population present. Nevertheless,long term fluvial sediment input could still influencelong term sedimentation patterns. It has beensuggested (IMER, 1974, p. 26), for example, that 'thebulk of the sedimentary material is retained within thesystem for periods of at least tens of years' and that'resuspension is the major source of particulate loadobserved at anyone time'. Such an interpretation isconsistent with flushing times in the Channel of theorder of 150 to 400 days.(c) A preliminary facies model for the macrotidalBristol Channel is presented.

ACKNOWLEDGEMENTS

I am indebted to Ms. J. Greengo and Keith Naylor(Department of Earth Sciences, Swansea) for theirtyping and draughting achievements, respectively.The manuscript was prepared whilst I was in receipt ofa Visiting Fellowship at St. Johns' College, Cambridgeand attached to the Department of Earth Sciences,University of Cambridge. This contribution repre­sents Cambridge Earth Science Contribution No. 993.

References

ABDULLAH, M. I., H. M. DUNLOP & D. GARDNER.1973. Chemical and hydrographic observations in theBristol Channel during April and June, 1971.J. mar. bioi.Ass ., 53, 299-319.

ADMIRALTY, 1973. Tidal Stream Atlas: The English andBristol Channels, Hydrographic Office, Tauton , U.K.,16 pp.

AVOINE, J. 1987. Sediment exchanges between the SeineEstuary and its adjacent shelf. JI. geol. Soc. Lond., 144,135-48 .

-- & C. LARSONNEUR. 1987. Dynamics and behaviourof suspended sediment in macrotidal estuaries along thesouth coast of the English Channel. Continental ShelfResearch (in press).

BANNER, F. T. 1980. Sediments of the north-westernEuropean shelf. In (Banner, F. T. et al .; eds), TheNorth-West European shelf seas: The sea bed and thesea in motion 1. Geology and sedimentology , ElsevierOceanography Series 24A, Elsevier , Amsterdam , 271-300.

--, M. BROOKS & E. WILLIAMS. 1971. The geology of

the approaches to Barry, Glamorgan. Proc. Geol. Ass.,82,231-47.

-- & M. B. COLLINS. 1975. 'Introduction toOceanography' at University College of Swansea. Proc.Geol. Ass ., 86, 87-93.

-, M. B. COLLINS & K. S. MASSIE. (eds.) 1980. TheNorth-West European shelf seas: the sea bed and the sea inmotion 11. Physical and chemical oceanography, andphysical resources, Elsevier Oceanographic Series 24B,Elsevier, Amsterdam.

BARRIE, J . V. 1980. Heavy mineral distribution in bottomsediments of the Bristol Channel , U.K. Estuar. est. mar.Sci., 11,369-81.

BELDERSON, R. H. & A. H. STRIDE. 1966. Tidalcurrent fashioning of a basal bed. Mar. Geol., 4,237-57.

BENNETT, A. F. 1975. Tides in the Bristol Channel.Geophys. J. R. astr. Soc., 40, 37-43.

B. G. S. (BRITISH GEOLOGICAL SURVEY) . 1986. SeaBed Sediments and Quaternary Geology: Bristol ChannelSheet 51°N-04°W, HMSO, London.

382 MI CHAEL COL LINS

BORTHWICK, I. & M. B. COLLINS. 1976. Tidalmo vements around Mumbles Head and along the CowerCoast , Final Report to Swansea City Counc il, 31 pp.

BRITTON, R . & S. R. BRITTON. 1980. Sedimentarybedforms and linear banks . In (Collins , M. B. et al. ; eds.),Industrialised embayments and their envi ronmentalproblems: a case study of Swansea Bay. Pergamon Press,Oxford, 177-92.

BROOKES, R . E. 1974. Suspended sediment and solutetransport for rivers entering the Seve rn Estuary . Unpubl.Ph .D . thesis, Univ. Bristol, U.K., 114 pp.

COLLlNS, M. B . 1981. Sediment yield stud ies of headwate rcatchments in Sussex, S. E . England. earth surfaceprocesses and landforms, 6, 517-39.

-- 1982. Discussion on Papers 18-20. In Severn Barrage ,Thomas Telford , London, 209-10.

-- 1983. Supply, distribution and tran sport of suspendedsediment in a macrotidal environment: Bristol Channel,U.K. Can. J. Fish. Aquat. Sci . , 4O(Suppl. 1) ,44-59.

-- & F. T . BANNER. 1980. Sediment transport by wavesand tides : probl ems exemplified by a study of SwanseaBay, Bristol Channel. In (Banne r, F. T . et al. ; eds.), TheNo rth-West European shelf seas: the sea bed and the sea inmotion. 11. Physical and chemical oceanography , andphysical resources. Elsevier Oceanography Series, 24B,Amsterdam, 369-89.

- , F. T. BANN ER, P. A . TYLER, S. 1. WAKEFI ELD& A . E . JAMES. (eds.) . 1980. Industrialised emb aymentsand their environmental problems : a case study of SwanseaBay , Pergam on Press, Oxford , 615 pp.

- - & J. V. BARRIE. 1979. The thre shold of movement ofthe Woodhead sea-bed drifter under unidirectional andoscillatory flow, separately and in combination . Dt.hydrogr. Z., 32, 133-18.

-- & F. FERENTINOS. 1984. Residual circulation in theBristol Channel , as suggested by Woodhe ad sea-beddrifter recovery patt erns. Oceanol. Acta, 7, 33- 42.

- , G. FERENTINOS & F. T. BANNER. 1979. Thehydrod yamics and sedimentology of a high (tidal andwave) energy embayment (Swansea Bay, northern BristolChannel ). Estuar. est. mar . Sci . , 8, 49-74.

CULVER, S. J. 1980. Differen tial two-way sedimenttran sport in the Bristol Channel and Severn Estuary, U .K.Mar. C eol. , 34, M39-M43.

-- & F. T. BAN NER . 1979. The significance of derivedpre-Quaternary foraminifer a in Holocene sediments of thenorth-central Bristol Channel. Mar. Geol., 29, 187-207 .

DAVIES, C. M. 1980. Evidence for the formati on and ageof a commercial sand depo sit in the Bristol Channel.Estuar. est. mar. Sci., 11, 83-99.

DYER, K. R . 1984. Sedimentation processes in the BristolChannel /Severn Estuary. Mar. Poll. Bull. , 15, 53-57.

EVANS , C. D . R. 1982. The geolog y and superficialsediments of the Inner Bristol Channel and SevernEstuary . In Severn Barrage , Thomas Telford , London,35- 42.

EVANS, D. J. & M. S. THOMPSON. 1979. The geology ofthe central Bristol Chann el and Lundy area , S.W.Approaches, British Isles. Proc. Ceol. Ass., 90, 1-14 .

FERENTINOS, G . & M. B. COLLINS. 1979. Tidally­induced secondary circulations and their associatedsedimentation processes. J. oceanogr. Soc. Jap. , 35,65-74.

-- & -- 1980. Effects of shoreline irregul arities on a

rectilinear tidal current and their signifiance in sedimenta­tion processes. J. sedim. Petrol. , 50, 1081- 94.

FREY, R. W. & J . D . HOWARD. 1986. Mesoscaleestuarine sequences: a perspective from the Georgia Bight.J. sedim. Petrol . , 56, 911-24.

FONG, S. W. & N. S. HEAPS. 1978. Note on quarter-wavetidal resonance in the Bristol Channel. Rep. Inst.oceanogr. Sci., 63.

FORTNUM , B. C. H . & P. J. HARDCASTLE. 1979.Waves recorded at the Scarweath er bank in the BristolChannel. Rep. lnst. oceanogr. Sci., 79, 7 pp.

HAMI LTO N, J. 1973. The circulation of the BristolChannel. Geophys. J. R. astr. Soc ., 32. 409-22.

HAMILTON, D. 1982. Discussion on Papers 18-20. InSeve rn Barrage, Thom as Telford , London, 210-12.

-, J. H. SOMERVILL E & P. N. STANFORD. 1980.Bottom currents and shelf sediments southwest of Britain.Sedim. Ceol ., 26, 115-38.

HARRIS, P. T. 1984. Patterns of sediment transport in theBrist ol Channel . Unpub\. Ph.D. thesis, Univ. of Wales,248 pp .

1987. Large-scale bedforms as indicators of'mutually-evasive' sand transport in wide-mouthed es­tuaries. (In preparation).

-- & M. B. COLLINS. 1984. Side-scan investigations intotemporal variation in sandwave morpholg y: Hel wickSands, Bristol Channel. Ceo -Marine Leuers , 4, 91-97.

- - & -- 1985. Bedform distributions and sedimenttransport paths in the Bristol Channel and Severn Estuary,U.K . Mar. Ceol . , 62, 153-66.

- , G . ASHLEY & M. B. COLLINS, 1986. Topograph icfeatures of the Bristol Channel sea-bed : a comparison ofSEAS AT (synthetic aperture radar) and side-scan sonarimages. Int. JI. Remote Sensing, 7, 119-36.

HEAPS, N. S. 1967. Storm surges , Oceanogr. Mar. Biol.A nn. Rev ., 5,11-47.

-- 1968. Estimated effects of a barrage on tides in theBristol Channel. Proc. Inst. Ciu. Eng .; 40, 495-509.

-- 1982. Pred iction of tidal elevations: model studies forthe Severn Barr age. In Severn Barrage , Thomas Telford ,London , 51- 58.

HEATHERSHAW, A. D . 1981. Comparisons in measuredand predicted sediment transpor t rates in tidal currents . In(Nittrouer, C. A .: ed.), Sedimentary dynamics ofcontinental shelves ,

-- & A . P. CARR. 1977. Measurements of sedimenttransport rates using radioactive tracers. CoastalSediments '77, ASCE , Charleston, S. Carolina, Nov. 2-4 ,1977, 399- 416.

- - & HAMMOND , F. D . C. 1980. Transport anddeposition of non-cohesive sediments in Swansea Bay. In(Collins, M. B. et al .; eds.) , Industrialised embayments andtheir environmental problems: a case study of Swans eaBay , Pergamon Press, Oxford , 215-48.

IMER, 1!J74. Institut e for Marine Environmental Research(NERC). Report 1973- 74, IMER, Plymouth , Devon ,71 pp.

JACKSON, W. H. & D. R. NORMAN . 1980. PortTalbot-Accretion and dredging in the harbour andChannel entrance. In (Collins, M. B. et al. ; eds),Industrialised embayments and their environmentalproblems: a case study of Swansea Bay , Pergamon Press,Oxford , 573-82.

KENYON, N. H . & A. H. STRIDE. 1970. The tide-swep t

SEDIMENT TRANSPORT IN THE BRISTOL CHANNEL 383

continental shelf sediments between the Shetland Isles andFrance. Sedimentology, 14, 159-73.

KIRBY, R. 1987. Sediment exchanges across the coastalmargins of N.W. Europe. Jl. geol. Soc. Lond., 144,121-26.

-- & W. R. PARKER. 1982. A suspended sediment frontin the Severn Estuary. Nature, Lond., 295, 396-99.

LEE, A. J. 1960. Hydrographical observations in the IrishSea, Jan-March, 1953. Fisheries Investigations, Series II,XXIII(2), London, HMSO.

METIAM, C. J. 1982. Tidal power from the SevernEstuary: an essay review. Nature in Wales, 1, 50-61.

MILES, G. V. 1982. Impact on currents and transportprocesses. In Severn Barrage, Thomas Telford, London,59-64.

MURRAY, J. W. 1987. Biogenic indicators of suspendedsediment transport in marginal marine environments:quantitative examples from S.W. Britain. J/. geol. Soc.Lond., 144, 127-34.

-- & A. B. HAWKINS. 1976. Sediment transport in theSevern Estuary during the past 8000-9000 years. Jl. geol.Soc. Lond., 132, 385-98.

MURRAY, L. A., M. G. NORTON, R. S. NUNNY & M.S. ROLFE. 1980 The field assesment of dumping wastesat sea: 7. Sewage, sludge and industrial waste disposal inthe Bristol Channel. Fish. Res. Tech. Report, 59, MAFF,Lowestoft, 40 pp.

NORTH, F. J. 1964. The evolution of the Bristol Channel,National Museum of Wales, Cardiff, 110 pp.

ODD, N. V. M. 1982. The feasibility of using mathematicalmodels to predict sediment transport in the SevernEstuary. In Severn Barrage, Thomas Telford, London,195-208.

OWEN, A. 1980. The tidal regime of the Bristol Channel: anumerical modelling approach. Ceophys. J. R. astr. Soc.,62,59-75.

--& N. S. HEAPS, 1979. Some recent model results fortidal barrages in the Bristol Channel. In (Severn, R. T. etal.; eds.), Tidal Power and Estuary Management,Scientechnica (Colston Papers No. 30), Bristol, 65-92.

PARKER, W. R. & R. KIRBY. 1982. Sources andtransport patterns of sediment in the inner BristolChannel. In Severn Barrage, Thomas Telford, London,181-89.

PATIIARATCHI, C. B. & M. B. COLLINS. 1984.Sediment transport under waves and tidal currents: a casestudy from the northern Bristol Channel, U.K. Mar.Ceo/., 56, 27-40.

-- & -- 1985. Sand transport under the combinedinfluence of waves and tidal currents: an assessment ofavailable formulae. Mar. Geol., 67, 83-100.

-- & -- 1987. Wave influence on coastal sand transportpaths. (In preparation).

-, T. M. HAMMOND & M. B. COLLINS. 1986.Mapping of tidal currents in the vicinity of an offshoresandbank, using remotely sensed imagery. Int. Jl. Remote

Sensing, 7, 1015-29.PINGREE, R. B. & D. K GRIFFITHS. 1979. Sand

transport paths around the British Isles resulting from M2and M4 tidal interactions. J. mar. bioi. Ass. U.K., 59,497-513.

-- & -- 1980. Currents driven by a steady uniform windstress on the shelf seas around the British Isles. Oceanol.Acta, 3, 227-36.

ROBINSON, A. H. W. 1973. The use of the sea bed drifterin coastal studies with particular reference to the Humber.Zeitsch. fur Ceomorph., 7,1-23.

ROBINSON, I. S. 1978. Tidal response of a wedge-shapedEstuary to the installation of a tidal power barrage: asimplified analytical approach. Proc. Inst. Civ. Eng., 65,773-90.

SHACKLEY, S. E. & M. B. COLLINS. 1984. Variations insublittoral sediments and their associated macro-infauna inresponse to inner shelf processes, Swansea Bay, U.K.Sedimentology, 31, 793-804.

SHAW, T. L. 1980. Physics of water movement in theSevern Estuary. In (Shaw, T. L.: ed.), An environmentalappraisal of tidal power stations: with particular referenceto the Severn Barrage, Pitman, London, 1-36.

-- (ed.) 1980. An environmental appraisal of tidal powerstations: with particular reference to the Severn Barrage,Pitman, London, 220 pp.

SHUTILER, R. M. 1982. The wave climate in the SevernEstuary. In Severn Barrage, Thomas Telford, London,27-34.

STEPHENS, C. V. 1986. A three-demensional model fortides and salinity in the Bristol Channel, Continental ShelfResearch, 6, 531-60.

STRIDE, A. H. 1963. Current-swept sea floors near thesouthern half of Great Britain. Q. Jl. geol. Soc. Lond.,119, 175-99.

TAYLOR, G. I. 1921. Tides in the Bristol Channel. Proc.Camb. phil. Soc., 20, 320-25.

TYLER, P. A. & S. E. SHACKLEY. 1980. The benthicecology of linear sandbanks: a modified Spisula sub­community. In (Collins, M. B. et al.; eds.), Industrialisedembayments and their environmental problems: a casestudy of Swansea Bay, Pergamon Press, Oxford, 539-52.

UNCLES, R. J. 1982. Computed and observed residualcurrents in the Bristol Channel. Oceanol. Acta, 5, 11-20.

WARICK, R. M. & J. R. DAVIES. 1977. The distributionof sublittoral macrofauna communities in the BristolChannel in relation to substrate. Estuar. est. mar. Sci., 5,267-88.

-- & R. J. UNCLES. 1980. Distribution of benthicmacrofauna in the Bristol Channel in relation to tidalstress. Mar. Ecol. Prog. Ser., 3, 97-103.

WILDING, A. J. & M. B. COLLINS. 1980. Tidal andnon-tidal variations in sea level. In (Collins, M. B. et al.;eds.), Industrialised embayments and their environmentalproblems: a case study of Swansea Bay, Pergamon Press,Oxford, 85-113.