Tllllllll:. 785
Total Page：16
File Type：pdf, Size：1020Kb
.Iournal of Coastal Research 774-7H5 Fort Laudr-rdalr-, Florid" Summer 1996 Lateral Grading of Beach Sediments: A Commentary Eric C.F. Bird Geostudies Pty Ltd., Black Rock Victoria 3193, Australia ABSTRACT _ BIRD, E. C. F., 1996. Lateral Grading of Beach Sediments: A Commentary. Journal of Coast«! Research. 12(3).774 .tllllllll:. 785. Fort Lauderdale (Florida), ISSN 0749-0208. ~ ~. Lateral grading of beach sediments can he achieved by downdrift attrition and/or longshore drifting. Chesil Beach (England) is graded from small to large pebbles and cobbles south-eastward in the direction of increasing wave energy. 1't t't 1" Attrition is very slow, and grading seems to have been achieved by alternations of longshore drifting. with selection of coarser from finer particles by the stronger south-eastward movement. Wave energy gradients may have contributed by moving smaller particles westward, and by conserving the lateral gradation. Hawke Bay beach (New Zealand) is graded in the reverse direction. It is fed with sand and gravel by the' Mohaka River, near tho western end, and lateral grading has been attributed to downdrift attrition eastward. Longshore sorting has probably also contributed, and a method of separating att.rition from sorting is proposed. The two systems are compared as a basis for elucidat.ing the causes of lateral drifting, which can evidently develop in different ways. ADDITIONAL INDEX WORDS: Latera! gradinr;, beach granulomctrv; !ol1r;shore drifting, II'{l1'e cncru» gradientn, at trition. INTRODUCTION the beach runs in front of vertical cliffs of layered sandstone, but at Burton Cliff, a kilometre east of the mouth of the River Lateral grading of beach sediments generally takes the Bride, it diverges to form a shingle barrier about 150 metres form of progressive longshore changes in the size, shape or wide, rising 7 metres above high tide level, fronting a series density of pebbles and sand grains, and has been observed of marshy meres and then the shallow tidal lagoon known as and discussed on beaches in various parts of the world. The The Fleet (Plate 21. The shingle barrier widens south-east topic was of much interest to the late RW.G. Carter, who wards to approximately 200 metres, increasing in crest discussed it on the Magilligan Foreland beach (CARTER, height to 14.7 metres above high tide level at the south-east 1975) and elsewhere on the coast of Northern Ireland, and ern extremity. It faces south-west, and receives ocean swell contributed a brief review of 'beach grading' in his book from the Atlantic as well as waves generated by wind action Coastal Environments (CARTER, 1988, pp. 240-244 J. It is ev in the English Channel. Mean spring tide range is 3.7 metres ident that several factors have contributed to lateral grading, at Lyme Regis, to the west, but diminishes eastward to about and that there is no single, simple explanation for the phe 3 metres on Chesil Beach. nomenon. Among the many beaches that show lateral grad ing by particle size the longest and most impressive are Ches Evolution of Chesil Beach il Beach on the south coast of England and the Hawke Bay beach on the east coast of North Island, New Zealand. Both The shingle barrier rests upon a gentle bedrock slope that are gently curved beaches with a southerly aspect, but they declines seaward to a depth of 15 metres below Ordnance show lateral grading in opposite directions. This commentary Datum, the shingle being underlain by lagoonal deposits. will refer primarily to these two beaches in considering some These indicate that Chesil Beach has moved landward during of the problems that have to be solved before a comprehensive and since the Late Quaternary r Flandrinn l marine trans explanation of lateral grading of beach sediments can be gression: the inner shore of The Fleet has only minor cliffing, achieved. and has never been exposed to the open sea at its present level, a point first noted by BAIJEN-POW~;LL (19301. There is CHESIL BEACH still intermittent landward movement as the result of over washing of beach material during occasional south-westerly Chesil Beach is a shingle beach on the Dorset coast in storm surges, and lagoonal deposits exposed low on the sea southern England, stretching about 28 kilometres from the ward side are' the source' of lumps of clay and peat thrown up harbour breakwaters at West Bay, near Bridport, and curv on the beach south-past of Abbotsbury during storms. The ing south-eastwards until it terminates beneath the high accompanying short-term variations in bench crest height limestone cliffs at Chesilton, on the west coast of Portland and location have been documented by CAlm and GLEASON Bill (Figure 1: inset, and Plate 1), Towards the western end (1972) and CAlm and S~:AWAHIJ (199()). Chesil Beach consists of well-rounded hrown pebbles, more .95145 received 22 Februarv 1.9.94: accepted ill renision ]() June 1.9.95. than 98.5'lr of which are flint and chert derived from Creta- Lat eral Gra ding of Beach Sedim ents 775 WINO RO SE N POIHand Bill . : . .,vJ f'/ § 0/ \ ) ~!, r:-~ (~ 1 t' LYM, REGIS ".BRIDPORT O\ ;f( ~·~E:: :~l~. ~~~\ ~ L_1J 1,) - 7.:;;K~ ~ ' -"~"'--------- " SID MO UTH _ '\6} I La ' I Ir~~ B""on ~e " ~\ "~ 2'"]1TI'"i C ·.-1 \ 5 Lr A \ ,;t!... Y 4 L Y ME 8A Y I " . A BBOTSSURY ~ ( BUDLEIGH ; I '. • \\ \. SALHAlON s mall pebbles C" • EXMQUTH \ L- t cm -S-" ", t I Incr ease In Size ....« A/. i.'----...-. J.'~r ( 0 . "'" •.••• • ~ / sooth- eastwards W EYMOUTH L~ ---7 -' "~ '" ( N '-... J A LONOON.- <lc-s- B" . De> ~ O~~ SHINGtE BEACH COMPARTMENTS '" locallon 01 Map Rn J ~ -, / " '.~ T r:-:=l ~....-:1:::Y I"'- - ....... "7 C A~S I BUO LEIG H 7 C HARTO N larqe co bbles /~ . ~ ---' '-V' \ 2 St DM OUTH 9 PINHA Y (( L, nd"" area L - 10 ' " em 3 WESTON 9 M ONM OUTH G:<~ .....I CHESIL BEACH HA NNEl ,,\ " Cl ilts S 18 Cks. bou lder s. J ) 4 BRANSCOMBE 10 CHARMO UTH \) NGLIS H C / Seo ov beach V E / 5 BEEA COVE t1 SEA TOWN I •0 5ll-'1'£....!.50km a , ~ 'C\' --"j<i,EHAVR E 6 SEATO N 12 EYPE " Sh,ngl. beach P~~ '" A N/J I J 10 Figure 1. Chesil Beac h and other shi ngle compartments in Lyme Bay. Dorset, on t he south coast of En glan d. The wind rose (inset) indic at es tha t south westerly wind -generated wave s are dominant. but tha t southerly, south-easterly and west erly waves also occur in coasta l wat ers. There is also a sout h westerly swell arrivin g from t he Atlant ic Ocean . ceous formations, which overlie Jurassic rocks in the Dorset cobbles from th e west, deri ved mainly from cliff-top weath hinterland, and are exposed in cliffs a long the coast to the ered gravelly Pleistocene dr ift deposits. Th ey are assembled west, capped by derived Pleistocene flint a nd chert gra vels in beach compa rtments on the coast of Lyme Bay, and as (BIRD 1995l. Th e remaining 1.5% a re Triassic quartzites, a intervening headl ands are cut back th ey are drifted ea st ward few pebbles of J ur assic lim estone an d sandstone, an d fra g by the predomin ati ng south-westerly waves towards Chesil ments of granite. vein quartz and metam orphic rock derived Beach . The building of the West Bay ha rbour br eakwater s at from formation s tha t outcrop fart her west, on th e coasts of the mouth of the River Brit in 1742 reduced beach dr iftin g, Devon and Cornw a ll. Towa rds th e south-eastern end of th e but some gravelly material is moving eastward acr oss the sea beach there is an increasin g proportion of limestone pebbles floor. Although th er e are none of the ir regul ar white-coated and cobbles derived from the clilTs and quarry wast e of th e flin t nodules or well-rounded blui sh flint cobbles of the kind Isle of Portl and . All of th e beach material has come from found, freshly deri ved from Chalk or Jurassic limestone out eroding cliffs beyond either end of th e beac h. and from the crops, on bea ches further west in Lyme Bay (BIRD, 1989). it sea floor: th e rivers of southern En gland are sma ll a nd deliv has been calculated that th e recession of cliffs to the west is er very little sed iment to th e coas t other th an fine ma te rial yielding over 4,000 mVyear of gravel to the beaches between (silt a nd clay), and th ere has been no fluvial input to Chesi l Lyme Regis and BridporUBR AY 1992 ). The proportion of this Beach. Petrological variations within the bea ch a re a fun ction movin g on to and along Chesil Beach is difficult to determine, of th e durability and initial size of th e stones, their source but th e preservation oflate ral grading indicates th at any ma and their history of migr ation (CAR R and BLACKLEY, 1969). terial arriving mu st move qu ickly to the sector dominated by Chesil Beach has been rega rded as essentially a relict for pebbles of similar size . mation, which origina ted du rin g Pleistocene oscillations of sea level. when succe ssive ma rine transgressions collected Lateral Grading gr avel from ea rlier fluvial and per iglacial deposits on what is now th e sea floor and built them into a bar rier forma tion.