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Evolutionof Coastallaridformsinthe Western Part of Srilanka

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Evolutionof Coastallaridformsinthe Western Part of Srilanka HiroshimaHiroshimaGeographicalAssociation Geographical Association Geographical Sciences vol, 43 no.1 pp, 18-36, 1988 Evolution of Coastal Laridforms inthe Western Part of Sri Lanka JINADASA KATUPOTHA* Key Words:evolution of coastal landforms, SriLanka, late Pleistocene, Holocene, landfOmi classMcation Abstract Geomorphic and geologic evidence shows four different stages {Stage I-IV} in the evolution of coastal landforrns on the west coast of Sri Lanka during the Iate Pleistocene and Holocene Epochs. The author assumes that the old ridges in Stage I at Sembulailarna, Kiriyanl(ailiya, Pambala, Wiraliena, Uluambalarna and Kadrana areas have been fonned precedng the Holocene transgression. Low hMs and ridges in the area were coated mainly by wind blown sand, fonowing the lower sea levels during the Late Pleistocene and Earty Holocene Epochs. Radiocarbon datings en the west and seuth coast$ reveal that the sea level remained 1rn or more above the present sea level between 6170± 70 and 535e± 80 yr B. P. During this transgression, the forTner drainage basins were submerged and headland bay beaches were ereated. Many wetlands aiid beach ridges, particularly in Stages ll, III, and IV were gradualy formed owing to rninor oscMations of sea levet after mid-Holocene. Most of these landiorTns haveaclose relationship with main climatic zenes of the country. 1987; Katupotha, 1988) also help to deterrnine their I. Introduction evolution. The island of Sri Lanka has a coastline over Coastal Iandform maps of the study area were 1920 km in length, exhibiting a diversity of coastal cornpiled by means of interpretation of aerial photo- landromis. The coastal lowlands with elevation graphs (1:40,OOO-Survey Department of Sri Lanka, from mean sea level (MSL) to 30m consist of 1956) and field observations. Boring logs data of barrier beaches, barrier islands, sand dunes, buried the text received from the department of Highways, and emerged coral reef patches, lagoons and lakes, State Development and Censtruction Corporation, marshes, mangrove swamps and beachrock shoals Water Resources Board and Multiplex Overseas which are mostiy related to the Holocene Epoch. Constmction (Pte) Ltd. of Sri Lanka. Soil auger The coral reets, beachrock and mangrove swamps observations also were carried out in selected areas. are specinc features particularly characteristic of II. Physicalbackground tropical coasts. This study deals with the prelirninary investiga- About nine-tenths of the island consists of crys- tions to identify coastal Iaiidforms and their evolution talline rocks from the Precambrian age. The rest based on weathering conditions, colour, constituents includes sedimentary formations which were depo- of sand and height of the barrier ridges, distributionsited in the Jurassic, Miocene and Quaternary of beachrock and wetlands on the west coast of Sri periods (Coates, 1935). The quatemary deposits "Older Lanka (Fig. 1). Radiocarbon datings on the west of Sri Lanka are designated as Group" and "Younger and southwest coasts (Katupotha and Fajiwara, Group" (Cooray, 1967; 1968a). The Old- `Red * er Group ismade up of Earth',basalferrugi- Postgraduate student, Hiroshjma University. Address nous and terrace deposit after of gravel graveltplateau fromAprl1988: Department Geography,University Wayland, 1919; cap after Wadia, 1941). of SriJayewardenepura,Gangodawila,Nugegoda,Sri gravel Lanka. Holocene deposits made up of alluvium, dune sand, -18- NII-Electronic Library Service HiroshimaHiroshimaGeographicalAssociation Geographical Association J. KATUPOTHA:Evolution ef Coastal Laiidfdrms in the Western Part of Sri Lanka 19 se. 81' 82' 1 t 9. EE 8. 7- 6' l Lowlands Uplands and highlands AZDZIZWZ(a)Cb)(c)(d)(e)= ARID ZONE :: DRY ZONE = INTERMEDIATE ZONE = WET ZONE Mundal Lake Chilaw Lake Negombo Lagoon Beira Lake Bolgoda Lakes Fig. 1. Location map of west coast, Sri Lanka, A, Between Udappu and the Dedum Oya outfal1 (deltaic topography); B. Between Mahawewa and Seeduwa (ridge and swale topography); C. Between Pamunugama ancl the Kelani Ganga outfal and the adjoining area of Colombo (wetlands and bedrock related topography), beachrock shoals and wetlands are widely distributed weathered bedrock and capped by lateritic soils are across the entire coastal lowlands (Fig. 2). North- located close to the sea (Fig. 1). ern and eastern coastal lowlands correlated with Marine processes, mainly waves, currents and "intermediate", "Dry" "Arid and Zones" are widely tides, are strongly govemed by the southwest and covered by sand dunes, sand bars and barrier nomheast monseons. The seasonal waves are im- `CWet" beaches with lagoons. Nong the southwest portant agents which control beach behaviour and and south coasts, residual hills and ridges formed of the pattem of littoral drift around the island. High -19- NII-Electronic Library Service HiroshimaHiroshimaGeographical Geographical Association 20 teegpt743-1,I988 AB III. Outline of study area h..p 4 The area betweenMundal Lake and the Kalu Ganga outfal1 extend over the about 1,675 km2 with- in the latitude of 6040' and 8000'N and the longitude of 79095' and 80002'E. The Deduru Oya (river), the Maha Oya, the Kelani Ganga and the Kalu Ganga are the main drainage systems of the area with runfoff of 1,130; 1,485; 5,582; and 8,187 by mi1lion m3! per year respectively (Wijesekara and Het- tiarachchi 1983). Approxirnately 75 km2 of the tota1 area is occupied by large water bodies namely Mun- dal Lake, Negombo Lagoon, Beira Lake and Bolgoda Lakes. The present shoreline, particularly to the north of the Kelani Ganga outfal consists of barrier beaches, barTier islands, spits, sand dunes and beachrock shoals. Low hills and ridges which are coated with wind blown sand extend inland with dfierent elevations. The wetlands such as mud flats, mangrove swamps, marshes and so on have o been developed between the present shoreline and 'marine, Fig. 2. Sirnplified landiorrns map of Sri Lanka (After the low hills and ridges by fluvial and bioge- Swan, 1982), nic processes. The shorelme to the south of the A: Holocene beaches and sand dune B: O;d beaches C: Lime- Kelani Ganga outfab is characterized by headland bay stone low[ying D: Fluvial, deltaic and marshy lands platforms beaches. and associated water bodies E: Undulating terrains F; Low hills, ridges and terraces scarps, and associ- G: Steep ridges The uplands, behind the coastal lowlands ex- ated plateaus H: High plateaus and plains tend above 30 m from MSL. They are also widely covered with Iateritic soils or weathered bedrock, waves (height 5m or over) are seen along the south- and have been subjected to fiuvial process resulting west and south coasts during the strong southwest in the erosional land-forms. Many strearns of the monseon season from May to September. The west and southwest part of Sri Lanka flow in a area of Hikliaduwa, on the southwest coast, is situ- northwesterly direction on the uplands and hilly ated at a nodal point of the littoral drift. North of lands in accordance with the geological structure, Hikkaduwa the drift from the southwestern waves is But in the lowlands these strearns fiow consequently toward the north, and south of that point it is to the in a westerly direction supplying a considerable fine material to the south (Gerritsen, 1974). The tidal range on the Sri volume of terrestrialwater and Lanka coast is smal, ranging from the mean low lowlands. -37cm water spring (MLWS) at and MHWS at The coastal area between Udappu and the De- +40cm from the MSL (data based on Colombo duru Oya outfal is deltaic, while at the mouth the datum level 38 cm below MSL, Tide Tables, vol. 2, Maha Oya the delta is negligible in size. The lower Pacfic and indian Oceans, 1982). The phases of part of the Maha Oya and the Kalu Ganga lack deltas the tides have an obvious influence on the existence of any significance size which shows a drowned of mud flats, tidal fiats, marshes and mangrove valey topography. A flood tidal delta is formed at swarnps. the mouth of Negombo Lagoon in significance size, -20- NII-Electronic Library Service HiroshimaHiroshimaGeographical Geographical Association 21 J, KATUPOTHA:Evolution of CoastalLandiormsinthe WesternPartef SriLanka but no ebb deltas are formed in the study area. A. Between Udampu and the De(h`n` (lya o"ij1ill IV. landforms of the study area Coastal (delimc mpQg?ropluij area comprises Coastal landforms in Sri Lanka have been This deltaictype coastal 1akes, marshes, man- forrned by combined processes of marine, fluvial and tidalcreeks and wetlands (patchesof biogenic, and the basement structure of the lowest grove swamps). Streams such as the Katagamuwa and the peneplain. Three regions have been selected from Oya, the RatarnbalaOya SenegalOya flow connected the study area based on diversity of coastal land- inte a 1argetidalchannel which is to forms for detailed discussion emphasizing their ex- Mundal Lake and the Deduru Oya outfal. From tension, underlying material and the above men- the Deduru Oya outfal to the north, a narrow recent tioned processes (Table 1 and 2). The diversity of sand barrier ridge extends about 15km leng and ecological conditions of mangroves on the west coast about 200m wide, and is coated with low creeping is shown in Table 3. vegetation mainly ipomea Pesccipre, SPenipc littorous out- and rosea. It becomes wider A. Between Udappu and the Deduru Oya Canavalia graduaky north from Udappu village 3). fal1 (deltaic topography) to the (Fig. Before B. Between Mahawewa and Seeduwa (ridge the formation of this barrier
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