Mengabang Telipot is a slough about 10 km north of Kuala-Terengganu on the east coast of Peninsular Malaysia. Its mouth is normally closed to free exchange of sea water by a sand "wash-over" bar. On 16 August, 1984 the mouth was opened artificially, the slough became tidal, and a delta of

sand was formed. The mouth remained open until the end of September. The long-shore current averaged 13 cm/sec toward the north-west. This modified the delta, and eventually led to the closing of the slough mouth. Deposition on the south-east side of the delta made it possible to estimate a

volume of long-shore sand transport of250 cu m/day. This is in agreement with other estimates

Phillips, R. P.

Universiti Putra Malaysia Institutional Repository

Pertanika 8(2), 273 - 279 (1985)Long-Shore Transport of Sediment During August and Septemberon the Terengganu CoastR.P. PHILLIPS*Faculty of Fisheries and OceanographyUniversiti Pertanian Malaysia,Serdang, Selangor, Malaysia.Key words: Long-shore transport; littoral drift; coastal processes; sedimentation; Terengganu;Malaysia; South-China Sea.ABSTRAKMengabang Telipot merupakan tanah berpaya yang terletak 10 km ke utara bandar KualaTerengganu di pantai timur Semenanjung Malaysia. Pertukaran air laut di muara paya ini biasanyatertutup oleh tambakan benteng pasir pantai. Pada 16hb. Ogos, 1984 muara ini telah digali. Akibat-nya paya dapat dzpengaruhi oleh air pasang surut dan sebuah delta pasir terbentuk. Muara paya initerbuka sehingga akhir bulan September. Kelajuan purata arus menyusuri pantai ialah 13 cmlsaatmenghala ke barat laut. Ini mengubahsuaikan delta dan seterusnya menutup muara tersebut. Pe-mendapan di sebelah tenggara delta membolehkan isipadu pemindahan pasir sepanjang pantaidianggarkan sebanyak 250 meterpadulhari. Angka ini selaras dengan anggaran-anggaran yang lain.ABSTRACTMengabang Telzpot is a slough about 10 km north of Kuala-Terengganu on the east coast ofPeninsular Malaysia. Its mouth is normally closed to free exchange ofsea water by a sand "wash-over"bar. On 16 August, 1984 the mouth was opened artificially, the slough became tidal, and a delta ofsand was formed. The mouth remained open until the end of September. The long-shore currentaveraged 13 cm/sec toward the north-west. This modified the delta, and eventually led to the closingof the slough mouth. Deposition on the south-east side of the delta made it possible to estimate avolume of long-shore sand transport of250 cu mlday. This is in agreement with other estimates.INTRODUCTIONMengabang Telipot (5 deg 25.0' N Lat.,103 deg 05.2' E Long.) is located on the SouthChina Sea coast of Peninsular Malaysia, 10 kmnorth-west of Kuala Terengganu and about 160km south-east of the Malaysia-Thailand border(Fig. 1). The word "mengabang" is used locallyto mean a slough near the sea, with no majorstream flowing into it and no direct connectionto the oc~an. Mengabang Telipot is a slough thatacts as the sump for a drainage area of about 72sq km. The drainage into it is poorly organized,and has been further upset by development androad building. During periods of heavy runoff,the water in the mengabang will stand at about1.8 m above sea level and almost 5 sq km aresubject to inundation. This area is crossed byroads, has been developed with housing and hasbeen used for agriculture. During the dry seasonthe water drops to as low as 0.3 m above meansea level and its quality tends to deteriorate,becoming a nuisance due to odor and healthconsiderations.The slough is separated from the SouthChina Sea by a low sand ridge which is a storm-'Visiting Fulbright professor from the Environmental Studies Program, University of San Diego, U.S.A.R.P. PHILLIPSFig. 1. Sketch map and location diagram ofMengabang Telzpot, Terengganu, Malaysia.Dashed line indicates shore at time ofopening.built, or wash-over bar. This distinction isimportant to 'the development of the coastalarea, since a sand dune implies wind action thatbuilds it above the active level of the sea while awash-over bar will be inundated by large stormwaves. Fresh water draining into the slough fillsit to a level as high as 2 m above mean sea level.This level is limited by seepage through the 50 mof sand which separates the slough from the sea.Most of the water which flows into themengabang is rain water, though housing deve-lopment has added a load of domestic wastewater. The climate of the Terengganu coast ismonsoonal. The year can be divided into fourseasons; the south-west monsoons, the north-eastITlonsoons, and the two transition (doldrum)periods in between. No season is without rain,but the three months of the south-west monsoons(May, June and July) have an average of appro-ximately 120 mm/month of rainfall while therainy season of the north-east monsoons(November, December and January) averages.close to 500 mm/month (Drainage and liriga-tion Division, 1977). The normal weatherpattern from June through September is hot,sunny days with the build-up of convectivestorms with lightning, thunder and often torren-tial rainfall in the late afternoon and early even-ing. Run-off can be heavy, but brief during theseafternoon storms, and the level of the slough canchange by as much as 0.25 rn. If the water level ishigh in the slough the water seeps out ratherquickly, and by the next morning the level isback to the equilibrium level of about 1.8 mabove mean sea-level. Even though the waterlevel in the mengabang may be quite constant,the inundated area of the upper slough may bequite variable. The slope of the land surface is sogradual that a change of only a few tens of centi-meters in the water level can cause an extensivechange in the inundated area.During the period of observation the swellof the South China Sea was small, with signi-ficant wave heights of 0.5 m or less. These wavestend to approach the coast at an angle of 10 to15 degrees from the south-east, forming a surfzone about 2 m wide. While the details are quitevariable' (Table 1) the average long-shore.current is 14 em/sec from the south-east towardthe north-west. It should be noted that theseconditions change drastically during the monthsof the north-east monsoons. The coast ofTerengganu has a mean spring tide range of 1.8m. This is just at the boundary between themicrotidal and mesotidal coasts of Davies (1973).The tide is mixed, with a strong diurnal compo-nent beating with a smaller semidiurnal phase.This iiv~s rise to asymmetric tides near thesprings when the tide may flood in six to eighthours and ebb over as long as 18 hours. Duringthe neaps, a common pattern is that of a "doublebottom" with the lower high water only a fewtenths of a meter above the low waters. So, in274 PERTANlKA VOL. 8 NO.2, 1985LONG·SHORE TRANSPORT OF SEDIMENT DURING AUG. & SEPT. ON THE TERENGGANU COASTTABLE 1Observation of waves, littoral currents, and winds at the beach of the Marine Station,Mengabang Telipot, Terengganu, Malaysia.Wave Drift Wind TideDate Time Hght. Period Rate Toward (Est. ) From1984 (- 8) met sec cm/sec0801 1935 0.5 3 29 NW light SE low slack0807 1930 0.5 3 13 NW light SW mid ebb0810 1830 0.5 2 19 NW light SW mid ebb0815 1935 0.2 6 10 NW calm low slack0816 1930 <.2 3.5 16 NW light SW low slack0817 1920 <.5 6 none light SE low slack0818 1930 <.5 4 18 NW light W low slack0820 1930 0.5 7.5 none calm low slack0822 1940 0.1 6 3 SE calm mid ebb0825 1940 <.5 6 9 NW 3 knots SW mid ebb0826 1930 <.5 7 3 NW calm mid ebb0903 1930 <.5 6.5 none calm low slack0908 1930 0.2 2.5 43 NW 5 knots E high slack0909 1000 <.2 4.5 5 NW calm mid ebb0924 1930 0.2 3 50 NW 6 knots S high flood0925 1930 0.1 3 32 NW 4 knots SE high slackeffect, the tide IS low and slack for 14 to 18hours.THE MENGABANG DELTAThe water level in the mengabang stoodabout 0.6 m above mean sea level in August1984. The water quality was poor, with a salinityof 2 ppt, a dissolved oxygen content of 3.5 mg/land a pH of 3.7. Mullet living in the slough weremoribund. With the increase in rainfall duringthe end of July and early August the water levelincreased to a stable level of 1.8 m above meansea level. It soon became obvious that the waterwould not top the bar that separated the sloughfrom the sea, so that the only way to drain itwould be to open it artificially. This was neces-sarybecause of the extensive flooding inland, thepoor water quality in the slough, and the failureof septic and drainage systems due to the highwater table. At about noon, on 16 August 1984the Drainage and Irrigation Division (D.I.D.) ofthe Ministry of Agriculture moved a back-hoeonto the bar and cut a channel that allowed themengabang to drain into the sea. The sand fromthe cut was piled on the bar text to the cut. Theflowing water quickly eroded a channel approxi-mately 35 m wide and 50 m long, graded to thebottom of the mengabang and the low tide rangeof the day (0.7 m below mean sea level). Anestimated 4,000 to 5,000 cu m of sand wereeroded, and most of it deposited as a delta at themouth of the channel.At about 1600 hours on 16 August a "tapeand compass" survey of the delta was conducted(Fig. 2). The sand had been deposited in asymmetrical semicircular delta with a radius of45 m. The northward setting current was reflect-ed in the slight asymmetry with a spit that wasPERTANlKA VOL. 8 NO.2, 1985 275R.P. PHILLIPS----- Spit Limit 16/08/84.-- Shore Line,Low Tide, 18/08/84. I~/&~ Added, 16/08/84-20/08/84• Sample location(Table 2)awash on the south flank of the delta, and themain ebb-tide channel turned to the north. Theouter edge of the semicircle dropped steeply to adepth of 1.5 m. The undisturbed beach profilehas a rather steep beach face to a depth of 1.5 mat a distance of 4 m offshore. The volume of thedelta, therefore, can be calculated as a semi-circular slab with a prism out of the shore edge.Volume = (Pi X r X r X h)/2-(2 X r X c X h)/2= (3.14 X 45 X 45 X 1.5)/2-(2 X 45 X 4 X 1.5)/2= 4438 cu m = approx.4500 cu mThis calculation indicates that the sand thatwas eroded from the sand bar was deposited inthe delta. This is not an unreasonable conclu-sion, since the sand of the delta had a meandiameter of about 1.0 0 (0.5 mm; Table 2) andwould only be transported by currents in theactive surf zone.Fig. 2. Changes in the delta at the mouth ofMengabang Telipot from 16 August to20 August, 1984.metres103 0 05.2' EoI50!100I The shape of the delta and the above-waterbars changed systematically during the next fewweeks. The spit increased in height rapidly untilit was just barely awash at high tide. It extendedits length and became obviously recurved so thatTABLE 2Sedimentary analysis of sand samples from the temporary delta of Mengabang Telipot, TerengganuSample No.1 (S. edge of ebb flow channelat washover bar)2 (rippled flat, behind spit)3 (crest of spit)4 (S. side of delta at water line)Mediancfi 50 (note 1)-0.1+0.9+ 1.3+ 1.9Sortinga i (note 2)1.0 Moderately to poorly sorted0.6 Moderately well sorted0.6 Moderately well sorted0.6 Moderately well sortednotes to table 2: (1) cfi,(Phi) is the negative log to the base 2ofthe diameter in mm. cp. of 0 = 1 mmcp of + 1 = 0.5 mm(2) a i (Sigma sub I) is the Inclusive Graphic Standard Deviation of Folk (1966):276a =iPERTAN1KA VOL. 8 NO.2, 1985LONG-SHORE TRANSPORT OF SEDIMENT DURING AUG, & SEPT_ ON THE TERENGGANU COASTSOUTH CHINA SEAFig. 3. The delta at the mouth of MengabangTellpot on 3 Sept. 1984.MEASUREMENT OF LONG-SHORETRANSPORTo 50 100~I=--==--==J,!__~__Imetres1030 OS. 2'. EThe evidence that the mouth had been open wasalmost destroyed,For the first few days after the formation ofthe delta, the basic shape remained the same(Fig. 2). The major changes were the filling ofthe area south-east of the spit, and the erosion ofThe long straight coastline of the State ofTerengganu clearly indicates that long-shoretransport is an important mechanism here,Studies (Raj, 1982, Shiozawa, undated) havepointed to the magnitude of this phenomenonand its importance to engineering projects. Thechanges in the shape of the delta seem to offer away of measuring this long-shore transport ofsand during August and September.50 25.0' Nit was restricting the mouth of the mengabangtwo days after it formed_ The inflow of freshwater and the tidal prism were great enough togive current velocities as high as 100 em/sec atthe mouth, which kept a channel open to adepth of 0.6 m. There was a broad channelmouth bar formed where the water flow wasslowed by the sea, The geomorphic landformsthat developed in the delta and adjoining baywere a miniature version of the typical formsfound in estuaries along a mesotidal coast(Boothroyd, 1978). It made an ideal teachinglaboratory.When the estuary was first opened, thesalinity measurements revealed a stratified situa-tion, with the sea water flowing in along thebottom and warmer fresh water flowing out overit with little mixing. Later, the water flow withinthe estuary was strongly affected by the tide.During a rising or high tide the salt water flowedwell into the mengabang, and could be detectedup to 1 km from the mouth. The water would betypically mixed, with little change in salinityfrom the surface to the bottom. With the fallingtide, the inflow of fresh water would displace thesalt water and flow out over the top of it, so thata typical salt wedge estuary would develop.There was no evidence for the formation of aninner-estuarine bar upstream from the mouth.With time a spit formed across the mouth ofthe estuary, sand accumulated on the south sideof the delta, and was eroded from the beachnorth of it. As a result, the channel draining theestuary became more and more nearly parallel tothe coast (Fig. 3), and the original sharp semi-circular shape of the delta was modified to blendwith the beach. By mid September the spit hadgrown so that it extended 150 m north-west ofthe original opening and effectively forced theflow to follow a longer path with a less steepgradient from the mengabang to the sea.Finally, on the spring tides of September 28 and29 the sand carried over the spit filled thechannel. The flow of fresh water into the menga-bang was unable to keep the channel open. Itclosed, and within two weeks the spit migratedinland to merge with the original wash-over bar.PERTANIKA VOL. 8 NO.2, 1985 277R.P. PHILLIPSthe beach to the north. It would appear that thedelta formed a barrier to the transport of sandalong the beach and caused its deposition at thesouth side. The resulting loss of sand in the long-shore system caused erosion north of the delta, ina pattern that is very familiar to coastalengineers. The delta seemed to be an efficientsediment trap. During the first four days, changesto the outer edge of the north side of the deltawere minor. After 20 August, the sand appa-rently had filled the trap of the south side of thedelta, and began bypassing it. The spit prograd-ed rapidly parallel to the beach.If it is assumed that the delta did form anefficient barrier, an estimate of the amount ofsand added to it during the first four days willgive an estimate of the long-shore transport inthe surf zone. The added sand formed a triangu-lar deposit (F£g. 2) which reached the tip of thedelta, 45 m from the shore line and extended for35 m along the coast. The slope of the seawardside of the delta and the seaward edge of thedeposit were similar so the volume deposited canbe calculated by assuming a triangular slab witha prism out at the beach side.Volume = (b X r X h)/2-(b X c X h)/2= (35 X 45 X 1.5)/2-(35 X 4 X 1.5)/21181 - i05 = 1076cum= approx. 1000 cu mor approximately 250 cu m/day.This estimate is close to that from otherstudies. Shiozawa (undated, 1983?), from amodel study of Cendering Fishing Port, 21 km tothe south, estimated the total long-shore drift atCendering to be "500 to 700 thousand cubicmetre/year, out of which about ... 11 % is fromthe south" (pg. 23). If the northward drift isrestricted to the 250 days between north-eastmonsoonal seasons then this averages to about250 cu m/day.It is difficult to correlate the drift ratesgiven by Raj (1982) to volumetric measurements.Based on a study of changes in coastal featuresdetected on aerial photographs, he estimated thedrift at Batu Rakit Gust north of MengabangTelipot) as an "average annual rate· < 5 m/y"directed to the northwest, and at Marang (southof Cendering) as 45 m/y toward the south. Thisratio, < 5/45, is similar to that determined byShiozawa and consistent with the figures derivedhere. An annual volume of 1000 cu m/y hasbeen used in the design of the harbour at TanjongBerhala near the southern border of TerengganuState (Danish Hydraulic Institute, Sept. 1982).This drift is much lower than the transportthat takes pIa-ce during the north-east monsoonswhich may be higher by an order of magnitudeand is one of the major factors that must betaken into account whenever work is suggestedalong the east coast of Peninsular Malaysia. Onthe other hand, 250 cu m/day is a significantamount of sediment. In this case, it closed themouth of the Mengabang in six weeks, anddefeated the drainage scheme. I am sure that ithas affected other projects along the coast. It ishoped that engineers and others whose respon-sibility lies with structures on or modifications tothe coastline will keep this high volume of driftin mind.ACKNOWLEDGMENTSI wish to thank Encik Mohd. Lokman binHusain for his help and advice during themeasurement and writing of this paper. Rosnanbin Yakkab was an invaluable help during allstages of this study, including salinity and depthmeasurements in the mengabang. Mr. Ng CheeKiat aided by collecting temperature and salinitydata. I am indebted to Hj. Umar bin Saleh fortranslating the abstract, and Ahmad Daud forhelping draft the figures.REFERENCESBOOTHROYD, J.C. (1978): Mesotidal Inlets andEstuaries. Ch. 6 in Coastal Sedimentary Environ-ments, R.A. Davis, ed. Springer-Verlag, NewYork. p. 287 - 360.DANISH HYDRAULIC INSTITUTE (Sept. 1982): Hydrau-lic Investigations, Tanjong Berhala Harbour(extension of Kenama Petroleum Supply Base).Final report to the State Economic Development278 PERTANlKA VOL. 8 NO.2, 1985LONG-SHORE TRANSPORT OF SEDIMENT DURING AUG. & SEPT. ON THE TERENGGANU COASTCorporation, Terengganu (through Sepakat SetalPerunding Sdn. Bhd., Malaysia). 121 p. +appen. and charts.DAVIES, J.L. (1973): Geographical Variation inCoastal Development. Hafner; New York. 204 p.DRAINAGE and IRRIGATION DIVISION (D.I.D.) (1977):Hydrological Data-Rainfall Records for Penin-sular Malaysia, 1970-1975. Min. of Agric.,Malaysia, '472 p.FOLK. R. L. (1966): Petrology of Sedimentary Rocks.Austin. Hemphill's. Texas.RAJ, J.K. (1982): Net directions and rates of present-day beach sediment transport by littoral driftalong the east coast of Peninsular Malaysia. Bull.Geol. Soc. Malaysia. 15: 57 -70.SHIOZAWA, T. (undated-1983?): The Study on Silta-tion Problems in Chendering Fishing Port.Colombo Plan Report toJ.K.R. [?] 73 p.(Received 10 April, 1985)PERTANIKA VOL. 8 NO.2, 1985 279

Long-shore transport of sediment during August and September

on the Terengganu coast

http://psasir.upm.edu.my/2438/1/Long-Shore_Transport_of_Sediment_During_August_and_September.pdf

Long-shore transport of sediment during August and September on the Terengganu coast

Abstract

Similar works

Full text

Available Versions

Universiti Putra Malaysia Institutional Repository