Sedimentation processes at the navigation channel of Mongla port on the Pussur-Sibsa river system in Bangladesh

Transcription

1 EUROPEAN ACADEMIC RESEARCH Vol. III, Issu 1/ April 2015 ISSN Impact Factor: (UIF) DRJI Valu: 5.9 (B+) Sdimntation procsss at th navigation channl of Mongla port on th Pussur-Sibsa rivr systm in Bangladsh TONMOY SARKER UNESCO IHE Institut of Watr Education Th Nthrlands Abstract: Mongla port is situatd in th south wstrn hydrological zon of Bangladsh at th conflunc of Pussur Rivr and Mongla Nullah approximatly 132km u/s of th Bay of Bngal and 45 km south of Khulna city. This is th scond sa-port of th country which facs problm of navigabl dpth. Th study finds out th prsnt navigational dpth scarcity from sa mouth to port jtty du to siltation of th Rivr Pussur, Mongla and Sibsa Rivr Systm by using HECRAS-4.0 mathmatical modl. Snsitivity by a mthod of rivr width contraction is to s th rosion dposition pattrn in th channl and rspons of th systm hnc thn analyzd. Ky words: navigation, port, morphology, width contraction, sdimntation tc. Introduction Southwstrn rgions of Bangladsh ar boundd by th Gangs and th Lowr Mghna in th ast and by th Indian Bordr in th wst and by th Bay of Bngal in th south. Th coastal rgion of Bangladsh and th rivrs in this rgion shows a continuing procss of siltation progrssing gnrally from northwst to southast. Th significant sourc of upstram frshwatr at Mongla Port is flow through Gangs to Pussur. 787

2 Mongla port consists of shor basd facilitis and a shltrd anchorag in th Pussur rivr. Th banks of th rivr hav continuous blt of mangrov forst of Sundarbans with small crks at placs throughout th passag from 5.75mils south of Hiron Point up to th Mongla Harbor ara. Th wathr in th port ara is tropical with minimum tmpratur is 8 0 c and humidity of 50%. During summr maximum tmpratur riss to 40 0 c and humidity is 95%. Southwstrly monsoon from Jun to August causs rainfall of avrag 200cm.Tropical storm is formd in th Bay of Bngal during April to May and Sptmbr to Octobr. Th intnsity of storm is lost on raching th port aftr crossing th coast and th Sundarban. Thundrstorms of vry short duration usually occur with squally winds up to 21m/s. (Rahman t. al 2000). Maximum lngth of vssl that can ntr in th anchorag is 225m and for mooring buoy it is 185m. Vssl of 8.5m, 8m and 7m maximum draught can tak brth in th anchorag, mooring buoys and jttis rspctivly. Entranc to th Pussur Rivr is about 6mils wid at th mouth and has a bar ovr about 5mils whr dpth is 6.2m. Ships having draft up to 7.5m can cross th bar in all sasons. (Malk & Ashraf 2004, MPA 2007, Womra 2006) As pr invstigation by IWM (2004), th dominant bd matrial can b classifid as fin sand. Thr is no distinct variation in th bottom sdimnt ovr th ara. (min D50=.098 mm max D50 =.126mm man D50=.11mm), although it is strongly suspctd that coarsr matrial would b ncountrd ovr th shoals, du to sorting action of th wavs that will brak in ths aras. Th varianc of th bottom grading is surprisingly small (typical D85/D15=2.6) which implis vry littl cohsiv matrial is sttling out of suspnsion in th main channl on ovr th outr bar. Th absnc of cohsiv matrial on th bd cannot b xplaind by th by th local wav conditions, as th wav climat was gnrally vry mild during th fild campaign (typically Hs<1m). Consquntly, th tidal 788

3 currnts along must b sufficint to maintain th cohsiv matrial in suspnsion and transport it out of th Pussur ntranc and ovr th outr bar, whr it will b dpositd in dpr watr. Th dominant suspndd matrial can b classifid as mdium silt, D50=.015mm. Thr is littl discrnibl diffrnc in th grading of th surfac and nar bd sampls, although, in gnral th surfac sampls hav a slightly finr grading. Th proportion of non-cohsiv matrial is lss than 10% for th sampls takn 1m blow th surfac. This implis that suspndd sand is confind to th ara vry clos to th bd. Th majority of th suspndd load is thrfor classifid as wash load which will hav a littl ffct on th morphology and th sdimntation charactristics of th ara. Th ky obsrvations from th sampl analysis ar: a) Th bd matrial along th main flow aras of th biggr rivrs is fin sand. Closr to th banks it is oftn mainly silt. b) Th suspndd solid fin matrials dos not contribut significantly to rosion/sdimntation procsss in th main flow rgions of th biggr rivrs including th navigation channl of Mongla port. c) Th transport of bd matrials is significantly smallr than th transport of fins. It contributs to th total transport of sdimnt by to approximatly on-third. Problm aspcts of navigational systm nar Mongla port Mongla Port is facing navigational problm mainly in th following aras (aras illustratd in figur 1): a) In front of th Jttis b) At th innr bar rach c) At th outr bar rach 789

4 In 1964 th lft bank of Pussur Rivr was slctd for 8.5m draught vssls, th sits for th brth in Mongla Port. But th rivr suffrd gradually dtrioration in subsqunt yars including th harbor aras du to Siltation. Th rduction of U/S flow aftr commissioning of Farakka barrag furthr rsultd in a dcras in navigational dpth. Sinc 1970, th dpths in th ara had bn rducd significantly and prsntly 6m draught vssls can tak brth at th jttis. Th svr flood of 1988 carrying larg sdimnt loads causd furthr shoaling particularly at th conflunc and th southrn anchorag. Sinc last two dcads from Maidara to Sabur Bacon th rquird 6m draught for continuous navigational channl ar not availabl. Th drdging works don ach yar from 1979 to 1997 in front of th jttis wrn t sustainabl. Th scarcity of navigabl dpth has worsnd aftr th flood of 1988 and Th innr bar is locatd 10km D/s from Mongla and lis btwn Dangr khal and southrn anchorag. Significant morphological changs hav occurrd from Mongla Port Jtty and to Joymonirgal du to innr bar. Th outr bar is locatd about 45km saward from Hiron Point. Sinc th bginning of th port outr bar is a problm for marin vssls ovr 8m. Th main problm is frqunt shifting of bar position on th lft right dirction. Th gradual loss in dpth is also happning day by day. (DRP Farligh 1981, 1984) Figur 1 Bd lvl chang nar Mongla port (IWM, 2004) 790

5 Data and mthodology for th problm analysis Data has bn collctd from diffrnt scondary sourc(s). Bathymtry data wr collctd from Hydrographic survy carrid out by Mongla Port Authority ( ). Discharg data at Mongla Nala (nar th conflunc of Mongla Nala and Pussur Rivr) and Sibsa Rivr nar th conflunc of Sibsa and Pussur ar usd for analysis. Data is availabl for April 2003 & August 2003 which covrs both dry and Monsoon sason nd spring and nap tid cycl (Womra 2006). Othr data usd in th rport wr collctd from statistical yar book of Bangladsh, Banglapdia and diffrnt wbsits. Suspndd sampls has bn collctd during discharg masurmnts at 8 cross-sctions. Th samplings wr don simultanously during th flow masurmnt. Th suspndd sdimnt sampls wr collctd in ach vrtical at Surfac, 0.2, 0.4, 0.6, 0.8 and Bottom (1m abov th bd) of th total dpth. Dtrmination of total suspndd sdimnt concntration and sparation of fin and coars sdimnt in suspndd was don by filtration mthod. Th filtration mthod was somwhat fastr for sampls of small concntrations. Usually Millipor filtr paprs ar usd in cas of watr sampls with rathr low contnt of sdimnts. This is du to th fact that th pors of th filtrs ar of th dimnsion 0.45 µm. Th concntration sampls ar sparatd by an lutriator into grain siz fractions abov and blow 0.05 mm. Hydrologic and morphological modlling st up is don by HECRAS 4.0. It can simulat both stady and unstady flows and includs laborat tratmnt for complx channl cross-sctions and structurs lik bridgs, culvrts and dams. Th HECRAS 4.0 has addd som nw functions to simulat sdimnt transport within th SIAM modul. SIAM is basd on dividing th sdimnt load in a gomorphic rach into wash load and bd matrial load componnts and thn accounting for th supply, movmnt and storag of ach componnt on a 791

6 rach by rach basis throughout th fluvial systm. It is assumd that transport of wash load is supply-limitd and that wash load sdimnt will pass through a stram rach, without intracting with th bd and bing dpositd to driv aggradations. Th movmnt of bd matrial load is assumd to b transport-limitd so that scour or fill will occur if th local transport capacity is ithr gratr than or lss than th supply from upstram (HEC RAS 4.0 usr manual 2009). HECRAS uss St.Vnant Equations (#rf) in th form of continuity to dscrib flow movmnt and quilibrium quations for sdimnt transport modling. Th flow is also solvd using th Prissmann four-point schm, with laborat considrations of complx cross-sctional gomtry and numrous typs of hydraulic structurs. For th morphological modl st up, Exnr quation, inbuilt in HECRAS sdimnt routing routins, is th sdimnt continuity quation for th analysis. Th whol approach towards th modlling is shown in th figur blow. Figur 2 Approach and mthodology Rsults and discussions Figur 3 rprsnts th dpths along th Pussur Rivr from wst point (Sa Mouth) to U/S of Mongla Port. Figur 6.1 also rprsnts th dpth along th Pussur rach. It is sn from th 792

7 dpth (m) Tonmoy Sarkr- Sdimntation procsss at th navigation channl of Mongla figur that th navigabl dpth is maximum (35m blow CD, ISLWL) nar Tinkona dwip and minimum (4m blow CD, ISLWL) nar Mongla whr th port is prsntly locatd as markd by circl. 40 P u s s u r Pussur Rivr-Middl Rach 10Apr :00:00 P u s s u r Lgnd 10APR :00:00-dpth (m ) R i v r - L o w r R a c h R i v R i v r - U p p r R a c h R i v Main Channl Dis tanc (m) Figur 3 Dpth orintation in th Pussur systm (Dpth in chart datum, CD, ISLWL) Navigational width is also a problm for th Mongla port. Th following obsrvation is mad from th navigation width. Ths facts ar illustratd in th figur 4 blow. For 6m draught vssls 250 width along th channl from Jafford point (RS 2) about 66km is availabl but from RS 85 to RS 95 nar Mongla 250m width is hardly availabl. For 8m draught vssl th channl has adquat width from Jafford Point (RS 2) to Joymonirgoal (RS 65 about 62km). At th upstram of Joymonirgoal th width is lss than 250m. For 10m draught vssl th channl is continuously navigabl from Jafford point to 34km upstram along th channl. Excpt this rach, thr ar svral placs whr 250m width is not availabl. 793

8 Vlocity (m/s) Availabl Surfac Width(m) Availabl Surfac Width (m) Availabl Width(m) Avail Width (m) Tonmoy Sarkr- Sdimntation procsss at th navigation channl of Mongla Availabl Surfac Width Graph Availabl Width for 6m Vssl m vssl width scarcity nar Mongla Port Distanc (m) (from Wst Point) Distanc from Wst Point (Sa Mouth) (m) Availabl Width for 8m draught Vssl Availabl surfac Width 10m draught Vssl Width Scarcity for 3000 Width Scarcity for 10m draught vssl 8m draught 2000 vssl Distanc from Wst Point (Sa Mouth) (m) Distanc from Wst Point (Sa Mouth) (m) Figur 4 Width orintation in th Pussur systm (top-lft), with scarcity for 6m vssl (top-right), 8m vssl (bottom-lft) and 10m vssl (bottom-right) Two typs of vlocity ar obsrvd hr. Ths ar bb vlocity and flood vlocity. Th calculatd vlocity along th channl is shown in Figur 5. Th avrag bb vlocity is 0.28m/s and it rangs from 0.09m/s to 0.5m/s and th avrag flood vlocity is 0.68m/s and it rangs from 0.9m/s to 3m/s. Th highst vlocity is found at th uppr rach station Pussur -140 whr th channl is narrow. Maximum discharg at Middl rach is 7382 m3/s (Ebb) (RS 20) and 8145m3/s(flood). Maximum discharg at uppr rach is m3/s (flood). Vlocity along th Pussur Channl Distanc from Wst Point (Sa Mouth) (m) Ebb Vlocity Flood Vlocity Figur 5 Flood and bb vlocity in th channl 794

9 Th avrag sdimntation rat is 0.57m3/day. Th maximum sdimntation rat is found 4.89 m3/day at th point whr Mongla port is locatd. Th sdimntation rat varis from 0m3/day to 4.89m3/day. It could b notd from th Figur 5 and Figur 6 that th watr vlocity nar th Mongla port rgion is so low which ultimatly lads to sdimntation and incras th bd lvl. Th incras of bd lvl also causing navigational problm in th particular ara. Figur 6 Sdimntation pattrn in th channl To larn th bhavior of sdimntation rspons, a furthr modling study is conductd. Th right bank sid nar th Mongla port rgion is slctd for width contraction. Th main focus is to larn to find th snsitivity in th sdimntation pattrn. For th particular study, th modling a simpl obstruction of 10% of xisting width is usd for th ffct of contraction. 10% contraction of th width producs an incras in vlocity from 12% to 22%. In th downstram of th contractd rach th ffct of contraction is ngligibl and xcpt at rachs from 27 to 38 and 44 to 46. Th vlocity mor than 1m/s (RS 85 to RS 90) may caus bank rosion. An incras in discharg btwn 14% to 21% is found in th Mongla port ara. Th chang in th sdimntation pattrn is shown in th figur 7 blow. 795

10 Figur 7 Sdimntation pattrn fluctuation in th channl by contraction nar channl of Mongla port It could b sn from th figur that th contraction shows th rosion-dposition pattrn. Th contraction in nar th Mongla port (RS 75 to RS 90) lads to rosion th plac port jtty to Joymonirgal in downstram which hlps th incras of navigational dpth. Morovr, th som pattrn also find a scop to know whr th masur rquird to b takn for th improvmnt of navigational dpth. Conclusion Mongla port is suffring significant dtrioration in dpth. Th maximum draft of vssls that can ntr prsntly th port varis btwn 6.0 m and 8.0 m, dpnding on th tid and wathr conditions. From Maidara to Joymonirgoal th rquird minimum dpth for 10m draught vssl, continuous navigation channl dos not xist. Th rivr from Joymonirgoal to Hiron Point maintains a good navigabl channl which is sufficint for vn 10m draught vssl. Th channl from Hiron Point to Fairway Buoy also lack of rquird navigabl dpth. For modrn ag th port, navigabl dpth mor than 12m, ar usful for modrn cargo handling which is th lack of Mongla Port. It can b statd that th prsnt study is mor qualitativ than quantitativ du to lack of fild data. If propr 796

11 fild data is availabl for calibration and validation of th modl HECRAS SIAM modl can also b satisfactorily usd for morphological changs of Pussur-Sibsa rivr systm hnc Mongla Port. Width contraction and drdging can b usd for improving th channl navigabl dpth. In this study it is sn that, contraction of rivr width for Mongla cannot b vry usful in improving of navigation condition. A combination of contraction and drdging may prov usful. REFERENCES DRP Farligh (1981), Pussur Rivr Study, Phas 1, Final Rport, Port of Chalna Authority, Fbruary, 1981 DRP Farligh (1984), Pussur Rivr Study, Phas 2, Final Rport, Port of Chalna Authority, July, 1984 HECRAS usr Manual (2009), HECRAS Rivr analysis systm, Hydrologic Enginring Cntr, US Army corps of Enginrs, January 2009 IWM (2004), Fasibility study for improvmnt of navigability of Mongla port, Institut of Watr Modlling, Dhaka, Jun 2004 Malk A. & Ashraf J. (2004), Improvmnt of Navigation of Mongla port, Bachlor of Scinc thsis, Dpartmnt of Watr Rsourcs Enginring, Bangladsh Univrsity of Enginring and Tchnology, April 2004 MPA (2007), Summary of Mongla port and information, Mongla port Authority, Khulna, Bangladsh, Dcmbr, 2007 Rahman M, Hassan M.Q. and Shamsad S.Z.K.M (2000), Environmntal impact assssmnt on watr quality dtrioration causd by th dcrasd Gangs outflow and salin watr intrusion in south-wstrn Bangladsh, Environmntal Gology, Dcmbr 2000, Volum 40, Issu 1-2, pp

12 Womra S.A (2006), Problms and prospcts of Mongla port, Bachlor of Scinc thsis, Dpartmnt of Watr Rsourcs Enginring, Bangladsh Univrsity of Enginring and Tchnology, Jun