Impact of bridges and diversion channels on flow situation in the Titas river loop

Abstract

Titas River is either a left bank distributary of the Meglma River or sourced by the Sonai River IUlming down from rripura hills and alter traversing 127 km in the south-east again discharges into the Meghna at River about 16 km downstream of Bhairab Bazar Railway Bridge. The river ha~ fonned a big meander loop south of Brahm anbari a town of about 42 km from the ofl:lake to the outfall of Korulia khal which run~ along the center of Brahmanbariu town. The river loop has been cut ,hort by two interconnecting ehannels- Kurulia (Anderson) khal and Town klml which were mainly constructed for early drainage oflow lying area~ a~ well as for navigation purposes. HislOrieal1y, litus River ha~ great importance because it has pwvlded soeio-economic benefits to thousands of rural people in the fom] of lIavigation, li~hcries, drainage of agricultural lands and a~ a source of irrigation waler s"pply m the region. But, the river has been dymg allhe middle of the loop between Akhaura bridges to Ujanishar. The nveT loop remain~ completely dry in this reach for at lea"t seven months from (November to May) m a year. This results ill loss of navigation channel along the loop, loss of inigation water supply for more than 20, 000 hectares ofland on botb sides of the reach and creates drainage congestion durillg monsoon. The present study was aimed at understanding the reason(s) behind Titas River Loop dying. Field imestigation~ as well as numerical simulalions were ~onduclcd to identify whether the construction of the Akhaura Railway Bridge (constructed in 1910) or excavation of the diven;ion channels (Kurulia khal was constructed in 1936) or some other rea,>oncau~cd the loop to dry up, Field observations during February 2008 revealed that di~chargc through the Kurulia khal was higher than the djs~haTge fwm the upstream part of the Titas River Loop. Abo It was observed that the ri\'er fwm the offtake 'of the Kurulia khal to the Akhaura Railway Bridge wa~ tlowing towards the offtake of the Kuwlia khal, the loop reach from Akhaura Rmlway Bridge to LJjanishar was dried up and tho river from several ki!omekr, downstream of Ujanishar to the nullaJI nf Kurulia khal was again flowing towards the outfall of Kurulia khal. There were three trib"lary rivers: Sheuli, Bijni and Ilawra, in the vicinity of the dried up reach of the loop which contrib"ted ~ignifieant scdiment discharge and also cau~ed nash 1100d in the region. Watcr surface slope \Vas calculated from the BWDB obs~rv~d water level data between Akhaura Railway Bridge and the ofrtake and outfall of the Kurulia khal whieh showed change offlow dire"uon from 1977. HEC-RAS, the one dimen~innall1nw simulation model by USACE, was u,~d tvr Rteady tlnw ~imulation of th~ water Rurfaee elevation and velocity at different cfll~~-~~<.:tionR ulong the Titas River Loop. Four dift",r~nt ReenarioR were developed to analyze the impact on water surface el~vution and velocity distribution along the river loop and identity the po~sible eause(s) of river loop dying. Th~~~ ~eenano~ were: scenario 1: exi~ting scenario at present. scenario 2: ,cenariv resembling the condition before the construdinn of Akhsuf3 Railway Bridge und ex<.:avation vI' the Kurulia khal (situation betvre 1910), scenario 3.- scenario 2 plus the Akhallra Railway Bridge (situation before 1(36) ,md scenario 4: scenario 3 pll.ls the Kurl.lliu khal. NeeeR5ary interpolations for bed ~lope and river width were made in the middle part of the loop to generate scenario 2. Th~ ~ilTIu1atlOnfor existing scenario (~c~nario 1) ~u<':c~l;}(leidn repre~enting the present conditions for water snrface elevation and velo<.:ityalong the river loop reasollllbly welL AR expected, the simulrrtion results IVT ~<.:enarin2 Rhowed ~ignifieant increase of flow velocity along the river loop becau~c of no flow diversion and no possible llow constriction due to bridg~~. Scenario 3 did not show any significant change in velocity distribution along the loop wmparcd to scenario 2 indicating that the cOll,lru<.:tlon of Akhaura Railway Bndge did not create major alteration in flow through th~ loop. ln contrary, ~c~nario 4 showed significant drop in flow velocity through the loop which signili~~ the impact of the diversion chmmel in reducing tlow veio<.:itythrough the river loop. So rrmn thc numerical simulatiolls it became clerrrly evident that the Rignificant drop in flow velocity along the riv~r loop after the excavation of the diversion channel (Kurulia khal) acc~krat~d the Redimcll1 deposition of the tributary rivers from Tripura hi11~and eau~ed the middle part (from Akhaura Railway Bridge to Ujllllishar) of the Titas River Loop to dry lip.