Study on lowcost river training works in a meander bend along the Madhumati river

Abstract

Bank erosion and scdimcnla(ion mc ll1~jor COlLstminlsto alluvial ri\'cr management in Bangladesh, Bank ero~Lon detelio'CLles the SOCLO-CCOnOl1clloCnditions of floodplain inhabitants dll~ 10 loss of ,alunble agrL~llllUmlhll1d and sctllelnenl. Bankline or the river Madhumati a( Mollickpm has been shilled appro>..imatcly 1 (one) kilometer during the las! twel\'e y~ars. Due (0 erosion, p~orlc lost th~ir land and becamc helple~~, jobless. Man)' of thcm having 110 buying capa~ll)' l\1igrated to towns/cities. Some of the destitulcs due 10 bank erosion no"" mostly dep~nd on rickshaw for their hvelihood, The l\1eal people arc very much conc~rncd about (h~ behavior of the river. They Ihink thai (he river will eontinu~ its erosi,~ dllll'octenslles at th~ bend if propcr countermeasures are no( adopted The pur(ial protective work aggluvat~d the downstrcam erosion situation. The incomplete protective meusure should immediately be completed They ~uggested that the whole !lcnd should be taken under proteetiv~ mcosures, Physical e~periment was earned Ollt to test whether erosion will propagate or not. The bend channel was then examined 10 det~rmine its response ulld~r low cost protective meJsure i.c. penneahlc spur whieh arc pilrtially c,;:ccuted in a portion of the actual bend. A ~ouplc of combination was tested to ligllTC01.1tits perrorman~e. 11was clearly observed thut th~ now tends to shift towurd, Ihc concave ~ldc from the mid seclion of tilC bend Ch'lnllel if there is no protective work along the concave side of the bend. In the peml~able eases th~ !low is dj",,~rtcd remarkably from the structure and also from the embayment .cone of the series or ,pm-s, Dikes fllllctioned as blockages to Ihe flow. Although the t10w wa, shllted and maintained lb direction almo.,t parallel to the bank, lhe magnitude or (he vclo~it) waS ,lgl1ificantly decreascd in the embaymcnt area and also in between lhc ~purs. The bed morphology in lh~ (cst wilhout permeable groin (TOO) shows thot Ihe bed dcgradatlOn occurs at the con~a\e side thruugh all over the channeL The bed morphology ill the permeable case is quite ditTeren!. At the head~ of the dikes, the r.ow diverted, whieh led to local scouring, Local seollring occms at the toes of all the dikes. The main channel in the dike zone degrades signtlicanlly. Although the main channel is deepened, deposition i, fonnd along the ncar-bank area lIl-betwcen the dikes. No signilicant deposition occur, bctween lsi and 2nd spur Lll bolh the perlneabk case~. But the lest with sev~n spur (T07) "i,'idly shows sedimentMion in b~twcen 2nd and 3rd, " • am13rd and 4th spur. The test wI[h len ,Plll CliO) also rc,ponses 111,cdimcntation in betwccn 2nd, 3rd and 4th ,pur that relatively Ie" with respcet to T07. The highest h,j/h for TOO is 0.24 and lh,,( I,'r.t til i., 0.2:'( where h" is deposiLion depth and h is wata depth, On the other hand moximum h,111for TOO,s 0.](, and lhm for 1'10 is 0.37. wh~re h, is for 8cour depth. After the 7th 'pur, depositoon occur,; along the Otlter bank in both the cases. But in 107 fresh SCOllring happen atkr a certain lcngth of depositcd near bank. Thcsc intuitivc eompunsons made it clear 111<\t the permcable dikes wcre able to decrcase (he velocity in the embaymcnt while increase it in Lhe main eh,mnel. As a result. they mig!lt bc ulih~cd to prol~ct the ri\'lT bank from Cl'osion and improve the navigability l'y deepcning (he m~in dwnnel. 'l'hL 1'07 is inLapable 01"preventing the erosion all through [he outer bank, The "1'10 response, in a rcli"ble manner ofproleet;ng the ~eour for the whole bend as indica[~d by the I()~alpeople during social ~urvey.