Abstract:
Riverbank erosion in the large rivers of Bangladesh has always been a difficult problem and toaddress this challenge, geobags are commonlyused now-a-days. It is criticized in some cases that sediment loss from the geobags occur due to use of unspecified sand as fillermaterial.Attemptshavebeentakentocarryoutanexperimentalstudytoinvestigate if there occurs any sediment loss from the sand filled geobags when placed in flowing water. A total of eight experimental runs were conducted in 70 ft. long tilting flume for two different condition (1 layer 1 bag and 2 layer 3 bag) with geobags weighing 78 kg each, filled with specified and unspecified sand ('Vit Bali') for two different discharges- 100l/sand200l/s.Duringstudy,observationsonflowcondition,sedimentconcentration, velocitydistributionetc.havebeencarriedout.Itisrevealedthatvelocityoftheflowdoes not depend on the geobag filler material whether it is specified sand or unspecified sand rather it depends on the layout of the bag, number of layers, number of geobags, flow conditionetc.Thelower the velocitythe lower will bethe stress.The more the layer the lesser will be the possibility of bank protection failure.Volumetric measurements and observations reveal that geobags filled with specified sand can contain the filler material satisfactorily and hence, it is sustainable when placed in flowing water.
Ontheotherhand,geobagsfilledwithunspecifiedsanddischargesminoramount offiller material (sand) from the geobags for different flow condition. For one layer one bag condition, the average sediment concentration adjacent to the geobag per square meter bag area becomes 4.5 gm/l for 100 l/s of flow discharge and 5.0 gm/l for 200 l/s of flow discharge. On the other hand, for two layer three bag condition, the average sediment concentrationadjacenttothegeobagpersquaremeterbagareabecomes 2.7gm/lfor100 l/s flow discharge and 3.0 gm/l for 200 l/s flow discharge. For one layer one bag filled with unspecified sand, the average sediment concentration is less when the flow (discharge) is high. For two layers three bags the average sediment concentration is less when the flow (discharge) is less. It appears that few parameter/phenomena (e.g. water depth adjacent to the geobag, shear force acting on the geobag, vortices generated etc.) mightberesponsibleforthenon-linearsedimentlossfromthegeobagforsinglelayerand doublelayersetup.Therefore,geobagsfilledwithunspecifiedsand,lossesminoramount of fine sediment for different flowcondition and hence,itis not sustainablewhen placed in flowing water.
Hence,itisapparentthatifthegeobagsarefilledwithsandaccordingtospecificationthen thepossibilityoffailureduringheavyflowintheriverisnegligible inrespecttosediment loss from the geobags. On the other hand, if it is difficult to provide specified sand and unwillinglyunspecifiedsandisused,ahighpossibilityisthere tofailthebankprotection works during heavy flow due to sediment loss from the geobags.
