Padma Multipurpose Bridge Project

country boats), to assure a controlled supply. Each supplier must produce environmentalclearance/permits for rock quarries from the environmental agencies of the corresponding countries tothe contractor. And the contractor will ensure the availability of the copy of these permits wheneverBBA or a third party requires.47. In summary, the different rock supply systems all included a substantial number of risks, andwere found to have cause interruptions/delays at Jamuna. The issues associated with theprocurement and supply of rock can be avoided if geobag revetments are used. From aconstructability perspective geobag revetments can provide a considerable cost and risk reducingalternative compared to the rock revetment, in particular a reduced risk of construction interruptionsand delays due to rock supply. Geobags require mainly on-site materials and assembly.48. Surface water quality: Surface water quality in the river will deteriorate from increased waterturbidity during dredging and the underwater slope protection works. The total underwater slopesurface area is estimated as 51,316 m 2 at Mawa side that will involve revetment type bank protectionwork; at Janjira side this is about 1,457,388 m 2 for the preferred option.49. Dredged Material: The preferred RTW option at both Mawa and Janjira side has been selected. InMawa side, 2km revetment is recommended. In Janjira side continuous river training works along theexisting bank involving guide bund at the bridge. The guide bund at the bridge will give strongerprotection at the bridge landing point and will ensure high level of bridge safety from bank erosion.The rest being of revetment type, will ensure safety for the approach roads, bridge end facilities,resettlement sites, etc. An estimate for north bank identified that about 10Mm 3 dredged materials willbe generated and in south bank it will be 34 million m 3 . Total quantities of dredged materials due toRTWs for north and south banks are estimated about 44 million m 3 . A part of the dredge material willbe used for geobag filling, construction works/earth-filling in the approach roads, bridge end facilitiesand backfilling behind the proposed guide bunds, while the rest needs to be disposed off.50. Dredging works will be carried out during consecutive dry seasons along river banks up to adepth of -25 m PWD (this corresponds to 26.5m water column in February and 28.5m water column inNovember) 10 to remove the river bed sediments for construction of RTW. Due to the high turbidity ofthe material, direct disposal in the river during the dry season will create negative impacts on theaquatic life. Disposal of the materials on the land, for reclamation purposes or filling of project sites for100 year flood level, generates a huge outflow of wet materials that contain very high turbidity andimpact the soil fertility of nearby agricultural lands. In addition, the dredging activities will generatelocal turbidity around the cutter heads of the dredges, but this turbidity will not significantly spreadbeyond the dredging tranches and channels because the bulk of the dredging takes place below thesurrounding riverbed.51. The settling rate is primarily dependent on the density of the suspension and the grain sizedistribution of the suspended material, assuming a two-layer fluid system around the dredger or fillarea outflow: an upper layer of water and a lower layer of suspension (water plus sediment). Sandsizeparticles may settle within the hour, but clay-size particles (