Padma Multipurpose Bridge Project

oundary between the two layers and thereby causes mixing/dilution, leading to quick removal of partof the suspended sediment from the area with the water current.34. Silt curtains are often mentioned by the BBA experts in discussions with the design teamregarding turbidity due to dredging. Their usual function is to contain an area of more turbid water sothat it is forced to flow out near the bottom. A density current tends to form and the bottom turbiditybecomes invisible from the surface; the fines tend to settle nearer to the inflow than without siltcurtains. For the Padma River, silt curtains are not considered useful for the following reasons: In a river with strong currents and/or considerably varying water depths they are difficultto control and even with proper control have reduced effectiveness.The silt curtains will need to be opened and closed all the time which is highly impractical.Silt curtains around each dredger are impossible to handle because of the communicationbetween the dredger and the auxiliary equipment (launch for shift change, for visits ofsuperintendent, technical inspector and other visitors and for bringing spares andconsumables, workboat for handling/shifting of anchors and floating pipeline, fuel barge).Silt curtains around the three working dredgers together are also impossible to handle inview of the occasional shifting of a dredger, the floating transport of fascine mattressesand the sailing of boulder pontoons, and dump barges in the trench.Silt curtains could be used to contain the outflow from a reclamation area where currentsare near-zero and there is sufficient water depth, say 5 m. If the water level is expected tovary the silt curtain height is determined by the minimum water level during its operationalperiod at that location.35. Turbidity due to dredging is not considered problematic as the dredging quantities are very smallcompared to the total sediment load of the Padma River. Total annual amount of dredging in theProject will be in the order of 20 M m 3 per season. It only amounts to around 3.6% of the Padma’stotal annual sediment transport of 1 Billion tons or 555 M m 3 . Hence dredging has no major short-termimpact on the river morphology and any long-term impact would be masked by natural changesdepending on the annually changing flood characteristics.8.5.2 Possible Uses of Dredge Materials36. Approximate requirement of borrow material for filling of all the Project sites to a 100 year floodlevel is given in Table 8-5. About 20.2 M m 3 (13.9 M m 3 on Janjira side and 6.3 M m 3 on Mawa side)of borrow material is required. Dredge materials from RTW areas are found to be free ofcontamination and hence there is a potential that most of the dredge materials can be used as borrowmaterial. However, due to differences in the sequence of dredging operations and filling activities (e.g.service areas and construction yards will be developed prior to the dredging works) it may not befeasible to use all the dredging material for filling up of the Project components.Table 8-5: Dredge Material Requirement for Filling up of Project SitesSl.NoProject ComponentArea(m 2 )Ave.FillingHeight(m)Side SlopeQuantity ofMaterial forFilling (m 3 )1Mawa Side:Approach Road 308,000 51:2 (Service Road)1:4 (Main Road) 1,016,4002 Toll Plaza 175,000 5 1:2 577,5003 Service Area 273,600 5 1:2 902,8804 Railway270,000 6 1:2 1,068,000Embankment5 Construction Yard 830,000 5 1:2 2,739,0006 Backfilling behind the 20,000 0.5 1:2 6.600revetmentSubtotal: 6,310,380Janjira Side:8-36