Padma Multipurpose Bridge Project

erosion at the Janjira side. Due to the increase of the length of the bridge, the original bridge endfacilities such as service area and construction yard are to be relocated on Janjira side. This hasresulted into more requisition of land (about 78ha) than originally planned during FS stage. Thealignment for the approach road with associated structures has not been changed from the feasibilitystudy alignment except the dimensions (23.6m width). Some features of the structures have beenupdated based on the detailed hydrological investigation and climate change considerations.25. The navigation clearance and provisions adopted for FS were also updated during design stagebased on the analysis of satellite imageries of last 41 years. The Padma River is classified as aClass I Waterway and in accordance with the Bangladesh Inland Waterway Transport Authority(BIWTA) standards, the navigational clearance is required to be (18.3m) vertically and 250ft (76.2m)horizontally.26. Twenty one years of satellite images at the bridge crossing, between 1967 and 2009, werereviewed to assess the effect of channel shifting on the location of the channel. A comparison ofthese images confirms that it is not possible to fix the navigation channel at a single location.Furthermore, chars and islands can develop at both the north bank (referring to the imageries of 2001and 2002) and south side of the River (referring to the imageries of 2005 to 2009), which wouldobstruct navigation, although the channel has been more persistent on the north side. Therefore,navigation has always been possible near the centre of the channel, and by centering the raisedsection of the bridge between the two abutments, this condition will most likely continue. Hence, thefollowing navigational clearances have been adopted for the Main Bridge: Minimum horizontal clearance of 76.2m; Minimum vertical clearance above Standard High Water Level (SHWL) of 18.3m; Provision of 18.3m vertical clearance over 4800m of bridge centred about the bridgeabutments.27. The FS suggested navigational clearances for secondary waterways as 20m horizontally and 5mvertically, except for the 30m span bridge at Km 25+430 (chainage), where the requirement for verticalclearance was 2 m. During the design stage, the navigation clearance for secondary waterways isrevised to 20 m horizontally and 5.4m vertically above SHWL (+5.81m PWD), except for the onelocated at Km 25+ 430 for which it is 2.4m.28. A provisional allowance of 0.4m has been added to the navigational clearance requirements forthe Padma River and the secondary water courses considering future climate change.6.6 River Training Works29. The key objective of River Training Works (RTW) on both sides of the River at bridge location isto protect the river bank from erosion so that the bridge, approach roads and railways can remainintegrated and functional during the operation and maintenance phase of the Project. For the northbank at Mawa, the risk of erosion is estimated to very low and hence only modifications andrefinement of FS suggested RTW option is recommended without conducting any alternative analysis.However for south bank at Janjira, due its vulnerability to frequent erosion, various alternatives aredeveloped with preliminary hydraulic and morphological analysis and presented to the BBA and POEfor discussion. Based on these discussions, finally the following 3 alternatives are considered fordetailed analysis.6.6.1 Alternative 1 - Revetment on Bank of Southside Channel30. A revetment (updated Feasibility Study concept) can meet the key objective of the RTW and canintegrate the protection of the main bridge, end facilities, viaduct and approach road in a singlesolution. The 2 km section near the bridge will be raised above the flood level to prevent overtoppingor outflanking from floodplain flows (Figure 6-3). The ground near the viaducts will also be raisedabove the flood level as part of other plans associated with the end facilities. The revetment will needto extend upstream (west) along the existing bank of the south side channel to the end of the projectnear Char Janajat. The overall total length of protection is about 12.53 km. For the upstream 6 km6-19