|Detailed Design of Padma Multipurpose Bridge, Bangladesh|
CLIENT: Bangladesh Bridge Authority
NHC designed the river training works for protection of the Padma Multipurpose Bridge as a specialist sub-consultant to AECOM/Maunsell from 2009 to 2011.
The purpose of the river training works is to guide the Padma River, the world’s third largest river (design discharge 150,000 m³/s, 70 m scoured depth) towards the bridge. A special emphasis of the works was to stabilize a 12 km stretch of the south bank and minimize turbulent flow, as well as upstream and downstream impacts. The work during the first Scheme Design phase included NHC-supervised collection of field data on geotechnical and river conditions, and background studies on hydrology, morphology, climate change impacts, natural scour, possible extreme flow velocities, performance of existing river training works in Bangladesh, and constructability issues. NHC also studied different concepts and developed scheme alternatives for comparison in a Multi-Criteria Assessment to identify a preferred scheme design alternative. In the second phase, Detailed Design, NHC evaluated a large number of modeling results from four distorted and undistorted physical models of different scales (three conducted in NHC’s facilities), 2-D numerical morphological models, and 3-D numerical models conducted in parallel and compared in real time (hybrid modeling). The results provided the design scour depth and the design flow velocities for sizing the river training works. NHC also designed the toe protection (launching apron), the performance of which is not fully understood and which have a mixed history of performance in the large rivers of Bangladesh. Key deliverables were design base, drawings, technical specifications, and bill of quantities.
The final design consists of about 2 km of protection at the north bank, to extend an erosion-resistant upstream clay layer and 12 km of protection along the south bank. The underwater slopes are dredged to 1V:6H and covered with rock on geotextile filter in critical areas near the bridge, while the upstream protection alongside the approach road on the south bank consists of multiple layers of geobags. The aprons consist of geobags, with width varying between 40 and 75 m set 15 to 25 m below low water level. The bridge design life is 100 years.