Padma Bridge Project Pier Scour

CLIENT: Bangladesh Bridge Authority

The Padma River carries the combined flow of the Ganges and Brahmaputra Rivers and is ranked third largest river in the world in terms of discharge and the second largest in terms of sediment transport. The Padma Multipurpose Bridge Design Project comprises a new fixed crossing of the Padma River in Bangladesh, and will consist of a new bridge approximately 6.15km long, major river training works, approach viaducts, and approximately 12km of approach roads and bridge end facilities.

As a sub-consultant to AECOM-Maunsell, NHC's role in the project was to design the River Training Works, to assess scour at the bridge crossing and to assess the hydraulic and morphological impacts of the project on the physical environment. As part of the design process, NHC was responsible for providing scour estimates for various bridge pier arrangements. To provide the scour estimates, NHC built and tested models of four possible bridge piers using a movable-bed flume model at a 1:80 scale. Each configuration was subjected to a range of velocities, angles of attacks, and water depths. Test results were used to select the preferred configuration and provide data required for final design of the pile groups.

The model bed consisted of a lightweight ground walnut shell material with a specific gravity of 1.3, which closely represented the prototype sand bed. The model bed allowed for local scour depths of up to 22 m.

The piers proposed for the Padma Bridge consisted of raised pilecaps supported by a group of large diameter piles. Local scour is caused by the interaction of the various scour-producing components (pier stem, pilecap, and pile group) and cannot be accurately predicted using existing analytical or numerical models.

A laser scanner bed profiler was developed by NHC using a computer based video imaging system. The system used video cameras to track the pattern of a laser beam projected across the width of the model. Each scan produced a bed cross-section, and by moving the scanner along the model, a digital elevation model of the scour pattern was produced.