Bank Protection Structures along the Brahmaputra-Jamuna River, a Study of Flow Slides

Abstract

The planform of the Brahmaputra-Jamuna River followed its natural path in Bangladesh until the construction of bank protection works started to save Sirajganj from bank erosion since the 1930s. Several so-called hardpoints such as groynes and revetments were constructed in the period 1980–2015 and the Jamuna Multipurpose Bridge was opened in 1998. The Brahmaputra Right Embankment and other projects had saved the western flood plain from inundation during monsoon floods. These river training works experienced severe damage by geotechnical failures, mostly flow slides. A flow slide is an underwater slope failure because of liquefaction or a breaching process in the subsoil or a combination of both. The design of most of these training works did not consider the risk of damage by flow slides. All descriptions of the observed damages show that scour phenomena in the channel close to a river training work are a cause of flow slides, besides pore water outflow. The research question was: how can the design of river training works be improved to reduce the risk of damage by flow slides? The main part of the investigation was focussed on reducing local scour holes near river training works. The most promising results are river training works with gentle bank slopes, permeable groynes, bed protections in dredged trenches with gentle side slopes, and methods to increase locally the bearing capacity of the subsoil. It is recommended to increase the knowledge of the failure mechanisms in the Brahmaputra-Jamuna River by improved monitoring in the field, the setup of a database with descriptions of all observed flow slides and the circumstances in which they occur. In addition to these recommendations, a field test facility is proposed to verify the knowledge of the failure mechanisms in that river. These activities will optimise the design of new river training structures with a very low risk of damages by flow slides and geotechnical instabilities and they will contribute to an improvement of the current design guidelines for river training structures.