Reporter 403, 2 June 1997

Research shows still rivers run deeper than expected

Bangladesh is one of the world’s poorest countries and, with a population of 110M, is the most densely inhabited. Floods inundate one quarter of the land every year. The rivers are large – up to 15km wide – unpredictable, and at present there are no roads or railways that can cross them. Without an understanding of how the main rivers behave little can be done to counter the effects of these devastating floods and plan for the construction of bridges and pipelines across these great rivers. A study by Leeds and other researchers has thrown some new light on the extent of the damage these rivers cause.

Just as the collision of two solid objects can produce a devastating impact, the result of two rivers meeting is also dramatic. The turbulent water on the surface is really only the tip of the iceberg – much more violent things are going on underneath. As two rivers collide, they create powerful vortices which plunge downwards towards the river bed and gouge large scour pits.

The extent of this scouring process is shown in a study by Dr Jim Best (Earth Sciences) and Dr Phil Ashworth (Geography) of the confluence of the Jamuna and Ganges rivers in Bangladesh, where they found one of the world’s deepest natural river scour holes. At 30 metres deep, this scour is five times deeper than the rivers feeding into it. The research also revealed that the pit migrates at an alarmingly fast rate – in the 28 months the confluence was studied the scour migrated 3.5km downstream. A 30m deep hole moving at a rate of 1km per year has obvious implications for any structure, whatever its size. Severe river bank erosion can displace thousands of people in a matter of days and bridges and pipelines in the river must be laid deep enough to allow for these scour pits.

The research also has important implications for how past variations in sea-level and climate are detected. Large erosive surfaces in the sedimentary rock record may be generated by falling sea level, which causes rivers to incise in order to flow into the sea. However, Best and Ashworth’s study suggests that similar scour surfaces can be generated by rapidly shifting river confluences, and their work proposes a more reliable way for the detection of sea-level fall and climatic change.

The collaborative research was funded by the European Union, as part of the Bangladesh Flood Action Plan, in association with Delft Hydraulics and the Danish Hydraulics Institute. The total grant was for 9M Ecu, and the Leeds team, which also included graduate student Julie Roden, were subcontracted with a grant of £120,000. Details of their work were published in the May 15 edition of Nature.

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