GUWAHATI, Aug 19 - The total experience gained in planning to manage the challenges of flood and sedimentation along with dredging operations in the Yellow River (Huang He) in China has many things to be learnt and reviewed, while undertaking dredging of the Brahmaputra bed, said Prof Nayan Sarma of the IIT Roorkee. A research team under Prof Sarma has done a preliminary modelling study on dredging and embanking in two reaches of the Brahmaputra river.
While talking to this correspondent, Prof Sarma maintained that reducing erosion and sediment delivery within the Loess Plateau is an example of success in the Yellow River basin in China. Since the mid-1950s, the Yellow River Conservancy Commission has implemented widespread land management change in the Yellow River basin, particularly in the Loess Plateau region, which included major dam construction, afforestation, terracing, check dam construction and conversion of cropland on steeper slopes into grazing land.
Engineering measures for sediment control were widely used and those measures included construction of structures to trap sediment on slopes and within pond and dam systems in gullies. Terracing of fields was used extensively in the Yellow River basin to mitigate soil loss from croplands. Moreover, vegetative controls have also been used extensively.
Significant progress in banning tillage in the steep slopes has been attained in the Yellow River basin and these slopes have been often converted into forests or improved pastures for livestock. Terracing of farmland and laying horizontal ditches to trap sediment are other ways in which agricultural soil loss is reduced in the Yellow River basin. Thus, the Yellow River Project was successful in reducing sediment loads transported by the Yellow River and this changed its behaviour favourably for the riparian people.
Because of siltation, the Brahmaputra is getting its waterways widened to around 14 km at places like Sadiya and Dibrugarh and over 20 km in Gumi-Sualkuchi section. Between 1971 and 1977, the overall aggradation of the Brahmaputra, throughout its entire Assam reach, was 0.16 metres with approximately 70 per cent suspended sediment inflow being retained in the river channel. Due to high sedimentation, the river bed is becoming shallow day by day, reducing thereby the height of the banks and the flood-carrying capacity of the river.
While conducting the IIT Roorkee study, available cross sections were used. The preliminary numerical modelling study was broadly focused on the key fluvial and morphological parameters due to dredging and dyking in the Brahmaputra reaches. The river width for the purpose was kept between five and seven kilometres, going by the government figures.
Taking into consideration the huge size of the Brahmaputra, dredging operations cannot practically be done in its entire reach in one working season. It has thus become imperative to examine prima facie the fluvial consequences of such partial dredging operations. For hydraulically analysing this, two dredging scenario have been considered – one dredging operation commencing from the upstream end and the second starting from the downstream end.
Due to constriction of the river width by raising embankments, the channel velocity, shear stress, stream power and sediment transport capacity have been increased and hence, the silt deposit rate is curtailed proportionately within the dredged reach.
The upstream and downstream impacts beyond the dredged reaches due to partial dredging were also investigated under the study. The numerical simulations have indicated occurrence of river bed degradation or lowering at the upstream end, whereas significant aggradation or river bed rise is seen occurring at the downstream end of the dredged reach in the model due to abrupt reduction of stream power. These morphological river responses need to be thoroughly quantified and accounted for, said Prof Sarma.
From the study, it was observed that selective dredging with dyking may be one of the measures for controlling the Brahmaputra flood, in conjunction with other complementary measures. But, comprehensive catchment area treatment measures to control soil erosion are prerequisites for sustainability of the selective dredging of the Brahmaputra river bed, he asserted.