Morphological changes due to construction of a barrage on the Teesta river

Abstract

The Teesta Barrage having a length of 615.0 meters have started operation from August 1990. This study has been conducted to investigate the impact of the Teesta Barrage on the morphology of the Teesta River. Assessment of the long-term impact due to construction of the Barrage on the Teesta River along the total reach was also carried out. The reach under study is 121.0 Km long starting from the Indian border to the outfall on the Jamuna River near Chilmari. Cross-sectional properties at 20 locations of the Teesta River for the years 1988-89, 1991-92, 1994-95 and, 1998-99 were analyzed. The results were used to update the work done by BUET in 1988 during the project "Review and Optimization of Planning and Design of Irrigation and Drainage System of Teesta Barrage Project". It is observed that there is consistency in aggradation in front of the Barrage during the past 30 years. Overall there has been about 1 meter on the average aggradation during this period. But considering the period of the last 8 years the aggradation is only about 0.30 meters. Considering the change during the last 12 years and also along the total reach of the Teesta River it is seen that appreciable deposition is absent but there is quite a large amount of erosion downstream of the Barrage in between chainage 18 to 24 Km and also downstream of the Kaunia Railway Bridge between chainage 70 to 84 Km. It was also observed from the analysis of the water level records between years 1990 and 2000 that the Teesta River bed level at Dalia and Kaunia stations both located at the downstream of the Barrage has degradated by about a meter. The planform analysis with the images of 1984, 1994 and 1997 have revealed that during the past 14 years the river has shifted quite appreciably (2840 meters) towards the west in the middle part of the reach between the Indian border and about 15 Km upstream of the Kaunia Railway bridge. At 15 Km upstream of the Katmia Railway Bridge the situation is just the opposite and the shifting is towards the northeast by about 3409 meters. The extent of erosion indicates that the river requires v 1 sediment from either the bed or banks of the river to maintain the equilibrium state after loosing sediment at the upstream of the Barrage due to sedimentation. Future prediction on the Teesta River sedimentation was made using the Mike II morphological module of DHI based on the channel geometry of 1997. The model results have shown that there would be I meter bed level rise in the 10 years period after the construction of the Barrage. This is similar to the findings of the average bed level change, which show that the extreme most upstream cross-section, the bed level change has been about 2.5 ft in the past 10 years. Also it may be seen that the degradation is more in the downstream part and the impact zone of the Barrage is the total reach upstream of the Barrage and also about IS-Km downstream of it.