Riverbed relaxation downstream of Novosibirsk dam

Abstract

The Novosibirsk hydroelectric complex, the largest hydropower project on the Ob River, was built ~60 years ago. It is located 680 km from the confluence of the Biya and Katun rivers and formed a 8 km³ reservoir. The paper considers the riverbed transformation processes followed after the hydroelectric complex construction caused by changes in the hydrological regime and sediment flux. The reservoir regulates daily and seasonal runoff, and intercepts 90% of the suspended sediment flux. The riverbed transformation reflecting daily discharge fluctuations could be detected for 70 km downstream from the dam. The intensive erosion rates up to 12 cm/year were documented along the 8–10 km stretch of the channel near the dam during the period of 20 years due to the influence of daily discharge regulation waves and sediments shortage. The lowering of the riverbed and water levels for 1.8 m is exponential. During the 20-year period, the initial sand deposits of 0.5 mm in size were removed exposing poorly eroded, rocky and coarse-grained soils and significantly decreasing the erosion rates. Down the stream, 10–40 km from the dam, erosion started with a delay of 3–5 years with the rate of less than 3 cm/year. The lowering of the riverbed and water level reached only 1.4 m, and was stabilized ~50 years after the dam construction as the grain size of the riverbed sediments increased by 5–6 times and the water surface gradient decreased. Additional mechanical disturbances; mining of at least 20 million m³ sand and gravel construction materials, dredging the channel which allowed in the 1960s–1970s to increase the navigable depth by 1.0–1.3 m, and other river engineering work also played a significant role in riverbed transformation.