The impact of a hydroelectric power plant on the sediment load in downstream water bodies, Svartisen, northern Norway

When the Svartisen hydroelectric power plant was put into operation, extens ive sediment pollution was observed in the downstream fjord area. This pape r discusses the impact of the power plant and the contribution from various sources of sediment. Computation of the sediment load was based on samples collected one to four times per day. Grain size distribution analyses of s uspended sediments were carried out and used as input in a routing model to study the movement of sediments through the system. Suspended sediment del ivered to the fjord before the power station was constructed was measured a s 8360 metric tons as an annual mean for a 12-year period. During the years 1995-1996 when the power plant was operating, the total suspended load thr ough the power station was measured as 32609 and 30254 metric tons, respect ively. Grain size distribution analyses indicate a major change in the comp osition of the sediments from 9% clay before the power plant was operative to 50-60% clay afterwards. This change, together with the increase in sedim ent load, is believed to be one of the main causes of the drastic reduction in secchi depths in the fjord. The effect of the suspended sediment load o n the fjord water turbidity was evaluated by co-plotting secchi depth and p ower station water discharge. Measurements during 1995 and 1996 showed that at the innermost of these locations the water failed to attain the minimum requirement of 2 m secchi depth. In later years secchi depths were above t he specified level. In 1997 and 1998 the conditions improved. At the more d istal locality, the conditions were acceptable with only a few exceptions. A routing model was applied to data acquired at a location 2 km from the po wer station in order to calculate the contributions from various sediment s ources. This model indicated that the contribution from reservoir bed erosi on dominated in 1994 but decreased significantly in 1995. Future operation of the power station will mostly take place with a high water level in the reservoir and is likely to result in acceptable water quality in the fjord. However, during periods of low drawdown, sediment pollution may again beco me a problem. (C) 2001 Elsevier Science B.V. All rights reserved.