engleski

ANALYSIS OF CHANGES IN SEDIMENTATION PROCESS IN THE BOLJUNČICA BASIN AND THE PLOMIN BAY DUE TO DRAINING OF THE ČEPIĆ LAKE AND BUILDING OF STORAGE RESERVOIR BOLJUNČICA - ISTRIA (CROATIA)

The Boljunčica river basin in Istria is one of basins exceptionally exposed to erosion processes. At the same time, this basin represents an area in which extensive water-management infrastructure works were performed followed by serious after- effects on watercourse regimen alternations and sedimentation process. Firstly, there is draining of former Čepić lake (average surface area of 6 km2 ), where waters and deposits of the upper part of the Boljunčica basin fl ew into. In 1932 a 4.5 km long drainage tunnel was dug taking the lake waters into the Plomin bay. This is how the former Čepić lake became the Čepić plain, while sedimentation processes dislocated the Plomin bay making new problems in the harbour waters. The other major intervention took place in 1970 when a multi-purpose storage reservoir Boljunčica (6.5 million m3 ) was built upstream from the Čepić plain thus causing changes to watercourse regimen of downstream Boljunčica as well as changes to the conditions of sedimentation in the storage reservoir. The above named paper reports on analysis of these interventions and changes as well as the consequent negative effects of erosion, spreading and sedimentation of deposits. The estimated values reach up to 0.730 m3 of deposits accumulated in 35-year period since the formation of the storage reservoir, which considerably exceed the anticipated 100-year volume of dead deposits of 0.500 million m3. Moreover, there was an estimate made amounting to 2.55 million m3 of total deposit infl ow into the Plomin bay following the Čepić drainage tunnel construction. This amount of deposit causes problems in regular harbour activities in Plomin harbour where there is also a quay for coal carriers supplying thermoelectric power plant Plomin. Due to this phenomenon there were 0.760 m3 of recent accumulated deposits removed from the Plomin bay in 1990s. These were the deposits accumulated following the draining of Čepić lake. This paper reports on applied comparison of production and infl ow estimates made according to different methodology and on wider regional scale - based on empirical method of estimate, monitoring results of hydrologic concentration, spreading of suspended deposit and dynamics of changes in geometry of sediment bodies. This is how various aspects of past experience regarding the process of deposit accumulation in the storage reservoir were evaluated, which represent one of the starting points for planning of further irrigation storage reservoir building in Istria.

sedimentation process ; Boljunčica basin ; draining ; storage reservoir

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