Bergerosion i utskovskanaler
Today?s climate changes will probably give rise to precipitation in Sweden, which will cause more floods in Swedish rivers. Many of the Swedish rivers are regulated and have lots of hydro-electro power plants. Higher floods in the rivers will give greater water loads on the dams, which mean that a higher discharge through the sluice gates and in the spillway channels is needed. High discharge of water in a spillway channel can create scour of the material in the channel. Usually, for spillways in Swedish hydropower plants, this material is rock and the scour is in form of rock blocks. Scour downstream of dams can in the worst case endanger the dam construction.The purpose of this study was to identify the extent and the type of rock scour that may appear in spillway channels of Swedish hydro power plants. The scour in rock material in a spillway channel is usually caused by pressure fluctuations in the water, which can cause large differences in pressure on the top and bottom surfaces of blocks. This pressure difference can be high enough to lift whole blocks. The main factors that affect this kind of scour are the crack pattern of the rock mass, the geometry of the spillway, the discharge of water and the surface fluctuation in the water. In this study a numeric model analyse, a field study and a literature study has been made in order to examine the influence on scour of the orientation of cracks in a rock mass. A rock mass containing a group of cracks with small or no dip is more likely to scour when it is exposed to discharging water. The orientation of the cracks in a rock mass relative to the direction of the water flow has also an impact on the capacity of the rock mass to resist scour.In the field study the scour pattern in the spillway channels of the hydro power plants in Ligga, Harsprånget, Porjus, Satisjaure and Seitevare was examined. The scour of rock mass in the spillway channels in Ligga, Harsprånget and Porjus has been extensive. The rock mass in these channels has bedding cracks of small distances. This is probably the reason why these rock masses are sensitive for loads from discharging water. The rock mass in the spillway channel in Satisjaure has lots of cracks and is also very sensitive for loads from discharging water. This channel has been rebuilt during 2004.There are different methods to predict the ability of a rock mass to resist scour. The Erodibility Index Method is a semi-empirical method that is verified by field observations in the USA. In this work the ability to apply this method on Swedish channels has been examined with field observations from Porjus, Harsprånget and Seitevare. As the Erodibility Index Method seemed to work in the case of these three spillways, it was also applied on the spillways in Satisjaure and Ligga.Measures to reduce the energy in the discharging water in the spillways are very effective to reduce the risk of scouring in the rock mass. Blasted stairs in the rock mass in a channel or a stilling basin downstream the sluice gate is something that can reduce the energy in discharging water. Further documentary of spillway channels, like mapping and laser scanning, would contribute to a further verification of the Erodibility Index Method and to get a general survey of the risk of scour in Swedish spillways.