INFLUENCE OF EXTREME DISCHARGE ON RESTORATION WORKS IN MOUNTAIN RIVER - A CASE STUDY OF THE KRZCZONOWKA RIVER (SOUTHERN POLAND)

The research was conducted on the Krzczonowka River channel, one of the gravel-bedded, regulated mountain river in Polish Carpathians. The main morphological and ecological problem of the river was lack of sediment and channel downcutting. The area is currently associated with an on-going project called the Upper Raba River Spawning Grounds. Lowering of an existing debris dam on Krzczonowka River is a part of the project. In 2013 twelve artificial riffles have been created by heaping up stones at points within the segment of the river channel below the debris dam. The riffles are to introduce variety to the longitudinal profile of the river and to reduce the river's slope. Consequently, these are to decrease sediment transport and to prevent further deepening of the river channel. Post-project monitoring of river restoration works is conducted to determine channel changes and development. In May, 2014, extreme flooding occurred, which caused unexpected changes in channel development. This paper describes maintenance work performed in the riverbed of the Krzczonowka River. Observations and calculations concerning changes in conditions of water flow and sediment transport are also presented. The main purpose is to characterize the influence of an extreme flow event on morphology and functioning of the recently restored gravel-bed river.