Author(s): Adam Lajczak
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Keywords: No Keywords
Abstract: Regulation of riverbeds in order to make rivers more navigable has been one of the dominating evidences of human impact in river valleys for almost two centuries (Gregory 1987, Lajczak 1995,2003). Such measures and melioration works in adjacent floodplain, are also carried out in order to diminish flood hazard in river valleys (Kok 1992, Zelazo 1993, Lajczak 1999). Regulation of riverbeds leading to shortening their channel and drying the floodplain by a dense network of deep ditches, are assumed as the most controversial method of diminishing flood hazard (Wolman 1967, Kok 1992, Jasnowska 1995, Tomialojc 1995). Many years’ experience in this matter shows the results, which are often contrary to the expected one (Kajak, Okruszko 1990, Andrews, Burgess 1991, Finlayson 1991, Angelstam, Arnold 1993). Shortening of riverbed, especially in case of meandering sandy river initiates process of rapid bed deepening, which results in the increase of its bankful discharge (Lajczak 1995,1999). Such process occurs only when the material from channel deepening is transported at large distances. If the river cannot transport the material far enough, for example due to small gradient of the channel or the occurrence of large water-swamp lands adjacent to river channel and being flooded during overbank water stages, the process of material aggradation resulting in rapid channel shallowing, occurs downstream the river, i. e. below the section of deepened river channel takes place. This causes the increase of frequency of overbank water stages in a long river section below the regulated course, which results in the extension of the time of floodplain flooding in the lower section of the river. Another result of the extended time of the inundation of the floodplain in such sections of river valleys is accelerated rate of overbank sedimentation, especially along river banks occupied by riparian forest (Cooper et al., 1987, Howard 1992, Kajak 1993).
Year: 2004