Changes of fluvial processes caused by the restoration of an incised mountain stream

The construction of a high check dam on mountain Krzczon ' owka Stream, Polish Carpathians, in the mid-20th century caused numerous detrimental changes to the downstream reach. In 2014 the check dam was lowered to make the structure passable for river biota. Before that, several block ramps were constructed in the deeply incised downstream reach to facilitate entrapment of the sediment expected to be released from the lowered check dam. When the check-dam lowering was underway, a flood flushed out from the dam reservoir a considerable amount of sediment that was efficiently trapped by the block ramps. To determine to what extent the environmental problems caused by the long-term sediment starvation of the stream were mitigated by the restoration works, one-dimensional hydraulic modelling of flood flows was performed for pre- (2013) and postflood conditions (2015) in ten study cross-sections. Moreover, hydromorphological quality of the stream was determined before the onset of restoration activities (2012) and after their completion (2015). The flood of 2014 deposited about 15,650 m3 of bed material in the downstream reach, which re-established an alluvial channel bed and increased bed elevation by 0.50 m on average. Bed aggradation reduced flow capacity of the channel and increased water stages attained at given flood discharges. This significantly decreased bed shear stress and entrainable grain size of bed material. The proportion of the total flow conveyed over the floodplain and retention potential of the floodplain increased, although these effects were largely dependent on the amount of bed aggradation in the study cross-sections. The hydromorphological quality of the stream improved in 4 out of the 5 evaluated cross-sections, with 3 cross-sections being upgraded from moderate to good quality class. The study demonstrated effectiveness of block ramps in mitigating problems in the physical functioning of an incised mountain stream.