Development of sediment slug upstream from the Czorsztyn Reservoir (southern Poland) and its interaction with river morphology

- Significant in space and time in-channel sedimentation occurred upstream from the CR.
- Dam construction and the large flood in 1997 were the main factors initiated this phenomenon.
- Bidirectional bar-bank interaction controls the development of in-channel sedimentation.
- This interaction resulted in channel widening, bend initiation and remobilization of sediment stored from nineteenth century.
- The conceptual model of backwater slug development upstream from the CR was proposed.

The effects of base-level rising upstream of dam reservoirs on in-channel sedimentation and interaction of the stored sediments with the gravel-bed channel morphology have received little attention so far. Previous studies, however, suggested that the feedback mechanism between in-channel sedimentation and bank erosion may affect channel morphology. Here, the pattern of the bar area, bank erosion, and morphology of the gravel-bed Dunajec River upstream from the Czorsztyn Reservoir (CR), constructed in 1997 in southern Poland were analyzed from aerial images (1982-2012) and LiDAR data (2013). In the part of the post-dam period with a large flood, the average bar area increased significantly in the backwater section, and at some distance upstream, and then extended in the upstream direction at an average rate of above 40 m/y, reaching 2.2 km upstream from the CR in 2012. The bar area variation was 40% and 77% explained by local bank erosion in the periods of large and low to moderate floods, respectively. The sum of bank erosion from the post-dam period explained 80% of the variation in the present width/depth ratio, significantly increased in the backwater section. The results showed that the large floods in 1997, in conjunction with backwater inundation, initiated intensive bank erosion and bar growth. Subsequently, in the period with low and moderate floods, the localized bar-bank interaction, connected with the flow divergence around the deposited bar, led to localized bank erosion and additional bar growth promoting bend development. These processes, propagating in the upstream direction, were facilitated by the existence of a large amount of easily remobilized sediment stored in the floodplain connected with the sedimentation zone from the end of the nineteenth century. Bend development was controlled by valley confinement causing downstream bend translation in the narrower valley-confined section and its extension in the wider unconfined section of past sediment storage. The results obtained from this site-specific location imply that the occurrence of similar past sediment storage zones of easily remobilized coarse sediments may favor intensive bank erosion and bar growth during large floods, and in the case of low transport competence backwater zones of gravel-bed rivers where the remobilized sediments cause localized bar growth; it may promote morphologically effective bar-bank interaction in the period with low and moderate floods.

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