River response to channel regulation: Case study of the Raba river, Carpathians, Poland

Processes induced by the channelization of the Raba River in the present century are examined to illustrate the response of a gravel‐bed stream following narrowing and straightening of its channel. Up to 3 m of incision has occurred. The change from a slow and relatively steady degradation in the lower reaches to separate degradation events in the higher reaches is attributed to the differential rate of headcut retreat and to the control exerted by mid‐channel bars upon the rate of river‐energy dissipation. Progressive outwashing of finer grains from bed material has followed the diminishing sediment yield of the basin and the increase in stream power. The ensuing growth in mean grain size and changes in sediment fabric have increased boundary resistance to flow and reduced particle susceptibility to entrainment. Downstream magnification of peak discharges has become increasingly pronounced with the advancing incision. The decrease in flood‐plain storage and self‐acceleration of flows passing the relatively deep and straight channel has caused flood waves to become progressively more flashy in nature. An increase in channel depth and reduction in gradient caused by downward and backward erosion, as well as bed material coarsening has promoted the re‐establishment of an equilibrium. Conversely, flow velocity increases due to flow concentration in the deepened channel. Reduction in grain mobility allows the river to attain a new equilibrium at flow‐velocity and stream‐power levels higher than before the channelization. Numerous disadvantages of the applied regulation scheme and its failure to reduce flood hazard raise the question of its maintenance. To be successful, any regulation design must take into account changes in sediment supply and flood hydrographs resulting from the simultaneous alterations in basin management.