Analysis of historical changes in planform geometry of a mountain river to inform design of erodible river corridor
Hanna Hajdukiewicz, Bartłomiej Wyżga
Abstract
The analysis of changes in planform geometry of the Biała River during the last 130 years is used to diagnose hydromorphological river degradation and thus to verify the need of river restoration through establishing an erodible river corridor. Outcomes from the analysis are then used for delimiting the erodible corridor and predicting the potential for future river widening in the corridor. The study analyses presentations of the active river zone and the structure of river geomorphic units on topographic maps from 1878, 1935 and 1962, aerial photos from 1967, 1977 and 1987, and orthophotos from 1998 and 2009. As a result of river channelization followed by channel incision, during the study period the width of the Biała was reduced to 16%–57% of its width in 1878. The river narrowing was associated with a reduction in the proportion of channel bars in the total river width and nearly complete elimination of islands. All these changes demonstrated the degradation of hydromorphological integrity of the river and the need for its restoration by allowing free channel development in an erodible corridor. The belt of the river migration during the last 130 years identified on the basis of overlays of the river position from all dates was ⁓5 times wider than the contemporary river. This justified delimiting the erodible river corridor on a substantially larger area of the valley floor than the area of the river in 2009. In 2009 the river was narrower than the largest river width recorded between 1878 and 1998 along nearly the whole length of the study river sections. This indicates that the concentration of flood flows in the narrow channel, increasing their unit stream power, is a factor increasing the potential for future bank retreat and channel widening in the erodible corridor of the river.