FLOOD EVENTS AS GEOMORPHIC THRESHOLDS FOR CHANNEL BED LEVEL CHANGE

Abstract

Channel bed levels are highly dynamic in time and across space, constantly adjusting under the influence of natural and anthropogenic controls. The aim of this study was to assess the role of flood events recorded between 1994 and 2018 in the bed elevation changes documented on the major tributaries of Trotuș River (Eastern Carpathians). To this purpose, the data from fifteen gauging stations was used, based on which the evolution trends of channel bed levels were determined for the investigated channel reaches. The role of geomorphic threshold was assigned to stream power. The daily maximum and total values of this parameter were used to explain the bed elevation shifts. The average channel bed deepening rate was 2.2 cm/yr. Depending on the variation trend of channel bed levels, three evolution patterns could be distinguished in relation to the reference year: (i) a category where channel degradation was nearly continuous; (ii) another one where initial aggradation of the channel bed occurred, ensued by continuous degradation, without reverting to the initial state, and (iii) a group with oscillating aggradation-degradation evolution resulting in the eventual recovery of the initial state. The recovery time for changes triggered by major flood events ranged between one and nearly 10 years, whereas in the case of shifts generated by high frequency, low magnitude events, the recovery time was maximum one year. Over short term flood events drive the direction and intensity of channel bed elevation adjustments.