Influence of tributaries on downstream bed sediment grain sizes under flysch conditions

Abstract

Tributaries are one of the most important factors contributing to variability in the downstream evolution of bed sediment grain size. The primary aim of this work is to evaluate the response of the bed sediment texture in the recipient channel induced by ten tributaries of the Cerna Ostravice stream and find reach-scale and catchment-scale parameters that would be able to predict this response. The research was based on collecting information on the grain size distributions at sites adjacent to confluence zones. A significant change in sediment texture occurred in the vicinity of five confluences. Considering the other factors contributing to grain size variability (e.g., local channel geometry, lithology, and lateral sediment sources), it was assumed that only four of them were associated with a sufficient bedload influx to alter the sediment calibre below the junction. Moreover, a significant morphological effect in the form of a large confluence bar was observed in one case. These tributaries had several common features: (i) they had a larger relative catchment area than that of nonsignificant tributaries; (ii) they were characterized by different bed grain sizes, with some exceptions; and (iii) they had a higher unit stream power close to the confluence in relation to that of the mainstream. These characteristics were represented by the proposed relative parameters, including the relative unit stream power and bed material texture, which allowed the best classification of significant and nonsignificant tributaries. In their simplified form, the parameters described the transport capacity and grain size distribution, which were generally considered to be primary factors responsible for a redefinition of the sediment texture in the recipient channel. However, it should be noted that these results are subject to some degree of uncertainty due to the relatively small sample size of only 10 tributaries.