Sediment flux during flood events along the Trotuș River channel: hydrogeomorphological approach

Abstract

The degree of the river channel change, and particularly the amount of transported sediment, defines the geomorphic effectiveness of a flood event. This study aims to improve the level of understanding of the control exerted by the streamflow and the flow energy on the sediment flux regimes during flood events, which are ranked depending on their geomorphic effectiveness. The study focused on the Trotuș River, Eastern Carpathians, Romania. Two data sources were used: gauging stations (discharge and suspended sediment load) and field observations (grain size, channel bed slope and channel width corresponding to bankfull discharge). The bedload sediment transport was estimated using the Bedload Assessment for Gravel-bed Streams (BAGS) program. The parameters and variables taken into consideration (magnitude, duration, stream power, total energy expenditure, total sediment load) were statistically analysed for each flood event. Just 15% of the total 76 investigated flood events ranked in the class which includes the flood events with the highest geomorphic effectiveness. Of the 41 × 106 t of sediment carried through the four gauging stations during flood events, approximately 74% was transported at stream power values above 300 W m−2. In the years with type B flood events, the sediment flux was much higher than the multiannual average value. Changes of the sediment rating curves were documented at all gauging stations subsequent to major flood events. Clockwise loops accounted for the highest percentages in all types of flood events. Major flood events generate significant changes in river channels, which are subsequently reflected in the sediment regime. The main cause of the changes occurring in the sediment regime likely lies in the channel adjustments subsequent to flood events. This investigation introduces a methodology for ranking flood events depending on their magnitude, duration, stream power and transported sediment.