GEOMORPHIC EFFECTIVENESS OF FLOODS ON TROTUŞ RIVER CHANNEL (ROMANIA) BETWEEN 2000 AND 2012 - Carpathian Journal of Earth And Environmental Sciences

We determined the hydrological and hydraulic characteristics of flood events occurring on the river Trotus between 2000 and 2012 based on data recorded daily at 6 gauging stations, direct measurements in the field (channel slope, grain size, channel width, and riverbank height) and data provided by orthophotos taken before and after flood events. The parameters we derived from these data (i.e., bankfull discharge, mean annual flood, catastrophic flood, geomorphic flood, specific stream power, shear stress, and critical shear stress) allowed us to assess the degree of geomorphological impact of each flood, according to their magnitude and duration. Of the 5 flood events (i.e., 2004, 2005, 2007, 2010, and 2012) during the study period, two (the 2005 flood, with a peak discharge of 2845 m³ s^-1, and the 2010 flood, with a peak discharge of 1567 m³ s^-1) stood out both in terms of the particular values of the aforementioned parameters and, in particular, the major changes they generated in the Trotus river channel. The bankfull discharge flood duration (72 hours in 2005 and 222 hours in 2010) and the duration above the Miller-Magilligan threshold (45 hours in 2005 and 48 hours in 2010), can to a large extent explain the high geomorphic effectiveness of moderate-magnitude flood events that span longer time frames. While the magnitude of the 2005 flood (with a recurrence interval of 200 years) was nearly double that of the 2010 flood, their geomorphological impact was rather similar. The response of the channel to these floods consisted of discontinuous spatial adjustments in the channel width and channel bed elevation. The assessment of the channel aggradation/degradation rates was performed based on the evolution over time of average multiannual discharge levels and channel survey data (cross-sections). Prominent changes in the channel occurred in the Târgu Ocna–Căiuţi sector (30 km long, nearly 20% of the entire length of the river), where, between 2005 and 2012, the channel bed deepened by an average value of 0.85 m (0.12 cm y^-1). In terms of their geomorphic impact, the 2005 and 2010 flood events can be considered severe floods, even catastrophic in certain sectors.