Volume XIV |

The temporal variation of suspended sediment transport according to the dominance of suspended sediment sources. Case study: the Trotuş river between 2000 and 2014

Abstract: Based on the data series of average daily streamflow and suspended sediment load recorded between 2000 and 2014 at four gauging stations (Lunca de Sus, Goioasa, Târgu Ocna and Vrânceni), the temporal variation of the suspended sediment transport was investigated according to the prevalence of source areas. Thus, a significant temporal variability (monthly, seasonal, annual) was determined, in close relation with the amount of precipitation and the streamflow. The following equation was determined between the mean monthly suspended sediment load (Ṝ) and the mean discharge (Ǭ) at Vrânceni section: R= 0,0035Q2,2895, r=0,899. We believe this relation has a high degree of confidence for the indirect determination of solid load and it is comparable with other equations of this type. Along the entire length of the river, July was the month during which the highest suspended sediment load was recorded, with an average percentage of 37% of the total amount. At the opposite end, December is the month with the lowest documented suspended sediment load, with just 0.5% of the total amount transported annually by the Trotuş. As regards the seasonal variability of the suspended sediment load, the following values were determined along the entire length of river Trotuş: during the winter season the volume of sediment carried by the river amounts to approx. 2.1% of the total annual transported suspended sediment, the spring season accounts for 33.7% of the annual volume, the summer season accounts for ca. 55.5%, and the fall for 8.7%. In order to plot the R-Q correlation, the wettest, as well as driest years were selected for every gauging station. On the resulting plots, there were identified the thresholds based on which the two sources were separated depending on the area of origin: dominant from the catchment or dominant from the river bed. Overall, during the investigated period on the Trotuş river, the river beds contributed with about 21% of the total volume of transported suspended sediment. Depending on the type of the year (wet, dry or normal), the average input of the beds to the annual volume of suspended alluvium was as follows: 4% in wet years; 43% in dry years; 15% in normal years. The total volume of suspended sediment transported through the four sections on the Trotuş river between 2000 and 2014 amounted to approx. 39×106 t, thus the average annual volume was 2,598,000 t. A large share of this suspended sediment yield was produced during major floods. For example, at Vrânceni ca. 61% of the total sediment yield for the 15 year-period under investigation resulted from just 3 flood events (2005, 2010 and 2012).

Volume XI |

The sediment transport of the Siret River during the floods from 2010

Abstract: Beginning with 2004 in the Siret River Catchment there occurred exceptional flash floods that exceeded the maximum historical values recorded, on the main river and on its tributaries (2004, the Trotuş River; 2005, the Siret River, the Trotuş River, the Bistriţa River, the Putna River; 2006 – the Clit River from the Suceava River Catchment; 2008, the Siret River, the Suceava River). The 2010 summer flood from the Siret River also falls in this category. This paper uses hydrological data (water discharge, suspended sediment discharge) between the 20th of June and 10th of July 2010 from 5 gauging stations located on the Siret River: Siret, Hutani, Lespezi, Dragesti, Lungoci, and also meteorological data (rainfall) measured at different gauging stations from the Siret River Catchment. The rainfall recorded in this time of the year in the catchment was very high, with values up to 210 l/m2 in approximately 10 days. The hydrographs of the flash flood indicate the fact that the transit of the water trough the reservoirs system from the Siret River (Rogojeşti – Bucecea and Răcăciuni – Bereşti –Călimăneşti – Movileni) reduced the maximum water discharge with values between 7-27%. The values of the maximum sediment discharge also recorded a reduction while transiting this reservoirs system with approximately 60%. The evolution of the Siret river bed channel during this flood (aggradations with values between 15-100 cm and degradations starting from 65 cm until 200 cm, in different moments of the flood) is influenced by the high values of the water and sediment discharge and by anthropogenic interventions on the river bed (pit-ballast, regularization of the river bed, reservoirs). Processing the hydrological and meteorological data recorded during the flood (20th of June – 10th of July 2010) indicates two important features of this event: the climatic variability – exemplified by the big values of precipitations from the catchment and the anthropogenic impact revealed by the transit of the flood wave and the evolution of the river bed.