Sanda, ROŞCA¹, Ştefan, BILAŞCO^1,2, Dănuţ, PETREA¹, Ioan FODOREAN¹ & Iuliu VESCAN^1
1"Babeş-Bolyai" University, Faculty of Geography, 400006 Cluj-Napoca, Romania, rosca_sanda@yahoo.com, dpetrea@geografie.ubbcluj.ro, fioan@geografie.ubbcuj.ro, vescan@geografie.ubbcluj.ro
²Romanian Academy, Cluj-Napoca Subsidiary Geography Section, 9, Republicii Street, 400015, Cluj-Napoca, Romania, e-mail: sbilasco@geografie.ubbcluj.ro

BANKFULL DISCHARGE AND STREAM POWER INFLUENCE ON THE NIRAJ RIVER MORPHOLOGY

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The concept of dominant discharge is used in geomorphologic studies concerned with stability of riverbeds and riverbed management. The results of our study involves the specific energetic conditions on the Niraj River during maximum river flow (by analysing the bankfull discharge and the stream power during peak flow) and during normal flow (by analysing the stream power at average flow). The bankfull discharge value is considered fundamental for the present riverbed formation, for Niraj river beeing situated between 20.8 m³/s and 928 m³/s, depending on local morphometrical riverbed condition. Therefore using the Manning-Strickler formula, the bankfull discharge was calculated due to the fact that it is considered to be the main estimative parameter of the riverbed hydraulic geometry. The values of stream power varied between 3.51 (W/m²) and 289 (W/m²) and It was determined by means of data gathered on seven cross sections, taking into consideration the upstream riverbed typology. In two sectors, considered to be representative for the riverbed typology, the return periods were calculated, probability of exceedance (66%) and non-exceedance as well (1.5 year) as the influence of bankfull discharge on the Niraj riverbed dynamics in the 1970-2012 interval. The case study was performed on two different sectors taking into account the evolution of the sinuosity index, which is dependent on the influence of natural and anthropic factors. The analysis enabled the comparison of meandering rates for 100 year interval, both in the sectors with a natural evolution of the riverbed and in the sectors with high anthropic impact as a resuls of constraints of the dams. The high lateral dynamics is mainly caused by the decrease of slope gradient and the decrease of bank resistance due to extreme events with high energetic values (varying between 30.2 and 105.3 W/m² in the upper and middle sectors of the Niraj river). The obteined bankfull discharge values with 1.5 year probability (41.6 m³/s for Cinta and 28.9 m³/s for Bereni) were validated by comparing the calculated data within the cross sections located near the Gălești and Cinta hydrometric stations and the data obtained by hidrometrical measurements in the two stations mentioned above. The results are included in the confidence interval of 5%.