Abstract
In the outlet of the Raba River to the Vistula, the biggest river in Poland, the morphology and morphodynamics of sand and fine-gravel subaquatic dunes were investigated. The site is situated in highland region just about the entrance to Polish Carpathians. The dunes formed on the Raba River bed estuary are composed of sand and fine gravel (d 50 up to 11 mm). Systematic observation (within the 2000-2005) were made of geometry, sediment composition and hydraulic climate under which the dunes grew and decomposed. The investigation focuses here mostly on the geometrical parameters of these bed forms such as height, length, as well as granulometric characteristics of the sediment. Based on in-site measurements different hydraulic parameters were calculated such as shear stresses, resistant coefficient, Froude and Reynolds numbers and roughness coefficient. It was found that the relation between height (H) and length (L) of the Raba estuary dunes describes the formula: H = 0.05L0.35. Also these dunes are steeper and flatter then classical H/L index is: H/L = 0.0518L0.622. During the field campaign, when the foot access to the estuary was possible and dunes were spotted on the river bed the range of measured water velocity was from v = 0.39 m∙s -1 to v = 0.81 m∙s -1 with the highest velocity over the dune crest. At the same time the measured range of shear stresses within the dune field formation were from t = 0.115 N×m -2 to t = 1.59 N×m -2 . On the field investigations the CCHE2D - two-dimensional unsteady flow and sediment transport model for non-equilibrium transport of non-uniform sediment mixtures – was applied. The model was used to simulate the morphodynamic changes along the outlet of the Raba River basing on field observations of the 2005 summer flood as well as calculate hydraulics parameters. It was also used to test and confirm the range of morphodynamic changes, which take place along the research reach where the dunes are being developed. The maximum water flow velocity for and the shear stresses for the summer flood caused the hydrodynamic conditions when water velocity as well as shear stresses values reached respectively up to v = 1,93 m×s -1 , and t = 19.74 N×m -2 which shows that critical conditions of bed load movement were exceeded for the Raba estuary
Highlights
Sand-gravel subaquatic dunes studies are very rare, especially those involving field measurements
The fact that sand-gravel dunes are extremely rare in subaquatic environment, secondly, an appreciation of the changes of river bed morphology, thirdly, the dunes are responsible for flow resistance, which is significant, for ex-ample, when designing hydraulics structures or calculating flow residence times for water supply or pollutant dispersion
Within the lights of above statements the authors were lucky to find in the field sand-gravel subaquatic dunes with foot access for study, which were situated at the outlet of the highland character of the Polish Carpathian river the Raba
Summary
Sand-gravel subaquatic dunes studies are very rare, especially those involving field measurements. Equilibrium and non-equilibrium gravel dunes range in length from less 0.6 m to greater 100 m Their heights range from less 0.1 m to 16 m (Carling, 1999; RadeckiPawlik et al, 2006). Height (H) and length (L) data for the gravel dunes reported by some researchers (Carling, 1999) are consistent with the H/L function reported by Ashley (1990) for equilibrium dunes developed in sand. Using Ashley's dune form criteria of height (>0.1 m) and length (>0.6 m) as a general guide it can be shown that gravel dunes have been correctly identified in the number of studies (Galay, 1967; Dinehart, 1989; Carling, 2005). The other bed forms, which develop on the river bed are ripples Their length is proportional to the grain size. Some scientists (Kuhnle, 1986; Iseya and Ikeda, 1987) report bed load sheets and define them as low-relief bed forms, that develop in mixture of GEOREVIEW, Vol 21 (2012)
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