Assessment of Methods Used in 1D Models for Computing Bed-Load Transport in a Large River: The Danube River in Slovakia

Abstract

Comprehensive measurements of bedload sediment transport through a section of the Danube River, located approximately 70km downstream from Bratislava, Slovakia, are used to assess the accuracy of bedload formulae implemented in 1D modelling. Depending on water discharge and water level, significant variations in the distribution of bedload across the section were observed. It appeared that, whatever the water discharge, the bed shear stress τ is always close to the estimated critical bed shear stress for the initiation of sediment transport τ cr . The discussion focusses on the methods used in 1D models for estimating bedload transport. Though usually done, the evaluation of bedload transport using the mean cross-sectional bed shear stress yields unsatisfactory results. It is necessary to use an additional model to distribute the bed shear stress across the section and calculate bedload locally. Bedload predictors also need to be accurate for τ close to τ cr . From that point of view, bedload formulae based on an exponential decrease of bedload transport close to τ cr appear to be more appropriate than models based on excess bed shear stress. A discussion on the bedload formula capability to reproduce grain sorting is also provided.

Discussion of “Understanding the influence of slope on the threshold of coarse grain motion: Revisiting critical stream power” by C. Parker, N.J. Clifford, and C.R. Thorne Geomorphology, Volume 126, March 2011, Pages 51-65

“…Indeed, as observed by (Camenen et al, 2011), large variations of the bed shear stress may be observed throughout the section; and as bedload transport is non linear, one should compute bedload locally to yield a proper estimation of the total bedload transport across the section. This remark is even more important if a variation of the grain size is found throughout the section, as is often observed (Habersack and Geomorphology 139-140 (2012) , 2001;Frings and Kleinhans, 2008).…”

Assessment of total sediment load in rivers using lateral distribution method

“…1 shows the location of the study reach. According to Camenen et al (2011), the Danube River has experienced a dramatic change due to flood protection projects, the construction of navigation channels, and instream structures in the upstream reach of the river. As a result, the downstream reach of the SlovakHungarian Danube River has become degraded.…”

“…As a result, the downstream reach of the SlovakHungarian Danube River has become degraded. To study the impact of the hydropower plant, Camenen et al (2011) made extensive measurements of the bedload with a basket-type bedload sampler. The sampling point of the bedload is located about 11 km downstream from the gauging station.…”

“…Sediment particles range from fine to coarse gravel with D 50 ¼ 9 mm and D 90 ¼ 22 mm, corresponding to medium and coarse gravels, respectively. The grain size distribution of the bed sediment particles can be found in Camenen et al (2011).…”

“…However, for rivers whose flow depth varies to a large extent, the 1D method may underpredict the bedload seriously (Bradely et al, 1999). This is why Camenen et al (2011) and de Almeida and Rodriguez (2011) computed the total sediment load in a large river by distributing the shear stress laterally based on the channel geometry. Previous studies, however, did not take into account the impact of flow dynamics on the lateral distribution of the discharge.…”