Experimental investigation of bedload transport processes under unsteady flow conditions

Abstract

Abstract:Hydraulic engineering is usually based on theoretical analysis and/or numerical modelling simulation. As the dynamic behaviour of sediment movement under unsteady flow is still unclear, and field measurement is comparatively difficult during a large flood, prior investigations through flume experiments are required. A series of flume experiments, conducted using different inflow hydrographs without sediment supply from upstream, was carried out to investigate the sediment transport process under unsteady flow conditions.A series of triangular hydrographs were performed in the experiments. The results indicate that a temporal lag was found between the flow hydrograph peak and the sediment hydrograph peak because large size sand dunes lasted for a short period in the falling limb of the flow hydrograph. The temporal lag was found to be about equal to 6-15% of the flow hydrograph duration. Owing to the temporal lag, the total bedload yield in the rising period was less than that in the falling period. Furthermore, the measured total bedload yield in the unsteady flow experiments was larger than the predicted value, which was estimated by using the results obtained from the equivalent steady flow experiment. The peak bedload transport rate for unsteady flow conditions was also larger than the predicted value. The ratios of the measured to the predicted quantities mentioned above were found to be constant values for different shapes of hydrographs. It is, therefore, expected that the analytical results of sediment transport from equivalent steady flow can be a good reference for sediment transport under unsteady flow conditions.

Laboratory studies on bedload transport under unsteady flow conditions

“…However, our knowledge on how matter, particularly sediment, is transported under unsteady flow conditions is still insufficient. This situation triggered a good deal of experimental, theoretical and computational efforts exploring how it works (Bombar, 2016;De Sutter et al, 2001;Julien et al, 2002;Lee et al, 2004;Mao, 2012;Mrokowska et al, 2016;Phillips and Sutherland, 1990;Wang et al, 2015). Nevertheless, the problem of bedload transport is still open for systematic research.…”

“…They varied in terms of various factors, such as the shape of unsteady flow hydrograph, e.g., triangular (Lee et al, 2004), trapezoidal (Bombar et al, 2011), step-wise (Mao, 2012), naturally shaped (Humphries et al, 2012); bed composition -uni-modal or bimodal (Wang et al, 2015); supply of sediment from upstream (Mao, 2012); initial bed conditions -armoured bed (Bombar et al, 2011) and not armoured bed (Humphries et al, 2012), or duration of flow from a few minutes (Guney et al, 2013) to a few hours (Mao, 2012). Some studies took into account or commented on scaling from natural to laboratory conditions, e.g., (Cooper and Tait, 2009;Wang et al, 2015).…”

Sediment transport during flood event: a review

“…In reviewing different studies, it can be concluded that many researchers have used the following unsteady parameter: (Graf and Suszka 1985;Yen and Lee 1995;Song and Graf 1996;Nezu et al 1997;Lee et al 2004;Ahanger et al 2008;Bombar et al 2011): where u *b is the bed shear velocity associated with the steady base flow before hydrograph passages. It can be seen that flow unsteadiness increases with increasing parameter P. However, this parameter takes into account the duration of the hydrograph only and not the steepness of the rising limb of the hydrograph.…”

“…Lee et al (2004) introduced two other parameters which influence bed load transport during hydrograph events:…”

“…2). Furthermore, Lee et al (2004) conducted a series of flume experiments, using different inflow triangular hydrographs to investigate the sediment transport process under unsteady flow conditions. These investigators also believed that unsteady parameters presented in Eqs.…”