Experimental and Numerical Investigation of Bed-Load Transport under Unsteady Flows

Abstract

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18 m in length, 0.80 m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8 cm in height consists of scraped uniform small gravel of D 50 ¼ 4:8 mm. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30 s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the increase in the value of the proposed unsteadiness parameter. Further analysis of the experimental results revealed that total flow work has an inverse exponential variation relation with the lag time. A onedimensional numerical model that employs the governing equations for the conservation of mass for water and sediment and the momentum was also developed to simulate the experimental results. The momentum equation was approximated by the diffusion wave approach, and the kinematic wave theory approach was employed to relate the bed sediment flux to the sediment concentration. The model successfully simulated measured sedimentographs. It predicted sediment yield, on the average, with errors of 7% and 15% of peak loads for the triangular and trapezoidal hydrograph experiments, respectively.

Laboratory studies on bedload transport under unsteady flow conditions

“…Parameter α is referred to as the rate of unsteadiness in this study. Additionally, unsteadiness parameter introduced by (Bombar et al, 2011) was evaluated:…”

“…Kuhnle (1992) observed a clockwise hysteresis in natural streams and attributed this pattern of transport to the formation and destruction of roughness elements. Bombar et al (2011) reported an anti-clockwise hysteresis in experiments with a static armour layer, where the maximum bedload transport was possible only after the peak flowrate, when the armour layer was destroyed by a high flow. When sediment is heterogeneous, sediment transport is affected by grain arrangement (Ockelford and Haynes, 2013): microforms (clusters) are formed (Mao, 2012), static or mobile armour layer appears (Guney et al, 2013), and partial transport may take place (Sun et al, 2015).…”

“…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).…”

A methodological approach of estimating resistance to flow under unsteady flow conditions

“…Both formulae underestimate the time instant at which ∂h ∂x = 0. As a matter of fact, from the practical point of view, the evaluation of the friction velocity is exceptionally important in this region, as intensified transport processes may occur just before the wave peak (Bombar et al, 2011;De Sutter et al, 2001).…”

Dynamic Lift on an Artificial Static Armor Layer During Highly Unsteady Open Channel Flow

“…While those studies are all based on steady flow, limited research has been performed to investigate the effect of unsteadiness in free surface flows. Some unsteady flow experiments focus on velocity measurements, sediment transport behavior or studied the river as an ecosystem [3,[12][13][14][15][16][17]. Others address different types of flows, like gravity currents [18,19].…”