Bedlaod sediment transport and bed evolution in steady and unsteady flows

Abstract

Until recently, bedload sediment transport has generally been investigated under steady flow conditions, which does not represent the typical sediment transport scenarios observed within natural fluvial channels. These channels often experience significant variations in bedload transport and bed morphological evolution under unsteady flow conditions, especially during flood hydrograph events. At present, however, there is a distinct lack of understanding of the interaction between hydrograph flow properties and the variability in bedload transport and bed evolution. Within the current study, different series of parametric experiments were conducted in two glass-walled, tilting flume facilities with the main aim of investigating the response of uniform and graded sediment beds to a range of different unsteady flow hydrograph conditions, and compare this to the corresponding response under equivalent steady flow conditions. The novelty of this research against previous studies is highlighted in terms of (i) the identification of the individual influence of hydrograph-related parameters (shape , unsteadiness HG and total water work Wk) on unsteady sediment transport characteristics (e.g. hysteresis and temporal lag) and the resulting bed evolution; (ii) the use of natural-shaped hydrographs (single and double peak) rather than stepped, triangular or trapezoidal hydrographs; (iii) the direct comparison between bedload sediment transport yields and corresponding bed evolution under unsteady hydrograph and equivalent (volume) steady flows; and (iv) enhanced understanding of influence of intergranular effects (e.g. particle exposure and/or hiding) on observed bedload sediment transport behaviour in unsteady and equivalent steady flows. Experiments conducted with both uniform (course sand, d = 1.95mm) and graded (fine gravel, d50 = 2.64mm, 5.00mm) beds not only confirm the clockwise and counter-clockwise hysteresis in measured bed load transport rates, but also reveal no/mixed hysteresis patterns as a transitional condition between these clockwise and counter-clockwise hystereses for bedload transport measured under unsteady flow hydrographs. For graded sediment transport, the intergranular effect on temporal lag and hysteresis patterns for transport of classified fine, medium and coarse size classes within graded (unimodal, bimodal and fine-grained) sediment beds in unsteady flow hydrographs is also revealed. Specifically, the coarse size class tends to respond preferentially to the changing flow and exhibits clockwise hysteresis in general; whereas the fine size class tends to become more active later and demonstrates reduced clockwise, no/mixed or counter-clockwise hysteresis. Corresponding temporal variations and hysteresis patterns in the median db50 size of the bedload transport load are also observed, with the db50 value generally reaching its peaking value during the rising hydrograph limb and indicating a clockwise hysteresis over the hydrograph duration. Bedforms (i.e. dunes and alternate bars) are found to develop only within uniform sediment bed (d = 1.95mm) under both steady and unsteady flows tested. A relevant analysis of the formative conditions under which these bedforms develop is presented. By contrast, no bedforms are observed to form within graded sediment beds; only slight bed evolution (e.g. bed aggradation up to +5mm at upstream) is identified for fine-grained sediment bed (d50 = 2.64mm) runs. Additionally, the median size of the bed surface ds50 is shown to coarsen during the antecedent flow conditions within all graded sediment beds (compared to d50 in specified particle size distributions). This surface coarsening persists over the duration of both steady and unsteady flow events tested in current study [e.g. run-averaged ds50 values: (i) unimodal grading, ds50 = 6.51mm (> d50 = 5mm); (ii) bimodal grading, ds50 = 6.74mm (> d50 = 5mm); and (iii) fine-grained grading, ds50 = 3.62mm (> d50 = 2.64mm)]. The temporal variations in the median bedload size db50 (< ds50 throughout) for both the steady and unsteady flow runs, are also responsible for explaining the development and persistence of the surface armour layer. Bedload transport and bed evolution for uniform and graded sediment beds under steady flows are measured to provide a benchmark for direct comparison with equivalent bedload rates and yields, bedform development, and variations in bedload and bed surface grain size composition under unsteady flows. The difference in effects of unsteady and steady flow is found to be significant for bedload rate (with the unsteady-steady bedload ratio Kq = 0.23 – 14.95), bedload yields (i.e. yield ratio us = 1.11 – 13.56), bedforms (i.e. dunes, alternate bars and etc.) and bedload composition (i.e. db50 ratio Kb = 0.38 – 1.56), but less significant for bed surface composition (i.e. ds50 ratio Ks = 0.92 – 1.33). Further, the individual influence of hydrograph related parameters (shape , unsteadiness HG, total water work Wk) on bedload yields, yield distribution over the rising and falling two limbs of single-peaked hydrographs, and on of bedform characteristics (i.e. type, dimensions) is also evaluated. In more complex flow hydrographs, such as double-peaked hydrographs, the equivalent bedload transport and bed evolution characteristics are also derived. Finally, the presence of specific grain size classes within the graded sediment beds and their influence on the transport of other six classes is considered from comparison of sediment transport characteristics for (i) the unimodal and bimodal sediment mixtures (with the same d50 = 5.0mm), and (ii) the uniform (d = 1.95mm) and fine-grained sediment mixture (d50 = 2.64mm), which arise from relative grain exposure and sheltering effects under the same steady and unsteady flow events. An idealised, theoretical analysis of the hydrodynamic forces and moments associated with these potential impacts is provided. Most importantly, the transport of fine particles is found to vary depending upon the presence state of coarser particle sizes in the surface layer (i.e. stationary or mobile to varying rates), which reveals the mechanism for selective fractional transport within the graded sediment bed under equivalent steady and unsteady hydrograph flow conditions.