Bedload transport and bed evolution under unsteady hydrographs

Abstract

Until recently, river channel response has generally been investigated under a single unsteady flow hydrograph that does not represent a typical flood condition. The river channel often experiences significantly different bedload transport and bed evolution under unsteady sequential flow conditions. At present, however, there is a significant lack of understanding of the interaction between hydrograph flow parameters and the response of the river channel over sequential flood hydrographs. In order to improve our understanding of a river channel response under sequential flow conditions, a comprehensive experimental study was conducted in a flume facility. The novelty of this research work lies in (i) the natural-shaped sequential hydrographs; (ii) the identification of the isolated and combined influence of hydrograph parameters on sediment transport characteristics and bedform development; (iii) linking the bedload transport characteristics to the bed evolution characteristics over sequential flow conditions; and (iv) development of a bedload yield prediction model for the sequential flow condition. The difference in the channel response for a single hydrograph and sequential hydrograph was found to be significant for the bedload rates (reducing trend over the subsequent subhydrographs), bedload yields (i.e., discrepancy ratio of 1.5 – 2), bedform changes (i.e., small scale to large scale bedform), and surface roughness (i.e., discrepancy ratio of 1.2 – 1.8), but less significant for channel slope and bedform dimensions. Further, the individual influence of hydrograph parameters (water work, and unsteadiness) on bedload yields was evaluated that confirmed water work parameter was an important parameter and had a primary influence on sediment transport while unsteadiness parameter had a negligible influence. An improved bedload yield model was developed for yield prediction over the sequential flow hydrograph that is applicable to a wide range of bed material and unsteady flow conditions. Finally, recommendations on the application of the sequential flood modelling approach for flood management are outlined.