As one of the most important components of river mechanics, sediment transport capacity of sediment-laden flows has attracted much attention from many researchers working on river mechanics and hydraulic engineering. Based on the time-averaged equation for a turbulent energy equilibrium in solid and liquid two-phase flow, an expression for the efficiency coefficient of suspended load movement was derived for the two-dimensional, steady, uniform, fully-developed turbulent flow. A new structural expression of sediment transport capacity was achieved. Using 115 runs of flume experimental data, which were obtained through two kinds of sediment transport experiments in the state of equilibrium, in combination with the basic rheological and sediment transporting characteristics of hyperconcentrated flow, the main parameters in the structural expression of sediment transport capacity were calibrated, and a new formula of sediment transport capacity for hyperconcentrated flow was developed. A large amount of field data from the Yellow River, Wuding River, and Yangtze River, etc. were adopted to verify the new formula and good agreement was obtained. These results above contribute to an improved theoretical system of river mechanics and a reliable tool for management of rivers carrying high concentration of sediments. hyperconcentrated flow, turbulent energy equilibrium, efficiency coefficient, sediment transport capacityIn the Yellow River, hyperconcentrated flood occurs frequently during the rainy seasons. Both the historical maximum concentration of 1600 kg/m 3 and the mean annual amount for sediment transport of 1600 million tons are the highest values worldwide. According to the statistical results coming from the sedimentation bulletin of Chinese Rivers and the Yellow River 1)2) , and the analysis of the field data from Tongguan, Huayuankou, and Lijin Hydrological Stations along the

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Sediment delivery ratio (SDR) for fluvial rivers was formulated with sediment rating curve. The observed data of SDR on flood event scale of the Lower Yellow River (LYR) were adopted to examine the formulation and to calibrate the model parameters. A regression formula of SDR was then established and its 95% prediction interval was accordingly quantified to represent its overall uncertainties. Three types of factors including diversity of the incoming flow conditions, river self-regulation processes, and human activities were ascribed to the uncertainties. The following were shown: (1) With the incoming sediment coefficient (ISC) being a variable, it was not necessary to adopt the incoming flow discharge as the second variable in the formulation of SDR; and (2) ISC=0.003 and therefore SDR=2 might be a threshold for distinguishing the characteristics of sediment transport within the LYR. These findings would highlight sediment transport characteristics on the scale of flood event and contribute to uncertainty based analysis of water volume required for sediment transport and channel maintenance of the LYR. sediment delivery ratio, Lower Yellow River, flood event, uncertainty, prediction interval Citation: Fu X D, Jiang L W, Wu B S, et al. Sediment delivery ratio and its uncertainties on flood event scale: Quantification for the Lower Yellow River. Sci

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