Study On Bankfull Discharge Prediction And Its Application To The Yellow River

Combining the basic theory and measured data analysis, studies are carried out on the adjustment of bankfull discharge (BD) responsing to the variation of incoming flow and sediment conditions (FSC) of Yellow River (YR). FSC to maintain an aimed value of BD are proposed. This study reveals the trends of fluvial river processes and provides a reference for river training in a better way.Three alluvial reaches of Yellow River basin, e.g. the Inner Mongolia reach, the lower YR, and the lower Wei River the biggest tributary of YR, are selected as samples to analyze the variations of flow, sediment and the characteristics of river channel morphologies in recent 5 decades. It presents visibly decrease of runoff and sediment load in the YR basin that led to a continuous shrinking of main channel, declining of BD, and degrading of capacity for flood conveyance and sediment transport.According to the 1-D equation of bed deformation and non-equilibrium sediment transport, a basic equation for the delayed response model for alluvial river is deduced. Besides, the model is further improved in the framework and its parameter determinations: as 1) Reapportioning the effect weights of FSC of previous years with a more reasonable distribution; 2) Introducing a variableβinstead of constant to reflect the different rate of channel adjustment for different years; 3) Considering the FSC of both flood and dry season; 4) Taking account the effect of flow process in to the BD. Methods of BD calculation for selected river reaches are provided based on the modified model. Tests against measured data indicate that the modified model can accurately describe the adjustment of BD.Result of analysis reveals that the adjustment of BD is affected by the FSC of present year and that of earlier years as well. However, the length of years should vary from case to case. It is equal to the time period of BD adjustment, on the base of the adjustment process of BD and relationship between"accumulated effects"and"delay response". Studies show that the annual sediment concentration of flood season plays an important role in determining the time period in the YR basin.The energy dissipation mechanism of sediment-laden flow and the energy allocation for both channel forming and sediment transport (CFST) is studied, and method for the calculation of water demands for CFST is developed based on the energy balance principle. Furthermore, a method for the calculation of water demand for CFST for the whole lower YR course is proposed by utilizing the FSC at San-Hei-Xiao as the input variables. As an example, the range of water demands for the forming and maintenance of BD of 4000m3/s is provided.