A Maximum Rating Method for Determination of Abandoned Floodwater in Hyper-concentration Rivers

Due to the existence of severe soil and water losses over arid,semi-arid and water-shortage areas in North China,the phenomenon of high sediment concentration in runoffs would occur.The floodwaters in flood season are however difficult to be controlled and utilized,which results in floodwater waste in flood season and an intense situation of water resources supply and demand.This study makes an attempt to seek a relatively optimal approach to evaluating the abandoned floodwater in flood period,that is,the flood volume in flood season that cannot be controlled and utilized by engineering measures.A case study on the Jinghe River,one branch of the Weihe River and a second tributary of the Yellow River,China,was carried out.Both the runoff characteristics and the water resources development and utilization of the Jinghe River basin were taken into account.In terms of the characteristics of runoff and water resources development and utilization in the river basin,three data sequences regarding runoff,sediment concentration in runoff and water diversion amount from the river to the Jinghuiqu Irrigation District in the lower reaches were employed.The data were measured monthly from June to October for the period of time 1933-2001 at the control section of the Jinghuiqu Irrigation District.Their inherent relationships were analyzed in detail.The sequential data about the ratio of water diversion of runoff and the sediment concentration in runoff were generally divided into several grades in an ascending order.The months of their occurrence at each grade were recorded.A rating relationship between the coefficient of abandoned floodwater in flood season and the corresponding influencing factors (e.g.,the sediment concentration in runoff) was obtained in the left upper triangular form.A maximum rating method for coefficient determination of abandoned floodwater in flood season was then presented.The product of the coefficient of abandoned flood water and the natural runoff in flood season can be taken as the volume of abandoned floodwater in flood season.Results for the Jinghe River basin indicate that the method is able to determine efficiently and exactly the amount of abandoned floodwater under different limits of sediment concentration conditions,which would be conductive to utilization of river water resources.Provided that the limit of sediment concentration in runoff is set as 10%,the mean annual amount of abandoned floodwater in flood season over the period 1957-2001 is calculated as 0.840 billion m3,0.049 billion m3 larger than that from the traditional method.Compared with the traditional method,it is proved that the method presented in this study is more rational and effective.The presented approach can provide an effective tool for utilization and evaluation of floodwater resources in rivers with high sediment concentration in North China.