Maximum Grade Approach to Surplus Floodwater of Hyperconcentration Rivers in Flood Season and its Application

The severe soil erosion in the Chinese Loess Plateau has resulted in a considerable wastage of surface runoff (floodwater) in flood season due to high sediment concentration in runoff. To address the water scarcity problem, it is a viable solution to utilize the floodwater. A maximum grade approach (MGA) is presented to calculate the coefficient of surplus floodwater in flood season. The raw data sequences are analyzed in a four-step process. An upper triangle of grades is obtained after the third step. A relationship between coefficient of surplus floodwater and sediment concentration in runoff is achieved in terms of the upper triangle. The surplus floodwater in flood season then can be determined. A case study of water diversion from the Jinghe River (the second tributary of the Yellow River, China) to the Jinghuiqu Irrigation District for irrigation was performed. The monthly data sequences of runoff volume, water diversion and sediment concentration in runoff from May to September for the period 1933–2001 are employed. A 16-grade upper triangle is obtained based on the MGA. A functional relationship between the coefficient of surplus floodwater (y, %) and the sediment concentration in runoff (x, %) is achieved as <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$y=0.0191x + 0.1516 (R^{2\thinspace }=0.9738)$</EquationSource> </InlineEquation>. Results demonstrate that the average annual surplus floodwater in flood season and available surface water resources in the Jinghe River over 69-year time span are 978 and 909 million cubic meter (mcm), respectively, when the limit of sediment concentration for river water utilization is set as 10%. The irrigation district still has a potential of available water diversion of 509 mcm over 1981–2001 time span. Compared with the traditional methods, the MGA produces more reasonable and effective results. The MGA has the ability of rapidly estimating the surplus floodwater under different limits of sediment concentration and is a useful tool for available surface water resources assessment. Copyright Springer Science+Business Media B.V. 2011