Step Change In The Flood Frequency And The Response Of Its Design Flood For Small And Medium-Sized Catchments In South China

In recent years,under the dual influence of climate change and human activities,extreme hydrological events have occurred frequently,and the flood control situation is particularly severe in small and medium-sized catchments due to low defense standards.Flood frequency curves are an important basis for flood design.Preliminary studies have found that there are hydrological non-liner and non-consistent step changes in the flood frequency curves at different flood magnitude,and such step changes have increased the uncertainty of flood forecasting.The non-liner response of catchments runoff production and confluence may lead to more complex or serious step changes of the flood frequency curve,which makes the calculation of design flood and flood prediction more difficult.In this context,this paper selects the Caojiang River basin,a typical small and mediumsized catchment in Guangdong Province,as the research object,and conducts a comprehensive analysis of the multi-year rainstorm and flood characteristics of the study area based on 274rainstorm-flood data in the past 50 years.Based on the river network of the catchment,redefines the variables of rainstorm spatial variation,establishes the spatial interpolation method of rainstorm in small and medium-sized catchments,and analyzes the effect of rainstorm center movement on flood.Based on the catchment underlying conditions,the hydrological model of the Caojiang River basin is constructed by using HEC-HMS to simulate and analyze the changes of flood characteristics and step changes of the flood frequency curve under different rainfall and underlying conditions.The study supports the reduction of design flood uncertainty in small and medium-sized catchments in the heavy rainfall areas of South China.The main research contents and results are as follows:(1)The changes in the characteristics of 274 rainstorm and flood in the Caojiang River basin between 1967 and 2013 were analyzed,including the changes of average,variance and maximum of maximum 1h,3h,6h,12 h and 24 h rainfall,the changes in the location of the rainstorm center,and the chronological changes in the number of floods in different return periods.The results show that the average,variance,and maximum values of maximum 3h,6h,12 h and 24 h rainfall in the Caojiang River basin gradually increase with the chronological changes,the number of times that the location of the rainstorm center falls on each station is bipolar at Dabai and Magui stations before 1990,and gradually tends to average out after 1990.The proportion of small return period floods tend to increase,and the proportion of large return period floods tend to decrease.(2)By comparing and analyzing five empirical frequency formulas: General Formula(a=0.25),Hazen Formula,Weibull Formula,Blom Formula,Cunnane Formula.Two probability distribution functions: P-III type distribution,GEV(Generalized Extreme Value Distribution).Two parameter estimation methods: moment estimation,Maximum Likelihood Estimation.The most suitable frequency calculation and the design flood distribution functions for small and medium-sized catchments in the heavy rainfall area of South China were selected.The estimation results of Cunnane Formula,Generalized Extreme Value Distribution and Maximum Likelihood Estimation methods are more suitable for the hydrological frequency analysis and design flood calculation in the study area.(3)Based on the HEC-HMS hydrological model,simulations were conducted to analyze the effects of the location of the basin rainstorm center,the average catchment size,the average catchment slope and the impervious area on the shape of the flood frequency curve for the 47-year annual maximum discharge corresponding to the field flood.The first-order derivative(slope)of the flood frequency curve was used as an index to quantify the magnitude of the step changes of the flood frequency curve,and the degree of influence of each factor on the magnitude of the step change and the return period in which it occurs was analyzed.The results show that for the large recurrence period level step change,the magnitude decreases from 55.64 to 41.20 with the shortening of the rainstorm center to the basin outlet,and the return period of occurrence shifts back from 13.11 to 29.5 years.The magnitude of the step change was positively proportional to the catchment average slope,the catchment size,and the impervious area,in addition,the change of the magnitude of the step change has a liner relationship with the convergence path length and the catchment size.The return period in which the step change occurs moves toward a large return period as the average catchment size and the impervious area of the catchment increase.(4)The spatial variation of rainstorm and the change of catchment underlying conditions are the key factors affecting the variation of flood characteristic values.Based on the HEC-HMS hydrological model,the responses of peak discharge and peaking time were examined by varying the location of rainstorm center,the average catchment slope,the catchment size,and the impervious area of the catchment under two conditions: single basin and the whole basin,where the location of rainstorm center was described by the catchment mass center convergence path.The results show that the average catchment slope,the catchment size,and the impervious area are positively correlated with the peak discharge,while the location of rainstorm center is negatively correlated with peak discharge.The peaking time is delayed with increasing distance of the rainstorm center location from the catchment outlet and advanced with increasing average catchment slope.The spatial variation of the rainstorm has the most significant effect on the peak discharge as well as the peaking time,while for the whole basin-wide condition,changing the catchment size has the most significant effect on the peak discharge.