Study On The Design Storm Flood Response Under Changing Environment

In recent years,the global water cycle system has undergone significant changes due to the combined effects of the changing external environment,such as climate change and human activities.This has led to an increase in water and drought disasters,resulting in various water security challenges for several countries and regions.To address this issue,this thesis focuses on the Jing River Basin in the Loess Plateau region and analyzes the evolution of hydro-meteorological elements and subsurface conditions in the study area.The research also explores the variation of design precipitation under non-conforming conditions and develops a Distributed Hydrological Model to analyze the design flood process under various design frequencies and conditions(climatic and surface conditions).Furthermore,this study aims to quantify the impact of climate change and anthropogenic disturbances on the variation of floods.The research findings highlight several important conclusions.(1)From 1971 to 2013,the Jing River Basin experienced changes in hydro-meteorological elements due to the combined effects of changing external factors such as climate change and human activities.An analysis of the average flood precipitation,average flood runoff,and average flood temperature revealed a non-significant upward trend,a significant downward trend,and a significantly upward trend,respectively.A nonstationary test conducted on the three elements showed that abrupt changes occurred in 1994,dividing the basin into a base period(1971-1994)and a change period(1995-2013).(2)The annual average NDVI values in the Jing River Basin from 1998 to 2018 exhibited an upward trend in time distribution.The minimum annual average NDVI value occurred in 1999 at 0.449,while the maximum value occurred in 2018 at 0.695.Spatially,the annual mean NDVI values were distributed with a characteristic of low in the north and high in the south.The areas with the highest NDVI values(0.75-0.86)were mainly found in the southeastern and southwestern parts of the basin,where woodlands and pastures dominated and vegetation flourished.Trend analysis of NDVI indicated that from 1998-2018,the area of improved vegetation cover was 98.61% while the area of degradation was 1.07%,with significant improvement.The land use types in the Jing River Basin in 1985 and 1990-2013,in descending order,were grassland,cropland,forest land,building land,water area,and bare land.The main land use types were grassland and cropland,with the sum of the two areas accounting for over 80%.(3)Between 1971 and 2013,the maximum flood peak,corresponding surface rainfall,and flood volume in the Jing River Basin showed a declining trend.Precipitation exhibited a non-significant downward trend,while flood peak and volume showed a significant downward trend.Using the conditional probability distribution calculation method and the Pearson type III(P-III)distribution method,the hydrological frequency of the daily maximum precipitation for 43 years at 12 rainfall stations in the basin was simultaneously calculated.It was discovered that the design rainfall calculated with the conditional probability distribution was generally larger than that calculated with the traditional Pearson type III distribution under the changing environment.The recurrence period was positively correlated with the design and precipitation.The design rainfall processes of 12 rainfall stations corresponding to the design frequencies were calculated based on the design rainfall of 12 rainfall stations.(4)A distributed hydrological model,namely HEC-HMS(Hydrologic Engineering Center-Hydrologic Modeling System),was developed to simulate design storm floods in the Jinghe River basin.To achieve this goal,a rigorous methodology was employed to select eight flood processes that exhibited good correspondence between rainfall and flood relationships.The parameters of the model,including those of the base and change periods,were meticulously determined,verified,and optimized utilizing the model’s own optimization algorithm.The design precipitation calculated by the conditional probability method is under the consideration of climate change conditions,which leads to the occurrence of hydro-meteorological non-conformity,therefore,the design precipitation process calculated by the conditional probability can be regarded as the design precipitation process affected by climate change,and the design precipitation process calculated by the P-III distribution method can be regarded as the design precipitation process not affected by climate change.The parameter-CN values in the SCS runoff curve method are used as a representation of the changes in surface conditions in the basin.The CN values of the baseline period can be regarded as the undisturbed subsurface conditions and the CN values of the change period can be regarded as the subsurface conditions disturbed by human activities.The study found that the flood peak flow and total flood volume calculated based on the conditional probability under changing climatic conditions were higher than those calculated using the traditional P-III distribution method.Specifically,at design probabilities of P=1%,P=2%,and P=5%,the flood peak flow was found to be 6.34%,0.54%,and 10.98% higher,respectively.Similarly,the total flood volume was found to be 6.11%,1.50%,and 11.11%higher,respectively.Moreover,the study investigated the impact of changes in surface conditions on the design flood.The flood peak flow and total flood volume of the design flood at the three design frequencies simulated by inputting the CN values of the change period were found to be smaller than those simulated by inputting the CN of the base period.Specifically,the design flood peak flow at the three design frequencies of P=1%,P=2%,and P=5% was found to be 7.19%,0.70%,and 4.98% smaller,respectively.Similarly,the total flood flows were found to be smaller by 7.14%,0.76%,and 5.38%,respectively.Under the combined climate change and subsurface change scenarios,the flood peaks and volumes for the design floods with design frequencies of P=1% and P=2% are reduced,and the flood peaks and volumes for the design floods with design frequencies of P=5% are increased.Climate change and subsurface change mainly affect small and medium floods,and the effect of climate change on design floods increases as the return period decreases.These findings are important for effective flood management and design of flood control infrastructure in the Jinghe River basin.The rigorous methodology employed in this study can serve as a template for similar studies in other regions.