Dynamic Effects Of Underlying Surface Changes On Storm Floods With Different Return Periods

Urbanization,as a typical pattern of human activity,altered underlying surfaces dramaticly and therefore incurred a series of storm floods,which,has currently become one of the most heated topics in water and hydrologic scientific researches.In the recent decades,scientists from around the world have conducted many researches on river system alterations,land use/cover changes and flood hazards,etc,and attained a wide range of fruitful and productive results.However,comprehensive study on streams from the perspective of its amount and structure currently lies at an early stage,especially the inveatigation into the influences of underlying cover,combined with land use pattern and river system,on flood events,besides,relevant research ideas and methods are still to be improved.As urbanization progresses,impervious cover area is gradually expanding,quickening the rainstorm-flood response process,thus resulting in more and more seriousness of flood's potential threats.Thereby,it is a rewarding task,in the long term,to perform scientific researches on the impacts of changing underlying surfaces on storm floods of different frequency that can indeed provide more accurate and sound supports for flood prevention to some extent and enrich new ideas of related flood management researches.Apparently,this research is of great scientific significance and application values.The Taihu basin,one of the most developed regions in China,also possesses dense rivers and lakes,especially in the center of Taihu basin.From the 1980s,the urbanization development started to overwhelmingly accelerate,meanwhile,led to a sharp decline in both amount and structure of rivers and lakes,and expansion in the construction land area at a large scale.The both altered rain-runoff and flow routing that affected storm floods,resulting in a series of variations in flood events at different magnitude in this region."Low rainfall intensity,severe flood disater" recently becomes especially frequent and typical,making regional safety and sustainable developments suffer from more and more serious pressure.It is necessary and forwarding-looking to investigate the impacts of river and landscape changes on storm floods of different return periods,which can promote water resources safety insurances and the work is obviously pretty meaningful to a certain extent.The center of Taihu basin was selected as the study region,the fluvial geomorphology,mathematical statistics and hydrology-geoography and other multidisciplinary approaches and means were applied into this paper,with supports of RS and GIS,according to the main framework:storm——flood response alterations,changes in the storm and flood's magnitudes,river system's and urbanization's individual impacts on flood peak water levels and their reoccurrence frequency,the combined effects of the underlying cover on flood events of different grades.The study concluded characteristics and regularities of magnitude changes in storms and floods under the progress of urbanization,analyzed and summarized the impact mechanisms of the underlying surface changes on flood events with different grades.The paper's main researches and results are as follows:(1)Alterations of the responses between characteristic rainfall and peak water levelContinuous maximum rainfall values of 1 day,3 days,7 days and 15 day,and annual maximum water levels in flood seasons were selected as the basic data,the thesis performed correlation analysis of the above characteristic rainfall and annual maximum flood level,respectively,then,comparatively analyzed variations in frequency of characteristic rainfall and peak water level and concluded changing regularities of them during urbanizing development periods.The results indicated as follows.Firstly,the maximum peak water level correlated best with three-day rainfall in Wuchenxiyu district,and best with seven days in Yangchendianmao district;Secondly,it tended to be increasingly apparent that annual maximum peak water level standed still high faced with low rainfall intensify,from 2000s on and it inclined to rise even although suffered from almost the same rainfall amount.;Thirdly,urbanization accelerated the responses between rainfall events and flood peak water levels,resulting in higher and higher peak water levels and sharply intensifying occurrence-risks of high flood water levels,especially in the late urbanization period.(2)The alterations of storm's and flood's return periodsThe P? hydrologic frequency curve fitting approach was applied to calculate the frequency of annual maximum peak flood water level and 24 hour rainfall.With the supports of the Kriging liner interpolation method,storm-flood frequency changing tendency during 1960-2014 years was worked out,changing characteristics and results were modeled and carried out.The results indicated as fllows.Firstly,changing tendency of both storm's and flood's frequency kept almost consistent,moreover,they displayed distinctive phase nature:altered significantly in 1960-1980s and 2000-2010s repectively,less obvious in 1980-2000s;Second,in contrast to the frequency of annual maximum 24h precipation,flood peak water level's frequency were fluctuating between a wider range;Secondly,urbanization made the frequency of storm floods with larger magnitude alter more significantly than that of lower-class floods;Thirdly,lower-grade floods underwent more complicated and marked alterations than higher ones,along with urbanizing development,besides,their frequency did not exhibit a transferring tendency as clear as the higher-levels' because underlying cover used to function on smaller floods more severely.(3)Effects of individual underlying surface indices on storm floodsBased on the conctruction of the indice system concluding water surface ratio,drainage density,box dimension,river evolution coefficient and impervious rate(urbanization rate),annual expansion intensity,the paper studied the individual influences of above underlying surface factors on flood peak water levels,the frequencies of peak water levels of different return periods and those of exceeding warning water levels.Based on the above,the impacts of single underlying indices on floods with typical return periods and their differences were summarized.The results indicated as follows.Firstly,with degradation in river network's quantity and structure,flood peak water levels ascended correspondingly.Besides,water indices attenuation led to a certain increase in frequency of those exceeding the warning water levels and peak water levels;Secondly,flood peak water levels were rising step by step under the same scale storm events and higher-class flooding events got more vulnerable to incur,as impevious rate and urbanization land expanding rate were increasing;Thirdly,reduction in river system networks and expansion in contruction land area and acceleration speed brought about an increase in the frequency of above 2-year-return-period floods,thus a decline in lower-class floods below 2-year-return period,correspondingly,thus intensifying flood risks.(4)Combined effect of the underlying surface on floods of different gradesThe Grey Correlation Analysis method(GCA)was put into effect to analyze the impacts of single underlying surface factors on flood peak water levels of different magnitudes and their frequencies,and revealed the siginicance differences between these indices on various-class floods,comparatively concluded the factors with the best correlation with flood events.On the above basis,rainfall,river system variations and urbanization expansion were all concluded into the multiple linear regression analysis models to reveal the comprenhensive effects of these factors on typical-scale floods.The results indicated as follows.Firstly,water surface ratio,drainage density,box dimension and impervious rate contributed to flood peak water levels and their frequencies more significantly and lower-class floods were more prone to suffer from the urdelying surface factors;Secondly,the increase in both impervious rate and rainfall accumulation amount and degaration in river system networks would bring peak water levels of all classes to ascend,differently,the impervious rate increase affected flood peak water levels more remarkably than river degaration in terms of the above 10-year-return-period floods;Thirdly,there appeared significant distinctions in underlying surface's effects on flood frequencies and risks of various scales:impervious cover increase might lead to an increase mainly in the frequency below 2-year-return-period floods.Water surface ratio reduction could increase the flood risks mainly by increasing the frequency of above 5-year-return-period floods,correspondingly,reducing the frequency of the below 2-year-reurn-period floods.