Analysis Of Flood Evolution And Its Influence On Vegetation In Floodplain In The Lower Reaches Of Nenjiang River

As an important disturbing factor for vegetation growth in floodplains,floods play a vital role in the composition and structure of vegetation.Studying the evolution of floods and analyzing the impact of floods on vegetation in floodplain areas is conducive to the formulation of flood control decisions and river ecological governance.In this paper,we use MIKE FLOOD to construct a one-and two-dimensional hydrodynamic coupling model,and analyze the evolution process of floods with four typical frequencies(P=1%,2%,5%,20%),taking the Jiangqiao-Dalai River section of the lower reaches of the Nenjiang River as the research area.At the same time,using geoaccumulation index method and geographic information spatial technology method,the influence of heavy metal content in river surface sediments and flood disturbance characteristics on vegetation growth in floodplains was discussed.The main conclusions are as follows:(1)After calibration and verification of the MIKE FLOOD one-and twodimensional hydrodynamic coupling model parameters,the model has high simulation accuracy and meets the requirements of Grade A forecast accuracy,that is,the model simulation results can better reflect the flood evolution law of the lower reaches of the Nenjiang River,and lay a model foundation for the numerical simulation of typical frequency flood evolution.(2)With the increase of the flood return period,the maximum submerged depth and maximum flood submerged range of the four typical flood frequencies(P=1%,2%,5%,20%)of the Nenjiang River Jiangqiao-Dalai River section have been increasing.The results of this study delineate the corresponding scope for the analysis of the impact of different frequency floods on the vegetation in the floodplain.(3)Flood disturbances of different frequencies have different degrees of impact on vegetation(vegetation coverage,land use types and landscape patterns).(1)Changes in vegetation index in different return periods have significant temporal and spatial differences: a)The vegetation index in different return periods in the lower reaches of the Nenjiang River has significant seasonal changes.b)There are significant differences in the vegetation indices(NDVI,EVI)in the floodplain at different return periods before and after a typical flood,indicating that floods have obvious damage to the vegetation in the floodplain.c)Floods of different frequencies have varying degrees of damage to vegetation,which conforms to the moderate interference hypothesis.(2)The disturbance of different frequencies of floods creates different spatial patterns of land use:The areas disturbed by higher frequency floods are mainly grassland and wetland,and the areas disturbed by lower frequency floods are mainly cultivated land,grassland and water bodies,the areas disturbed by moderate-frequency floods are mainly cultivated land,wetland and grassland.The results of the study conform to the principle of my country's three-zone division method.(3)The landscape pattern index in different recurrence periods shows significant spatial heterogeneity:the patch distribution in the moderately frequent inundation area is relatively complex,the degree of fragmentation and spatial heterogeneity of the landscape is high,the landscape type is balanced,and the landscape shape is relatively simple.The patch distribution in frequently inundated areas and less frequently inundated areas is relatively simple,the degree of landscape fragmentation and spatial heterogeneity is low,the landscape types are not balanced,and the landscape shape is more complicated.(4)Through sampling and analysis,it was found that the heavy metal content in the river surface sediments of the Songhua River Basin was generally close to or lower than the background value,and the floodplain showed no pollution or moderate to light pollution.Among them,the geoaccumulation index of the lower reaches of the Nenjiang River from the Jiangqiao to the Dalai River is less than 0,indicating that the soil environment(heavy metals)in the lower reaches of the Nenjiang River is not the main factor affecting the vegetation growth in the floodplain.