Study On Eco-environmental Response And Effect Evaluation Of Rural River Restoration Project In Cold Area

Rural rivers generally refer to the rivers flowing through the rural landscape,mostly referring to the tributaries and end channels of small and medium-sized rivers,with large numbers and wide distribution.Due to the lack of control,there are many problems,such as serious siltation,non-point source pollution,shrinking water bodies,ecological degradation and low flood control standards,which have become the weak link in rural areas.In northern China,due to severe winters and unequal distribution of seasonal precipitation,the ecological environment problems of rural rivers in cold areas are more prominent.Huaide River is a typical rural river in the cold region of northeast China.In order to improve the ecological environment of the river,The local government has launched a series of nature-based ecological restoration projects in 2019.Among the many river ecological restoration practices carried out in China,there are few projects based on natural solutions to restore rural rivers in the northern cold region.The ecological environment effects and evaluation results after restoration are rarely reported.The suitability of existing river ecological restoration technologies in the application process of cold areas needs to be clarified.In this study,the ecological restoration section of the Huaide River,a tributary of the Dongliao River,is taken as the research object.Firstly,the ecological environment problems in the Huaide River basin and the ecological restoration projects implemented by the government are clarified.Secondly,the ecological and environmental effects of ecological restoration projects were analyzed from the aspects of water quality,benthic macroinvertebrates fauna,phytoplankton,river channel and coastal vegetation.Finally,the analytic hierarchy method is used to construct an evaluation index system for the ecological restoration effect of rural rivers suitable for cold regions,and the restoration effect of Huaide River is evaluated.The suitability of the restoration technology was analyzed based on the measured data of ecological environment and the comprehensive evaluation results.The main results are as follows:（1）In this study,the ecological restoration engineering measures based on nature mainly included plant allocation measures to prevent and control non-point source pollution and soil erosion;Ecological revetment measures that prevent soil erosion and have flood control effects;Measures to adjust hydrological forms to increase river hydrodynamic power and improve river self-purification capacity.The scale and location of the specific project measures are determined mainly according to the ecological environment of the river itself,and the length of the restored river section is nearly 3 kilometers.（2）Ecological restoration projects help to improve the self-purification capacity of rivers and improve the river water environment.After the completion of the project,the water quality of the river was significantly improved（p<0.05）,the concentration of typical pollutants in the river was significantly reduced,and the dissolved oxygen content was significantly increased from 3.37mg/L to 5.93mg/L（p<0.05）.In addition,the self-purification capacity of water pollution in the restoration project section was significantly improved.The maximum reduction rate of NH₄⁺-N,NO₃^--N,TN and COD can reach 96.49%,43.92%,49.69%and 25.95%,and the maximum increase rate of DO is 77.96%.The operation effect of the project is affected by seasonal factors.Except the autumn at the end point of the external restoration project,the WQI value of the water quality increases significantly.Affected by the background value of the river water quality,the water quality after the restoration is at the level of average[50-70)and poor[25-50).The combination of hydrological morphological adjustment and aquatic vegetation allocation had a better restoration effect than the single hydrological morphological adjustment.The WQI value of downstream water quality was significantly increased between summer and autumn（p<0.05）.（3）Ecological restoration projects can significantly improve the benthic macroinvertebrates community structure.The Shannon-Wiener diversity index,Margelef richness index and Pielou evenness index of benthic macroinvertebrates fauna at the end of the project were significantly higher than those at the beginning of the project（p<0.05）:Shannon-Wiener diversity index,Margelef richness index and Pielou evenness index increased by174.03%,85.37%and 110.71%,respectively.Shannon-Wiener diversity index,Margelef richness index and Pielou evenness index increased by 310.34%,187.23%and 237.50%,respectively.The Shannon-Wiener diversity index,Margelef richness index and total density of benthic macroinvertebrates fauna were significantly higher in the lower reaches of the river with only hydrological morphological adjustment measures in summer and autumn than in the upper reaches（p<0.05）.The Margelef richness index was significantly higher in the lower reaches than that in the upper reaches（p<0.05）in the river reach combined with hydrological morphology adjustment and vegetation allocation in summer and autumn.（4）Ecological restoration projects significantly affected aquatic plant community structure.The number of phytoplankton species,Shannon-Wiener diversity index,Margelef richness index and Pielou evenness index were significantly higher at the start point than at the end point（p<0.05）.The effects of different measures on phytoplankton were significantly different,and the effects were greatly affected by seasons.The Shannon-Wiener diversity index,Margelef richness index and Pielou evenness index of river reach showed opposite trend in different seasons.（5）An evaluation index system of ecological restoration effect of rural rivers in cold region including 18 sub-indexes was constructed by AHP and applied in Huaide River.According to the evaluation results,the ecological restoration project improved the health status of river ecosystem,and the comprehensive score of the ecological system increased after restoration.In the multi-index comprehensive evaluation,the comprehensive score of the non-engineering section in the upper reaches of the river is 2.27（general）,the comprehensive score of the initial engineering section is 2.81（good）,and the comprehensive score of the final engineering section is 3.34（great）.After ecological restoration,various indicators of river ecosystem developed well,including the ecological function of river water body,the ecological function of riparian zone,the river safety function and the social and economic function were restored.（6）The techniques used in the restoration of rural rivers in cold areas,such as hydrological form adjustment,ecological bank protection and vegetation allocation,have good suitability.Among them,the combination of hydrological morphology adjustment and aquatic vegetation allocation is better than the single hydrological morphology adjustment measure.Field investigation results show that after the project was put into use,the vegetation could adapt to the cold climate and grow normally.No frost heaving and cracking occurred in the measures of ecological bank protection and hydrological adjustment.After the implementation of vegetation allocation measures the function of the coastal zone is improved,and the highest interception rates of NH₄⁺-N,TP,TN,COD and particulate matter can reach 79.93%,82.16%,23.98%,27.71%and 84.89%.;Flow through the hydrologic form adjustment measures water environment improved,the maximum reduction rate of NH₄⁺-N and NO₃^--N can reach 26.25%and 58.18%,and the maximum increase rate of DO can reach 94.45%;Ecological bank protection effectively prevents flood erosion and effectively intercepts the loss of soil from farmland.The results of eco-environmental effect and comprehensive evaluation show that after restoration,the self-purification capacity of the river was increased,water quality was improved,coastal zone function was enhanced,soil and water loss was reduced,and biodiversity including benthic animals,phytoplankton,river channel and coastal zone vegetation was increased.The restoration project can adapt to the natural environment of the cold area,effectively solve the ecological environment problems of the rural rivers,and is suitable for popularization and application in the vast northern cold rural areas.