Study On Eco-environmental Effects Of The Bank Slope Stability Structures In The Middle Reaches Of The Yangtze River

Many bank slope stability projects have been implemented in the middle reaches of the Yangtze River.Soft mattress（SM）,permeable tetrahedron（PT）and riprap（RI）are the common types of bank slope stability structures.The ecological transformation of structures is an inexorable trend in ecological channel construction,but there exist two problems of insufficient research in engineering technology and basic theory.In the aspect of engineering technique,the insufficient ecological environment monitoring data in engineering areas with different structures areas block the process of the ecological transformation of structures.In the aspect of basic theory,the insufficient research of evaluation system of the bank slope stability structures blocks ecological channel becoming a quantitative research object.In order to preliminarily solve these two problems,the investigations of water environment,sedimentary environment and biological community in engineering areas with different engineering structures（SM,PT,RI）and recovery times（time from project completion to initial sampling are 2 years（R2）,3 years（R3）,4 years（R4）and 5 years（R5）,respectively）were conducted in the middle reaches of the Yangtze River in 2017,2018 and 2020.The reasons for the differences of biological communities in different structure areas were analyzed,The ecological environment effect evaluation system based analytic hierarchy process（AHP）and entropy weight method（EWM）was established,and was applied to evaluate the eco-environmental effects of structures in the middle reaches of the Yangtze River.The results of this study are of great significance to the transformation of engineering technology in the construction and improvement of basic theoryand in evaluation of ecological channel bank slope stability project.The main research contents and results are as follows:1.the effects of bank slope stability structures on water environment:The water environment indexes in the engineering area were investigated,including current velocity,dissolved oxygen（DO）,turbidity,total nitrogen（TN）,total phosphorus（TP）,chemical oxygen demand（COD）and ammonia nitrogen（NH₃-H）.Pearson correlation analysis was used to study the correlation between water environmental factors,and principal component analysis（PCA）was used to analyze the water quality in engineering areas with different structures and recovery time.The result showed that current velocity and dissolved oxygen（DO）were the main two different water environment indexes in engineering areas with different structures.There was no significant difference in water environmental indexes among the engineering areas with different recovery times（P>0.05）.Dissolved oxygen and total nitrogen had a significant negative correlation with current velocity（P<0.05）,turbidity had a very significant negative correlation with current velocity（P<0.01）,dissolved oxygen had a very significant positive correlation with turbidity（P<0.01）,total phosphorus had a significant negative correlation with total phosphorus（P<0.05）.Three principal components were extracted from seven water environment indexes,and the three principal components account for 67.73%of the total variation.Dissolved oxygen,turbidity and total nitrogen had a large load on the first principal component,ammonia nitrogen and current velocity had a large load on the second principal component,and ammonia nitrogen,total phosphorus,total nitrogen and chemical oxygen demand had a large load on the third principal component.The principal component comprehensive value（F value）of water quality in R5 engineering areas<R4 engineering areas<R2 engineering areas<R3 engineering areas,the results indicated better water quality occured in engineering area with long recovery time.The comprehensive value of principal components（F value）of water quality in soft mattress area<control area<permeable frame area<riprap area,the results indicated that the water quality in soft mattress area was the best and that in riprap area was the worst in the four engineering areas with different structures.2.the effects of bank slope stability structures on depositional environment:The depositional environment indexes in the engineering area were investigated,including sediment grading characteristics,nutrient distribution characteristics（TN,TP and TOC）and heavy metal distribution characteristics（Hg,Cd,As,Cu,Pb,Cr and Zn）.Pearson correlation analysis was used to study the correlation between sediment average particle size and nutrient content,heavy metal content.The pollution degree of nutrients and heavy metals were evaluated by nutrient pollution index（PI）and potential ecological risk assessment index of heavy metals（RI）,respectively.The result showed that the content of silt was the most,followed by sand and clay in riprap area and permeable frame area.The content of sand was the most,followed by sand and clay in soft mattress are.The average particle size of sediments in soft mattress area was the largest,followed by control area,permeable frame area and riprap area.There was no significant difference in the average particle size of sediment particles in R2,R3,R4 and R5 engineering areas（P>0.05）,but the average particle size gradually decreased with the increase of recovery time.There was no significant difference in the nutrient content of sediments in the engineering areas with different recovery times（P>0.05）,and the nutrient content in engineering areas with long recovery time was higher than that in engineering areas with short recovery time.The content of nutrients in the sediment in riprap area and permeable frame area were significantly higher than that in the control area,while the nutrients content in soft mattress was significantly lower than that in the control area（P<0.05）.The content of heavy metals in the sediments in R3 engineering area was the highest,while the content of heavy metals in R4 engineering area was the least among the four engineering area with different recovery times.The content of heavy metals in sediments in permeable frame area was the highest among the four engineering area with different structures,followed by riprap area,control area and soft mattress area.the content of Total nitrogen,total phosphorus and total organic matter were significantly positively correlated with the content of clay and silt,and were significantly negatively correlated with the content of sand and the average particle size（P<0.01）.There was no significant correlation between heavy metal content and sediment particle grading characteristics（P>0.05）.The comprehensive pollution degree of nutrients（PI）in R2,R3,R4 and R5 engineering areas were all at the level of severe pollution.The comprehensive level of nutrients in riprap area was in severe pollution,and that in permeable frame area and control area were in moderate pollution,and that in soft mattress area was in mild pollution.Cd（0.81 mg/kg）and Pb（33.81mg/kg）are the two elements exceeding the standard,exceeding 79.08%and 25.68%of the background value.The potential ecological risk level（RI）of heavy metals in R2,R4 and R5engineering areas were at a slight level,and R3 engineering area were at a medium level.The potential ecological risk level（RI）of heavy metals in the control area,soft mattress area,permeable frame area and riprap area were all at a slight level.3.the effects of bank slope stability structures on biocoenosis:The biological communities were investigated in the engineering area,including zooplankton,epiphytic algae,sediment bacteria,zoobenthos and fish.The indexes of species number,density,biomass,diversity indexes and the difference of community structure were measured.The results showed that there was no significant difference in the number of species in different structure areas（P<0.05）,but there were significant differences in density,biomass and diversity index（P<0.05）,and there were differences in biological communities in different structure areas（P<0.05）.The density,biomass and diversity index of aquatic organisms in riprap area and permeable frame area>control area>soft mattress area.There are differences in biological species,density,biomass and diversity index,as well as biological community structure in the project area at different restoration times.As a whole,various indicators increased with the increase of recovery time.The results of RDA analysis showed that dissolved oxygen,Cd,sand content and benthos density were the driving factors for the differences of zooplankton,sediment bacteria,benthos and fish communities,respectively.4.simulation experiment on the effects of flow velocity and environmental lead（Pb）exposure on demersal fishes:（1）The critical swimming speed(U_crit)of yellow catfish（Pelteobagrus fulvidraco）and Chinese long-snout catfish（Leiocassis longirostris）were measured,and the velocity groups of 20%,40%,6%,80%,100%and 120%critical swimming speed were set to study the changes of oxygen consumption rate（MO₂）and blood biochemical indexes of the two demersal fishes exposed different current velocity for 30minutes.The results showed that the critical swimming speed(U_crit)and optimum swimming speed(U_opt)of yellow catfish were 4.35±0.06 BL/s and 42.19 cm/s,and that of Chinese longsnout catfish were 4.81±0.10 BL/s and 37.45 cm/s,respectively.The optimal critical swimming speed(U_opt)of yellow catfish and Chinese longsnout catfish can be approximately regarded as the preferred swimming speed(U_perf).When the swimming speed exceeded 80%of the critical swimming speed,the oxygen consumption rate,the contents of blood lactic acid（LAD）,creatine kinase（CK）and lactate dehydrogenase（LDH）increased significantly（P<0.05）,and the content of blood glucose（Glu）decreased significantly.（2）yellow catfish was exposed to environmental related lead（Pb）concentrations（0,0.5,5 and50 mg/L）for 7,14 and 28 days.The content of Pb in immune organs（head kidney and spleen）,the degree of tissue damage,relative gene expression（stress,antioxidation,inflammation,immunity and apoptosis）and the activities of antioxidant and immune related enzymes in blood were measured.The results showed that with the increase of exposure concentration and exposure time,the content of lead in head kidney and spleen increased significantly（P<0.05）,induced oxidative stress,inflammatory response and apoptosis response,and then inhibited immune function and damaged the tissue structure of immune organs.The increase of exposure time changed the activitives of immune related enzymes and the expression of immune related genes in yellow catfish.The concentration of Pb in the environment exceeding 50μg/l could damage the immune function and the integrity of tissue structure of immune organs of yellow catfish.5.the evaluation of eco-environmental effect of bank slope stability structures:Based on the investigation of the environmental and biological data of the bank slope stability structures in the middle reaches of the Yangtze River,an eco-environmental effect evaluation system with 24 indexes including three criteria layers of water environment,sedimentary environment and biological community was constructed by using the fuzzy comprehensive evaluation method.Analytic hierarchy process（AHP）and entropy weight method（EWM）were used to determine the weight of each evaluation element in the criterion layer and index layer,respectively.Based on the national quality standards,literature and monitoring data of the control area,the evaluation and classification standards of each index were determined.The ecological environment effect evaluation index（EEEI）was used to characterize the ecological environment effect in engineering areas with different structures and different recovery times.The results showed that the contribution of water environment,sedimentary environment and biological community indexes in the criterion layer to the target layer were10.62%,26.05%and 63.34%,respectivley.The current velocity（32.02%）,total nitrogen（17.67%）and turbidity（14.91%）contributeed greatly to the water environment;The average particle size（16.22%）,total phosphorus（10.40%）and total organic carbon（10.03%）contributed greatly to the sedimentary environment;Fish density（26.77%）and macrobenthos density（18.72%）contributed greatly to aquatic community.The comprehensive evaluation index of eco-environmental effect（EEEI）in riprap area was the highest,followed by permeable frame area,control area and soft mattress area,successively.The soft mattress area and control area were in the middle grade,while the riprap area and permeable frame area were in the good grade.The EEEI index of R5 project area was the highest among the four engineering areas with different recovery times,fllowed by R4 engineering areas,R3engineering areas and R2 engineering areas,successively.The grades of the four engineering areas with different recovery times were good,good,good and medium,respectively.On the whole,there were differences in water environment,sedimentary environment and biological community indicators among the three engineering areas（SM,PT,RI）with diffetent structures.Riprap（RI）and permeable framework（PT）with pore structure were friendly to aquatic organisms,and hardened cement board laying（SM）had adverse effects on aquatic organisms.With the increase of restoration time,the habitat conditions in the engineering areas had improved gradually.issolved oxygen,Cd,sand content and benthos density were the driving factors for the differences of zooplankton,sediment bacteria,benthos and fish communities,respectively.The results of this study are of great significance to the transformation of engineering technology in the construction and improvement of basic theoryand in evaluation of ecological channel bank slope stability project.