Numerical Simulation Research On "Green Embankment Effect" Of Tidal Flat Vegetation

With climate change and rising sea levels,flood risks in coastal areas are increasing,traditional projects(such as sea walls,dams)are plagued by problems such as long-term disrepair and ground subsidence,and flood control capabilities are challenged.This has led to interest in the use of nature-based approaches to reducing flood risk and the exploration of a whole new set of flood prevention concepts to address the shortcomings of traditional levees.This paper coupled atmospheric model(Holland typhoon)-ocean model(TELEMAC tidal–TOMAWAV wave)-land model(MIKE21 hydrodynamic)implement floods drown simulation in the study area,combining with the field survey and integration of ecology,geomorphology and water conservancy project research,analysis of the tidal flat of salt alga vegetation flood control ability,establish salt alga,vegetation types and shoal roughness quantitative vegetation under the condition of storm surge on the reduction effect of the waves,to calculate the green dikes in flood control ability of the dam and artificial equivalent conversion relationship,and two measures of the costs and benefits of doing a comprehensive evaluation,Provide new solutions to future storm and flood threats.The main research contents and conclusions are as follows:(1)numerical modelThe model coupling technology is adopted to realize the data and driving force transmission of multiple models,and the hydrologic numerical model coupled with the atmospheric model-ocean model-land model is constructed to realize the flood inundation simulation in the study area.The biological characteristics of dongtan vegetation in chongming were obtained by field measurement,the research status and main achievements of conversion relationship between vegetation and roughness at home and abroad were summarized,the corresponding manning roughness coefficient was set,the community composition of dongtan tiantan vegetation was analyzed,and the water resistance characteristics of different vegetation were discussed.In order to study the impact of beach vegetation on storm surge and compare the flood control effect of beach and seawall,three sets of scenario simulation were designed:Assuming all dongtan vegetation disappears,the manning value n in the wetland area outside the embankment is set to 0.025;Assume that all tidal flat vegetation disappears;Base vegetation disappears to increase the seawall height.By comparing the simulation results of three sets of scenarios,the effects of the presence or absence of tidal flat vegetation on the reduction of storm surge were analyzed and the benefits/costs of the reinforcement of wetlands and seawall were calculated.(2)assessment of flood control capacityThe maximum storm water level reduction rate(MSWLRR)is generally between0.3%and 40.7%,and the corresponding maximum storm water level reduction(MSWLR)is 0.02m to 2.62m.Numerical simulation experiments of different bottom friction coefficients were established by setting different vegetation cover types(reeds,reeds and ribes)to simulate the flood control effect of tidal beach vegetation outside chongming dongtan seawall.The results showed that the reduction rates of interfloridas,reeds and ribes were 0.19m/km,0.26m/km and 0.07m/km,respectively.Using ArcGIS10.6 software for flood disasters occur under different scenarios covered scope,submerged depth and submerged time 2,and 3 d simulation show,draw the submerged area distribution,the results show that the existing in vegetation and vegetation disappear scenario,submerged depth is greater than 0.2 m were 89.6km~2and inundation area 111.6 km~2,disappear in vegetation and seawall overall heightening 1 m scenario,submerged depth is greater than 0.2 m of inundated area of59.2 km~2.(3)benefit and cost assessmentIn this paper,the relationship between levee engineering and natural foundation engineering under extreme storm surge is evaluated by combining numerical simulation with cost-benefit mathematics.The results showed that the tidal beach vegetation in the east bank of chongming was 54.2km~2,which roughly protected the land area of 25.7km~2within the seawall from the threat of storm surge.The cost-benefit ratio of levee engineering and natural foundation engineering under extreme storm surge condition was evaluated with cost-benefit method.It is found that the cost-effectiveness of coastal ecosystem flood control measures depends on four main factors:1)the economic loss value under the influence of the ecosystem,and the degree to which storm and flood are affected by the vegetation in the tidal flat at the beginning and after the rise of sea level;2)the cost of constructing and maintaining the wetland compared to the engineering measures;3)restoration cost of tidal beach vegetation after the occurrence of storm and flood;4)the economic value of the protected areas is relatively low,and the flood control measures based on the coastal ecosystem are more attractive.