Assessment Of Flow Attenuation In Salt Marshes In The Intertidal Zone Of The Yangtze Estuary Based On Remote Sensing And In-situ Observation

The coastal zone is one of the most important areas for human activities,economic and social development,however,it is vulnerable to natural disasters such as storm surge and coastal erosion.In the context of sustainable development,governments and scholars of various countries have begun to construct protection engineering by coastal salt marshes in recent years to resist natural disaster.Previous studies focused on wave attenuation while the quantitative research on flow attenuation in salt marshes are still insufficient.Low-middle tidal flats are the areas where tidal currents and vegetation interact most frequently,salt marshes in thses areas can attenuate flow velocity,promote suspended sediment,reduce erosion risk and contribute significantly to the coastal area.In order to reveal the capacity of flow attenuation under different seasons,above-ground biomass,flow velocity and water depth condition,and evaluate the attenuation capacity of salt marshes in Chongming Dongtan,the Yangtze Estuary.The in-situ observation and data processing system were constructed at first.From June to November in 2019 and 2020,we took Scirpus mariqueter and Spartina alterniflora as research objects,transects were established in Chongming Dongtan and Nanhui East Shore to collect data of hydrology,vegetation growth and terrain.Then we filled the missing data of water depth,eliminated outlier of flow velocity and marked the characteristics of data,then we built a data warehouse to store the processed data.After all above these steps,we quantified the flow attenuation rate under different seasons,above-ground biomass,flow velocity and water depth conditions.Based on models of land use classification and vegetation aboveground biomass estimation through remote sensing image,equation of relationship of flow attenuation rate with flow velocity,water depth and aboveground biomass,we established a spatial assessment model,the spatial grids were used in the model to evaluate the spatial difference of flow attenuation capacity of salt marsh vegetation in the typical coastal area of the Yangtze Estuary,simulated the process of velocity value attenuation procession with distance under specific vegetation and hydrological conditions,then built a Web GIS platform to provide online data and assessment services.The main conclusions were as follows.(1)Flow attenuation rate of salt marshes varied throughout seasons,and the rates were directly proportional to the above-ground biomass.Flow attenuation rates of S.mariqueter ranked with an order in summer(1.18%/m)>in autumn(0.98%/m)>in winter(0.56%/m),while rates of S.alterniflora ranked with an order in winter(1.41%/m)>in summer(1.28%/m)>in autumn(1.25%/m).The reason why rate of S.mariqueter decline rapidly was due to the loss of its above-ground parts,while above-ground parts of S.alterniflora were almost still remained in winter,with less biomass loss and lower water depth condition.The salt marshes with higher above-ground biomass had stronger flow attenuation rates,when the above-ground biomass of S.mariqueter with 0.39 kg/m~2was 1.76 times that of 0.18 kg/m~2,and when the above-ground biomass of S.alterniflora with 0.89 kg/m~2was 1.36 times that of 0.61 kg/m~2.(2)The flow attenuation rates were positively correlated with the velocity conditions in the vegetation edge,and negatively correlated with the water depth conditions in the vegetation edge.The correlation coefficients between flow attenuation rates and velocity conditions of S.mariqueter in summer,autumn and winter were 0.88,0.72 and 0.97,while the correlation coefficients between rates and velocity conditions of S.alterniflora were 0.99,0.97 and 0.69,respectively.Flow attenuation rates and water depth conditions were negatively correlated except S.mariqueter in winter,the correlation coefficients between flow attenuation rates and water depth conditions of S.mariqueter in summer,autumn and winter were-0.96,-0.92,and 0.97,and the correlation coefficients between rates and water depth conditions were-0.87,-0.91,and-0.82,respectively.(3)The assessment results showed that salt marshes in Chongming Dongtan could attenuate flow velocity obviously,the middle and low risk parts took up more than 87%of total.The simulation showed that the velocity in vegetation zone attenuated exponentially with distance,fully attenuation required longer distance in S.mariqueter zone.In August,2020,the areas of S.mariqueter and S.alterniflora were864.12 hm~2and 142.91 hm~2,respectively,and the above-ground biomass were 0.33kg/m~2?0.86 kg/m~2and 1.39 kg/m~2?6.70 kg/m~2,respectively.In the southeastern part of Chongming Dongtan and the areas covered by S.alterniflora vegetation,the flow attenuation rates were high,while some parts in the northeastern of Chongming Dongtan were lower.Segments with low risk(attenuated over 75%through 50 m),medium risk(attenuated 25%to 75%through 50 m)and high risk(attenuated less than 25%through 50 m)took up 4.88%,83.11%and 12.00%of total,respectively.The high-risk parts were mainly located in the areas where S.mariqueter grew sparsely.The simulation results showed that the velocity in the vegetation zone attenuated exponentially with distance,under the same condition of flow velocity(18cm/s),water depth(0.76 m),above-ground biomass condition.