Deformation Monitoring And Failure Risk Assessment Of Seawalls In Shanghai Based On Multi-platform MT-InSAR

Coastal area is the most concentrated and active area of human activities,and also the core region for China’s social-economic development.Under the background of sea-level rise and frequent storm surges induced by global climate change,the flood risks of this area are increasing.As the most important safety barrier of Shanghai,floodcontrol seawalls can greatly reduce the flood risks.However,in case of failure of seawalls（including levee-breach and overtopping）,it will lead to rapid seawater intrusion and flooding,which will have a great impact on coastal communities and important infrastructures.The long-term uneven deformation of flood-control seawalls will lead to seawall height decreased,which will affect their stability and safety,and meanwhile weaken their protection capability and then increase the risk of overtopping.Besides,severe uneven deformation will trigger the collapse of seawalls and increase the risk of levee-breach.Realizing large-scale,high-precision and periodic seawall deformation monitoring is crucial to know the stability of seawalls and assess their risks in time.Traditional monitoring practices,such as levelling and GPS,have the disadvantages of high costs and discrete monitoring results.Multi-temporal In SAR（MT-In SAR）is a newly developed deformation monitoring method,which can obtain the continuous deformation of wirelike seawalls with a millimeter accuracy.As a deformation decomposition algorithm,Minimum Acceleration Approach（Min A）can generate the two-dimensional（i.e.,Up-Down and East-West）deformation field of the monitoring target by processing multi-platform LOS-projected deformation time series.The deformation components obtained by Min A have high accuracy.Moreover,for this method,the combination of multi-platform deformation time-series is carried out in the post-processing stage,making it has good practicability.Specifically,taking Shanghai as a study area,based on 167 Sentinel-1 images with ascending orbits and 78 COSMO-Sky Med images with descending orbits from 2018 to2021,the LOS-projected deformation time-series of two satellites were retrieved by using MT-In SAR method.Subsequently,the LOS-projected deformation products were jointly combined and the two-dimensional deformation field of coastal Shanghai was obtained.After extracting the deformation velocity of all the pixels on the seawalls,the deformation velocity of the whole seawalls in the Up-Down and East-West directions were obtained.It was found that the subsidence mainly occured in the newly-reclaimed land that located in eastern Shanghai.The subsidence velocity of the 5th runway of Pudong Airport and its surrounding seawalls are the highest,which reaches 20 mm/year.In this paper,we divided the seawall system of Shanghai into 40 segments,with the length of every section being about 2 km.Using the deformation results of the seawalls and combined with the extreme water-level dataset,the overtopping probability of each segment and the levee-breach index that representing the levee-breach probability were calculated.The results show that the seawalls in southern Shanghai have both higher probability of overtopping and levee-breach.Hydrodynamic model LISFLOOD-FP was employed to assess the hazard of 100-year flood under the failure scenarios of different segments.The permanent scatterers（PSs）refer to targets that have strong backscattering signals in SAR images and retain high phase stability over long periods（i.e.,not affected by spatial-temporal decorrelation and atmospheric delay）.By using Azimuth Split-spectrum Algorithm（ASS）and setting coherence threshold of 0.35,the high-coherent PSs,that are coherent at both low and up Doppler bands,were extracted.The high-coherent PSs are mainly distributed in artificial building areas.In this paper,the density of high-coherent PSs was used as the vulnerability index to estimate the potential flood loss induced by seawall failure.On the whole,the inundation area induced by the seawall failure in southern Shanghai is relatively small,but the density of high-coherent PSs in this area is higher,resulting in more serious loss.The inundation area induced by the seawall failure in eastern Shanghai is larger,which is showed as continuous seawater intrusion from Dishui Lake to the Pudong Airport.However,in the inundation area,due to the large area of cultivated land,water and wetland and the small area of artificial buildings,the assessed potential loss is small.In this study,the risks of seawall failure have five levels: high,medium-high,medium,medium-low and low.The risk of the southern seawall is high,while the risk of the eastern seawall is at a medium to low level.Specifically,the 6th segment that near the Luchao Port located in southern Shanghai,has a relatively high risk of overtopping and levee-breach.By analysing the inundation time-series,it was found that after the failure of this segment,the seawater spread rapidly from south to north at the beginning,and then spread to the East-West directions.The correlation coefficient of the density of high-coherent PSs and building areas in the inundation areas is 0.96,which proves the reliability and substitutability of high-coherent PSs in characterizing the distribution of artificial buildings.