Soil Types and Construction Activity as Drivers of Surface Subsidence in Coastal Urban Areas. The Case of Hue in Central Vietnam

Many coastal cities in Southeast Asia are increasingly threatened by flooding through sea-level rise, but also extreme rainfall and flash floods from the hinterlands. Surface subsidence additionally increases the flood risks, especially in river delta areas. However, the drivers and consequences of subsidence are little analyzed and understood in many cases. This study investigates the relationship between surface subsidence and land-use in the city of Hue, Central Vietnam. Time-series of Sentinel-1 imagery are utilizing a persistent scatterer approach. This resulted in deformation rates between -25 and +10 millimeters per year. While large parts of the urban area show a stable condition, there are concentrated patterns of high subsidence in the outskirts of the city. In order to identify drivers of these subsidence patterns, the deformation rates are compared to two external datasets: A map of soil types showing the most important soil types in the study area. An analysis of variance (ANOVA) was calculated to confirm a significant difference between the soil types at a 0.001 confidence level, with Plinthic Acrisols as the soil type with the largest negative average surface velocity. This soil type is present in areas of rice cultivation. As the persistent scatterers are retrieved from solid targets, such as roads or buildings, the results indicate that these parts of the city show a higher risk for subsidence because of the subsurface water from the rice fields intrudes the neighboring residential areas. Secondly, a multi-temporal classification of urban areas was conducted based on Landsat imagery to determine the extent of the city for the periods 1980-1999, 2000-2009, and 2010 to 2016. Results indicate higher deformation rates for later stages of urbanization, again at a tested confidence level of 0.001. This means that parts of the city which were built between 2010 and 2016 are more vulnerable to surface subsidence, most likely because they are at the peripheral areas of the city where the ground is not as solid as in the center of the city. Results of this study help to understand drivers and hotspots of surface subsidence for the development of adapted strategies of flood risk reduction