Natural Compaction of Sediments

Natural compaction of Holocene deposits is one of the main processes contributing to land subsidence in low-lying coastal environments. Its quantification has been challenging until now because of difficulties inherent to monitoring land displacements in coastal and transitional environments and disentangling the various processes that superpose to produce the observed total displacement. A significant step forward has been made possible by interferometric processing of Synthetic Aperture Radar (InSAR), specifically Persistent Scatterer Interferometry (PSI). InSAR retrieves land displacement from satellites without needing direct access to the coastland where inner water basins, watercourses, wetlands, and the lack of infrastructure make ground-based monitoring techniques almost impossible to use. PSI detects thousands of measurable radar reflectors per square kilometer over large areas. This high spatial resolution helps disentangle the causes of the observed displacements and, consequently, understanding where natural compaction is a key factor. In this chapter, we review relevant case studies and discuss the outcomes of the research carried out in the coastland of the Po River plain, northern Adriatic Sea, Italy. Specifically, we focus on the low-lying farmlands and wetlands in the Po delta and the tidal marshes of the Venice Lagoon. L-, C-, and X-band SAR data are integrated to increase the effectiveness in retrieving information on scattered small infrastructures, vegetation, and wet zones. Integration with in situ data, such as the sediment deposition age, differential compaction at a few depth intervals, and accretion rates, reveals a significant correlation between natural compaction rates and the marsh age. Such dataset is then used to develop a specific accretion-natural compaction model to forecast the resilience of these transitional environments to anthropogenic stressors and sea level rise due to climate change.