2.2.2. Relative Sea Level Change

Coastal plains have always been subject to subsidence, as sediments recently deposited are subject to compaction. This process is faster on river deltas, where deposition rates are higher, but can reach massive values where oil, gas, and water extraction is carried out. The high coastal erosion rate found at Ravenna, Italy, is mostly induced by oil and gas extraction both inland and offshore that produced a 40 mm/year subsidence in 1970–1977, the period of most intensive activity [65]. Satellite interferometry measured a value of 30 mm/year at the Gudao Oilfield, on the Yellow River Delta [66], and in the Piombino alluvial plain, natural subsidence rate shifted from 1 mm/year in the Holocene to the present 10 mm/year, as a consequence of water pumping for agriculture and a steel mill [67].

Sea level rise, as a cause of sediment loss, has its rationale in the Bruun Rule [68] which was firstly enthusiastically accepted, and later strongly rejected; it passed through several modifications, see, e.g., in [69–72], and is related to those sediments deposited near the depth of closure that subsequently exit the active beach following the "apparent deepening" caused by the rise in sea level. Although suggestions have been made that it is time to abandon the Bruun Rule [73], no operative alternatives have been proposed, but it is evident that if water depth increases, sediments moved to the limit of the active beach enter the "inactive" part of the profile. However, sea level rise can also induce sediment shifting to the coast, as occurred during the last 18,000 years, when barrier islands emerged and moved ashore [74,75], somehow contradicting the Bruun Rule. This input had been significant during the Holocene Sea Level Rise, but there have been no studies to investigate if it is still operating.
