Dynamics and Vulnerability of Delta Systems - loicz

CS3: Subsidence of the Po River Delta
Albert Kettner
Deltas are almost always prone to subsidence and the Po delta is no exception. Tectonics, sediment
loading and sediment compaction are the main components driving natural subsidence rates for the
Po Delta, which is only partly compensated by post-glacial rebound (Carminati & Martinelli 2002).
The net natural subsidence rate for the Po Delta is estimated to be ~ 2-4mm/y (Simeoni et al. 2007).
However, observed subsidence rates have been significantly higher since the 1950s. Anthropogenic
factors like groundwater extraction accelerated as the area became more populated, but natural gas
mining was central to increasing subsidence rates to 40-60 mm/y on average in the late 1950s, early
1960s (Fig. CS3.1a). Gas extraction in the Po Delta stopped at the end of the 1970s, which almost
instantly decreased the subsidence rate to 10-20mm/yr (Fig. CS3.1b). It took till the 1990s to slow
subsidence to its natural rate (Bondesan et al. 1995).
Fig. CS3.1: Decreasing subsidence rates (A-C) of the Po Delta. A) Subsidence rates during active mining of
methane rich water (after Caputo et al. 1970), B) Recovered subsidence rates when most mining activity
stopped (after Bondesan & Simeoni 1983), and C) Simulated natural subsidence rates (after Simeoni et al.
2007).
Over the same time, the demand for sediment for building material increased with the rapidly growing
population. Between 1958 and 1981, 90-95 x 10 6 m 3 of sediment was mined in the lower course of
the river (Simeoni et al. 2007). In addition there was significant sediment trapping in the upper drainage
basin by dams and reservoirs. Elevated subsidence rates, decreasing sediment supply due to
dam and reservoir interception, and sediment mining (Fig. CS3.2) caused major delta erosion after
the 1950s.
Fig. CS3.2: Sediment load decreased by a factor 2.9 in less than 60 years (after Syvitski & Kettner 2007).
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