The Po Delta is restarting progradation: geomorphological evolution based on a 47-years Earth Observation dataset

Abstract

From the 1950s, the Po delta, one of the largest anthropogenic world deltas, has been subjected to a fast degradation and shoreline retreat due a marked reduction of sediment supply, mainly controlled by human impacts/factors, including subsidence. Through the interpretation of satellite images, coupled with the analysis of the flow discharge, and of the annual frequency of marine storms, we show that recently (>2010) the Po River has resumed delta progradation, especially in its northern portion. This happens after decades of erosion, followed by alternating regrowth and degradation phases, indicating conditions of substantial stability (1970-2000). Today the delta shows aggradation of new mouth-bars at the main distributary mouth, a clear evidence of active constructive processes. The ongoing trend marks a countertendency compared to many deltas worldwide.

Figure 1

Sentinel 2a scene (L1C_T32TQQ_A009743_20170504T101349), acquired on 4 May 2017, visible false color composition (B432+ Gaussian_stretch, 10 m cell size). In this normal low-tide image the prograding landforms are clearly shown: notice the mouth bars in front of the main Pila mouth, and on the secondary distributary channel (N oriented) a series of three spit recurves. The satellite image was processed by the authors using Copernicus Sentinel 2017 data (L1C level of processing), downloaded from https://eros.usgs.gov/about-us/data-citation under EU open access policy (https://sentinel.esa.int/web/sentinel/sentinel-data-access). The map in the inset was obtained from Open Street Map under Open Data Commons Open Database License (ODbL) V1.0: https://opendatacommons.org/licenses/odbl/. The location map of Italy was obtained from http://d-maps.com/carte.php?num_car=14528&lang=it under Open Access policy http://d-maps.com/conditions.php?lang=it.

Figure 2

Landsat satellite images: (a) L1MSS-12 August 1972 (60 m); (b) L5TM 8 May 1987 (30 m); (c) L5TM 14 July 1994 (30 m); (d) L7ETM 23 April 2002 (30 m); (e) L5TM 17 April 1991 (30 m); (f) L8OLI 3 April 2015 (30 m). Landsat images were obtained by processing datasets downloaded from the Glovis USGS server (https://glovis.usgs.gov) under open-access policy https://eros.usgs.gov/about-us/data-citation.

Figure 3

Sediment transport capability, effective discharge (Q_k) time series. It shows that in the last 50 years only three periods of potential high sediment flux occurred around 1960, 1976–1977 and 2000–2002. The interval between the last two peaks, 26 years, is the largest of the time series and coincides with the phase of higher retreat rates of the delta. The red dotted line is the five-years moving average.