Isostatic rebound between the riverbanks and the discharge of the Po River (northern Italy) by wavelet coherence analysis of high-resolution remote sensing and discharge data

Land movements cause changes in topographic gradients which could favor flooding of urban, agricultural or coastal areas. Moreover, sea level rise linked to recent climate changes has increased awareness that the ground deformation, although slow over time, can become hazardous both for humans and environment. The understanding of the causes of land movements is a not easy task due to the complex relation between natural and anthropic mechanisms. In order to improve the knowledge of natural versus anthropogenic variability of vertical land movements of the Po River Delta (northern Italy), in this work advanced data analysis techniques were used for finding possible correlations between high-resolution remote sensing data and river discharge records. In particular, focus was placed on the isostatic rebound between the river banks and the discharge of the Po River within its Delta area. Daily ground deformation measurements obtained with Continuous Global Positioning System (CGPS) were used, referring to the stations of Porto Tolle (44°57'07.2"N - 12°20'02.4"E) and Taglio di Po (45°00'10.8"N - 12°13'40.8"E) between 2012 and 2015, and the daily Po River discharge record at Pontelagoscuro (44°53'19.68"N - 11°36'29.52"E). It has been found that the vertical displacement functions between the two river bank stations, which are located at about 10 km apart, are strongly correlated. This suggests that the Po riverbanks in this area could be characterized by similar dynamics. On average, CGPS measurements indicate a downward movement of about 4 mm/yr. The use of cross wavelet and wavelet coherence analysis between the CGPS data and the Po River discharge data put in evidence that these records present strong negative correlation between 3- to 12-month scales. In particular, three main correlated frequency ranges with periods of 3 months, 4 - 6 months and 8 - 14 months were identified. These ranges correspond to the seasonal, semiannual and annual natural meteorological variations observed in the entire Po River area. The found negative correlations indicate that the increase/decrease of the river discharge induces the simultaneous decrease/increase of the near riverbank. The observed oscillations have an amplitude of a few millimeters (at most 5 mm). A possible cause of this regular seasonal oscillating movement could be an isostatic rebound of the riverbank in response to the change of water pressure on the river bed, which is directly related to the river discharge volume.