Quantifying the Impact of Spatiotemporal Resolution on the Interpretation of Fluvial Geomorphic Feature Dynamics From Sentinel 2 Imagery: An Application on a Braided River Reach in Northern Italy

Abstract

Machine learning algorithms applied on the publicly available Sentinel 2 images (S2) are opening the opportunity to automatically classify and monitor fluvial geomorphic feature (such as sediment bars or water channels) dynamics across scales. However, there are few analyses on the relative importance of S2 spatial versus temporal resolution in the context of geomorphic research. In a dynamic, braided reach of the Sesia River (Northern Italy), we thus analyzed how the inherent uncertainty associated with S2's spatial resolution (10 m pixel size) can impact the significance of the active channel (a combination of sediment and water) delineation, and how the S2's weekly temporal resolution can influence the interpretation of its evolutionary trajectory. A comparison with manually classified images at higher spatial resolutions (Planet: 3 m and orthophoto: 0.3 m) shows that the automatically classified water is ∼20% underestimated whereas sediments are ∼30% overestimated. These classification errors are smaller than the geomorphic changes detected in the 5 years analyzed, so the derived active channel trajectory can be considered robust. The comparison across resolutions also highlights that the yearly Planet‐ and S2‐derived active channel trajectory are analogous and they are both more effective in capturing the river geomorphic response after major flood events than the trajectory derived from sequential multiannual orthophotos. More analyses of this type, across different types of river could give insights on the transferability of the spatial uncertainty boundaries found as well as on the spatial and temporal resolution trade‐off needed for supporting different geomorphic analyses.

Remote sensing assessment of anthropogenic and climate variation effects on river channel morphology and vegetation: Impact of dry periods on a European piedmont river

“…However, given the relatively low resolution of the used images, information about plant-specific dynamics might be hardly retrievable. At the same time, sporadic vegetation instead did not have a sufficiently strong spectral signal to be classified in the vegetation class (Bozzolan et al, 2023).…”

“…At the same time, plant growth and distribution depend on local environmental conditions that vary largely with the flow regime and the morphodynamic processes occurring during extreme events (Camporeale & Ridolfi, 2006;Caponi et al, 2023;Gurnell et al, 2012Gurnell et al, , 2016. As past studies pointed out, vegetation development, changes in river morphological structure and channel aggradation/degradation might increase flood risk by augmenting the resistance to flow, reducing cross-section width and river connectivity and endangering the stability of levees (e.g., Boothroyd, Williams, et al, 2021;Bozzolan et al, 2023;Darby, 1999;Heritage & Entwistle, 2020;Mossa & Chen, 2022;O'Donnell & Thorne, 2020). In the case of the Po River, its channel narrowing, and its profile degradation have occurred since the 1950s, because sediment transport reduction and planform oversimplification are mainly consequences of anthropogenic activities.…”

Leveraging Sentinel-2 and Geographical Information Systems in Mapping Flooded Regions around the Sesia River, Piedmont, Italy

“…The Sesia River rose rapidly, destroying the valley floor. Estimations of the maximum discharge sit at about 3000-5000 m 3 /s [54].…”

“…The Sesia River (Figure 1) is located in the Sesia-Val Grande geopark and is one of the longest rivers in the Piedmont (Piemonte) region, as it also has short tributaries in Lombardy (138 km) and is an important tributary of the Po [51][52][53][54]. It emerges from Monte Rosa at an altitude of about 2500 m from the homonymous glacier.…”

“…The heavy rainfall (450 mm [54]) caused many rivers to overflow and caused flooding in many areas. The Sesia River overflow in the Piedmont region caused road closures, the collapse of the bridge, and flooding in several areas.…”