Short-Term GIS Analysis for the Assessment of the Recent Active-Channel Planform Adjustments in a Widening, Highly Altered River: The Scrivia River, Italy

Abstract

From the 1990s onwards several Italian rivers have experienced a recent phase characterized by active-channel widening and, generally, by bed-level stability or slight aggradation. However, its triggering factors and its diffusion, along with the relationship between active-channel planform dynamics and vertical adjustments, are still quite debated and only few studies are available. This research deals with the active-channel planform changes occurred along the Scrivia River floodplain reach (NW Italy) over the period 1999–2019 and it aims at investigating in detail the ongoing geomorphological processes under the river management perspective. The study is based on a quantitative multitemporal analysis of aerial photographs and satellite images performed in a GIS environment and supported by field surveys. The outcomes revealed a generalized trend of gentle active-channel widening together with widespread bank instability and several (26% of total banks) intense and localized bank retreats involving both the modern floodplain and the recent terrace. In the investigated 20-year period, the active-channel area has increased by 22.7% (from 613.6 to 753.0 ha), its mean width by 25% (from 151.5 to 189.3 m), whereas no relevant length variations have been noticed. These morphological dynamics have been more or less pronounced both at reach scale and over time. The extreme floods occurred in the investigated period can be considered the most important triggering factor of the active-channel planform changes, most probably together with an increase of the reach-scale unit stream power due to changes in the channel geometry occurred over the 20th century.

Urban geomorphology of a historical city straddling the Tanaro River (Alessandria, NW Italy)

“…Therefore, the Tanaro urban reach experienced a generalized riverbed narrowing over the last few centuries (Arattano et al, 1995;Bellardone et al, 1998). This evolutionary trend generally follows the morphological evolution already documented by previous researches for other neighboring rivers (Mandarino et al, 2019b;Mandarino et al, 2020;Pellegrini et al, 2008;Rinaldi et al, 2010;Tropeano, 1989).…”

Flood-induced ground effects and flood-water dynamics for hydro-geomorphic hazard assessment: the 21–22 October 2019 extreme flood along the lower Orba River (Alessandria, NW Italy)

“…In general, an intense in-channel sediment mobilization, a widespread reactivation of bank protection scouring, and a locally-relevant channel widening were documented. However, in contrast with other cases (Cencetti et al, 2017;Clerici et al, 2015;Mandarino et al, 2020c;Pellegrini et al, 2008), the active channel did not experience severe planform changes despite the very high magnitude of the event. This aspect is most probably related to the progressive incision and channelization occurred along the LOR over the twentieth century (Mandarino, 2018), that restricted the natural dynamics of the river.…”

Urban geomorphology of Genoa old city (Italy)

“…Even if the early development of the city adapted to the geomorphological and hydrographical conditions, over time and particularly since the Middle Ages, Man has drastically modified the natural riverbeds. In fact, more than 70% of the current watercourses have been regulated: narrowed, rectified, cemented and culverted (Brandolini et al, 2018;Luino et al, 2019;Mandarino et al, 2020;Piana et al, 2019).…”