Assessment of geomorphic effectiveness of controlled floods in a braided river using a reduced-complexity numerical model

Most Alpine rivers have undergone strong alteration of flow and sediment regimes. These alterations have notable effects on river morphology and ecology. One option to mitigate such effects is the flow regime management, specifically by the re-introduction of channel-forming discharges. The aim of this work is to assess the morphological changes induced in the Piave River (Italy) due to two different controlled flood strategies, the first characterized by a single artificial flood per year and the second by higher magnitude, but less frequent, floods. The work was carried out applying a 2D reduced-complexity morphodynamic model (CAESAR-LISFLOOD) to a 7 km-long reach, characterized by a braided pattern and highly regulated discharges. The numerical modelling allowed the assessment of morphological changes for four long-term scenarios (2009–2034). The scenarios were defined taking into account the current flow regime and the natural regime, which was estimated by a stochastic physically-based hydrologic model. Changes in channel morphology were assessed by measuring active channel width and braiding intensity. Comparing controlled flood scenarios to a baseline scenario (i.e., no controlled floods) it turned out that artificial floods had small effects on channel morphology. The highest channel widening (13.5 %) was produced by the release strategy with higher magnitude floods, while the other strategies produced lower widening (8.6 %). Negligible change was observed in terms of braiding intensity. Results pointed out that controlled floods may not represent an effective solution for morphological recovery in braided rivers strongly impacted in their flow and sediment regimes

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

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

Virtual Velocity Approach for Estimating Bed Material Transport in Gravel-Bed Rivers: Key Factors and Significance

In large gravel-bed rivers bed material transport estimation is challenging since theoretically-based formulas often fail to accurately predict sediment fluxes, and it is difficult to carry out field measurements. A viable alternative to direct measurement is provided by the virtual velocity approach representing a hybrid solution to calculate the bed material transport based on a theoretical framework and use of tracers. This work aims to improve the methodology and to assess the role of input factors through a case-study application carried out in the Parma River (Italy). Two tracer types and scour chains were deployed at four sections. Data on water level, transport processes, particle travel distances and active layers were collected over 17 months and 6 events. The transport that occurred during two events was calculated applying different configurations taking in account for several input factors (i.e. grain size, water stage, topography). Applying simple or more complex configurations led to significant differences in transport estimates: in relation to channel morphodynamics, different factors (e.g. variability of water level within the cross-section in multi-thread channels) play a key-role on transport processes. Results indicate that it is crucial to collect and process field data developing reach-specific transport rating-curves and to combine different type of tracers for monitoring the clast displacement lengths. Based on the methodological improvements and sensitivity analysis addressed in this study, we developed a decision tree in order to design future applications of the virtual velocity approach for estimating the bed material load in different gravel-bed river contexts

Pre-Alpine and Alpine deformation at San Pellegrino pass (Dolomites, Italy)

In this work, we present the geological map of the San Pellegrino pass, inserted in the spectacular scenario of the Dolomiti region (Southern Alps, Italy), at a scale of 1:10.000 and accompanied by geological cross-sections. The detailed distinction of lithological thin units allowed to achieve a consistent interpretation of the local structural setting by drawing brittle and ductile Alpine tectonic deformations. The differential deformation and structural styles within the geological map are the result of the different rheological nature of volcanic and sedimentary rocks, as well as of the superimposition of compressional Alpine tectonics over Permo-Mesozoic extensional tectonic phases, and consequent reactivation of inherited structures

Supplementary Material - "Assessment of geomorphic effectiveness of controlled floods in a braided river using a reduced-complexity numerical model"

This Supplementary Material provides supplementary text concerning the CAESAR-LISFLOOD model (hereafter C-L) detailing several issues about the hydrodynamic and sediment transport numerical scheme, the lateral erosion and the vegetation component modules (Text S1). It also includes details concerning the C-L model calibration to the Piave River case study (Text S2) in addition to a specific focus on the sediment budget estimation carried out to enforce the calibration achievements (Text S3)

River widening in mountain and foothill areas during floods: Insights from a meta-analysis of 51 European Rivers

River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures