Lorenzo Picco

Large Wood And Flash Floods: Evidence From The2007 Event In The Davča Basin (Slovenia)

This paper presents the evidence gathered during a post-flood survey in a Slovenian mountain basin (Davca basin, drainage area of 32 km) following the catastrophic flood of September 2007. Channel avulsion, debris flows and landslides delivered large volumes of wood into the channel, and massive wood accumulations were found at the basin outlet. Wood-induced dam-break flows were therefore hypothesised to be responsible for most of the damages. Field observations and approximate discharge estimations indicate that the damages suffered in the Davca basin can be attributed to excessive wood load only to a limited extent, and that the critical factors were narrow road crossings which acted as traps for sediment and wood.

Medium-term fluvial island evolution in a disturbed gravel-bed river (Piave River, Northeastern Italian Alps)

Abstract River islands are defined as discrete areas of woodland vegetation surrounded by either water‐filled channels or exposed gravel. They exhibit some stability and are not submerged during bank‐full flows. The aim of the study is to analyze the dynamics of established, building, and pioneer islands in a 30‐km‐long reach of the gravel‐bed Piave River, which has suffered from intense and multiple human impacts. Plan‐form changes of river features since 1960 were analyzed using aerial photographs, and a was used to derive the maximum, minimum and mean elevation of island surfaces, and maximum and mean height of their vegetation. The results suggest that established islands lie at a higher elevation than building and pioneer islands, and have a thicker layer of fine sediments deposited on their surface after big floods. After the exceptional flood in 1966 ( > 200 years) there was a moderate increase in island numbers and extension, followed by a further increase from 1991, due to a succession of flood events in 1993 and 2002 with > 10 years, as well as a change in the human management relating to the control of gravel‐mining activities. The narrowing trend (1960–1999) of the morphological plan form certainly enhanced the chance of islands becoming established and this explains the reduction of the active channel, the increase in established islands and reduction of pioneer islands.

Riparian forest structure, vegetation cover and flood events in the Piave River

Until very recently, rivers have been considered as the result of the interaction between water and sediments, thus simplifying this very complex system. In doing so, one important component was missed. This is vegetation, namely trees growing on banks, floodplains, and bars/islands. The pattern of riparian vegetation in gravel bed rivers depends on the climate, hydrological regime, floods, sediment transport and the morphological settings of the river. Also, the marked spatial variability of density, height, species diversity, age, and rates of growth reflects the very complex nature of bed colonization, the strong influence of sequences and magnitude of floods, and the feedbacks between morphology, vegetation and hydraulics. Furthermore, a wide array of human impacts acting at either the basin or river network scales can influence substantially the morphodynamics and thus the characteristics, types and distribution of vegetation within the river corridor. The aim of the work is to analyze the relationship between the vegetation structure and the morphological characteristics of two sub-reaches of the Piave river which suffered both floods and intense and multiple human impacts especially due to dam building and inchannel gravel mining. Six cross sections were surveyed and vegetation, soil variables and sediment deposited were measured on 214 plots, (4 x 4 m each one and 10 m spaced). Each vegetation plot were recognized on a recent aerial photo (2009) and its ages were calculated performing both a multi-temporal analysis of older photos (1960, 70, 80, 91, 99, 2006 and 2009) and a dendro-chronological , Monitoring, Simulation, Prevention and Remediation of Dense and Debris Flows IV 137 www.witpress.com, ISSN 1743-3533 (on-line) WIT Transactions on Engineering Sciences, Vol 73,

Medium Term Fluvial Island Evolution InRelation With Flood Events In The Piave River

River islands could be defined as discrete areas of woodland vegetation surrounded by either water-filled channels or exposed gravel which exhibits some stability, and remain exposed during bunk-full flows. Islands are important from morphological, functional, and ecological points of view. The aim of the present study is to analyze the dynamics of stable, young and pioneer islands in the Piave River, which suffered intense and multiple human impacts especially due to dam building and in-channel gravel mining. Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces and the maximum and mean height of the island vegetation. The results suggest that stable islands lie at higher elevation than young and pioneer islands, and are characterized by a thicker layer of fine sediments deposited on their surfaces after big floods. There was a progressive decrease in the active corridor area from 1960 to 2006. After the 1966 exceptional flood (RI> 200 years) there was a moderate increase of island extend and numbers, followed by a further increase from 1991, due to a succession of flood events, in 1993 and 2002 with RI> 10 years, as well as a change in the anthropic management characterized by a control relating to mining activities. The narrowing trend (1960-1999) of the morphological plan-form, certainly enhanced the chance of the island to become stable and this explains the reduction of active channel, the increase of stable island and the reduction of pioneer island. Even though a general channel Monitoring, Simulation, Prevention and Remediation of Dense and Debris Flows IV 161 www.witpress.com, ISSN 1743-3533 (on-line) WIT Transactions on Engineering Sciences, Vol 73,

Assessing short term erosion-deposition processes of the Brenta river using LiDAR surveys

The extent and amount of erosion and deposition processes experienced by the Brenta river after the flood events of November and December 2010, have been assessed on the basis of DTMs derived from LiDAR surveys made before (2010) and after (2011) these floods. The study, carried out on three sub-reaches of this regulated gravel-bed river took advantage, beside LiDAR elevation data, two relatively novel approaches for channel bathymetry and uncertainty calculation. The first one allowed us to obtain the elevations within the wetted channels while the second method allowed the quantification of uncertainty related to the various phases of the topographic representation process. The erosion-deposition volumetric results derived from the two years DoD differencing models highlighted the impact of the two floods. In all the sub-reaches, erosion processes have dominate and featured a decreasing downstream trend, being the lower reach in a nearly equilibrium condition. The quantitative results of morphological changes have emphasized the evidence that relevant flood events (RI > 10 years) in high regulated gravel bed-rivers, encourage erosion rather than deposition patterns.

Role and management of in-channel wood in relation to flood events in Southern Andes basins

This paper analyses the connection among wood dynamics, channel morphology and flood risk in mountain basins of the Southern Chilean Andes and of the Argentinean Tierra del Fuego. Guidelines for the management of in-channel wood and of the riparian buffer strip are suggested, along with considerations about possible input from landslides and debris flows. A selection of low-impact check dams specifically designed for the retention of floating LW elements is finally provided.

Bed material transport estimate in large gravel‐bed rivers using the virtual velocity approach

Abstract This paper reports on a first attempt of using the virtual velocity approach to assess sediment mobility and transport in two wide and complex gravel‐bed rivers of northern Italy. Displacement length and virtual velocity of spray‐painted tracers were measured in the field. Also, the thickness of the sediment active layer during floods was measured using scour chains and post‐flood morphological changes as documented by repeated survey of channel cross‐sections. The effects of eight and seven floods were studied on the Tagliamento and Brenta Rivers, where 259 and 277 spray‐painted areas were surveyed, respectively. In the Tagliamento River 36% of the spray‐painted areas experienced partial transport, whereas in the Brenta River this accounted for 20%. Whereas, full removal/gravel deposition was observed on 37% and 26% of these areas on the Tagliamento and Brenta Rivers, respectively. The mean displacement length of particles, the thickness of the active layer and the extent of partial transport are well correlated with the dimensionless shear stress. The virtual velocity approach allowed calculation of bed material transport over a wide range of flood magnitudes. Annual coarse sediment transport was calculated up to 150 for the Tagliamento, and 30 × 103 m3 yr−1 for the Brenta. The outcomes of this work highlight the relevance of partial transport condition, as it could represent more than 70% of the total bed material transported during low‐magnitude floods, and up to 40% for near‐bankfull events. Results confirm that bed material load tends to be overestimated by traditional formulas. Copyright

Analysis of Morphological Processes in a Disturbed Gravel-Bed River （Piave River）： Integration of LiDAR Data and Colour Bathymetry

The magnitude of river morphological changes are better analyzed through the use of quantitative approaches, wherein resolution accuracy and uncertainty assessment are treated as crucial key-factors. In this sense, the creation of precise DEMs (Digital Elevation Models) of rivers represents an affordable tool to analyze geomorphic variations and budgets, except for wetted areas, where reliable channel digitalization can normally be obtained only using expensive bathymetric surveys. The proposed work aims at improving channel surface models without having available bathymetric sensors, by deriving dry areas elevations from LiDAR data and water depth of wetted areas from aerial photos through a predictive depth-colour relationship. The methodology was applied to two different sub-reaches of the Piave River, a gravel-bed river which suffered severe flood events in 2010. Erosion and deposition patterns were identified through DEM differencing, showing a predominance of scour processes which can lead to channel instability situations. The bathymetric output was compared to other previously-derived models confirming the accuracy of the in-channel elevation estimates. Finally, a discussion on the role played by longitudinal protections during the studied flood events is proposed, focusing the attention on the incidence of two major bank erosions that removed significant volumes of stable areas.

Analysis of Channel Morphology and Large Wood Characteristics Through Remote Images in the Blanco River After the Eruption of the Chaitén Volcano (Southern Chile)

Volcanic eruptions drastically affect fluvial systems, modifying channel morphology and processes of supply and transport of sediment and wood material, destroying vegetation and reducing infiltration rates on hillsides. This work aims to study the morphological changes and the distribution of large wood (LW) material along the Blanco River after the eruption of the Chaiten volcano in May 2008 using aerial and satellite images. Four different images from pre and post-eruption conditions were used: a satellite image from 2005, an aerial one from 2009, and two satellite images from 2012 and 2013. To assess river morphology, the active channel was digitized in all the images; and LW assessment was carried out by digitizing every individual element as polyline and wood jams as polygons. Results showed a general increase in the channel width, being the most notable change between 2005 and 2009. The same occurred in the case of LW supply, both individual elements and wood jams within the active channel increased significantly between 2005 and 2009 but there were no statistically significant differences comparing 2009, 2012 and 2013 images. This drastic event provided an opportunity to study post-eruption situation in the Blanco River, and with the results obtained it was possible to verify that the shape of the channel and presence of LW were strongly affected by the volcanic eruption.