Diego Ravazzolo

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,

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.