Exploring short-term geomorphic changes and bank retreat evolution in fluvial systems

The geomorphological effectiveness of flood events may highly vary depending on several factors, including discharge, duration, frequency, ordering, environmental conditions, human interference, etc. Precise rates and patterns of change produced by flooding remain largely unknown and unpredictable and multiple sources of error undermine the achievement of accurate estimates of geomorphic variations. Focusing on channel processes, the complex control exerted by riparian vegetation at multiple scales and, in particular, on riverbank dynamics further registers a lack of understanding that may lead to possible misleading approaches to manage fluvial erosion issues. This study primarily seeks to analyse and interpret the geomorphological response of two differently-impacted gravel-bed rivers (Piave and Tagliamento rivers, Italy) to flood events slightly below bankfull discharge. Furthermore, a focus on bank erosion processes on an Australian stream characterized by extensive tree abutments and cohesive scalloped riverbanks (the King River, Victoria) will try to explain the control exerted by riparian vegetation on the evolution of bank retreat. New findings and relationships will help to create a more comprehensive conceptual model describing bank migration past forested riverbanks that will be further tested on the Piave River, offering the possibility of comparing fluvial systems with very different environmental conditions (primarily, cohesive versus non-cohesive sediment). For the detection of geomorphic variations, enhanced DoD models including a precise evaluation of flowing channels, are derived by differencing pre- and post-flood integrated DEMs (LiDAR, dGPS and colour bathymetry). The reliable estimates of change have contributed to effectively quantify planimetric and volumetric adjustments of main channels and major bank removal. On the other hand, bank migration processes were examined on the King River by surveying the characteristics of trees and bank profile spaces to uncover significant relationships explaining the retreat of vegetated riverbanks. Results on geomorphic impacts of floods have confirmed a prevalence of erosion processes in the study reaches of the Piave and Tagliamento rivers, even with a general tendency toward sediment equilibrium. Flooding has caused a nearly total reconfiguration of the main flowing channel network, contributing to erode considerable areas of stably vegetated banks. The analysis on bank migration processes has further found elements and relationships previously unconsidered in bank retreat modelling. Major findings on the King River include the proof that riparian trees mostly grow on the bank face as well as the identification of more complex patterns of hydrological erosion around riparian trees (overcutting and undercutting) suggesting tree toppling by fluvial scour than by mass failure. Further, the progression of scallop depth along riverbanks has confirmed to be fairly influenced by tree spacing, both on the King and the Piave rivers. Discussions debating the impacts of floods have highlighted the ability of events below bankfull discharge to effectively rework the geomorphological structure of river channels at all levels. River changes appear to be also influenced by the physical characteristics of river reaches and human interventions. Moreover, the new findings on bank migration have allowed to fill some gaps of knowledge related to the complex effect of trees on riverbank erosion, leading to propose a more comprehensive conceptual model describing bank retreat evolution in forested cohesive riverbanks. Further parallelisms between King and Piave rivers have demonstrated that scallop depth features similar average magnitudes in both fluvial systems, even though the average effect of tree spacing within individual morphological units (bend- or arc-level) explains better erosion development in the Piave River. The significance of the study lies on the attempt of providing a reliable approach to meaningfully evaluate river geomorphic changes, revealing the high potential of floods also below bankfull discharge to revolutionize channel forms and patterns. The enhanced understanding of crucial channel processes as the reconfiguration of main branches and the control of riparian vegetation on bank retreat can further represent a decisive help to river experts within the complicated issue of managing fluvial systems

integration of colour bathymetry lidar and dgps surveys for assessing fluvial changes after flood events in the tagliamento river italy

The estimation of underwater features of channel bed surfaces without the use of bathymetric sensors results in very high levels of uncertainty. A revised approach enabling an automatic extraction of the wet areas to create more accurate and detailed Digital Terrain Models (DTMs) is here presented. LiDAR-derived elevations of dry surfaces, water depths of wetted areas derived from aerial photos and a predictive depth-colour relationship were adopted. This methodology was applied at two different reaches of a northeastern Italian gravel-bed river (Tagliamento) before and after two flood events occurred in November and December 2010. In-channel dGPS survey points were performed taking different depth levels and different colour scales of the river bed. More than 10,473 control points were acquired, 1107 in 2010 and 9366 in 2011 respectively. A regression model that calculates channel depths using the correct intensity of three colour bands (RGB) was implemented. LiDAR and water depth points were merged and interpolated into DTMs which features an average error, for the wet areas, of ±14 cm. The different number of calibration points obtained for 2010 and 2011 showed that the bathymetric error is also sensitive to the number of acquired calibration points. The morphological evolution calculated through a difference of DTMs shows a prevalence of deposition and erosion areas into the wet areas

Medium- and short-term channel and island evolution in a disturbed gravel bed river (Brenta River, Italy)

The timing and extent of the morphological and island changes that have occurred in the last thirty years in a gravel bed river that has been heavily impacted by human activities were analysed by nine sets of aerial photographs, repeated topographical measurements and morphological- vegetation surveys. Dam operations and gravel mining activities have produced modifications in the natural sediment regime that have generated important morphological responses in the channel. Large areas of the formerly active channel were colonised by riparian forest, both as islands and as marginal woodlands. The cessation of gravel extraction in the late 1990s seems to be causing incipient reversion of this pattern, with evidence of vegetation erosion/channel widening. Alteration of sediment regime has played a major role in the medium- and short-term channel evolution. However, only relevant flood events (recurrence interval >10 years) appear to determine substantial island erosion and, therefore, the proportion of islands versus channel fluctuates depending on flood history. Smaller scale analysis (sub-reach level) was more effective in describing morphological responses and relationships with the sediment dynamics within the 20 km study reach

An update of the sediment fluxes investigation in the Rio Cordon (Italy) after 25 years of monitoring

Quantification of bed-load transport in high-gradient mountain streams is important, but the field data needed to test transport models are scarce and difficult to obtain. In the present study, we describe the experimental station for monitoring water and sediment fluxes built in 1985 on the Rio Cordon, a small step-pool channel in the eastern Italian Alps. The measuring station consists of an inclined frame that separates fine from coarse sediments (D>20 mm), which are continuously measured by a series of ultrasonic sensors fitted above a storage area. The acquired 25-year dataset, which comprises a high-magnitude/ low-recurrence flood event, has allowed a magnitude-frequency analysis of bed-load volumes to be performed. Results from a combined frequency analysis of peak water discharge and total bed-load volumes are presented. In addition, the integration between the sediment transport dataset and the repeated surveys of sediment sources and of channel changes allowed us to assess the geomorphological effectiveness of different flood events. Despite the importance of the experimental station for making these bed-load observations possible, its maintenance costs are not low and these may have an impact on its future existence. At the same time, improving current instrumentation and future installations with novel technology would make the station an ideal location for calibrating surrogate techniques for sediment transport monitoring

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

Exploring short-term geomorphic changes and bank retreat evolution in fluvial systems

The geomorphological effectiveness of flood events may highly vary depending on several factors, including discharge, duration, frequency, ordering, environmental conditions, human interference, etc. Precise rates and patterns of change produced by flooding remain largely unknown and unpredictable and multiple sources of error undermine the achievement of accurate estimates of geomorphic variations. Focusing on channel processes, the complex control exerted by riparian vegetation at multiple scales and, in particular, on riverbank dynamics further registers a lack of understanding that may lead to possible misleading approaches to manage fluvial erosion issues. This study primarily seeks to analyse and interpret the geomorphological response of two differently-impacted gravel-bed rivers (Piave and Tagliamento rivers, Italy) to flood events slightly below bankfull discharge. Furthermore, a focus on bank erosion processes on an Australian stream characterized by extensive tree abutments and cohesive scalloped riverbanks (the King River, Victoria) will try to explain the control exerted by riparian vegetation on the evolution of bank retreat. New findings and relationships will help to create a more comprehensive conceptual model describing bank migration past forested riverbanks that will be further tested on the Piave River, offering the possibility of comparing fluvial systems with very different environmental conditions (primarily, cohesive versus non-cohesive sediment). For the detection of geomorphic variations, enhanced DoD models including a precise evaluation of flowing channels, are derived by differencing pre- and post-flood integrated DEMs (LiDAR, dGPS and colour bathymetry). The reliable estimates of change have contributed to effectively quantify planimetric and volumetric adjustments of main channels and major bank removal. On the other hand, bank migration processes were examined on the King River by surveying the characteristics of trees and bank profile spaces to uncover significant relationships explaining the retreat of vegetated riverbanks. Results on geomorphic impacts of floods have confirmed a prevalence of erosion processes in the study reaches of the Piave and Tagliamento rivers, even with a general tendency toward sediment equilibrium. Flooding has caused a nearly total reconfiguration of the main flowing channel network, contributing to erode considerable areas of stably vegetated banks. The analysis on bank migration processes has further found elements and relationships previously unconsidered in bank retreat modelling. Major findings on the King River include the proof that riparian trees mostly grow on the bank face as well as the identification of more complex patterns of hydrological erosion around riparian trees (overcutting and undercutting) suggesting tree toppling by fluvial scour than by mass failure. Further, the progression of scallop depth along riverbanks has confirmed to be fairly influenced by tree spacing, both on the King and the Piave rivers. Discussions debating the impacts of floods have highlighted the ability of events below bankfull discharge to effectively rework the geomorphological structure of river channels at all levels. River changes appear to be also influenced by the physical characteristics of river reaches and human interventions. Moreover, the new findings on bank migration have allowed to fill some gaps of knowledge related to the complex effect of trees on riverbank erosion, leading to propose a more comprehensive conceptual model describing bank retreat evolution in forested cohesive riverbanks. Further parallelisms between King and Piave rivers have demonstrated that scallop depth features similar average magnitudes in both fluvial systems, even though the average effect of tree spacing within individual morphological units (bend- or arc-level) explains better erosion development in the Piave River. The significance of the study lies on the attempt of providing a reliable approach to meaningfully evaluate river geomorphic changes, revealing the high potential of floods also below bankfull discharge to revolutionize channel forms and patterns. The enhanced understanding of crucial channel processes as the reconfiguration of main branches and the control of riparian vegetation on bank retreat can further represent a decisive help to river experts within the complicated issue of managing fluvial systems.Gli ambienti fluviali sono caratterizzati naturalmente dal susseguirsi di eventi di piena che svolgono un ruolo fondamentale nel mantenere il "dinamico equilibrio" di questi sistemi, integrando un gran numero di processi. Flussi idrologici, trasferimento di sedimento, interferenza di legname vivo o morto, impatto di opere antropiche ed altri fattori ancora, contribuiscono a ridisegnare la morfologia di un corso d'acqua, in un continuo cambiamento strutturale (Tockner et al., 2000). Sebbene l'effetto di piene provochi una naturale evoluzione delle forme fluviali, a partire dal secolo scorso si è registrata una consistente accelerazione del grado di impatto di questi eventi nella maggior parte dei sistemi fluviali dei "paesi occidentali" (Europa, America, Australia, ecc). Questi effetti, sempre più irregolari ed imprevedibili, oltre ad alterare le dinamiche evolutive di molti corsi d’acqua, rappresentano fattori di rischio che possono provocare ingenti danni in luoghi caratterizzati da centri abitati od opere infrastrutturali. Anche se le cause di questi cambiamenti imprevisti si pensa siano da ricollegare in gran parte all'influenza di opere antropiche, il ruolo di elementi naturali, come la vegetazione riparia, è sempre più dibattuto nell’ambito di interventi per proteggere le sponde da fenomeni erosivi e ridurre situazioni di rischio. Infatti, sebbene gli apparati radicali delle piante situate lungo le sponde aumentino la coesione del terreno, aumentando la resistenza allo scavo ed asportazione da parte dei deflussi (Hubble et al., 2010), la loro presenza in alveo e rimozione può portare a situazioni di pericolo, come per esempio blocchi a ridosso di ponti o infrastrutture. Strutture o azioni perpetrate a varia scala, di bacino, di corridoio o di alveo attivo, per scopi di protezione, regimazione, produzione di energia elettrica o prelievo di inerti, hanno compromesso per decenni direttamente o indirettamente le dinamiche evolutive di gran parte dei corsi d’acqua, creando ingenti problemi di riqualificazione ora che una nuova politica di salvaguardia naturale è in continua affermazione (Kondolf et al., 2007; Gurnell et al., 2009). Infatti, le modificazioni ai processi fluviali messe in atto negli ultimi decenni hanno così mutato le dinamiche evolutive di questi ambienti, che la volontà di ripristinare queste aree ad una condizione naturale antecedente a questi impatti è pressoché impossibile, dato che questa non si può più identificare (Wohl, 2005). In ambito italiano, questi impatti hanno provocato, in gran parte degli ambienti fluviali, fasi prolungate di restringimento ed incisione dell’alveo attivo, apportando cambiamenti anche a livello di forme planimetriche (da canali intrecciati a canale singolo). Un esempio è il fiume Piave che ha registrato un restringimento pari al 50% ed un’incisone fino a 10 m dell’alveo attivo e le cui cause sono state ricondotte ad un’alterazione abnorme del regime di sedimento dovuto al passato prelievo intensivo di inerti e alla presenza di alcune dighe (Comiti et al., 2011). Recentemente, grazie all’accresciuta attenzione ambientale e all’esigenza di raggiungere obbiettivi designati da nuove direttive nazionali ed europee (per esempio, Water Framework Directive), molti corsi d’acqua stanno sperimentando una nuova fase sviluppo verso un ritrovato equilibrio di funzionamento. In questo quadro, la disponibilità di strumenti accurati per valutare efficacemente gli effetti di eventi di piena ed una maggiore comprensione del ruolo della vegetazione riparia nei processi morfologici all’interno dell’alveo attivo, con particolare attenzione all’erosione spondale, svolgono un ruolo fondamentale nell’identificazione e spiegazione di dinamiche fluviali chiave per azioni di riqualificazione. Questo studio si propone di quantificare in modo preciso gli effetti geomorfologici provocati da eventi di piena al di sotto della portata bankfull che nel 2010 hanno interessato due fiumi italiani caratterizzati da un diverso impatto antropico, il Piave ed il Tagliamento. Oltre ad isolare volumi e tendenze morfologiche dominanti nei tratti di studio, verranno valutati i processi di riconfigurazione planimetrica del canale principale e le dinamiche di erosione spondale riguardanti porzioni di vegetazione stabile per comprendere meglio le conseguenze delle piene sui diversi settori del corridoio fluviale. Inoltre, il ruolo della vegetazione riparia nell’evoluzione delle dinamiche di erosione spondale, verrà approfondito in un ambiente fluviale caratterizzato da sedimento coesivo e profili spondali fortemente modificati dalla resistenza all’erosione da parte di piante adiacenti al canale attivo: il fiume King (Australia). La ricerca di relazioni che spieghino la progressione di processi di erosione laterale in sponde caratterizzate da vegetazione riparia a diversa densità avrà come obbiettivo finale la creazione di un modello onnicomprensivo che spieghi l'evoluzione dell'erosione spondale lungo anse fluviali vegetate. Un ulteriore confronto con il fiume Piave, grazie alle numerose differenze fisiche ed idrologiche (in particolare il substrato composto da materiale non coesivo), offrirà spunti di discussione interessanti per la riconsiderazione della funzione della vegetazione riparia nelle strategie di protezione e riqualificazione fluviale. La prima parte, riguardante la stima accurata degli effetti geomorfologici provocati dagli eventi di piena del 2010, è stata sviluppata grazie alla disponibilità di modelli digitali del terreno (Digital Elevation Model – DEM) che, integrando rilievi GPS, LiDAR e provenienti da batimetria da colore, hanno permesso di riprodurre in modo puntuale la superficie complessiva (aree bagnate e non bagnate) dei tratti fluviali analizzati prima e dopo gli eventi. Attraverso un processo, modificato ad hoc, di produzione di modelli digitali di input collegati fra loro da script linguistici creati in MatLab (Fuzzy Inference System files), si è stati in grado, utilizzando l’applicazione Geomorphic Change Detection, di elaborare modelli digitali differenziali (DEM od Difference – DoD), che hanno permesso di valutare accuratamente gli impatti delle piene analizzate. I volumi ottenuti da questi modelli sono stati associati ad un calcolo preciso dell’errore che, contribuendo a fornire risultati quanto più vicini alla realtà possibile, ha permesso di estrapolare processi e tendenze evolutive dei due fiumi e di valutare la loro situazione rispetto ad una condizione di equilibrio. In seguito, la nostra attenzione si è focalizzata sul comportamento dei collettori fluviali principali che, attraverso una digitalizzazione e conseguente quantificazione dei processi erosivi e di deposizione, hanno dimostrato di aver subito una riconfigurazione morfologica pressoché completa in tutti i tratti analizzati. Infine, la stima dell’impatto di queste piene su erosioni localizzate di aree spondali caratterizzate da vegetazione stabile ed opere antropiche (fiume Piave), ha mostrato il potenziale di questi flussi idrologici nell’asportare porzioni di superficie teoricamente più resistenti grazie all’effetto stabilizzante degli apparati radicali, ipotizzando una possibile interferenza degli interventi di sistemazione idraulica. I risultati degli effetti geomorfologici a varia scala prodotti dagli eventi di piena hanno sottolineato il fatto, peraltro ribadito da altri studi (Chappell et al., 2003; Bertoldi et al., 2010), che anche eventi al di sotto della portata bankfull possono essere in grado di influenzare e riconfigurare in modo pronunciato lo sviluppo morfologico degli ambienti fluviali, oltre che aumentare talvolta il grado di rischio quando opere antropiche interferiscono con essi. I riscontri volumetrici hanno mostrato una predominanza complessiva di processi erosivi nei sottotratti analizzati, alludendo ad un deficit di sedimento ancora presente e da colmare per ottenere una condizione di equilibrio. Lo spostamento pressoché totale dell’asta fluviale principale ha infine confermato l’imprevedibilità degli effetti di piena che inoltre, asportando importanti quantitativi di aree vegetate stabili, hanno aperto nuovi interrogativi sul ruolo della vegetazione riparia nell’evoluzione dell’erosione spondale. Questo ruolo è stato approfondito nel bacino del fiume King (Australia) che, offrendo particolari processi di interazione fra vegetazione riparia e profilo spondale, ha permesso di indagare e comprendere le dinamiche evolutive dell’erosione spondale. Il tratto studiato del suddetto fiume presenta infatti profili spondali caratterizzati dal susseguirsi di piante di diversa grandezza e densità, intervallate da concavità prevalentemente erosive, che isolano progressivamente gli appartati radicali delle piante stesse fino a farle cadere nel collettore principale. L’analisi dei parametri associati alla vegetazione riparia e a queste concavità ha permesso di trovare varie relazioni che spiegano l’avanzamento dell’erosione spondale in ambienti caratterizzati da effetti di resistenza e stabilizzazione prodotti da piante e radici. In particolare, risultati sull’importanza della densità vegetativa hanno sottolineato l’influenza di piante poco spaziate sul profilo longitudinale della sponda nel ridurre l’ampiezza delle concavità erosive. Queste incoraggianti conclusioni hanno portato alla creazione di un modello concettuale di evoluzione di profili spondali caratterizzati da sedimento coesivo e vegetazione riparia. Questi esiti sono stati poi confrontati con i tratti analizzati del fiume Piave, portando all’individuazione di alcune dinamiche simili e altre moderatamente diverse. Alcune caratteristiche di sviluppo dei profili spondali erosivi hanno dimostrato di essere comuni nei due sistemi fluviali, come ad esempio il ruolo della densità della vegetazione riparia attiva sulla sponda, che promuoverebbe una limitazione dell’avanzamento dell'erosione laterale. Al contrario altri aspetti, fisici e di scala di processo, hanno riportato profonde differenze, probabilmente date dalle diverse caratteristiche climatiche, di portata, di funzionamento e, non da meno, di substrato (coesivo contro non coesivo) dei due ambienti fluviali. Concludendo, il presente studio ha indagato con successo le ripercussioni geomorfologiche causate da eventi di piena moderati, ottenendo stime verosimili di processi erosivi e di deposizione che hanno altresì permesso di valutare la condizione attuale degli ambienti fluviali analizzati. Piene anche al di sotto della portata bankfull hanno avuto effetti su tutte le componenti geomorfologiche a livello di tratto, includendo la riconfigurazione dell’asse fluviale principale e l’erosione laterale di aree vegetate stabili. L’approfondimento del ruolo della vegetazione riparia nel ridurre i processi di migrazione erosiva e il successivo confronto fra ambienti fluviali diversi, ha incrementato la nostra conoscenza su queste dinamiche che sono alla base di azioni di protezione e riqualificazione fluviale. I significativi risultati raggiunti da questa ricerca possono infine rappresentare un importante arricchimento per gli esperti del settore che, a fronte delle tendenze emerse, potranno usufruire di un ulteriore base su cui progettare nuove e più efficaci strategie di gestione degli ambienti fluviali

Field-Based Estimates of Floodplain Roughness Along the Tisza River (Hungary): The Role of Invasive Amorpha fruticosa

River flooding has become a major issue in the last decades, placing at risk wide portions of the planet. Modeled scenarios are the most helpful tools to reduce flood hazard; nevertheless they can lead to significant over-or underestimation of flood discharge and peak flood level if field attributes, especially floodplain roughness, are not carefully surveyed and reasonably included in their design. Besides, floodplains provide optimal habitats for invasive species, which can form dense stands, influencing roughness and flow velocity considerably. The floodplains of the Tisza River and its tributaries (Hungary) are highly invaded by invasive plants, however their existence is not considered during flood modeling. Among these plants Amorpha fruticosa has the greatest impact and creates impenetrable shrubbery on the floodplain, having 5–10 branches sprouting from its stump. The aim of this study is to give a precise and field-based estimation of vegetation density and floodplain roughness of different vegetation categories along the floodplain of the Tisza River, to overcome inaccuracies caused by the use of reference values. Through the Parallel Photographic Method, digital images are used to create photo mosaics and derive accurate estimates of vegetation density and floodplain roughness. Further, the role of invasive A. fruticosa in increasing vegetation density of the floodplain is also investigated. Results confirm a considerable underestimation of vegetation density and floodplain roughness (average difference: 0.04–0.06) by using simple reference values, as invasive Amorpha contributes for 55% of vegetation density on average, and on abandoned fields it constitutes 100% of the woody vegetation. These values suggest that modeled flood water levels can be remarkably underestimated if density of invasive species is not considered. The results of this study stress the importance of including field-based assessments of vegetation characteristics and the influence of invasive species in river modeling, to generate realistic predictions and address effective flood hazard strategies

Hybrid DTMs derivided by LiDAR and colour bathymetry for assessing fluvial geomorphic changes after flood events in gravel bed rivers (Tagliamento, Piave and Brenta Rivers, Italy)

Risk management and flood protection are frequently assessed through geo-morphometric evaluations resulting by fl oods events. If we aim at elevation models with high resolutions and covering large areas, airborne laser imaging detection and ranging (LiDAR) surveys can represent a good compromise among costs, time and uncertainty. The major limitation of the non-bathymetric LiDAR surveys consists in the detection of wet areas. Indeed, accounting for more than 20 cm of water depth, LiDAR signal increases its error exponentially. In this article we present a comparison of the results concerning the application of a colour bathymetry methodology for the production of hybrid digital terrain models. These elevation models were derived by merging LiDAR data for the dry areas and colour bathymetry for the wet areas. The methodological approach consists in a statistical regression between water depth and RGB band intensity values from contemporary aerial images. This methodology includes the use of fi lters to reduce possible errors due to the application of the model and to estimate precise \u2018in-channel\u2019 points. The study areas are three different human-impacted gravel-bed rivers of the north-eastern Italy. This methodology has been applied in three sub-reaches of the Brenta River, two of the Piave River and two of the Tagliamento River before and after relevant fl ood events with return intervals of 6510 years. Potentials and limitations of the applied bathymetric method, the comparison of its use in different fl uvial contexts and its possibility of employment for geo-morphometric evaluations, were then tested. DGPS control points (1841, 2638 and10 473 for the Brenta, Piave and Tagliamento Rivers, respectively) were fi nally used to evaluate the accuracy of the wet areas. The results showed that, in each model, the wet areas\u2019 vertical errors were comparable with those featured by LiDAR data for the dry areas

Evaluation of short-term geomorphic changes using improved dems of difference: a comparison between differently impacted rivers, Piave and Tagliamento

The evaluation of the morphological dynamics of rivers is increasingly focusing, in recent years, on the achievement of quantitative estimates of change in order to identify geomorphic trends and forecast targeted restoration actions. Thanks to the development of more effective and reliable survey technologies, more accurate Digital Elevation Models (DEM) can be produced and, through their consequent differencing (DoD), extremely useful geomorphic analyses can be carried out. In this situation, a major role is played by uncertainty, especially in the final volumetric rates of erosion and deposition processes, that may lead to misinterpretation of spatial and temporal changes. This paper aims at achieving precise geomorphic estimates derived from subsequent hybrid (LiDAR and bathymetric points) surface representations. The study areas consist of gravel-bed reaches of two differently impacted fluvial environments, Piave and Tagliamento rivers, that were affected by two severe flood events (Piave, R.I. of 7 and 10 years and Tagliamento, R.I. of 15 and 12 years) in the inter-surveys period. The basic Hybrid Digital Elevation Models (HDTM) were processed accounting for spatially variable uncertainty and considering, beside slope and point density input variables, a novel component measuring the quality of the bathymetric derived points. In fact, since the major changes occur within river channels, the integration of this variable evaluating the precision of the bathymetric channel elevations in the HDTMs, has allowed, through the creation of targeted FIS (Fuzzy Inference System) rules, to obtain reliable geomorphic estimates of change. Volumes and erosion and deposition patterns were then analyzed and compared to outline the different dynamics among the sub-reaches and the two river systems