1,720,981 research outputs found
An evaluation of sediment yield reduction induced by small irrigation reservoirs
No full textA GIS Based method has been applied for the determination of the reduction in fluvial sediment transport, induced by small agricultural reservoirs. The method involves the Revised Universal Soil Loss Equation (RUSLE) for the estimation of the soil erosion (gross soil erosion) and the concept of sediment delivery ratio to route surface erosion from each cells to the catchment outlet. The SDR is often used coupled with RUSLE in order to reduce the over estimation of soil erosion provided by the RUSLE method itself. The process of sediment delivery from grid cells to the catchment outlet is represented by the topographical index derived from Digital Elevation Model (DEM). This procedure has been applied in the Upper Tiberian Valley (Tuscany-Umbria boundary, central Italy) in order to evaluate the "pitfall effect" of numerous small reservoirs present in this area
Q-RAS: Simple tool for HEC-RAS Geometry data preparation. As QGIS can become an HEC-RAS preprocessor
No full textEvaluation of landslide dams hazard and risk: an application in Upper Tiber Valley (central Italy)
No full textThe note presents the results of a project, carried out by the Engineering Geology Group of Department of Physics and Geology of University of Perugia and funded by the Umbria Region, aimed at evaluating the hazard of occlusion of riverbeds by landslide (landslide dams) in the Upper Tiber Valley. The authors explain the used methodology that, starting from the data available in the catalogs of IFFI (Inventario dei Fenomeni Franosi in Italia, i.e. Italian Landslides Inventory) and PAI (Piano di Assetto Idrogeologico, i.e. Hydrogeological Assessment Plan), concerning the active and quiescent landslides, selects, through different steps (preliminary identification of landslides for which it is assumed the interference with riverbeds; photointerpretation; geomorphological survey; simulation of the total occlusion of riverbed; simulation of the collapse of the natural dam) the landslides for which is considered realistically possible the occlusion of riverbed due to a reactivation or persistence of the mass movement. The methodology also provides for a first evaluation of conditions of risk related to the complex phenomenon of interference between landslide and dynamics of riverbeds, for both areas located upstream of the dam, at risk of drowning due to the formation of the dam lake, both for those located downstream, where it is possible the transit of an abnormal flood wave, resulting from the eventual collapse of the natural dam and the rapid emptying of the lake
The stream power variation in a GIS environment as an index to evaluate the most 'sensitive' points of a river
No full textThe note deals with the problem of measurement and variability, along the fluvial reaches of a hydrographic network, of the stream power, understood according to the definition originally provided by Bagnold (1966) “The available power supply, or time rate of energy supply, to unit lenght of a stream is clearly the time rate of liberation in kinetic form of the liquid’s potential energy as it descends the gravity slope S”. In formula: Ω = ρ g Q S
where: Ω = stream power per unit of flow length (W/m) ρ g = density x gravity acceleration = specific weight of the fluid (kg/m3) Q = liquid discharge (m3/s) S = slope of the considered reach (m/m). Recent digital elevation models allow the calculation of channel gradient and consequently stream power with a finer spatial resolution, opening promising and novel opportunities to investigate river geomorphological processes and forms. The work carried out consisted in defining and implementing a methodological approach that could be automated within a geographic information system and that meets two requirements: 1) use input data that is easy to find as DEM at a low resolution; 2) estimate, in the best possible way and on the basis of the available data, the stream power and its variability along the considered stream. In particular, the methodological approach has been implemented in the GIS environment (GRASS Gis, Qgis), and it has been applied to a sample basin, highlighting its variability along the streams of a higher order (in practice the main stream and its most important tributaries). The sudden and more substantial variations of stream power were then related to the processes acting in the fluvial system. This approach has made it possible to highlight how the erosion, solid transport and sedimentation phenomena occurring along the fluvial reaches (and the geomorphological and geological-applicative problems that these involve, especially in anthropized areas), are correlated precisely to abrupt variations (increase/decrease) of the ”power” available. Hence the idea that automated and standardized screening of stream power variability along a stream can be used as a preliminary diagnostic element to identify the most ”sensitive” points of the same, on which to concentrate subsequent investigations (field checks, to verify the causes), aimed at mitigating the risk due to the dynamics of the riverbed
Analysis and characterization of historical and recent landslide dams in the Umbria-Marche Apennine area.
No full textAs part of a project finalized to evaluate landslide dams hazard in Umbria (central Italy), an analysis of recent and historical events occurred in the Umbria-Marche Apennines and neighboring (on the boundaries with Tuscany, Emilia-Romagna, Lazio and Abruzzo) was carried out. The aim of this analysis is to characterize the phenomenon of landslide dam, identifying the parameters (geometrical, geological, hydrological and hydrogeomorphological) that could, if properly combined, make it possible or not the occurrence of a landslide dam in the study area. Available data are related to 49 cases of landslides that have interfered with the river dynamics. 35 of these leaded to the formation of a dam lake, totally occluding the riverbed – floodplain system (landslide dams s.s.). The analysis indicates that over half of those 35 events (60%) involved the turbiditic sediments in facies of flysch (alternation of marls and sandstones) outcropping in the area; prevailing landslide types are slumps, slides, flows and combination between the previous (complex or composite landslides); 83% has a volume of material involved less than 5 million cubic meters; the total width of the floodplain affected by the damming does not exceed 200 meters in 86% of examined cases. In 29% of the total cases, the formation of a dam lake did not occurred; in the remaining cases, 49% of them was naturally drained by overflow and progressive erosion of the threshold; in 46% of cases the dam has resisted, so as to cause the filling of the lake with sediments or its current persistence (in the most recent cases). For the evaluation of the landslide dams hazard, two representative parameters for the examined cases were chosen: the landslide volume (which provides informations on magnitude of the landslide) and the area of the watershed outlet at the section of blockage (related to the stream power of the river, to its flow discharge and then to its ability to erode the natural barrier). The data indicate that most of the landslides (97%) that have interfered with the stream, producing a complete occlusion of the riverbed – floodplain system and the formation of a dam lake are characterized by a high index ratio: landslide volume / subtended watershed area. However, this condition appears to be necessary, but not sufficient to make certain the total occlusion: the values of this index, in cases of partial occlusion (i.e. without that the landslide has produced the formation of a dam lake), are extremely dispersed, which does not allow to use the same index to determine with certainty the possibility of total occlusion. Other parameters (the most important: velocity of the landslide and width of the involved riverbed - floodplain system) play, obviously, an important role that has to be properly evaluate
Analysis of the evolution of a riverbed using CAESAR, a cellular automata model
No full textL'evoluzione storica del F. Paglia (affluente di destra del F. Tevere nell'Umbria meridionale) ha messo in evidenza un generalizzato approfondimento dell'alveo, iniziato fin dagli anni '50 e causato soprattutto dall'intensa attività estrattiva di inerti. Questo stato, unito a una pianificazione che ha aumentato la vulnerabilità territoriale dell'intera valle del Paglia (soprattutto nel tratto finale, prossimo alla confluenza con il F. Tevere) è alla base delle condizioni di rischio oggi presenti, come i recenti eventi alluvionali del novembre 2012 hanno dimostrato. L'analisi storica, durante la quale sono stati reperiti utili documenti cartografici, contenenti informazioni di tipo plano-altimetrico, ha permesso di valutare l'entità di tale approfondimento, grazie al confronto tra modelli digitali del terreno, elaborati sulla base dei dati cartografici e fotografici pregressi (1951) ed attuali (2008). Gli stessi set di dati sono stati utilizzati per la calibrazione di un modello ad automi cellulari (CAESAR, COULTHARD et alii, 2002) che simula l'evoluzione morfologica di un corso d'acqua, a scala di bacino e a scala di alveo. I valori dei parametri di input che simulano con la migliore approssimazione possibile l'evoluzione di un periodo di circa 60 anni (1951-2008) possono così essere utilizzati anche per simulare l'evoluzione futura dell’alveo, condizione essenziale per qualsiasi opera di pianificazione a livello di bacino, ivi compresa la realizzazione di opere idrauliche in alveo finalizzate alla mitigazione del rischio geologico-idraulico
A toolkit for identifying and measuring physical streams features from high resolution DEMs and digital image
No full textIn fluvial dynamics remote sensing using airborne LiDAR (Light Detection and Ranging) may provide an alternative basis for identifying and measuring physical-geomorphological stream features that are traditionally recorded by field surveys. The ability to remote sense bankfull elevations was of particular interest in this study because bankfull mapping depends on topographic indicator. Bankfull is defined as the elevation at which flow spills onto the floodplain. In order to objectively define the bankfull elevation, we plotted the hydraulic depth as function of flow height for each cross section. The local maxima values indicate a sudden increase in flow width where water spills across floodplain. Such approach have been used to create a GIS tool that runs as plugin in QGIS software and provides as result a series of river indices (sinuosity index, braiding index, bankfull channel width, formative discharge) very helpful for fluvial geomorphologists and the polygon map of the bankfull limits. The algorithm was applied on a reachs of River Paglia an Umbrian rivers (in central Italy) affected by a strong disequilibrium
Geoinformatics in morphological study of River Paglia, Tiber River basin, Central Italy
No full textThis work reports the results of a geomorphological study on the River Paglia (Tiber basin, Central Italy), to analyse the historical evolution of the riverbed/floodplain system in its lower valley reaches, upstream the confluence of River Tiber. A morphological–sedimentological approach has been used, starting with an historical analysis and a survey of the forms and sediments in the riverbed/alluvial plain system, in order to reconstruct the current evolutionary trend and to identify the causes and processes leading to changes in natural trends. This approach is not in contrast to the hydrologic–hydraulic one, normally applied in engineering fields, but it is integrated with it in a complementary and parallel manner, in order to achieve the complete knowledge of the river system, result of an investigation multidisciplinary, aimed to define its physical state. Historical data (cartographic documents, digital terrain models—DTM, topographic surveys, aerial photographs, etc.) indicate clear-cut narrowing and deepening of the active channel, manifested after the Second World War and due to the considerable changes which affected the entire fluvial system (from the recovery of land for agriculture near the river, to sediments taken from its bed over the past few decades). The result is a state of great disequilibrium, which is shown in the passage from a braided morphology, observed in the 1950s, to a single channel with low sinuosity (wandering type). This type of morphology is certainly more unstable: in fact, the narrowing and deepening of fluvial sections provide a better hydraulic efficiency for the upstream, so the peak discharge does not overflow. Such peak discharge arrives downstream, next to the confluence with the Tiber River, not laminated, causing serious flooding effects. An increased vulnerability is observed in the last reach, in the areas pertaining to the river, where most of the industrial-economic activities are concentrated and where transportation infrastructures of nationwide importance (Italy’s most important motorway, the A1, and the Rome–Florence–Milan high-speed rail links) are located. The evolution of the Paglia (which is not substantially different from that of many streams in Italy and in general throughout Europe) is affected by severe anthropic constraints and intense exploitation of resources, which have produced a state of disequilibrium approaching irreversibility in an already vulnerable system. Finally, some suggestions for the correct management of the river system are pointed out, with the aim of mitigating the flood risk in the lower Paglia valley
A GRASS tool for the Sediment Delivery Ratio mapping
No full textThis work involves a tool implementation for evaluating Sediment Delivery Ratio (SDR) in a river basin, through GRASS GIS software. The definition of a spatially distributed value of SDR is a very important task as the sediment routing can affects solid transport in stream channels, water quality degradation, and frequency increase of natural disasters such as debris flows and mudflows. For such reason the SDR evaluation, coupled with GIS approach, has been extensively used in scientific literature. Geographic information systems provide a fundamental support for a better prediction of SDR, since it can consider the space variability of factors influencing the sediment routing processes. Actually a specific GIS module to estimate the spatial variability of SDR does not exist. We implemented a GRASS GIS module (in python language) called r.sdr where the sediment delivery ratio is evaluated by GIS procedure using several SDR equations available in literature. We applied the tool to the Feo Creek watershed, located in the Apennines area of northeastern Umbria (Italy).</jats:p
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