Unitus DSpace

Investigating the spatial variability of hillslope flow velocities in the Width-Function

2011-06-27T18:05:58Z

Title: Investigating the spatial variability of hillslope flow velocities in the Width-Function Authors: Petroselli, Andrea; Santini, Monia; Nardi, Fernando; Grimaldi, Salvatore; Vivoni, Enrique Abstract: The width function (WF), measuring the number of channel links or the portion of contributing area at the same hydrologic distance (i.e. the distance measured along the downhill flow path) to the outlet, is an important river basin and stream network parameter for hydrogeomorphic applications. The rescaled WF, obtained weighting the hydrologic travel distances with the corresponding surface flow velocity, is commonly used in the so-called WF-based Instantaneous Unit Hydrograph or simply WFIUH. The basic assumption of assigning two different constant flow velocities respectively to the hillslope and to channel network has already proven to improve the ability of the WFIUH in characterizing the runoff timing formation and the behaviour of the river basin hydrologic response. Nevertheless, a complete spatially variability of hydraulic conditions at the river basin scale has never been broadly investigated. Aim of this work is to investigate the impact of WF rescaled using a fully spatially distributed field of flow velocities as related to the different terrain morphologic (slope) and land cover properties(vegetation, soil type).

Flow routing algorithms and landslide modelling

2011-06-27T17:38:31Z

Title: Flow routing algorithms and landslide modelling Authors: Rulli, Maria Cristina; Nardi, Fernando; Santini, Monia; Petroselli, Andrea; Grimaldi, Salvatore Abstract: The physical processes governing shallow landslide triggering can be ascribed to hydrological and geomechanic forcings. Modelling shallow landslide triggering via hydrogeomorphic spatially distributed model requires to extract for any location in the river basin the geomorphic and hydrologic attributes. The Digital Elevation Models allow an easy quantification of several morphologic and hydrologic landscape properties e.g.primary attributes such as slope, aspect, plan and profile curvature, flow path lengths and secondary attributes such as topographic index and drainage area per unit contour length. Although all of these attributes are used when assessing the hydrological forcings to shallow landslide triggering, it has to be stressed that the flow direction and accumulation algorithm plays a significant role in the accurate physical representation of those waterdriven slope instability phenomena. In this study three flow direction algorithm (D8, D8- LTD and D ) are applied in conjunction with two DEM correction techniques for the treatment of pits and flat areas (ArcInfo standard approach and PEM4PIT), in order to determine the terrain attributes needed as input for the hydrogeomorphic model Shalstab. Quantitative results corresponding to the implementation of the different hydrogeomorphic terrain analysis algorithms on different case studies provide an objective framework for testing the accuracy and performance of the different schemes in relation to the DEM-based hydrogeomorphic prediction of shallow landslides.

Evaluating topographic and hydrologic attribute sensitivity to upscaled resolution DEMs from LIDAR data

2011-06-27T17:25:41Z

Title: Evaluating topographic and hydrologic attribute sensitivity to upscaled resolution DEMs from LIDAR data Authors: Petroselli, Andrea; Santini, Monia; Nardi, Fernando; Tarolli, Paolo; Grimaldi, Salvatore

Width-Function based Instantaneous Unit Hydrograph rainfall runoff model: relationship between Width Function, hillslope flow velocities and concentration times

2011-06-27T20:27:14Z

Title: Width-Function based Instantaneous Unit Hydrograph rainfall runoff model: relationship between Width Function, hillslope flow velocities and concentration times Authors: Nardi, Fernando; Petroselli, Andrea; Santini, Monia; Bocchiola, Daniele; Grimaldi, Salvatore Abstract: The Width Function (WF), measuring the number of channel links or the portion of contributing area at the same hydrologic distance (i.e. the distance measured along the downhill flow path) to the outlet, is an important river basin hydrologic parameter. The rescaled WF, obtained by weighting the flow distances with the corresponding flow velocity, characterizes the residency time distribution, that represents the WF Instantaneous Unit Hydrograph (IUH) or WFIUH, a useful tool for the prediction of the hydrologic response in ungauged basins. The rescaled WF is usually obtained by hypothesizing two constant flow velocities, one for the channel part of the basin and one for the hillslope part of the basin, neglecting the heterogeneity of surface flow patterns throughout the basin. This works aims to verify the eventual improvements in the characterization of the flow hydrograph of the WFIUH considering the spatial distribution of hillslope flow velocities as a function of local terrain properties (e.g. slope, land use etc) and river basin lumped parameters (e.g. lag time).

Correcting DEMs for hydrologic applications: the PEM4PIT model and its parameterization

2011-06-27T17:16:37Z

Title: Correcting DEMs for hydrologic applications: the PEM4PIT model and its parameterization Authors: Santini, Monia; Grimaldi, Salvatore; Nardi, Fernando; Petroselli, Andrea

Flow Time estimation in ungauged basins

2011-06-27T17:39:43Z

Title: Flow Time estimation in ungauged basins Authors: Grimaldi, Salvatore; Petroselli, Andrea; Alonso, Gustavo; Santini, Monia; Nardi, Fernando Abstract: The aim of this work is to focalize the attention on the design flood estimation on small ungauged river basin of limited extension (<150 Km2). In this contest we refer to the Width Function Based Instantaneous Unit Hydrograph (WFIUH) model, which optimizes, through the DEM, the distributed morphological basin information. The Width Function (WF) is defined as the distance-area function or the probability measure obtained by dividing the number of cells at given hydrologic distance from the outlet by the total number of basin cells (the distance is measured along the flow path and normalized by the maximum distance from the divide to the outlet). WF is easily obtained using common flow direction algorithms on elevation data, but in order to obtain the basin travel time distribution (IUH, or FT, Flow Time) two parameters have to be assigned: the channel and hillslope velocities (Vc, Vh). Indeed these two values, rescaling the WF expressed in terms of length, provide the travel time of each cell of the basin. The WFIUH model is not largely applied since these two velocity values have to be calibrated while this IUH approach should be adopted on small and almost ungauged basin. Further improvements can be obtained considering the spatial variability of hillslope velocities with the aim to reduce the number of parameter and to better determine the basin IUH. Overland flow velocities are recognized to vary with slope length, flow depth, land use and other geomorphic hillslope characteristics. Several approaches for the variability velocity field estimation applied for FT definition can be found in literature, for instance starting from classic Manning’s law and making assumptions on the hydraulic ratio, or linking hillslope velocity with power laws or local geomorphic properties such as slope or contributing area. Although several studies have already focused on the relationships between WF, channel flow velocity and hillslope flow velocity, in literature it was not deeply investigated the spatial variability of overland flow velocity and how this variability affects the basin hydrologic response. So, after a brief review of the main methods, aim of this work will be: A) to highlight if it is useful and what are differences in using a fully spatial distributed hillslope flow velocity field to rescale the FL, as respect to the standard approach which considers constant hillslope and river network velocity values. B) to evaluate approaches useful for the flow velocity estimation in term of the capability to reproduce appropriate values and in term of the number and type of parameter introduced.

Confronto tra alcuni metodi per l’estrazione del reticolo idrografico da DEM affetti da aree piane

2011-07-18T13:18:20Z

Title: Confronto tra alcuni metodi per l’estrazione del reticolo idrografico da DEM affetti da aree piane Authors: Petroselli, Andrea; Nardi, Fernando; Santini, Monia; Grimaldi, Salvatore; Ubertini, Lucio Abstract: In questo lavoro si analizzano alcuni metodi per la definizione delle direzioni di deflusso e per il trattamento delle aree piane dei Digital Elevation Model (DEM). La procedura presente nei più utilizzati pacchetti software (ESRI ArcInfo e GRASS) viene confrontata con alcune tecniche recentemente proposte in letteratura, analizzando i risultati ottenuti su un sottobacino del Fiume Tevere. Il confronto viene svolto sia stimando alcuni parametri idro-geomorfologici, ma soprattutto valutando diverse grandezze di progetto delle analisi idrologiche più comuni.; In this manuscript some DEM-based flow-direction methods and flat area treatment approaches are investigated. The standard procedure of widely used GIS software packages (ESRI ArcInfo and GRASS) is compared to some new recently proposed techniques analysing results obtained on a small sub-basin of the Tiber river. Differences of the new experimental procedures as respect to the standard approach are evaluated in term of some hydrogeomorphic variables and other hydrological design parameters.

Metodologie per l’estrazione automatica del reticolo: confronto degli effetti sulla risposta idrologica

2011-06-27T18:23:03Z

Title: Metodologie per l’estrazione automatica del reticolo: confronto degli effetti sulla risposta idrologica Authors: Di Lazzaro, Michele; Nardi, Fernando; Petroselli, Andrea; Santini, Monia; Grimaldi, Salvatore Abstract: In questa memoria vengono confrontate diverse procedure per l’estrazione automatica del reticolo idrografico a partire da DEM. Sono analizzate in particolare quattro procedure derivanti dalla combinazione di due tecniche per la rimozione dei pit e di due metodi per la definizione delle direzioni di drenaggio. Il confronto è effettuato sulla base della metrica del reticolo riprodotto, ed in particolare dei momenti di primo e secondo ordine della rete di versanti e canali. Viene infine evidenziato l’effetto delle diverse tecniche sulla risposta idrologica, utilizzando l’approccio geomorfologico basato sulla funzione d’ampiezza.

Procedura di stima dei parametri di un modello di rimozione dei pit e delle aree piane di un DEM

2011-06-27T19:02:54Z

Title: Procedura di stima dei parametri di un modello di rimozione dei pit e delle aree piane di un DEM Authors: Santini, Monia; Petroselli, Andrea; Nardi, Fernando; Grimaldi, Salvatore Abstract: I modelli digitali del terreno (DEM) sono affetti da errori di varia natura. I più comuni, dal punto di vista idrologico, sono gli avvallamenti (o pit) e le aree a pendenza nulla poiché impediscono la propagazione dei deflussi superficiali. L’origine di tali imperfezioni deriva sia dalle tecniche di rilievo o interpolazione utilizzate per costruire il DEM, sia dai metodi di trattamento preliminare applicati per ottenere DEM idrologicamente corretti. Un’impropria analisi di tali punti può influenzare significativamente la planimetria ed il relativo profilo altimetrico dei percorsi di drenaggio, nonché la stima di indici idrologici e geomorfologici che dipendono in genere dalla distribuzione delle pendenze e delle aree contribuenti. Recentemente è stato introdotto in letteratura un metodo di correzione del DEM fisicamente basato PEM4PIT (Physically Erosion Model for PIT removal), che sembra fornire risultati più realistici rispetto ai metodi tradizionali. In questa memoria si presenta una procedura per stimare i tre parametri del PEM4PIT e si mostrano alcune applicazioni sia per validare tale procedura di stima, sia per evidenziare le caratteristiche peculiari di tale metodo di correzione.

Modello afflussi – deflussi WFIUH: metodologia innovativa per l’applicazione in bacini non strumentati

2011-06-27T18:56:05Z

Title: Modello afflussi – deflussi WFIUH: metodologia innovativa per l’applicazione in bacini non strumentati Authors: Petroselli, Andrea; Santini, Monia; Nardi, Fernando; Grimaldi, Salvatore Abstract: Il modello afflussi - deflussi di tipo WFIUH (Width Function Instantaneous Unit Hydrograph) si basa sulla determinazione della Funzione d’Ampiezza o Width Function (WF), ovverosia la distribuzione dei punti aventi la stessa distanza idrologica dall’outlet. La Funzione d’Ampiezza (definita tramite supporto topografico digitale, DEM: Digital Elevation Model), riscalata rispetto alle velocità con cui il deflusso avviene sulle singole, fornisce la distribuzione dei tempi di arrivo all’outlet e quindi l’idrogramma unitario istantaneo del bacino (IUH). Per quanto il metodo sia stato ben sviluppato in letteratura ha incontrato difficoltà nella fase applicativa soprattutto a causa della determinazione delle velocità di deflusso. Tali velocità, infatti, sono assunte costanti nelle celle di tipo canale e nelle celle di tipo versante, non considerando quindi la variabilità spaziale del fenomeno. Inoltre si è verificato esse siano dei parametri di calibrazione del modello non desumibili da ipotesi di natura fisica. Scopo di questo lavoro è considerare la distribuzione spaziale delle velocità di deflusso sul terreno, proponendo e testando una metodologia speditiva per il calcolo delle velocità di deflusso per poter cosi calibrare il modello in bacini idrografici “poco strumentati” per i quali non sono disponibili misure contemporanee di precipitazione e portata.

A new GIS-based spatial modeling approach for desertification risk assessment in the Mediterranean Area. An Italian case study: Sardinia Island

2011-04-29T00:32:05Z

Title: A new GIS-based spatial modeling approach for desertification risk assessment in the Mediterranean Area. An Italian case study: Sardinia Island Authors: Santini, Monia Abstract: Desertification risk assessment has become a prior goal for environmental protection, recently even in the Mediterranean area. Assuming this phenomenon as interaction among predisposing (i.e. geographic location), triggering (i.e. extreme climatic events) and quickening (i.e. human activities) factors, a new model-based methodology was developed, combining synergically a wide range of desertification indicators developed by previous projects in Mediterranean area and included within the DPSIR (Driving force, Pressure, State, Impact, Response) scheme. The presented approach was applied for desertification risk evaluation in Sardinia (Italy). A large multi-thematic dataset has been acquired, processed and re-arranged in a GIS environment in order to supply the input to five models regarding several degradation processes typical in the study area: soil erosion, soil degradation, vegetation degradation, coastal aquifer salinization and overgrazing. The quantities resulted by modelling were normalized as indices varying from 0 (no degradation) to 1 (irreversible degradation) and they supplied a map of risk for each degradation process. These procedures were applied for two different periods according to the largest availability of data: the early nineties and the present. A total of ten single degradation indices were then integrated into a final one, combining in this way different factors of desertification, their magnitude, their changes, their development rate and giving them different weights according to the significance of each process and to the quality and detail of its input data. The final Integrated Desertification Index (IDI) has the same range of values and has been classified into 5 levels of increasing severity. The desertification risk map obtained using this methodology well reflects conditions observed on the ground. Further spatial statistic analyses allowed a comprehensive knowledge of features in most critical areas. This methodology allowed to highlight the spatial and temporal variability of desertification phenomenon, besides to estimate the land vulnerability in respects to each single degradation process. Starting from environmental vulnerability estimation and jointly with socio-economic aspects regarding probable land use scenarios, we will identify areas at different risk for the future, again in a GIS-oriented approach, supplying an useful instrument for prevention, adaptation and mitigation countermeasures.; La valutazione del rischio di desertificazione è diventato un obiettivo primario per la tutela ambientale, recentemente anche nell’area Mediterranea. Assumendo il fenomeno di desertificazione come l’interazione tra fattori predisponenti (es: localizzazione geografica), scatenanti (es: eventi climatici estremi) ed acceleranti (es: le attività umane), è stata una sviluppata una nuova metodologia che combina insieme, nella struttura di un modello integrato, numerosi indicatori di desertificazione, precedentemente sviluppati da vari progetti relativi all’area Mediterranea, integrandoli nello schema DPSIR (Driving forces, Pressure, State, Impact, Response). Il presente approccio è stato applicato per valutare il rischio di desertificazione in Sardegna. E’ stato realizzato un database multitematico, che in seguito è stato processato ed organizzato in ambiente GIS allo scopo di fornire gli input per cinque modelli riguardanti diversi processi di degrado tipici dell’area di studio: erosione idrica, erosione eolica, degrado biologico del suolo, perdita della produttività della vegetazione, salinizzazione degli acquiferi costieri e sovrapascolamento. Le quantità risultanti dalla modellazione di tali processi sono state in seguito normalizzate in indici variabili da 0 (nessun degrado) a 1 (degrado irreversibile: desertificazione) che hanno fornito una mappa di rischio per ciascun processo di degrado. L’intera procedura è stata applicata per due differenti periodi secondo la massima disponibilità di dati: l’inizio degli anni novanta ed i nostri giorni. I dieci indici di degrado così ottenuti sono stati successivamente integrati in un indice finale, combinando in questo modo differenti fattori di desertificazione, la loro intensità, e la loro rapidità di variazione, e dando pesi diversi sia secondo l’importanza di un singolo processo sia secondo la qualità dei dati input. L’indice integrato di desertificazione finale (IDI – Integrated Desertification Index) ha un range di variazione da 0 (nessun degrado) a 1 (degrado irreversibile). Tale metodologia ha permesso di evidenziare la variabilità spaziale e temporale del fenomeno di desertificazione, oltre a stimare la vulnerabilità del territorio rispetto a ciascun singolo processo di degrado. Dalla stima della vulnerabilità ambientale ed unitamente ad aspetti più strettamente socio-economici riguardanti possibili scenari di cambiamenti di uso del suolo, sarà possibile identificare aree a differente rischio per il futuro, di nuovo in un approccio GIS-orientato, fornendo un utile strumento per predisporre misure di prevenzione, adattamento o mitigazione contro la desertificazione. Description: Dottorato di ricerca in Ecologia forestale

Hydrogeomorphic properties of simulated drainage patterns using digital elevation models: the flat area issue

2011-06-27T18:05:06Z

Title: Hydrogeomorphic properties of simulated drainage patterns using digital elevation models: the flat area issue Authors: Nardi, Fernando; Grimaldi, Salvatore; Santini, Monia; Petroselli, Andrea; Ubertini, Lucio Abstract: Flat areas are a critical issue for the characterization of drainage patterns using digital elevation models (DEM). In this work, flat area removal and flow direction algorithms are implemented, and also a physically-based DEM correction model is introduced, for investigating their influence on the topological properties of the channel network, the Hortonian parameters and the hillslope width function. Differences of results, as compared to the standard procedures implemented in widely-used GIS-based hydrological packages, show the importance for hydrogeomorphic modellers to consider the use of more detailed approaches.; Les zones planes sont problématiques pour l’interprétation des modèles numériques de terrain (MNT) en termes de caractérisation des réseaux de drainage. Dans cette étude, des algorithmes de suppression des zones planes et d’analyse des directions d’écoulement sont implémentés, dont un nouveau modèle à bases physiques de correction de MNT, afin d’étudier leur influence sur les propriétés topologiques du réseau hydrographique, les paramètres de Horton et la fonction largeur de versant. Les différences de résultats, lors de la comparaison avec les procédures standard implémentées dans les modules hydrologiques courants sous SIG, montrent l’importance pour les modélisateurs hydro-géomorphologues d’envisager d’utiliser des approches plus détaillées. Description: L'articolo è disponibile sul sito dell'editore http://www.tandfonline.com/

Pre-processing algorithms and landslide modelling on remotely sensed DEMs

2011-06-27T19:01:04Z

Title: Pre-processing algorithms and landslide modelling on remotely sensed DEMs Authors: Santini, Monia; Grimaldi, Salvatore; Petroselli, Andrea; Nardi, Fernando; Rulli, Maria Cristina Abstract: Terrain analysis applications using remotely sensed Digital Elevation Models (DEMs), nowadays easily available, permit to quantify several river basin morphologic and hydrologic properties (e.g. slope, aspect, curvature, flow path lengths) and indirect hydrogeomorphic indices (e.g. specific upslope area, topographic wetness index) able to characterize the physical processes governing the landscape evolution (e.g. surface saturation, runoff, erosion, deposition). Such DEMs often contain artifacts and the automated hydrogeomorphic characterization of the watershed is influenced by terrain analysis procedures consisting in artificial depression (pit) and flat area treatment approaches combined with flow direction methods. In shallow landslide deterministic models, when applied using topographic dataset at medium scale (e.g. 30 m of resolution), the choice of the most suitable DEM-processing procedure is not trivial and can influence model results. This also affects the selection of most critical areas for further finer resolution studies or for the implementation of countermeasures aiming to landslide risk mitigation. In this paper such issue is investigated using as topographic input the ASTER DEMs and comparing two different combinations of DEM correction and flow routing schemes. The study areas comprise ten catchments in Italy for which hydrogeomorphic processes are significant. Aims of this paper are: 1) to introduce a parameter estimation procedure for the physically-based DEM correction method PEM4PIT (Physical Erosion Model for PIT removal); 2) to investigate the influence of different terrain analysis procedures on results of the slope stability model SHALSTAB (SHAllow Landsliding STABility) using remotelysensed ASTER DEMs; 3) trying to assess which of terrain analysis methods is more appropriate for describing terrain instability. Description: L'articolo è disponibile sul sito dell'editore http://www.sciencedirect.com/