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Hydrologic control on the triggering and magnitude of debris flows in alpine catchments
Full text linkThe present work analyzes the hydrologic conditions leading to the triggering
of debris flows in an alpine region. The overall analysis has been split in two parts: the first part of the analysis has been carried out at a regional and decadal scale to improve our knowledge of rainfall thresholds for debris-flow occurrences, of the uncertainty related to rainfall estimation at debris-flow initiation sites, and of the main morphometric characteristics of debris-flow triggering locations; in the second part a focus has been devoted to the analysis of the hydrologic response of some watersheds for a selection of events with the help of detailed input information, both topographic and hydrological. The study area is upper Adige River basin (Northern Italy), which occupies a distinctive hydrometeorological niche, characterized by high frequency of orographic thunderstorms.
The first part of the thesis analyzes the debris-flow triggering issue at regional and decadal-scale (2000-2010). A morphometric characterization of debris-flow triggering locations has been carried out in terms of slope-area analysis. The characterization was carried out dividing considered debris flows in classes of rainfall duration, rainfall intensity and considering also the potential relation with the different geologic settings. Another issue under investigation is related to rainfall threshold estimation (from raingauges network) for debris-flow occurrence and the uncertainty related to this estimation. First, a set of rainfall thresholds has been derived adopting a frequentist approach. The rainfall estimation procedure used for threshold identification was based on two major sources: records from the nearest gauge and an Inverse Distance interpolation of all the available records at regional and decadal scale. Thresholds have been analyzed considering all the available points and also trying to group debris-flow occurrences according to seasonal occurrence, different storm characteristics and homogeneous geologic setting. The level of uncertainty related to rainfall threshold estimation has been analyzed in detail The estimation problem is especially severe for the location of the debris flows, with raingauges that are commonly located at low elevation (e.g., in the valley floors) and debris flows that originate at high elevations, in the head part of the mountain catchments. The rainfall estimation procedure used for the uncertainty assessment, is the one based on the nearest raingauge, which is used as a proxy to estimate the rainfall that has resulted in debris flows. The objective is to investigate the impact of the uncertainty inherent in the estimation of rainfall on the definition of a threshold for possible debris-flow occurrence, and on the operational use of the threshold for prediction purposes. These effects are likely to depend on two factors: the space-time rainfall variability and the distances between the debris-flow locations and the raingauges on both the horizontal and the vertical planes. Three main problems are considered: (i) the effect of the rainfall sampling problem on the estimation of the parameters of the threshold model, (ii) the effect of applying a number of procedures to filter the rainfall information on the threshold model estimation, and (iii) the examination of the impact of the precipitation sampling problem on the performance of the threshold as a predictor of debris-flow occurrence. These aspects are examined here based on a simulation experiment. The methodology examines the intensity-duration thresholds derived from a set of raingauge locations that is assumed to represent debris flow/landslide points (DFR) and an equivalent set of raingauges assumed to have the role of closest available measurement (MR). A set of reference rainfall thresholds is used to identify the rainfall events at DFR that “triggered debris flows (i.e. exceed the threshold). For these same events, the corresponding rainfall thresholds are derived from MR observations. Comparison between the rainfall thresholds derived from DFR and MR, revealed that uncertainty in rainfall estimation has a major impact on estimated intensity-duration thresholds. Specifically, results showed that thresholds estimated from MR observations are consistently underestimated. Evaluation of the estimated thresholds for warning procedures, showed that while detection is high, the main issue is the high false alarm ratio, which limits the overall accuracy of the procedure. Overall performance on debris-flow prediction was shown to be good for low rainfall thresholds and poor for high rainfall thresholds examined. Finally different interpolation techniques have been applied to a set of gauges close to the DFR to assess the eventual improvement brought by the interpolation procedure. Results on this showed that interpolation can improve estimates specifically in the case of poor DFR-MR correlation, while in the other cases, especially considering intense and/or localized storms, the benefits of considering interpolated value in respect to only one near gauge is not so significant.
In the second part of the thesis the hydrometeorological and hydrological controls of these events are examined through analyses of three storm systems occurred on October 3-4, 2006, June 20-21 2007 and September 3-4, 2009. The first storm system generated a moderate flash flood along the main streams with almost no debris flows in the tributaries, the second triggered a large number of debris flows and was characterized by a minor runoff response for the major streams, and the third resulted in both a relevant flash flood response and widespread debris flows. These events have been examined by using per-event calibrated radar rainfall data, providing an important input for the evaluation of a distributed hydrological model, and by using a database reporting location and information related to the events. Raingauge and streamflow data, debris flow spatial information and observations are used along with the distributed model to analyze the hydrological and geomorphic responses to these
rainstorms. The striking contrast in flood and geomorphic responses between the three floods is related to contrasts in the antecedent moisture conditions and in the space-time structure of the triggering storm. Rainfall-runoff modeling has permitted estimating discharge and runoff volumes in a number of small ungauged catchments in which debris-flow volumes had been assessed by means of post-event surveys. The computation of sediment concentration by volume using model-estimated water runoff has resulted in realistic values. The relations between water peak discharge and debris-flow volumes show higher correlation coefficients than those involving morphometric variables and rainfall amounts.Il presente lavoro ha analizzato le condizioni idrologiche associate all’innesco di
colate detritiche in una regione alpina.
L’analisi è stata suddivisa in due rami principali: la prima parte è stata effettuata a scala regionale e decennale (2000-2010) per approfondire le tematica
delle soglie pluviometriche per l’innesco di colate e delle incertezze legate alla determinazione della pioggia nelle zone di innesco delle colate detritiche. Si è cercato, inoltre, di caratterizzare, dal punto di vista morfometrico, le zone di innesco delle colate stesse. Uno studio approfondito è stato dedicato successivamente all’analisi della risposta idrologica di alcuni bacini, per tre eventi di piena avvenuti fra il 2006 ed il 2009. L’area di studio è l’intera Provincia Autonoma di Bolzano (Alto Adige, Nord Italia). Tale area occupa una nicchia idrometereologica peculiare, caratterizzata da un’elevata frequenza di fenomeni temporaleschi con forzante orografica, che, specialmente nel periodo estivo, possono attivare colate detritiche e piene improvvise.
La caratterizzazione morfometrica dei siti di innesco è stata effettuata esaminando l’area contribuente e la pendenza locale dei punti stessi. La caratterizzazione è stata effettuata dividendo il dataset in classi di durata di precipitazione, introducendo ulteriori filtri sull’intensità di pioggia e considerando anche la relazione con la litologia. Un’altra questione in esame è legata alla stima di soglie pluviometriche per
l’innesco di colate detritiche unitamente all’incertezza legata a questa stima. In primo luogo, un insieme di soglie pluviometriche è stato derivato mediante un approccio frequentista. La procedura di stima di precipitazione per l’identificazione della soglia è basata su due fattori principali: dato del pluviometro più vicino e una interpolazione (Inverse Distance) di tutti i dati orari disponibili a scala regionale e decennale. Le soglie sono state analizzate prendendo in considerazione tutti i punti disponibili ed anche cercando di raggrup-pare i fenomeni di colata in base alla stagionalità, alle diverse caratteristiche dell’evento meteorico ed in base a zone geologicamente omogenee. Il livello di incertezza relativo alla stima di soglie di pioggia è stato analizzato in dettaglio. Il problema della stima di risulta essere particolarmente importante proprio a causa della posizione delle zone di innesco di colate detritiche, con pluviometri che sono comunemente situati a bassa quota (e.g., nei fondovalle) e colate detritiche che hanno origine ad altitudini elevate, nella parte di testata di bacini montani. La procedura di stima di precipitazioni adottata per la valutazione dell’incertezza, è quella basata sull’utilizzo del pluviometro più vicino, che viene considerato
come proxy per stimare la pioggia innescante sul sito di colata. L’obiettivo è quello di studiare l’impatto dell’incertezza insita nella stima delle precipitazioni sulla definizione di una soglia per l’innesco di colate detritiche, e l’uso operativo della soglia per scopi di previsione. Due sono le condizioni che maggiormente influenzano l’icertezza di stima: la variabilità spazio-temporale delle precipitazioni e le distanze tra le zone d’innesco ed i relativi pluviometri più prossimi sia sul piano orizzontale che sul piano verticale. Tre sono gli effetti principali considerati: (i) l’effetto del campionamento delle precipitazioni sulla stima dei parametri del modello di soglia, (ii) l’effetto dell’applicazione di una serie di procedure per filtrare le informazioni di precipitazione sul modello di stima della soglia, e (iii) l’incidenza del campionamento della precipitazione sulle performance della soglia come predittore di accadimento di colate. Questi aspetti vengono esaminati mediante una simulazione condotta a scala regionale. La metodologia adottata esamina le soglie di intensità-durata mediante la selezione di una serie di pluviometri che si assumono rappresentare un sito di innesco di colata, denominati DFR, ed una serie di pluviometri prossimi a questi (in rapporto 1:1), denominati MR, che vengono usati per stimare la precipitazione sul sito DFR. Una serie di soglie pluviometriche di riferimento viene utilizzate per identificare gli eventi di precipitazione che “innescano” colate de-
tritiche su DFR (cio` eventi che superano la soglia). Per questi stessi eventi, le corrispondenti soglie pluviometriche sono derivate da osservazioni MR. Il confronto tra le soglie pluviometriche derivate da DFR e MR, ha rivelato che l’incertezza nella stima di precipitazione ha un impatto importante sulle soglie di intensità-durata. In particolare, i risultati hanno mostrato che le soglie stimate dalle osservazioni MR presentano stime in difetto. La valutazione delle soglie stimate per procedure di allarme, ha mostrato che, mentre la probabilità di rilevamento è alta, il problema principale è l’elevato rapporto di falsi allarmi, che limita la precisione complessiva del procedimento. Le performance generali sulla previsione di colate detritiche si sono dimostrate buone per soglie pluviometriche moderate e scarse per soglie elevate.
Infine, diverse tecniche di interpolazione sono state applicate ad una selezione di pluviometri prossimi al DFR per valutare l’eventuale miglioramento portato dagli interpolatori. I risultati hanno dimostrato che l’interpolazione può migliorare le stime, in particolare i miglioramenti più significativi si sono notati nei casi di correlazione debole tra DFR e MR, mentre per gli altri casi ed in particolare considerando eventi intensi e/o localizzati, i benefici derivanti dal considerare i valori interpolati rispetto al valore del pluviometro più prossimo non sono così significativi.
Nella seconda parte della tesi viene esaminata la forzante idrometeorologica ed idrologica relativa all’innesco di colate, attraverso l’analisi dei tre eventi meteorici avvenuti il 3-4 ottobre 2006, 20-21 giugno 2007 e 3-4 Settembre 2009 per capire come la variabilità meteorologica si rifletta nelle diverse risposte dei bacini. Il primo evento ha generato infatti piene improvvise di modesta intensità lungo le aste torrentizie principali con quasi nessuna colata registrata nei corsi d’acqua secondari, il secondo ha innescato un gran numero di colate in bacini secondari ed è stato caratterizzato da una risposta idrica minore nei collettori principali, il terzo ha visto sia piene improvvise importanti che colate detritiche diffuse. Tali eventi sono stati studiati utilizzando dati di pioggia calibrati da radar sul singolo evento, fornendo un input importante per la calibrazione di un modello idrologico distribuito, ed utilizzando inoltre informazioni derivanti dall’analisi di un ampio database di fenomeni di instabilità (incluse le colate detritiche) che copre l’intero territorio provinciale e che viene mantenuto costantemente aggiornato. L’integrazione di dati radar e da rete pluviometrica, dati di portata, informazioni spaziali e volumetriche sugli eventi di colata assieme ad osservazioni sul campo sono usati, unitamente al modello idrologico distribuito, per analizzare le risposte idrologiche e geomorfologiche agli eventi innescanti. Il forte contrasto in termini di risposta idrica ed effetti morfologici fra le tre piene risulta legato a differenze nelle condizioni di umidità antecedenti e nella struttura spazio-temporale degli eventi innescanti. La modellazione afflussi-deflussi ha permesso la stima e di portate al picco e di volumi di deflusso in una serie di piccoli bacini non strumentati in cui i volumi depositati dalle colate sono stati stimati mediante indagini post-evento. Il calcolo della concentrazione volumetrica di sedimento eseguito utilizzando i risultati dell’approccio di modellazione scalato ai bacini da colata ha portato a valori realistici. Le relazioni tra portate al picco e volumi mobilizzati dalle colate mostrano coefficienti di correlazione più elevati di quelli che considerano le variabili morfometriche ed i volumi di precipitazione
Hydrological control on the triggering of debris flows in alpine catchments: storm analysis and basin response variability
No full textThree storm events, occurred in 2006, 2007 and 2009 in the upper Adige River basin (Eastern Alps, northern Italy) have been analyzed. The first storm system (4 October 2006) generated a flash flood with almost no debris flows and landslides, the second (21 June 2007) triggered a large number of debris flows and was characterized by a relatively minor runoff response, and the third (4 September 2009) resulted in both a relevant flash flood response and debris flows in minor streams. A strong interest both for civil protection and research purposes has been devoted by local authorities and researchers to such events. The study methods include radar rainfall analysis, hydrological modeling and {GIS} processing of spatial rainfall data and debris-flow locations. Precise information on debris-flow location and related volumes were derived from a geo-spatial database of instability phenomena implemented and managed by the Autonomous Province of Bolzano. Patterns of rainfall distribution and relations between the main hydrological variables (cumulative rainfall, intensity and antecedent moisture) have been analyzed to explain differences in catchments responses between the three studied events. Radar rainfall data have permitted to analyze rainfall fields with high spatial resolution, taking into account also the elevation variability of rainfall rates. The striking responses' contrast among the three events is related to differences in antecedent moisture, space-time structure of the rainstorms, cumulative rainfall and intensity distributions and temperature regime. The frequency analysis of the main hydrological variables revealed to be a powerful tool capable of distinguishing, within a synoptic framework, the space-time-magnitude variability of the events, so as to highlight the differences in flood and debris-flow response
A semi-automatic derivation of channel network from high-resolution DTMs: the example of an italian alpine region
No full textHigh-resolution digital terrain models (HR-DTMs) of regional coverage open interesting scenarios for the analysis of landscape, including derivation and analysis of channel network. In this study, we present our experience in the derivation of the channel network from a HR-DTM for the Autonomous Province of Trento. This region (about 6200 km2) has a complex orography, with a high variety of lithotypes and different types of land use (forests, bare ground at the highest elevation, rocky outcrops and debris, mountain grasslands and agricultural areas mostly located in valley floors).
A preliminary automatic extraction of the raw channel network was conducted using a curvature-based morphological algorithm (Tarboton and Ames, 2001) applied to a 2 m resolution DTM derived from an airborne LiDAR survey carried out in 2006. This algorithm shows a good performance in terms of drainage density and channel heads location when preliminarily compared to available cartography.The raw channel network automatically extracted from the HR-DTM needs a supervised control to check the spatial pattern of the channels and to classify every reach of the hydrographic network. This analysis was carried out by means of different informative layers grouped into three main classes: i) geomorphometric indexes derived from HR-DTM encompassing 2 m resolution shaded relief map, openness (Yokoyama et al., 2002), local anomalies, and plan curvature; ii) high-resolution orthophoto imagery (panchromatic and near-infrared); iii) technical cartography (1:10000, 1:25000 scale).The result is an accurate and scale independent channel network definition of the study area that therefore needs to be complemented and corrected via field surveys and expert supervision, especially in urban areas.
Summary statistics are presented to relate the drainage density to different topological and geomorphological parameters
Regional-scale debris-flow modelization for hazard mapping in alpine basins using a high-resolution DTM and events geodatabase
No full textGIS tools for preliminary debris-flow assessment at regional scale
No full textThe assessment of the areas endangered by debris flows is a major issue in the context of mountain watershed management. Depending on the scale of analysis, different methods are required for the assessment of the areas exposed to debris flows. While 2-D numerical models are advised for detailed mapping of inundation areas on individual alluvial fans, preliminary recognition of hazard areas at the regional scale can be adequately performed by less data-demanding methods, which enable priority ranking of channels and alluvial fans at risk by debris flows. This contribution focuses on a simple and fast procedure that has been implemented for regional-scale identification of debris-flow prone channels and prioritization of the related alluvial fans. The methodology is based on the analysis of morphometric parameters derived from Digital Elevation Models (DEMs). Potential initiation sites of debris flows are identified as the DEM cells that exceed a threshold of slope-dependent contributing area. Channel reaches corresponding to debris flows propagation, deceleration and stopping conditions are derived from thresholds of local slope. An analysis of longitudinal profiles is used for the computation of the runout distance of debris flows. Information on erosion-resistant bedrock channels and sediment availability surveyed in the field are taken into account in the applications. A set of software tools was developed and made available (https://github.com/HydrogeomorphologyTools) to facilitate the application of the procedure. This approach, which has been extensively validated by means of field checks, has been extensively applied in the eastern Italian Alps. This contribution discusses potential and limitations of the method in the frame of the management of small mountain watersheds
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Response time and water origin in a steep nested catchment in the Italian Dolomites
Full text linkIn this study, we investigate the surface flow time of rise in response to rainfall and snowmelt events at different spatial scales and the main sources originating channel runoff and spring water in a steep nested headwater catchment (Rio Vauz, Italian Dolomites), characterized by a marked elevation gradient. We monitored precipitation at different elevations and measured water stage/streamflow at the outlet of two rocky subcatchments of the same size, representative of the upper part of the catchment dominated by outcropping bedrock, at the outlet of a soil-mantled and vegetated subcatchment of similar size but different morphology, and at the outlet of the main catchment. Hydrometric data are coupled with stable isotopes and electrical conductivity sampled from different water sources during five years, and used as tracers in end-member mixing analysis, application of two component mixing models and analysis of the slope of the dual-isotope regression line. Results reveal that times of rise are slightly shorter for the two rocky subcatchments, particularly for snowmelt and mixed rainfall/snowmelt events, compared to the soil-mantled catchment and the entire Rio Vauz Catchment. The highly-variable tracer signature of the different water sources reflects the geomorphological and geological complexity of the study area. The principal end-members for channel runoff and spring water are identified in rainfall and snowmelt, which are the dominant water sources in the rocky upper part of the study catchment, and soil water and shallow groundwater, which play a relevant role in originating baseflow and spring water in the soil-mantled and vegetated lower part of the catchment. Particularly, snowmelt contributes up to 64 ± 8% to spring water in the concave upper parts of the catchment and up to 62 ± 11% to channel runoff in the lower part of the catchment. These results offer new experimental evidences on how Dolomitic catchments capture and store rain water and meltwater, releasing it through a complex network of surface and subsurface flow pathways, and allow for the construction of a preliminary conceptual model on water transmission in snowmelt-dominated catchments featuring marked elevation gradients
Analisi idrologica della la piena improvvisa del 2 agosto 2014 in un piccolo bacino delle Prealpi Venete
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