Marco Borga

Abstract: This work analyses the prominent characteristics of flash flood regimes in two Mediterranean areas: the North-Western Mediterranean region, which includes Catalonia, France and Northern Italy, and the South-Eastern Mediterranean region, which includes Israel. The two regions are characterized by similarities in the hydro-meteorological monitoring infrastructure, which permits us to ensure homogeneity in the data collection procedures. The analysis is articulated into two parts. The first part is based on use of flood peak data, catchment area and occurrence date for 99 events (69 from the North-Western region and 30 from the South-Eastern region). Analysis is carried out in terms of relationship of flood peaks with catchment area and seasonality. Results show that the envelope curve for the South-Eastern region exhibits a more pronounced decreasing with catchment size with respect to the curve of the North-Western region. The differences between the two relationships reflect changes in the effects of storm coverage and hydrological characteristics between the two regions. Seasonality analysis shows that the events in the North-Western region tend to occur between August and November, whereas those in the South-Eastern area tend to occur in the period between October and May, reflecting the relevant patterns in the synoptic conditions leading to the intense precipitation events. In the second part, the focus is on the rainfall-runoff relationships for 13 selected major flash flood events (8 from the North-Western area and 5 from the South-Eastern area) for which rainfall and runoff properties are available. These flash floods are characterised in terms of climatic features of the impacted catchments, duration and amount of the generating rainfall, and runoff ratio. Results show that the rainfall duration is shorter and the rainfall depth lower in the South-Eastern region. The runoff ratios are rather low in both regions, whereas they are more variable in the South-Eastern area. No clear relationship between runoff ratio and rainfall depth is observed in the sample of floods, showing the major influence of rainfall intensity and the initial wetness condition in the runoff generation for these events.

Abstract: The low risk awareness of the residents living in flood-prone areas is usually considered among the main causes of their low preparedness, which in turns generates inadequate response to natural disasters. In this paper, we challenge this assumption by reporting on the results of a sociological research in four communities exposed to flood risk in the Eastern Italian Alps. The research design included semi-structured interviews and focus groups with key local stakeholders and a standardized questionnaire submitted to 400 residents. Results revealed that residents felt both slightly worried about flood risk and slightly prepared to face an event. Considerable differences were found between the evaluations of individual subjects as opposed to overall communities. There was also a clear discrepancy between the actual adoption of household preparatory measures and the willingness to take self-protection actions. Overall, the risk awareness was significantly higher among those residents who had been personally affected by a flood in the past, were living in isolated (vs. urban) communities, in the most risky areas or had a lower level of trust in local authorities. The improvement of residents' knowledge about their environment and the residual risk seemed to be crucial to increase risk awareness, and the same was true for the strengthening of local support networks to foster preparedness. The link between risk awareness and preparedness was not at all straightforward. Results revealed instead the complexity of residents' perspectives, attitudes, behaviours and decisions about risk-related issues.

Abstract: The 2 June 2008 flood-producing storm on the Starzel river basin in South-West Germany is examined as a prototype for organized convective systems that dominate the upper tail of the precipitation frequency distribution and are likely responsible for the flash flood peaks in Central Europe. The availability of high-resolution rainfall estimates from radar observations and a rain gauge network, together with indirect peak discharge estimates from a detailed post-event survey, provided the opportunity to study in detail the hydrometeorological and hydrological mechanisms associated with this extreme storm and the ensuing flood. Radar-derived rainfall, streamgauge data and indirect estimates of peak discharges are used along with a distributed hydrologic model to reconstruct hydrographs at multiple locations. Observations and model results are combined to examine two main questions, (i) assessment of the distribution of the runoff ratio for the 2008 flash flood and how it compares with other less severe floods; and (ii) analysis of how the spatial and temporal distribution of the extreme rainfall, and more specifically storm motion, controls the flood response. It is shown that small runoff ratios (less than 20%) characterized the runoff response and that these values are in the range of other, less extreme, flood events. The influence of storm structure, evolution and motion on the modeled flood hydrograph is examined by using the "spatial moments of catchment rainfall". It is shown that downbasin storm motion (in the range of 0.7-0.9 m s(-1)) had a noticeable impact on flood response by increasing the modeled flood peak by 13 %.

Abstract: No abstract available.

Abstract: This work analyses the estimation of winter season (October 1 to May 31) precipitation over a high altitude catchment (Val di Peio) of the Eastern Italian Alps. The extrapolation of precipitation over ungauged areas is problematic in this basin due to the combined effect of measurement errors and orographic effects. The study is based on the availability of long term series of weather data, snow observations and glacier mass balance measurements. The error in precipitation measurement at the uppermost weather station was assessed by comparison with snow water equivalent data. The error estimates were consistent with the outcomes of a precipitation correction model, with the aerodynamic correction as its main component. This correction procedure was used to compute correction factors for the entire precipitation gauge network. Both corrected and uncorrected precipitation data were used to estimate the spatial distribution of precipitation over the study area by means of a technique that accounts for the precipitation-altitude relationship. Winter balance observations over Careser glacier (the longest mass balance series in Italy), which is within the study area, were used to assess the improvement given by the precipitation correction procedure, showing a reduction of bias from -38% to 2.5% and a reduction of the RMSE from 410 to 171 mm water equivalent. The use of corrected precipitation data led to a 31.7% (182 mm) increase of the winter season basin-averaged precipitation, whereas the winter season average vertical gradient increased from 4.0% km(-1) to 21.9% km(-1). Overall, these results highlight the need for precipitation correction in precipitation analyses over snow-dominated mountain areas. This paper also provides evidence of a considerable interannual variability in the correction factors for snow at the uppermost weather station. Further improvements in precipitation estimations would require an analysis of the dominant processes controlling this variability.

Abstract: This letter assesses the quality of temperature and rainfall daily retrievals of the European Climate Assessment and Dataset (ECA&D) with respect to measurements collected locally in various parts of the Euro-Mediterranean region in the framework of the Hydrological Cycle in the Mediterranean Experiment (HyMeX), endorsed by the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research Program (WCRP). The ECA&D, among other gridded datasets, is very often used as a reference for model calibration and evaluation. This is for instance the case in the context of the WCRP Coordinated Regional Downscaling Experiment (CORDEX) and its Mediterranean declination MED-CORDEX. This letter quantifies ECA&D dataset uncertainties associated with temperature and precipitation intra-seasonal variability, seasonal distribution and extremes. Our motivation is to help the interpretation of the results when validating or calibrating downscaling models by the ECA&D dataset in the context of regional climate research in the Euro-Mediterranean region.

Abstract: This study investigates the role of soil moisture on the threshold runoff response in a small headwater catchment in the Italian Alps that is characterised by steep hillslopes and a distinct riparian zone. This study focuses on: (i) the threshold soil moisture-runoff relationship and the influence of catchment topography on this relation; (ii) the temporal dynamics of soil moisture, streamflow and groundwater that characterize the catchment's response to rainfall during dry and wet periods; and (iii) the combined effect of antecedent wetness conditions and rainfall amount on hillslope and riparian runoff. Our results highlight the strong control exerted by soil moisture on runoff in this catchment: a sharp threshold exists in the relationship between soil water content and runoff coefficient, streamflow, and hillslope-averaged depth to water table. Low runoff ratios were likely related to the response of the riparian zone, which was almost always close to saturation. High runoff ratios occurred during wet antecedent conditions, when the soil moisture threshold was exceeded. In these cases, subsurface flow was activated on hillslopes, which became a major contributor to runoff. Antecedent wetness conditions also controlled the catchment's response time: during dry periods, streamflow reacted and peaked prior to hillslope soil moisture whereas during wet conditions the opposite occurred. This difference resulted in a hysteretic behaviour in the soil moisture-streamflow relationship. Finally, the influence of antecedent moisture conditions on runoff was also evident in the relation between cumulative rainfall and total stormflow. Small storms during dry conditions produced low stormflow amounts, likely mainly from overland flow from the near saturated riparian zone. Conversely, for rainfall events during wet conditions, higher stormflow values were observed and hillslopes must have contributed to streamflow.

Abstract: This paper describes a set of spatial rainfall statistics (termed "spatial moments of catchment rainfall") quantifying the dependence existing between spatial rainfall organisation, basin morphology and runoff response. These statistics describe the spatial rainfall organisation in terms of concentration and dispersion statistics as a function of the distance measured along the flow routing coordinate. The introduction of these statistics permits derivation of a simple relationship for the quantification of catchment-scale storm velocity. The concept of the catchment-scale storm velocity takes into account the role of relative catchment orientation and morphology with respect to storm motion and kinematics. The paper illustrates the derivation of the statistics from an analytical framework recently proposed in literature and explains the conceptual meaning of the statistics by applying them to five extreme flash floods occurred in various European regions in the period 2002-2007. High resolution radar rainfall fields and a distributed hydrologic model are employed to examine how effective are these statistics in describing the degree of spatial rainfall organisation which is important for runoff modelling. This is obtained by quantifying the effects of neglecting the spatial rainfall variability on flood modelling, with a focus on runoff timing. The size of the study catchments ranges between 36 to 982 km(2). The analysis reported here shows that the spatial moments of catchment rainfall can be effectively employed to isolate and describe the features of rainfall spatial organization which have significant impact on runoff simulation. These statistics provide useful information on what space-time scales rainfall has to be monitored, given certain catchment and flood characteristics, and what are the effects of space-time aggregation on flood response modeling.

Abstract: The sensitivities of runoff generation to rainfall variability and initial wetness conditions were examined for a major flash flood event that occurred during August 29, 2003 on the upper Tagliamento river basin in the eastern Italian Alps, The Triangulated Irregular Network (TIN)-based Real-time Integrated Basin Simulator (CRIBS) distributed hydrologic model was used to simulate the hydrologic response over a range of sub-basins. The model was calibrated for the single flash flood event based on the observed hydrograph at the outlet of the basin examined (Fella basin) and was validated based on the available observed hydrographs at interior points of the basin. A series of hydrologic simulations were performed for different initial soil moisture conditions and rainfall forcing resolutions in order to evaluate the sensitivity of runoff generation to those variables. Evaluation of the results suggests that both initial wetness and rainfall resolution affect significantly the simulated peak flow and runoff volume during the flash flood event. Sensitivity to initial wetness exhibits a scale dependence with the sensitivity increasing with basin scale. The bias introduced to the basin-averaged rainfall due to aggregation had a significant effect on runoff generation for all basin scales, while the effect of variability smoothing was important only for the larger scale basins. Finally, the sensitivity of the flood hydrograph to rainfall aggregation was shown to be more important for drier initial wetness states. (C) 2010 Elsevier B.V. All rights reserved.

Abstract: We present an index-based shallow landsliding susceptibility model which allows explicit incorporation of local heavy rainfall statistical properties. The model, called Quasi-Dynamic Shallow Landsliding Model (QD-SLaM), is developed upon a theory for coupled shallow subsurface flow and landsliding of the soil mantle. The model uses a 'quasi-dynamic' wetness index to predict the spatial distribution of soil saturation in response to a rainfall of specified duration, and can take into account the spatial variability of soil properties. The rainfall predicted to cause instability in each topographic element is characterized by intensity and duration. The incorporation of a scaling model for the rainfall frequency-duration relationship provides a parsimonious and robust way to include heavy rainfall statistical properties into shallow landsliding modeling. The model is used to determine the return period of the critical rainfall needed to cause instability for each topographic element. The model is validated over six different study sites, where detailed inventories of shallow landslides are available. Two study sites are located in the north of Taiwan, and four are located in the Italian Alps. The sites are characterized by different climates and by different duration of the landslide-triggering storms. Model results are evaluated against the surveyed landslides and compared to those obtained by using a steady-state model, resembling SHALSTAB. It is shown that QD-SLaM improves significantly over the steady-state approach in predicting existing landslides as represented in the considered landslide inventory. Moreover, the improvement is higher for the cases where the landslide-triggering storm duration is short with respect to the length of time required for every point on a catchment to reach subsurface drainage equilibrium. The results of our work highlight the capability of the model to incorporate a robust description of the heavy rainfall properties in the analysis and mapping of shallow landsliding susceptibility by using an index-style approach. (C) 2011 Elsevier B.V. All rights reserved.

Abstract: The management of flash flood hazards and risks is a critical component of public safety and quality of life. Flash-floods develop at space and time scales that conventional observation systems are not able to monitor for rainfall and river discharge. Consequently, the atmospheric and hydrological generating mechanisms of flash-floods are poorly understood, leading to highly uncertain forecasts of these events. The objective of the HYDRATE project has been to improve the scientific basis of flash flood forecasting by advancing and harmonising a European-wide innovative flash flood observation strategy and developing a coherent set of technologies and tools for effective early warning systems. To this end, the project included actions on the organization of the existing flash flood data patrimony across Europe. The final aim of HYDRATE was to enhance the capability of flash flood forecasting in ungauged basins by exploiting the extended availability of flash flood data and the improved process understanding. This paper provides a review of the work conducted in HYDRATE with a special emphasis on how this body of research can contribute to guide the policy-life cycle concerning flash flood risk management. (C) 2011 Elsevier Ltd. All rights reserved.

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Abstract: A generalised framework of space-time variability in flood response is used to characterise five flood events of different type in the Kamp area in Austria one long-rain event two short-rain events one rain-on-snow event and one snowmelt event Specifically the framework quantifies the contributions of the space-time variability of rainfall/snowmelt runoff coefficient hillslope and channel routing to the flood runoff volume and the delay and spread of the resulting hydrograph The results indicate that the components obtained by the framework clearly reflect the individual processes which characterise the event types For the short-rain events temporal spatial and movement components can all be important in runoff generation and routing which would be expected because of their local nature in time and particularly in space For the long-ram event the temporal components tend to be more important for runoff generation because of the more uniform spatial coverage of rainfall while for routing the spatial distribution of the produced runoff which is not uniform is also important For the rain-on-snow and snowmelt events the spatio-temporal variability terms typically do not play much role in runoff generation and the spread of the hydrograph is mainly due to the dui anon of the event As an outcome of the framework a dimensionless response number is proposed that represents the Joint effect of runoff coefficient and hydrograph peakedness and captures the absolute magnitudes of the observed flood peaks (C) 2010 Elsevier B V All rights reserved

Abstract: This study alms to assess the feasibility of assimilating carefully checked radar rainfall estimates into a numerical weather prediction (NWP) to extend the forecasting lead time for an extreme flash flood The hydro-meteorological modeling chain Includes the convection-permitting NWP model COSMO-2 and a coupled hydrological-hydraulic model Radar rainfall estimates are assimilated into the NWP model via the latent heat nudging method The study is focused on 26 September 2007 extreme flash flood which Impacted the coastal area of North-eastern Italy around Venice The hydro-meteorological modeling system is implemented over the 90 km(2) Dese river basin draining to the Venice Lagoon The radar rainfall observations are carefully checked for artifacts including rain-induced signal attenuation by means of physics-based correction procedures and comparison with a dense network of raingauges The impact of the radar rainfall estimates in the assimilation cycle of the NWP model is very significant The main individual organized convective systems are successfully Introduced into the model state both in terms of timing and localization Also high-intensity incorrectly localized precipitation is correctly reduced to about the observed levels On the other hand the highest rainfall intensities computed after assimilation underestimate the observed values by 20% and 50% at a scale of 20 km and 5 km respectively The positive impact of assimilating radar rainfall estimates is carried over into the free forecast for about 2-5 h depending on when the forecast was started The positive impact is larger when the main mesoscale convective system is present in the initial conditions The improvements in the precipitation forecasts are propagated to the river flow simulations with an extension of the forecasting lead time up to 3 h (C) 2010 Elsevier B V All rights reserved

Abstract: The aim of this study was to analyse the reproducibility of off-axis integrated cavity output spectroscopy (OA-ICOS)-derived delta H-2 and delta O-18 measurements on a set of 35 water samples by comparing the performance of four laser spectroscopes with the performance of a conventional mass spectrometer under typical laboratory conditions. All samples were analysed using three different schemes of standard/sample combinations and related data processing to assess the improvement of results compared with mass spectrometry. The repeatability of the four OA-ICOS instruments was further investigated by multiple analyses of a sample subset to evaluate the stability of delta H-2 and delta O-18 measurements. </br > Results demonstrated an overall agreement between OA-ICOS-based and mass spectrometry-based measurements for the entire dataset. However, a certain degree of variability existed in precision and accuracy between the four instruments. There was no evident bias or systematic deviations from the mass spectrometer values, but random errors, which were apparently not related to external factors, significantly affected the final results. Our investigation revealed that analytical precision ranged +/- from +/- 0.56 parts per thousand to +/- 1.80 parts per thousand for delta H-2 and from +/- 0.10 parts per thousand to +/- 0.27 parts per thousand for delta O-18 measurements, with a marked variability among the four instruments. The overall capability of laser instruments to reproduce stable results with repeated measurements of the same sample was acceptable, and there were general differences within the range of the analytical precision for each spectroscope. Hence, averaging the measurements of three identical samples led to a higher degree of accuracy and eliminated the potential for random deviations.

Abstract: This paper aims to clarify the dependence existing between spatial rainfall organisation basin morphology and runoff response This is obtained by applying a spatial rainfall metric which describes the spatial rainfall organisation in terms of concentration and dispersion statistics as a function of the flow travel time measured along the river network The metric is based on the observation that runoff routing through branched channel networks imposes an effective averaging of spatial rainfall excess at equal travel time in spite of the inherent spatial variability High resolution radar rainfall fields and a distributed hydrologic model are employed to examine how effective are these statistics in describing the degree of spatial rainfall organisation which is important for runoff modelling and in quantifying the effects of neglecting the spatial rainfall variability on flood modelling The investigation focuses on three extreme flash flood events occurred on the Carpathian range (Romania) in the period 2005-2007 The size of the study catchments ranges between 36 and 167 km(2) The analysis reported here shows that neglecting the spatial rainfall variability results in a considerable loss of simulation Nash-Sutcliffe (NS) efficiency in almost 30% of the cases (NS less than 0 8) with NS less than 0 6 in one of the cases This provides a significant documentation of the Influence of the spatial rainfall variability on runoff modelling for catchment size less than 160 km(2) Moreover It is shown that these rainfall statistics used in combination are able to isolate and describe the features of rainfall spatial organisation which have significant Impact on runoff simulation An alternative rainfall spatial variability index which is computed irrespective of flow travel time structure provides a comparatively poor description of the influence of neglecting rainfall spatial variability on flood modelling Overall this implies that rainfall organisation measured along the river network by using the travel time coordinate may be a significant property of rainfall spatial variability when considering flood response modelling (C) 2010 Elsevier B V All rights reserved

Abstract: The study presents a data-based numerical experiment performed to understand the scale relationships of the error propagation of satellite rainfall for flood evaluation applications in complex terrain basins. A satellite rainfall error model is devised to generate rainfall ensembles based on two satellite products with different retrieval accuracies and space-time resolutions. The generated ensembles are propagated through a distributed physics-based hydrologic model to simulate the rainfall-runoff processes at different basin scales. The resulted hydrographs are compared against the hydrograph obtained by using high-resolution radar rainfall as the "reference" rainfall input. The error propagation of rainfall to stream runoff is evaluated for a number of basin scales ranging between 100 and 1200 km(2). The results from this study show that (i) use of satellite rainfall for flood simulation depends strongly on the scale of application (catchment area) and the satellite product resolution, (ii) different satellite products perform differently in terms of hydrologic error propagation, and (iii) the propagation of error depends on the basin size; for example, this study shows that small watersheds (<400 km(2)) exhibit a higher ability in dampening the error from rainfall to runoff than larger-sized watersheds, although the actual error increases as drainage area decreases.

Abstract: A Mesoscale Convective System in North-Western Slovenia produced up to 350-400 mm in 12 h on 18 September 2007 The region impacted by the storm shows significant differences in climatic and geologic properties at short distances Owing to such variability extreme flooding concentrated over the Selska Sora watershed at Zelezniki (103 3 km(2)) outside the area which received the highest precipitation Hydrometeorological analyses of the storm are based on accurate analysis of C-band weather-radar observations and data from a rain gauge network Detailed surveys of high-water marks and channel/floodplain geometry carried out two months after the flood are used for hydrologic analyses of the Selska Sora flood These Include estimation of peak discharge at 21 sites Unit peak discharges range from 5 to 7 m(3) s(-1) km(-2) in basins characterised by size up to approximately 25 km(2) Higher unit peak discharges (>10 m(3) s(-1) km(-2)) estimated in a few smaller basins are influenced by Intense sediment transport Observed rainfall estimated peak discharges and observer notes on timing of peak discharge are used along with a distributed hydrologic model to reconstruct hydrographs at multiple locations Examination of the rainfall distribution and flood response shows that the extent and the position of the karst terrain provided a major control on flood response in the region impacted by the storm Use of the distributed hydrological model together with the post-flood survey observations is shown to provide an accurate description of the flood Water balance and response time characteristics are examined for selected catchments showing that event runoff coefficient ranged between 17% and 24% for different catchments The quality of the peak discharge simulation at the 21 surveyed sites is substantially degraded when using spatially-uniform rainfall over the area covering all the surveyed sub-catchments mainly due to rainfall volume errors Introduced by using the spatially uniform value On the other hand the influence of rainfall spatial averaging at the scale of the sub-catchments generally has a very limited effect on runoff modelling showing that rainfall spatial organisation was not able to overcome the catchment dampening effect for this flood (C) 2010 Elsevier B V All rights reserved

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Abstract: A survey has been conducted to understand what are the perceived barriers to the free exchange of hydrometeorological data in Europe A total of 127 questionnaires were received of which 61 were completed by data providers and 66 by data users in 32 Europe in countries with a total 631 entries (i e assessments of barriers affecting one data type) in the questionnaires The responses have been analysed in terms of what barriers are perceived to exist whether there ire differences between research industry and administration between the West and East of Europe and between different data types The responses suggest that the most important barriers are of economic nature The majority of the data users think there exist economic barriers to the free exchange of the data (significant at the 0 01% level) while the data providers give mixed results Out of the types of institutions the research institutions give the most significant response of the existence of economic barriers followed by Industry and administration For the East European countries economic barriers are considered a much more serious problem than for the West Out of the data types surveyed precipitation and get spatial data are considered to be the most critical in terms of costs The differences between the perceptions of data providers and data users depend strongly on the type of barrier The differences are smallest for legal barriers (such as licensing of data) followed by the economic barriers and are largest for the practical barriers Conflict of Interest is another potential barrier examined in the survey Suggestions are given on how to address the economic barriers in a European context (C) 2010 Elsevier B V All rights reserved

Abstract: Different relations between surface rainfall rate R and high-resolution polarimetric X-band radar observations were evaluated using a dense network of rain gauge measurements over complex terrain in Central Italian Alps The specific differential phase shift K(DP) rainfall algorithm (R(KDP)) although associated with low systematic error It exhibits low sensitivity to the spatial variability of rainfall as compared to the standard algorithm (R(STD)) that is based on the reflectivity-to-rainfall (Z-R) relationship On the other hand the dependence of the reflectivity measurement on the absolute radar calibration and the ram-path radar signal attenuation introduces significant systematic error on the Rem rainfall estimates The study shows that adjusting the Z-R relationship for mean-field bias determined using the R(KDP) estimates as reference is the best technique for acquiring unbiased radar-rainfall estimates at fine space-time scales Overall the bias of the R(KDP)-adjusted Z-R estimator is shown to be lower than 10% for both storm cases while the relative root-mean-square error is shown to range from 0 6 (convective storm) to 09 (stratiform storm) A vertical rainfall profile correction (VPR) technique is tested in this study for the stratiform storm case The method is based on a newly developed VPR algorithm that uses the X-band polarimetric information to identify the properties of the melting layer and devices a precipitation profile that vanes for each radar volume scan to correct the radar-rainfall estimates Overall when accounting for the VPR effect there is up to 70% reduction in the systematic error of the 3 degrees elevation estimates while the reduction in terms of relative root-mean-square error is limited to within 10% (C) 2010 Elsevier B V All rights reserved

Abstract: The aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine-Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes This is supported by cluster analyses to identify areas of similar flood processes both in terms of precipitation forcing and catchment processes The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt In many regions of the Alpine-Carpathian range there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation (C) 2010 Elsevier B V All rights reserved

Abstract: High-resolution data enabling identification and analysis of the hydrometeorological causative processes of flash floods have been collected and analysed for 25 extreme flash floods (60 drainage basins ranging in area from 95 to 1856 km(2)) across Europe Most of the selected floods are located in a geographical belt crossing Europe from western Mediterranean (Catalunia and southwestern France) to Black Sea covering northern Italy Slovenia Austria Slovakia and Romania Criteria for flood selection were high intensity of triggering rainfall and flood response and availability of high-resolution reliable data Hydrometeorological data collected and collated for each event were checked by using a hydrological model The derivation and analysis of summarising variables based on the data archive has made it possible to outline some characteristics of flash floods in various morphoclimatic regions of Europe Peak discharge data for more than 50% of the studied watersheds derive from post-flood surveys in ungauged streams This stresses both the significance of post flood surveys in building and extending flash flood data bases and the need to develop new methods for flash flood hazard assessment able to take Into account data from post-event analysis Examination of data shows a peculiar seasonality effect on flash flood occurrence with events in the Mediterranean and Alpine-Mediterranean regions mostly occurring in autumn whereas events in the inland Continental region commonly occur in summer revealing different climatic forcing Consistently with this seasonality effect spatial extent and duration of the events is generally smaller for the Continental events with respect to those occurring in the Mediterranean region Furthermore the flash flood regime is generally more intense in the Mediterranean Region than in the Continental areas The runoff coefficients of the studied flash floods are usually rather low (mean value 0 35) Moderate differences in runoff coefficient are observed between the studied climatic regions with higher values in the Mediterranean region Antecedent saturation conditions have a significant impact on event runoff coefficients showing the influence of initial soil moisture status even on extreme flash flood events and stressing the importance of accounting soil moisture for operational flash flood forecasting The runoff response displays short lag times (mostly <6 h) The identified relations between watershed area stream length and response time enable determination of a characteristic mean velocity of the flash flood process (at basin scales less than 350 km(2)) defined as the ratio of characteristic length (mean river length) and time (response time or lag time) equal to 3 m s(-1) This is related to the celerity with which the flood wave moves through the catchment The analysis of the response time provides information on the time resolution and the spatial density of the networks required for monitoring the storms that generate flash floods (C) 2010 Elsevier B V All rights reserved

Abstract: The objective of this paper is to examine how Current techniques for flash-flood monitoring and forecasting can meet the needs of the Population at risk to evaluate the flood severity and anticipate its danger. To this end, the social response time for different social actions in the course of two well studied flash flood events which Occurred ill France and Italy is identified. The event management activities are broadly characterized into three types according to their main objective (information, organisation and protection). The activities are also classified into three other types according, to the scale and nature of the human group involved (individuals, communities and institutions). The conclusions reached relate to (1) the characterisation of the social responses according to watershed scale and to the information available, and (2) to the appropriateness of the existing surveillance and forecasting tools to Support the social responses. Results suggest that representing the dynamics of the social response with just one number representing the average time for warning a population is an oversimplification. It appears that the social response time exhibits a parallel with the hydrological response time by diminishing in time with decreasing size of the relevant watershed. A second result is that the human groups have different capabilities of anticipation, apparently based oil the nature of information they use. Comparing watershed response times and social response times shows clearly that at scales of less than 100 km(2), a number of actions were taken with response times comparable to the catchment response time. The implications for adapting the warning processes to social scales (individual or organisational scales) are considerable. At small scales, and for the implied anticipation times. the reliable and high-resolution description of the actual rainfall field becomes the major Source of information for decision-making processes Such as deciding between evacuations or advising to stay home. This points to the need to improve the accuracy and quality control of real time radar rainfall data, especially for extreme flash flood-generating storms. Copyright (C) 2009 Royal Meteorological Society

Abstract: In this study we analyse space-time variability of soil moisture data collected at 0-6, 0-12 and 0-20 cm depth over three hillslopes with contrasting steep relief and shallow soil depth in the Dolomites (central-eastern Italian Alps). The data have been collected during two summer seasons (2005 and 2006) with different precipitation distribution. Analysis of soil moisture data shows that different physical processes control the space-time distribution of soil moisture at the three soil depths, with a marked effect of dew on the 0-6 cm soil depth layer. The range of skewness values decreases markedly from the surface to deeper layers. More symmetric distributions, characterised by relatively low skewness, are found for mid-range soil moisture contents, while highly skewed distributions (generally with more log-normal shape) are found at dry and wet conditions. Scatter plots drawn for the whole data set and the analysis of the correlation coefficients suggest a good persistence of soil moisture with depth: the highest degree of correlation was observed between data collected at 0-12 and 0-20 cm. Examination of correlation between soil moisture fields and topographical attributes shows that, notwithstanding the steep relief and the humid conditions, terrain indices are relatively poor predictors of soil moisture spatial variability. The slope and the topographic wetness index, which are found here the best univariate spatial predictors of soil moisture, explains up to 42% of the time-averaged moisture spatial variation. A negative relationship between the soil moisture spatial mean and the corresponding spatial standard deviation is found for mean water contents exceeding 25-30%, while a transition to a positive relationship is observed with drier conditions. Overall, soil moisture variability shows the highest values at moderate moisture conditions (23-29%) and reduced values for wetter and drier conditions for all depths. A negative linear relationship between mean soil moisture content and the coefficient of variation was observed. A soil moisture dynamics model proved to successfully capture the soil moisture variability at the hillslope scale. The simulated time series of hillslope-averaged soil moisture are in good agreement with the observed ones. Moreover, the model reproduces consistently the observed relationships between soil moisture spatial mean and corresponding variability. (c) 2008 Elsevier B.V. All rights reserved.

Abstract: High resolution radar rainfall fields and a distributed hydrologic model are used to evaluate the sensitivity of flash flood simulations to spatial aggregation of rainfall at catchment scales ranging from 10.5 km(2) to 623 km(2). The case study focuses on the extreme flash flood occurred on 29 August 2003 on the eastern Italian Alps. Four rainfall spatial resolutions are considered, with grid size equal to 1-, 4-, 8- and 16-km. The influence of rainfall spatial aggregation is examined by using the flow distance as a spatial coordinate, hence emphasising the role of river network in the averaging of space-time rainfall. Effects of rainfall spatial aggregation are quantified by using a dimensionless parameter, represented by the ratio of rainfall resolution (L(r)) to the characteristic basin length (L(w)), taken as the square root of the watershed area. Increasing the L(r)/L(w) parameter induces large errors on the simulated peak discharge, with values of the peak discharge error up to 0.33 for L(r)/L(w) equal to 1.0. An important error source related to spatial rainfall aggregation is the rainfall volume error caused by incorrectly smoothing the rainfall volume either inside or outside of of the watershed. It is found that for L(r)/L(w)<1.0, around 50% of the peak discharge error is due to the rainfall volume error. Remaining errors are due to both the distortion of the rainfall spatial distribution, measured with respect to the river network, and to the reduced spatial variability of the rainfield. Further investigations are required to isolate and examine the effect of river network geometry on the averaging of space-time rainfall at various aggregation lengths and on simulated peak discharges.

Abstract: This paper investigates the use of the Flash Flood Guidance (FFG) method and a method of model-based threshold runoff computation to improve the accuracy of flash flood forecasts at ungauged locations. The methodology proposed in this paper requires running a lumped hydrological model to derive flood frequencies at the outlet of the ungauged basin under consideration, and then to derive the threshold runoff from these model-based discharges. The Study examines the potential of this method to account for the hydrological model's Uncertainty and for biases originated by lack of model calibration. which is the typical condition in ungauged basins. Experiments to validate this approach involve the implementation of a semi-distributed Continuous rainfall-runoff model and the operation of the FFG method over four basins located in the central-eastern Italian Alps and ranging in size from 75.2 to 213.7 km(2). The model is calibrated on two larger basins and the model parameters are transposed to the other two basins to Simulate operations in ungauged basins. The FFG method is applied by using the 2 year discharge as the threshold runoff. The threshold runoff is derived both by using local discharge statistics and the model-based approach advocated here. Examination of the results obtained by this comparison shows that the use of model-based threshold leads to improvements in both gauged and ungauged situations. Overall, the Critical Success Index (CSI) increases by 12% for gauged basins and by 31% for ungauged basins by using the model-based threshold with respect to use of local data. As expected, the increase of CSI is more remarkable for ungauged basins, due to lack of local model calibration and the greater likelihood of occurrence of a Simulation bias in model application over these basins. This shows that the method of threshold runoff computation provides an inherent bias correction to reduce systematic errors in model applications to ungauged (and gauged) basins. Copyright (C) 2009 Royal Meteorological Society

Abstract: This article is dedicated to radar rainfall estimation for the post-event analysis of a flash flood that occurred on 18 September 2007 in Slovenia. The utility of the Mountain Reference Technique is demonstrated to quantify rain attenuation effects that affect C-band radar measurements in heavy rain. Maximum path-integrated attenuation between 15 and 20 dB were estimated thanks to mountain returns for path-averaged rain rates between 10 and 15 mm h(-1) over a 120-km path. Assuming the reflectivity-attenuation relationship to be known, the proposed technique allows for estimating an effective radar calibration correction factor to be accounted for in the parameterization of the attenuation correction. Screening effects are quantified using a geometrical calculation based on a digitized terrain model of the region. The vertical structure of the reflectivity is modeled with a normalized apparent vertical profile of reflectivity. Implementation of the radar data processing indicates that: (1) the combined correction for radar calibration and attenuation effects allows for obtaining satisfactory radar rain estimates (Nash criterion of 0.8 at the event time scale); (2) due to the attenuation equation instability, it is however compulsory to limit the maximum path-integrated attenuation to be corrected to about 10 dB; (3) the results also prove to be sensitive on the parameterization of reflectivity-attenuation-rainrate relationships.

Abstract: High resolution radar rainfall fields and a distributed hydrologic model are used to evaluate the sensitivity of flood and flash flood simulations to spatial aggregation of rainfall and soil properties at catchment scales ranging from 75 to 983 km(2). Hydrologic modeling is based on a Hortonian infiltration model and a network-based representation of hillslope and channel flow. The investigation focuses on three extreme flood and flash flood events occurred on the Sesia river basin, North Western Italy, which are analysed by using four aggregation lengths ranging from 1 to 16 km. The influence of rainfall spatial aggregation is examined by using the flow distance as a spatial coordinate, hence emphasising the role of river network in the averaging of space-time rainfall. The effects of reduced and distorted rainfall spatial variability on peak discharge have been found particularly severe for the flash flood events, with peak errors up to 35% for rainfall aggregation of 16 km and at 983 km(2) Catchment size. Effects are particularly remarkable when significant structured rainfall variability combines with relatively important infiltration volumes due to dry initial conditions, as this emphasises the non-linear character of the rainfall-runoff relationship. In general, these results confirm that the correct estimate of rainfall volume is not enough for the accurate reproduction of flash flood events characterised by large and structured rainfall spatial variability, even at catchment scales around 250 km(2). However, accurate rainfall volume estimation may suffice for less spatially variable flood events. Increasing the soil properties aggregation length exerts similar effects on peak discharge errors as increasing the rainfall aggregation length, for the cases considered here and after rescaling to preserve the rainfall volume. Moreover, peak discharge errors are roughly proportional to runoff volume errors, which indicates that the shape of the flood wave is influenced in a limited way by modifying the detail of the soil property spatial representation. Conversely, rainfall aggregation may exert a pronounced influence on the discharge peak by reshaping the spatial organisation of the runoff volumes and without a comparable impact on the runoff volumes. (C) 2008 Elsevier Ltd. All rights reserved.

Abstract: On 29 August, 2003. an intense convective storm system affected the Fella River basin, in the eastern Italian Alps, producing rainfall peaks of approximately 390 mm in 12 h. The storm triggered an unusually large debris flow in the ungauged Rio Cucco basin (0-65 km(2)), with a volume of approximately 78000 m(3). The analysis of the time evolution of the rainstorm over the basin has been based on rainfall estimates from radar observations and data recorded by a raingauge network. Detailed geomorphological field surveys, carried out both bel ore and after the flood of August 2003, and the application of a distributed hydrological model have enabled assessment of flood response, estimation of erosion volumes and sediment supply to the channel network. The accounts of two eyewitnesses have provided useful elements for reconstructing the time evolution and the flow processes involved in the event. Liquid peak discharge estimates cluster around 20 m(3) s(-1) km(-2), placing this event on the flood envelope curve for the eastern Italian Alps. The hydrological analysis has shown that the major controls of the flood response were the exceptional cumulated rainfall amount, required to exceed the large initial losses, and the large rainfall intensities at hourly temporal scales, required to generate high flood response at the considered basin scale. Observations on the deposits accumulated on the alluvial Pan indicate that, although the dominant flow process was a debris flow, sheetflood also contributed to fan aggradation and fluvial reworking had an important role in winnowing debris-flow lobes and redistributing sediment on the fan surface. This points out to the large discharge values during the recession phase of the flood, implying an important role for subsurface flow on runoff generation of this extreme flash flood event. Copyright (c) 2009 John Wiley & Sons. Ltd.

Abstract: Spatial and temporal scales of occurrence of flash floods, combined with the space and time scales of conventional measurement networks of rain and discharge, make these events particularly difficult to observe. The effective documentation of flash floods requires post-flood survey strategies encompassing accurate radar rainfall estimation, field observations of the geomorphic processes associated with the flood, indirect reconstruction of peak discharges and interviews of eyewitnesses. This paper describes the methods applied and the results achieved in the survey of a flash flood that occurred on 18th September 2007 in the Selska Sora watershed (Western Slovenia). Hydrometeorological analyses of the storm are based on radar reflectivity observations. The documentation of the flash flood reveals high peak flood discharges and a complex flood response. Peak discharges were estimated at 22 cross sections, with drainage areas ranging from 0.2 to 147 km(2). Among the lessons learned from the field study of the Selska Sora flash flood, there are three key conclusions that can inform similar studies. Firstly, geomorphological surveys are an important prerequisite for flood discharge reconstruction in mountainous watersheds affected by debris flow and intense sediment transport. Secondly, the accounts of eyewitnesses of the flood provide a unique contribution to event reconstruction. Finally, it is necessary to have quality controlled weather radar data, which may permit coupling field observations with rainfall-runoff modelling. Copyright (C) 2009 John Wiley & Sons, Ltd.

Abstract: Flash floods are one of the most significant natural hazards in Europe, causing serious risk to life and destruction of buildings and infrastructure. This type of flood, often affecting ungauged watersheds, remains nevertheless a poorly documented phenomenon. To address the gap in available information, and particularly to assess the possible ranges for peak discharges on watersheds with area smaller than 500 km(2) and to describe the geography of the hazard across Europe, an intensive data compilation has been carried out for seven European hydrometeorological regions. This inventory is the first step towards an atlas of extreme flash floods in Europe. It contains over 550 documented events. This paper aims at presenting the data compilation strategy, the content of the elaborated data base and some preliminary data analysis results. The initial observations show that the most extreme flash floods are greater in magnitude in the Mediterranean countries than in the inner continental countries and that there is a strong seasonality to flash flood occurrence revealing different climatic forcing mechanisms in each region. (C) 2009 Elsevier B.V. All rights reserved.

Abstract: Analyses of event runoff coefficients provide essential insight on catchment response, particularly if a range of catchments and a range of events are compared by a single indicator. In this study we examine the effect of climate, geology, land use, flood types and initial soil moisture conditions on the distribution functions of the event runoff coefficients for a set of 14 mountainous catchments located in the eastern Italian Alps, ranging in size from 7.3 to 608.4 km(2). Runoff coefficients were computed from hourly precipitation, runoff data and estimates of snowmelt. A total of 535 events were analysed over the period 1989-2004. We classified each basin using a "permeability index" which was inferred from a geologic map and ranged from "low" to "high permeability". A continuous soil moisture accounting model was applied to each catchment to classify 'wet' and 'dry' initial soil moisture conditions. The results indicate that the spatial distribution of runoff coefficients is highly correlated with mean annual precipitation, with the mean runoff coefficient increasing with mean annual precipitation. Geology, through the 'permeability index', is another important control on runoff coefficients for catchments with mean annual precipitation less than 1200 mm. Land use, as indexed by the SCS curve number, influences runoff coefficient distribution to a lesser degree. An analysis of the runoff coefficients by flood type indicates that runoff coefficients increase with event snowmelt. Results show that there exists an intermediate region of subsurface water storage capacity, as indexed by a flow-duration curve-based index, which maximises the impact of initial wetness conditions on the runoff coefficient. This means that the difference between runoff coefficients characterised by wet and dry initial conditions is negligible both for basins with very large storage capacity and for basins with small storage capacity. For basins with intermediate storage capacities, the impact of the initial wetness conditions may be relatively large. (C) 2009 Elsevier B.V. All rights reserved.

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Abstract: The objective of this work is to analyse the storage-flux hysteretic behaviour of a simplified model for subsurface flow processes. The subsurface flow dynamics is analysed by means of a model based on the kinematic wave assumptions and by using a width weighting/depth averaging scheme which allows to map the three-dimensional soil mantle into a one-dimensional profile. Continuity and a kinematic form of Darcy's law lead to a hillslope-storage kinematic wave equation for subsurface flow, solvable with the method of characteristics. Adopting a second order polynomial function to describe the bedrock slope and an exponential function to describe the variation of the width of the hillslope with hillslope distance, we derive general solutions to the hill slope-storage kinematic wave equations, applicable to a wide range of hillslopes. These solutions provide a physical basis for deriving two geometric parameters alpha and psi which define the hydrological similarity between hillslopes with respect to their characteristic response and hysteresis. The hysteresis eta, quantified by the area of the hysteretic dimensionless loop, has been therefore computed for a range of values of parameters alpha and psi. Slopes exhibit generally clockwise hysteretic loop in the flux-storage plot, with higher groundwater mean volume for given discharge on rising limb than at same discharge on falling limb. It has been found that hysteresis increases with decreasing alpha and psi, i.e. with increasing convergence (for the shape) and concavity (for the profile), and vice versa. For relatively large values of alpha and psi the hysteresis may take a complex pattern, with combination of clockwise to anticlockwise loop cycles. Application of the theory to three hillslopes in the Eastern Italian Alps provides an opportunity to examine how natural topographies are represented by the two hillslope hydrological similarity parameters. (c) 2007 Elsevier Ltd. All rights reserved.

Abstract: A model for the prediction of topographic and climatic control on shallow landsliding in mountainous terrain is enhanced to analyse the impact of upslope rocky outcrops on downslope shallow landsliding. The model uses a 'generalised quasi-dynamic wetness index' to describe runoff propagation on bare rock surfaces connected to downslope soil-mantled topographic elements. This approach yields a simple enhanced model capable of describing the influence of upslope bedrock outcrops on the pattern of downslope soil saturation. The model is applied in both diagnostic and predictive modes to a small catchment in the eastern Italian Alps for which a detailed inventory of shallow landslides in areas dominated by rocky outcrops is available. In the diagnostic mode, the model is used with satisfactory results to reproduce the pattern of instability generated by an intense short-duration storm occurred on 14 September 1994, which triggered a large percentage of the surveyed landslides. In the predictive mode, the model is used for hazard assessment, and the return time of the critical rainfall needed to cause instability for each topographic element is determined. Modelling results obtained in the predictive mode are evaluated against all the surveyed landslides. It is revealed that the generalised quasi-dynamic model offers considerable improvement over the non-generalised quasi-dynamic model and the steady-state model in predicting existing landslides as represented in the considered landslide inventory. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: This paper presents the evidence gathered during a post-flood survey in a Slovenian mountain basin (Davca basin, drainage area of 32 km 2) following the catastrophic flood of September 2007. Channel avulsion, debris flows and landslides delivered large volumes of wood into the channel, and massive wood accumulations were found at the basin outlet. Wood-induced dam-break flows were therefore hypothesised to be responsible for most of the damages. Field observations and approximate discharge estimations indicate that the damages suffered in the Davca basin can be attributed to excessive wood load only to a limited extent, and that the critical factors were narrow road crossings which acted as traps for sediment and wood.

Abstract: The main objective of this paper is to evaluate a threshold-based flash flood warning method, by considering a wide range of climatic and physiographic conditions, and by focusing on ungauged basins. The method is derived from the flash flood guidance (FFG, hereafter) approach. The FFG is the depth of rain of a given duration, taken as uniform in space and time on a certain basin, necessary to cause minor flooding at the outlet of the considered basin. This rainfall depth, which is computed based on a hydrological model, is compared to either real-time-observed or forecasted rainfall of the same duration and on the same basin. If the nowcasted or forecasted rainfall depth is greater than the FFG, then flooding in the basin is considered likely. The study provides an assessment of this technique based on operational, quality data from 11 mountainous basins (six nested included in five larger parent basins) located in north-eastern Italy and central France. The model used in this study is a semi-distributed conceptual rainfall-runoff model, following the structure of the PDM (probability distributed moisture) model. Two general. questions are addressed: (1) How does the efficiency of the method evolve when the simulation parameters can not be calibrated but must be transposed from parent gauged basins to ungauged basins? (2) How sensitive are the results to the method used to estimate the initial soil moisture status? System performances are evaluated by means of categorical statistics, such as the critical success index (CSI). Results show that overall CSI is equal to 0.43 for the parent basins, where the hydrological model has been calibrated. CSI reduces to 0.28 for the interior basins, when model parameters are transposed from parent basins, and to 0.21, when both model parameters and soil moisture status is transposed from parent basins. Performance differences between FFG and use of time-constant soil moisture status are very high for the parent basins and decrease with decreasing the system accuracy. The percent difference amounts to 53% for the parent basins, to 25% for interior basins with parameter transposition, and to 19% for interior basins with parameter and soil moisture status transposition. These results improve our understanding of the applicability and reliability of this method at various scales and under various scenarios of data availability. (C) 2008 Elsevier B.V. All rights reserved.

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Abstract: Post-event survey and investigation is one way to gain experience on natural hazards. The importance of the systematisation and standardisation of such investigations and re-analysis is progressively recognised in all the geophysical sciences as shown by the growing number of scientific papers and programs on the subject. But how to proceed in the case of a flash floods, what type of data should be collected for what type of analyses and to explore which particular issues? To give a first answer to these questions, a methodology for post-flash flood field investigations has been developed under the EC FLOODsite project and tested under the EC HYDRATE project. This paper presents shortly the principles of this methodology and illustrates its application for the study of two major flash floods that occurred in November 1999 and September 2002 in the South of France.

Abstract: The 29 August 2003 storm on the upper Tagliamento River basin in the eastern Italian Alps is examined as a prototype for organized convective systems that dominate the upper tail of the precipitation frequency distribution and are likely responsible for the majority of flash flood peaks in this area. The availability of response observations derived from stream gauge data and post-event surveys, provides the opportunity to study the hydrometeorological and hydrological mechanisms associated with this extreme storm and the associated flood. The flood occurred at the end of a climatic anomaly of prolonged drought and warm conditions over Europe and the Mediterranean region. A characteristic of the event is its organization in defined banded structures, some of which persisted in the same locations for the duration of the event. The steadiness of these rainbands led to highly variable precipitation accumulations and, associated with orographic enhancement, played a central role in the space - time organization of the storm. Two dominant controls on extreme flood response are recognized and analyzed: steadiness of convective bands and dry antecedent soil moisture conditions.

Abstract: The main objective of the study is to characterize the severity of a flash flood generating storm occurred on August 29, 2003 on the upper Tagliamento river basin, in the eastern Italian Alps. This storm was characterized by extraordinary rainfall amounts and large spatial variability. Regional frequency analysis based on the index variable method and L-moments is utilized to analyse short duration annual maximum precipitation for the Friuli-Venezia Giulia region, in north-eastern Italy, which includes the storm location. It is shown that the regional growth curves based on the Kappa distribution may be useful for the subregions specified. This analysis provides a framework to investigate the frequency characteristics of the August 29, 2003 flash flood generating storm for various rainfall durations. Radar rainfall estimates, adjusted by using a physically-based methodology and data from a raingauge network, are used to characterize the return period of the storm rainfall amounts, highlighting the importance of considering its spatial dimension. Severity graphs are developed to visualise the return periods and their variability for different rainfall durations within the storm. It is shown that adjusted radar rainfall estimates may suffer for considerable uncertainty and that the uncertainty magnifies in the evaluation of the relevant return periods. The analysis shows also that (i) attributing a single return period to a storm event is not realistic, and (ii) the severity of flash flood generating storms is poorly captured by using conventional raingauge networks. The reported results show that estimates obtained by using careful adjustment of radar observations may be useful to evaluate the severity of the storm for ungauged basins and to evaluate the spatial dimension of the frequency characterization. (C) 2007 Elsevier B.V. All rights reserved.

Abstract: [ 1] This study aims to assess the impact of a class of radar rainfall errors on prediction uncertainty of a conceptual water balance model. Uncertainty assessment is carried out by means of the Generalized Likelihood Uncertainty Estimation procedure ( GLUE). The effects of model input and structural error are separated, and the potential for compensating errors between them is investigated. It is shown that the radar rainfall bias term operates in a multiplicative sense on the model structural uncertainty, by either magnifying or reducing it according to the sign of the bias. The results show also that adjustment of radar rainfall, aimed to remove local biases and to reduce random errors, allows ensuring that a larger percentage of the observed flows are enclosed by the uncertainty bounds, with respect to nonadjusted radar input. However, this is obtained at the price of increasing the wideness of the uncertainty bounds. This effect is emphasized with increasing the radar beam elevation. As a second step, the issue of the impact of radar rainfall estimation error on model parameter distribution and parameter transferability across sites under the radar umbrella is examined. Radar data at different radar beam elevations are used to simulate radar estimation errors at different distances from the radar site and to analyze the impact of these errors on prediction uncertainty. The results show that distortion of parameter distribution due to radar error may be considerable and that adjustment of radar rainfall estimates improves the regionalization potential of radar-based precipitation estimates ( at least for ranges less than 70 km).

Abstract: In this paper the scaling hypotheses are applied to annual maximum series of rainfall depth for different rainfall duration to derive the depth-duration-frequency (DDF) curve. It is shown that, based on the empirically observed scaling properties of rainfall and some general assumptions about the cumulative distribution function for the annual maximum of the rainfall depth, it is possible to derive a simple DDF relationship. This general framework provides a basis for the generation of maps that can be used to infer DDF curves at any point of a particular area. Data from a dense raingauge network in a mountainous region in north-eastern Italy (the Trentino Province) are used to clarify the methodology for the construction and regionalization of the DDF relationship. The geographical variation of short-duration (i.e., less than 60 xmin) rainfall extremes is also evaluated by using the same framework. It is found that depth-duration ratios, defined as the ratios of the t-min to the 60-min rainfall depths of the same return period, may be considered independent of return period and geographical location for any storm duration less than 60 min.

Abstract: This study investigates how subsurface flowpaths are altered by forest roads and how these changes influence shallow landsliding susceptibility in steep, forested landscape. A simple conceptual model of the effect of forest roads on hillslope subsurface flow is developed. The model is incorporated into a hydro-geomechanical, threshold-based model for slope instability. In the model, the occurrence of shallow landsliding is evaluated in terms of drainage areas, ground slope and soil properties (i.e., hydraulic conductivity, bulk density, and friction angle). Model results allow to quantify the influence of roads on shallow landsliding hazard across a landscape and to generate hypotheses about the broader geomorphic effect of roads. Modelling results are compared with field data collected in four sites located in north-eastern Italy. Observed landslide patterns are broadly consistent with model estimates, a finding that underscores the utility of this simple approach for predicting the geomorphic effects of forest roads constructed on steep slopes. The approach used in this study may be useful for defining criteria for road design that reduce the effects of roads on geomorphic processes. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: Radar estimates of rainfall are being increasingly applied to flood forecasting applications. Errors are inherent both in the process of estimating rainfall from radar and in the modelling of the rainfall-runoff transformation. The study aims at building a framework for the assessment of uncertainty that is consistent with the limitations of the model and data available and that allows a direct quantitative comparison between model predictions obtained by using radar and raingauge rainfall inputs. The study uses radar data from a mountainous region in northern Italy where complex topography amplifies radar errors due to radar beam occlusion and variability of precipitation with height. These errors, together with other error sources, are adjusted by applying a radar rainfall estimation algorithm. Radar rainfall estimates, adjusted and not, are used as an input to TOPMODEL for flood simulation over the Posina catchment (116 km(2)). Hydrological model parameter uncertainty is explicitly accounted for by use of the GLUE (Generalized Likelihood Uncertainty Estimation). Statistics are proposed to evaluate both the wideness of the uncertainty limits and the percentage of observations which fall within the uncertainty bounds. Results show the critical importance of proper adjustment of radar estimates and the use of radar estimates as close to ground as possible. Uncertainties affecting runoff predictions from adjusted radar data are close to those obtained by using a dense raingauge network, at least for the lowest radar observations available. Copyright (C) 2004 John Wiley Sons, Ltd.

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Abstract: Road networks in mountainous forest landscapes have the potential to increase the susceptibility to shallow landsliding by altering subsurface flow paths. In this paper a road interception/rerouting grid-based model is developed and coupled with a hydrogeomechanical, threshold-based model for slope instability. Model results allow quantification of the influence of roads on shallow landsliding hazard across a landscape and generation of hypotheses about the broader geomorphic effect of roads. The model is applied to three sites in northeastern Italy to study the influence of three different procedures for upslope drainage area computation on model results. The three procedures are a single-flow direction procedure (D8), a multiple-flow direction procedure (MF), and an algorithm based on proportioning flow between two downslope pixels (DINF). Model results are highly sensitive to upslope drainage area algorithm choice. It is shown that D8 and MF are unsuitable for road impact evaluation due to grid bias (D8) and excessive dispersion (MF). Model implementation with the DINF procedure results in a reasonable compromise between the artifacts that characterize the other two algorithms. Observed landslide patterns are broadly consistent with model estimates, a finding that underscores the utility of this simple approach for predicting the geomorphic effects of forest roads constructed on steep slopes. The approach used in this study may be useful for defining criteria for road design that reduce the effects of roads on geomorphic processes.

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Abstract: A model for the simulation of shallow landsliding triggered by heavy rainstorms is analysed and discussed. The model is applied in two mountainous catchments in the Dolomites (Eastern Italian Alps): the Cordon catchment (5 km(2)) and the Vauz catchment (1.9 km(2)), where field surveys provided a description of hydraulic and geotechnical properties of soils and an inventory of landslide scars is available. The stability mapping procedure, which is similar to that proposed by Montgomery and Dietrich (1994 Water Resources Research 30: 1153), combines steady-state hydrologic concepts with the infinite slope stability model. The model provides an estimate of the spatial distribution of the critical rainfall, which is the minimum steady-state rainfall predicted to cause instability. The comparison of the landslides observed in the study basins with model predictions shows that the distribution of critical rainfall obtained from the model provides a surrogate for failure initiation probability as a function of topographic location. Copyright (C) 2002 John Wiley Sons, Ltd.

Abstract: A comprehensive analysis of precipitation estimation by weather radar operated over hilly terrain is performed. The study is based on a nearly continuous 3-year database of weather radar observations and hourly rain accumulations from a dense rain gauge network located in southwest England. Radar rainfall processing scenarios associated with correction for systematic and range dependent errors are investigated. The focus of the study is in demonstrating the impact of radar range effects at short to medium (less than 70 km) distances and determining the significance of coupling mean-field and range-related bias adjustment in radar rainfall estimation. The investigation is performed on the basis of climatological radar rainfall statistics for selected radar sectors and radar-rain gauge comparisons over a densely instrumented medium size (135 km 2) basin located at 35-40 km from the radar. The application of the combined adjustment is shown to reduce overall error over the catchment by 24%. The analysis suggests that gauge-based radar adjustment can be properly applied only when homogeneity in the accuracy of the radar rainfall estimates with respect to range and scanning elevation is ensured.

Abstract: The aim of this work is to analyse the impact of errors in radar rainfall estimates on rainfall-runoff modelling. This issue is addressed through application of radar rainfall estimates for continuous, lumped rainfall-runoff modelling of the Brue catchment, a mid-sized basin in South-West England, over a two and a half-year period. The study is focused on radar rainfall errors associated with range-related bias due to the vertical profile of reflectivity and with mean-field bias due to systematic errors in the radar calibration and biased reflectivity-to-rainrate relationship. Streamflow is simulated through a conceptual hydrological model based on mean areal rainfall estimates obtained by using various radar rainfall processing scenarios. These simulations are evaluated and compared with corresponding streamflow simulations from a dense raingauge network. The comparisons show that radar errors may preclude the use of unadjusted radar estimates for runoff modelling. Radar rainfall adjustment significantly improves model results with simulation efficiency increasing up to 30% after adjustment. Comparison of radar-driven simulations with observed discharge data reveals a simulation efficiency of 0.75 for the lowest radar scan (adjusted), whereas simulation efficiencies are lower for higher radar scans. The results reveal the critical importance of using radar rainfall estimates as close as possible to the ground and the considerable impact that effects of vertical variability of reflectivity have on runoff simulation. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: A model for the prediction of both topographic and climatic control on shallow landslide initiation processes in hilly mountainous terrain is proposed. The model develops upon a theory for coupled shallow subsurface flow and landsliding of the soil mantle previously proposed by Montgomery and Dietrich [Water Resour. Res, 30 (1994) 1153], The model uses a 'quasi-dynamic' wetness index to predict the spatial distribution of soil saturation in response to a rainfall of specified duration. The rainfall predicted to cause instability in each topographic element is characterised by duration and frequency of occurrence. The incorporation of a rainfall frequency-duration relationship into assessment of landslide hazard provides a practical way to include climate information into estimation of the relative potential for shallow landsliding. The model is applied to a mountain experimental basin where high-resolution digital elevation data are available: the Cordon catchment (5 km(2)), in north-eastern Italy. An inventory of landslide scars is used to document sites of instability and to provide a test of model performance by comparing observed landslide locations with model predictions. The model reasonably reproduces the observed distribution of landslides, although spatial variability of soil properties and hydrologic complexities not accounted for by the model complicate prediction of where landslides occur within areas of similar topographic-climatic control. Model predictions from the quasi-dynamic model are compared with those provided by the steady-state model [Water Resour, Res. 30 (1994) 1153], These results suggest that the quasi-dynamic model offers significant improvement over the steady-state model in predicting existing landslides as represented in the considered landslide inventory. (C) 2002 Published by Elsevier Science B.V.

Abstract: This paper investigates a multicomponent radar-based rainfall estimation algorithm that includes optimum parameter estimation and error correction schemes associated with radar operation over mountainous terrain. Algorithm preprocessing steps include correction for terrain blocking, adjustment for rain attenuation, and interpolation of reflectivity data from polar radar coordinates to a three-level ( 1, 2, and 3 km) vertically integrated Cartesian grid. The error correction schemes investigated herein include a simple but efficient approach to correct for the vertical variation of reflectivity and a stochastic filtering approach for mean-field radar-rainfall bias adjustment. The primary algorithm parameters are estimated through a global optimization scheme. Eight major flood-inducing storm events observed coincidentally by a C-band weather radar and 39 rain gauge stations over an alpine region of northeast Italy are used. We describe sensitivity analysis of the parameter values obtained from global optimization, the improvements in accuracy owing to the implementation of the different preprocessing and error correction schemes, and the overall improvement achieved as compared with previous algorithm studies in the area. The advantages of performing vertical integration versus using the lowest-available-elevation radar field, applying stochastic filtering versus the deterministic approach for mean field bias, and estimating the algorithm parameters through optimization are demonstrated.

Abstract: Due to the vertical variability of reflectivity and radar sampling geometry, radar rainfall estimation is necessarily subject to biases that are range-related. These range-related errors mirror the vertical structure of reflectivity field as it is perceived by the radar at various ranges. This study focuses on range-dependent biases that arise at mid-ranges (30-70 km) due to the requirement of using relatively high elevation radar scans to avoid ground effects. Rainfall data for the Warden Hill weather radar in UK, both unadjusted and adjusted for range-related effects, are evaluated for a three-year period. Quantitative analysis is splitted into two distinct stages: i) radar-only evaluation, and ii) radar-gauge comparison along the range. Effects of bright band are recognizable at close ranges, while at farther ranges the effects of beam sampling the ice region lead to strong underestimation. It is shown that improvements obtained by applying the range-dependent bias adjustment procedure lead to remarkable reduction of inter-elevation bias. The coupling of range-dependent bias adjustment with a procedure for mean-field bias removal allows a significant reduction of radar-raingauge differences. (C) 2000 Elsevier Science Ltd. All rights reserved.

Abstract: This paper investigates the effect of systematic mean-field and range-dependent radar rainfall errors on the accuracy of runoff simulation in mountainous basins. Statistical analysis of radar rainfall and runoff simulation error is performed on six flood events for two medium size watersheds in northern Italy, located at 38 and 60 km basin-to-radar distances, respectively. We show significant range-related rainfall biases, which are due to the high elevation radar scans used to minimize the interception of the radar beam with the topography. These biases are corrected by converting radar reflectivity measurements at a given altitude into their equivalent surface values, using real-time identification of the mean vertical reflectivity profile. The mean-field bias is adjusted using a multiplicative factor determined based on real-time radar-rain gauge comparisons. The impact of the above radar rainfall biases, and the improvements obtained from the proposed corrections, on areal-rainfall estimation and runoff simulation are evaluated. It is shown that radar rainfall biases magnify through the rainfall-runoff transformation and preclude the accurate simulation of runoff, particularly for the distant basin. The combined correction procedure results in significant reduction of the hydrologic prediction error, especially at the distant basin.

Abstract: The aim is to evaluate the use of infrared satellite precipitation estimates for nowcasting purposes in the context of a real-time flood-warning scheme. A radar-based calibration technique is described which is applied to the Negri-Adler-Wetzel scheme. This procedure employs radar data over a defined calibration area to estimate, for each satellite image, actual min-rates to be used in the Negri-Adler-Wetzel scheme. Calibrated satellite estimates obtained from this procedure can be used to diagnose areas of precipitation beyond radar range, thus allowing an extension of precipitation nowcasting lead time. Calibrated estimates are compared with radar rainfall measurements and results are discussed for various sizes of integration area. Calibration reduces consistently both bias and variance of the error of the original Negri-Adler-Wetzel estimates, even for integration areas as small as 2000 km(2). This indicates the capabilities of the new technique for nowcasting purposes over medium-sized river basins.

Abstract: A model for the analysis of topographic influence on shallow landslide initiation is applied to an experimental mountain basin where high-resolution digital elevation data are available: the Cordon catchment (5 km(2)) located in northern Italy. The model delineates those areas most prone to shallow landsliding due to surface topographic effects on hydrologic response. The model is composed of two parts: a steady-state model for shallow sub-surface runoff and an infinite-slope Coloumb failure model which assumes that the soil is cohesionless at failure. An inventory of landslide scars is used to document sites of instability and to provide a test of model performance by comparing observed landslide locations with model predictions. The model reproduces the observed distribution of landslide locations in a consistent way, although spatial variations in soil strength and transmissivity, which are not accounted for in the model, influence specific distribution of landslide areas within regions of similar topographic control.

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Abstract: The use of digital elevation models (DEMs) allows the automatic derivation of channel networks and the quantitative description of the geomorphic characteristics of basins. A common method of channel network extraction from DEM data is based on the specification of a threshold area (A(t)) that is the minimum support area required to drain to a point for a channel to form. Usually, an arbitrary constant threshold area value is chosen for channel network extraction. In this study the effects of threshold area selection, both on the morphometric and scaling properties (such as drainage density, total channel length, Horton laws and fractal dimension) of a channel network and the associated hydrological response function are analysed. The response is obtained following the geomorphological instantaneous unit hydrograph theory. Two different probabilistic models are used. They both relate the characteristic response function of the basin to its DEM data derived networks: one is derived assuming a Strahler stream ordering system and the other is obtained by averaging a flow equation with respect to the network structure (described by the width function). Applications are shown for three mountainous basins in the Italian Alps. A sensitivity analysis is performed to study the influence of the variability of morphometric properties, with respect to A(t), on the hydrological response obtained. It is shown that the model based on the width function is able to reduce the effects of this variability on the simulated response. (C) 1997 by John Wiley & Sons, Ltd.

Abstract: This paper investigates the adaptive use of a simple conceptual lumped rainfall-runoff model based on a Probability Distributed Model complemented with a Geomorphological Unit Hydrograph. Three different approaches for updating the model and for its use for real time flood forecasting are compared: the first two are based on a parameter updating approach; in the third procedure the model is cast into a state-space form and an Extended Kalman Filter is applied for the on-line estimation of the state variables. The comparison shows that the procedure based on the filtering techniques provides more reliable results; acceptable results are also obtained by using a parameter updating approach based on the on-line adjustment of the initial conditions.

Abstract: The paper focuses on the ties of kriging with a deterministic interpolation procedure, known as multiquadratic surface fitting. The two methods are compared, first from a theoretical point of view, then using a practical example. It is shown that kriging equations with a linear variogram model are identical in form to equations of multiquadratic surface fitting with cone surfaces. The issue of the accuracy of both estimators is discussed through a case study where hourly rainfall maps of real storm events collected by radar provided the reference rainfall. Random point sampling of the accumulation pattern simulated gauge returns. Eight sampling densities were used and for each density rainfall spatial distributions were estimated for a large number of realisations. It is shown that kriging performs better at lower gauge density, while at higher gauge density the accuracy of both estimators is similar.

Abstract: Brightband effects are one of the more important causes of vertical variability of reflectivity and severely affect the accuracy of rainfall estimates from ground-based radar. Monte Carlo simulation experiments are performed to investigate the efficiency of a procedure for the correction of errors related to the vertical variability of reflectivity. The simulation model generates three-dimensional radar reflectivity fields. Brightband effects are simulated through a physically based model of melting-layer reflectivity observations. Sensitivity of the correction procedure for a number of different precipitation scenarios and radar systems is analyzed. Overall, the identification method is found to be a robust procedure for correction of brightband effects. Results indicate a dependence of the effectiveness of the correction procedure on mean altitude and spatial variability of the melting layer.

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Abstract: The assumption of a particular type of distribution of rainfall cells in space is needed for the formulation of several space-time rainfall models. In this study, weather radar-derived rain rate maps are employed to evaluate different types of spatial organization of rainfall cells in storms through the use of distance functions and second-moment measures. In particular the spatial point patterns of the local maxima of rainfall intensity are compared to a completely spatially random (CSR) point process by applying an objective distance measure. For all the analyzed radar maps the CSR assumption is rejected, indicating that at the resolution of the observation considered, rainfall cells are clustered. Therefore a theoretical framework for evaluating and fitting alternative models to the CSR is needed. This paper shows how the ''reduced second-moment measure'' of the point pattern can be employed to estimate the parameters of a Neyman-Scott model and to evaluate the degree of adequacy to the experimental data. Some limitations of this theoretical framework, and also its effectiveness, in comparison to the use of scaling functions, are discussed.

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