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Flash Flood Guidance (FFG) is a rainfall threshold which initiates flooding in streams. It merely provides a binary output (yes or no) which has large uncertainties in forecasting. In this paper, we propose a new method by combining FFG with the Frequentist method to present the probability of flash flood occurrence based on historical rainfall events. We first calculated deviation from the log transform rainfall data leading to flash floods. Kernel Density Estimation (KDE) was used to describe the deviation. Normal Distribution Function (NDF) was chosen to fit the KDE output and to calculate probabilities of flooding as per the Frequentist FFG. In order to aid decision making, three probability thresholds (10, 20 and 60%) were used for defining four flood risk classes, namely very low, low, significant and high, and were colour coded respectively as green, yellow, orange and red. The proposed Frequentist FFG method was then applied to the Posina River basin in Italy. Comparison of forecasts from the conventional FFG (with probability 0 or 1) and Frequentist FFG for 94 6-hourly rainfall events, including 23 flood events, shows that the Frequentist FFG presented a probability of flooding varying from 0 to 100% and the corresponding risk class can be used to reduce false alarms while still reducing the disaster risk. The application of the developed approach to the Posina basin shows that decision making regarding flash forecasting is easier with the presented approach compared to the traditional FFG approach. [ABSTRACT FROM AUTHOR]

The evaluation of sediment budgets of flash-flood prone torrential catchments is important for the assessment of flood susceptibility and hazard in piedmont areas. Here, we developed and tested a predictive, quantitative geomorphological technique for estimating the magnitude of debris flows related to flash floods triggered by extreme rainfall events. We applied our method to steep torrential catchments that allowed testing of our technique through quantification of the effects of recently-occurred flash floods. The three study catchments are located in southern Italy and experienced flash flood-related debris flows triggered by a rainstorm with a recurrence interval > 200 yrs. The study basins are small size, steep, narrow and the stream network has a low order (maximum 3rd order). We estimated the volumes of loose sediment stored in the bottoms of the mainstem valleys of the investigated mountainous catchments prior to the flash flood and compared our results with the sediment volumes entrained by the debris flows. The latter volumes were estimated by post-event data collected by means of field surveys. The results were mutually consistent so supporting our approach. The approach is fit to the analysis of torrential catchments with geomorphological features comparable to those of our study basins, provided (i) the stream is confined by hard rock slopes not affected by channel lateral erosion, (ii) all stored sediment is eroded and transported downstream during the flood, and (iii) no landslide occurs along the hillslopes. For these types of ungauged catchments, our method can be applied extensively, even in the absence of field data and measurements, for flash flood hazard assessment and planning of mitigation strategies. [Display omitted] • Flash-flood prone torrential catchments are source of hazard for piedmont areas. • Evaluation of sediment budget is crucial to assess flood-related debris flow hazard. • We use a geomorphological method to estimate sediment budgets of three small basins. • We test the results by post-event evaluations of the eroded sediment volumes. • The method allows prediction of the magnitude of debris flows for small catchments. [ABSTRACT FROM AUTHOR]

Large floods may produce remarkable channel changes, which determine damages and casualties in inhabited areas. However, our knowledge of such processes remains poor, as is our capability to predict them. This study analyses the geomorphic response of the Nure River (northern Italy) and nine tributaries to a high-magnitude flood that occurred in September 2015. The adopted multi-disciplinary approach encompassed: (i) hydrological and hydraulic analysis; (ii) analysis of sediment delivery to the stream network by means of landslides mapping; (iii) assessment of morphological modifications of the channels, including both channel width and bed elevation changes. The spatial distribution of rainfall showed that the largest rainfall amounts occur in the upper portions of the catchment, with cumulative rainfall reaching 300 mm in 12 h, and recurrence intervals exceeding 100–150 years. The unit peak discharge ranged between 5.2 and 25 m 3  s −1  km −2 . Channel widening was the most evident effect. In the tributaries, the ratio between post-flood and pre-flood channel width averaged 3.3, with a maximum approaching 20. Widening was associated with channel aggradation up to 1.5 m and removal of riparian vegetation. New islands formed due to the fragmentation of the former floodplain. In the Nure River, the average width ratio was 1.7, and here widening occurred mainly at the expenses of islands. Bed level dynamics in the Nure were varied, including aggradation, incision, and overall stability. The flood geomorphic effectiveness was more pronounced in the middle-higher portions of the basin. Planimetric and elevation changes were well correlated. Regression analysis of the relationship between widening and morphological/ hydraulic controlling factors indicated that unit stream power and confinement index were the most relevant variables. The study provides useful insights for river management, especially with regard to the proportion of the valley floor subject to erosion and/or deposition during large events. [ABSTRACT FROM AUTHOR]

In this paper, a 1:5000 map of the geomorphic consequences of the 14th–15th October 2015 flash flood is presented. The most severely affected area (about 13 km2) was located at the footslope of Mt. Camposauro (Southern Italy), between the municipalities of Solopaca and Paupisi. In this area high-intensity and very localized cloudbursts of short duration induced hillslope erosion and sediment delivery from the slopes to the Calore River, activating several alluvial fans. The rainstorm lasted 7 h with a maximum recorded rainfall peak of nearly 140 mm in 2 h. The event map resulting from a geological and geomorphological post-event field survey was supported by satellite images and unmanned aerial vehicle image interpretations, respectively taken one day and one week after the event. The flash flood resulted in no casualties but severely affected the local economy, mainly based on agriculture and wine production. [ABSTRACT FROM PUBLISHER]

On 3 July 2006, an exceptionally heavy convective rainfall affected a small area in Calabria, Italy. A rainfall amount of 202 mm was recorded in 2.5 h, producing considerable damage and causing a localized flash flood. The Weather Research and Forecasting (WRF) model was used to analyze the instability present in the event and the related triggering mechanisms. The high-resolution simulation is able to correctly identify the position of the precipitation peak and to clarify the mesoscale processes involved, although it significantly underestimates the total amount of precipitation. Some sensitivity experiments confirm the importance of the choice of planetary boundary layer and microphysics parameterization schemes for a correct simulation of the event, showing a strong sensitivity to these numerical tests. Also, the need for high horizontal resolution emerges clearly: an accurate representation of the orography at small scales, is required to simulate the event in its correct location. Instability indices identified an extremely favorable environment for convection development, with very high values of CAPE and high moisture content at low levels. The low mountains near the rainfall peak play an important role in triggering the release of instability and controlling the location of rainfall; in particular, the peculiar morphology of the orography creates low-level wind convergence and provides the uplift necessary for the air parcels to reach the level of free convection. In this framework, nondimensional parameters, such as the Froude number, have been calculated to better understand the interaction of the flow with the orography. [ABSTRACT FROM AUTHOR]

The hydrological and hydraulic processes of the flash flood that affected Atrani village (Amalfi Coast of Southern Italy) on 9 September 2010 are analysed using post-flood surveys and numerical modelling. Observations in this case are particularly challenging because of the small spatial and temporal scales of the event, relative to the sampling characteristics of conventional rain and discharge measurement networks. Given these aspects, some of the relevant characteristics of the event were deduced by analysing field data that were collected immediately after the event.A width function instantaneous unit hydrograph (WFIUH) rainfall-runoff model is applied to derive a flow hydrograph for the area immediately upstream of the village. This hydrograph is inputted as an upstream boundary condition for the simulation of the propagation processes. A two-dimensional innovative hydraulic model is used to reproduce the flood within the village. The use of an unstructured mesh enabled a detailed simulation of water flow within the narrow and winding alleyways of the village. The results of the simulations indicate agreement with the field observations. The implemented approach is suitable for simulating flash floods in similar contexts and, therefore, may be used to predict potential future events. [ABSTRACT FROM AUTHOR]

Abstract: The city of Genoa, which places between the Tyrrhenian Sea and the Apennine mountains (Liguria, Italy) was rocked by severe flash floods on the 4th of November, 2011. Nearly 500mm of rain, a third of the average annual rainfall, fell in six hours. Six people perished and millions of Euros in damages occurred. The synoptic-scale meteorological system moved across the Atlantic Ocean and into the Mediterranean generating floods that killed 5 people in Southern France, before moving over the Ligurian Sea and Genoa producing the extreme event studied here. Cloud-permitting simulations (1km) of the finger-like convective system responsible for the torrential event over Genoa have been performed using Advanced Research Weather and Forecasting Model (ARW-WRF, version 3.3). Two different microphysics (WSM6 and Thompson) as well as three different convection closures (explicit, Kain–Fritsch, and Betts–Miller–Janjic) were evaluated to gain a deeper understanding of the physical processes underlying the observed heavy rain event and the model's capability to predict, in hindcast mode, its structure and evolution. The impact of forecast initialization and of model vertical discretization on hindcast results is also examined. Comparison between model hindcasts and observed fields provided by raingauge data, satellite data, and radar data show that this particular event is strongly sensitive to the details of the mesoscale initialization despite being evolved from a relatively large scale weather system. Only meso-γ details of the event were not well captured by the best setting of the ARW-WRF model and so peak hourly rainfalls were not exceptionally well reproduced. The results also show that specification of microphysical parameters suitable to these events have a positive impact on the prediction of heavy precipitation intensity values. [Copyright &y& Elsevier]

Abstract: The island of Ischia is particularly susceptible to landslides and flash floods due to its particular geological and geomorphological context. Urbanization in recent decades coupled with the development of tourism has increased the risk. After the November 10, 2009 event occurring in the northern sector of the island (the town of Casamicciola), a detailed geo-morphological survey was conducted to ascertain the evolution of the phenomenon. In the watersheds upstream of Casamicciola, many landslides were mapped and the volume of material involved during detachment and sliding was estimated. In the lower course area, near the town and towards the sea, flow pathways were reconstructed with the aid of extensive video footage taken during the event. Rainfall data were also analyzed and a relationship was established between the hourly rainfall rate and the flash flood. The phenomenon was found to be quite complex, with many upstream landslides stopping before reaching the urban area. In the lower course the alluvial event occurred as a flood with a very small sediment discharge, which left a very thin layer of sediment. Reconstruction of the flash flood phenomenon suggested possible action for future risk mitigation, early warning and civil protection plans. [Copyright &y& Elsevier]

Piemonte region, in the north-western Italy, is characterized by complex orography and Mediterranean influence that often causes extreme rainfall event, during the warm season. Although the region is monitored by a dense gauge network (more than one gauge per 100 km²), the ground measurements are often inadequate to properly observe intense and highly variable precipitations. Polarimetric weather radars provide a unique way to monitor rainfall over wide areas, with the required spatial detail and temporal resolution. Nevertheless, most European weather radar networks are operating at C-band, which may seriously limit quantitative precipitation estimation in heavy rainfall due to relevant power signal attenuation. Phase measurements, unlike power measurements, are not affected by signal attenuation. For this reason, polarimetric radars, for which the differential phase shift measurements are available, provide an additional way in which to estimate precipitation, which is immune to signal attenuation. In this work differential phase based rainfall estimation techniques are applied to analyze two flash-floods: the first one occurred on the Ligurian Apennines on 16 August 2006 and the second occurred on 13 September 2008, causing rain accumulations above 270 mm in few hours. [ABSTRACT FROM AUTHOR]

Summary: This study aims 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 90km2 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 20km and 5km, respectively. The positive impact of assimilating radar rainfall estimates is carried over into the free forecast for about 2–5h, 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 3h. [ABSTRACT FROM AUTHOR]

Summary: 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 rain-path radar signal attenuation introduces significant systematic error on the R STD 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 0.9 (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 varies 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° elevation estimates, while the reduction in terms of relative root-mean-square error is limited to within 10%. [ABSTRACT FROM AUTHOR]

The article presents a study on the hydrometeorological controls and erosive response of an extreme debris flow in Rio Cucco basin with a volume of about 78, 000 m3> in the Italian alps after it was hit by a storm on August 29, 2003. It has shown that aside from the dominant debris flows, sheetflood also contributed to fan aggradation. Moreover, it states that the large discharge values during the flood phase recession implies a vital role for subsurface flow of the flash flood.

Summary: 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. [Copyright &y& Elsevier]