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Flash flooding is a significant societal problem, but related precipitation forecasts are often poor. To address, one can try to use output from convection‐parametrising (global) ensembles, post‐processed to forecast at point‐scale, or convection‐resolving limited area ensembles. The new methodology described here combines both. We apply "ecPoint‐rainfall" post‐processing to the ECMWF global ensemble. Alongside we use 2.2 km COSMO LAM ensemble output (centred on Italy), and also post‐process that, using a scale‐selective neighbourhood approach to compensate for insufficient members and to preserve consistently forecast local details. The two resulting scale‐compatible components then undergo lead‐time‐weighted blending, to create the final probabilistic 6 h rainfall forecasts. Product creation for forecasters, in this way, constituted the "Italy Flash Flood use case" within the EU‐funded MISTRAL project; real‐time delivery of open access products is ongoing. One year of verification shows that, of the five components (2 ×$$ \times $$ raw, 2 ×$$ \times $$ post‐processed and blended), ecPoint is the most skilful. The post‐processed COSMO ensemble adds most value to summer convective events in the evening, when the global model has an underprediction bias. In two typical heavy rainfall case studies we observed underestimation of the largest point totals in the raw ECMWF ensemble, and overestimation in the raw COSMO ensemble. However, ecPoint elevated the ECMWF maxima and highlighted best the most affected areas and merged products seemed to be the most skilful of all. Even though our LAM post‐processing does not include (or arguably need) bias‐correction, this study still provides a unique blueprint for successfully combining ensemble rainfall forecasts from global and LAM systems around the world. It also has important implications for forecast products as global ensembles move ever closer to having convection‐permitting resolution. [ABSTRACT FROM AUTHOR]

The mesoscale dynamics of a record‐breaking Atmospheric River (AR) that impacted the Middle East in mid‐April 2023 and caused property damage and loss of life are investigated using model, reanalysis and observational data. The high‐resolution (2.5 km) simulations revealed the presence of AR rapids, narrow and long convective structures embedded within the AR that generated heavy precipitation (>4 mm hr−1) as they moved at high speeds (>30 m s−1) from northeastern Africa into western Iran. Gravity waves triggered by the complex terrain in Saudi Arabia further intensified their effects. Given the rising frequency of ARs in this region, AR rapids may be even more impactful in a warming climate, and need to be accounted for in reanalysis and numerical models. Plain Language Summary: Atmospheric Rivers (ARs) are narrow and long bands of high water vapor content, which largely originate in the tropics or subtropics and propagate into mid‐ and high‐latitudes. They can bring beneficial rain and snow but, in particular the most intense, can lead to catastrophic flooding and loss of life. One of such occurrences in the Middle East in mid‐April 2023 is investigated using model and observational data. The high‐resolution (2.5 km) simulation put in evidence narrow (5–15 km) and long (100–200 km) convective structures within the AR, known as AR rapids, which produced heavy precipitation (>4 mm hr−1), further enhanced by gravity waves that developed over the high terrain in western Saudi Arabia, and propagated at high speeds (>30 m s−1). ARs are occurring more frequently in the Middle East as they are globally, and with increased atmospheric water vapor in a warming climate, AR rapids may be even more destructive. Key Points: The exceptional and impactful Atmospheric River of mid‐April 2023 in the Middle East is investigated with model and observational dataA 2.5 km simulation reveals the presence of "Atmospheric River rapids," narrow along‐flow structures that generated rain rates >4 mm hr−1Model simulations and ground‐based observations underlined the effects of the Atmospheric River and associated trailing cold front [ABSTRACT FROM AUTHOR]

It is increasingly acknowledged that the acceleration of the global water cycle, largely driven by anthropogenic climate change, has a disproportionate impact on sub‐daily and small‐scale hydrological extreme events such as flash floods. These events occur thereby at local scales within minutes to hours, typically in response to high‐intensity rainfall events associated with convective storms. In the present work, we show that by employing physically based representative hillslope models that resolve the main gradients controlling overland flow hydrology and hydraulics, we can get reliable simulations of flash flood response in small data‐scarce catchments. To this end, we use climate reanalysis products and transfer soil parameters previously obtained for hydrological predictions in an experimental catchment in the same landscape. The inverted mass balance of flood reservoirs downstream is employed for model evaluation in these nearly ungauged basins. We show that our approach using representative hillslopes and climate data sets can provide reasonable uncalibrated estimates of the overland runoff response (flood magnitude, storm volume, and event runoff coefficients) in three of the four catchments considered. Given that flash floods typically occur at scales of a few km2 and in ungauged places, our results have implications for operational flash flood forecasting and open new avenues for using gradient resolving physically based models for the design of small and medium flood retention basins around the world. Plain Language Summary: Flash floods have become increasingly common worldwide, with catastrophic damages to both human life and the economy. While the extent of global warming and climate change impacting these events is still under much debate, it is almost certain now that we need to be better equipped to understand and model these extremes to prevent and mitigate the possible risk to human life and infrastructure in a warming climate. To test, if we can use first principles derived from thermodynamic conservation laws and process based hydrological models for the same, we modeled flash flood response in four headwater catchments over Southern Germany using the concept of "representative hillslope." Since the regions considered in our work are poorly gauged, we made use of global climate reanalysis products and parameter transfer from past experiments. The encouraging results obtained in predicting the flood magnitude and volume speak to the overall applicability of our approach. We are able to get decent uncalibrated predictions in three out of the four catchments considered with minimum computational effort. Understanding and managing the adverse impacts of such extreme hydroclimatic events remains one of the crucial hurdles facing humanity toward the sustainable development goals (SDG17) in this decade. Key Points: Physically based representative hillslope models can be used for flash flood predictions in small data‐scarce and rural catchmentsClimate reanalysis data enable the initialization of a process‐based model, helping to reduce the uncertainties in estimating antecedent soil conditionsTransfer of model parameters within the same hydrological landscape is feasible [ABSTRACT FROM AUTHOR]

Flash flood risk management is among the key topics of the European Flood Directive. Design hydrographs of small river basins could underestimate the hydraulic risk in case of intense and short‐duration events because of the intensification of hourly rainfall and the design rainfall depths uncertainty in often ungauged river basins. The use of synthetic hyetographs obtained from past observed events can be useful for a better evaluation of the flash flood hazard at the river basin scale. For this reason, a simple approach based on rainfall scaling and storms transposition of occurred events is presented to identify areas where design discharges underestimate the flash flood risk. The methodology considers the spatial distribution of the observed storms over a specific zone and the areal reduction factors to scale the observed hyetographs for different river basins extensions. Hyetographs of past observed events are used as input of a hydrological model in a set of river basins located in Northern Tuscany (Italy). The results show how peak discharges of short‐duration events are usually greater than the design floods of the small river basins with an area generally less than 30 km2. [ABSTRACT FROM AUTHOR]

This study focuses on the physical vulnerability of buildings to flash floods using an indicator‐based methodology. A physical vulnerability index (PhVI) that combines intrinsic vulnerability (IV) of buildings and flash flood intensity (FFI) is proposed. IV evaluates the propensity to suffer damage, resulting from indicators related to building properties. FFI estimates the potential to cause damage, resulting from indicators related to flow parameters. PhVI was applied to a critical section of a small drainage basin in Portugal where flash floods are frequent. Evaluating IV and the intensity of natural hazards is essential in physical vulnerability assessments. This study addresses two problems found in the literature: the lack of flash flood‐dedicated physical vulnerability assessments and the difficulties in assembling building properties and the intensity of natural hazards in a vulnerability index defined from indicator‐based methodologies. PhVI is a useful tool where damage records are rare or non‐existent, allowing the prioritisation of resources and application of local protection measures. This index can be adapted to other study areas and natural hazards, although more research is needed to improve the knowledge on the indicators and weights of IV and FFI. [ABSTRACT FROM AUTHOR]

Estimation of the infiltration/natural recharge to groundwater from rainfall is an important issue in hydrology, particularly in arid regions. This paper proposes the application of The Natural Resources Conservation Service (NRCS) mass balance model to develop infiltration (F)–rainfall (P) relationship from flash flood events. Moreover, the NRCS method is compared with the rational and the Ф‐index methods to investigate the discrepancies between these methods. The methods have been applied to five gauged basins and their 19 sub‐basins (representative basins with detailed measurements) in the southwestern part of Saudi Arabia with 161 storms recorded in 4 years. The F–P relationships developed in this study based on NRCS method are: F = 39% P with R2 = 0.932 for the initial abstraction factor, λ = 0.2. However, F = 77% P with R2 = 0.986 for λ = 0.01. The model at λ = 0.01 is the best to fit the data, therefore, it is recommended to use the formula at λ = 0.01. The results show that the NRCS model is appropriate for the estimation of the F–P relationships in arid regions when compared with the rational and the Ф index methods. The latter overestimates the infiltration because they do not take λ into account. There is no significant difference between F–P relationships at different time scales. This helps the prediction of infiltration rates for aquifer recharge at ungauged basins from monthly and annual rainfall data with a single formula. [ABSTRACT FROM AUTHOR]

To understand the climatology of lightning affecting Britain and Ireland and to investigate the large‐scale environments which produce lightning in this region, relationships between mean sea‐level pressure (MSLP) patterns and thunderstorm occurrence are investigated. Using daily data between 2010 and 2019, instances of lightning activity are examined against a set of 30 pre‐defined weather patterns based on MSLP. Metrics relating to thunder area, thunder intensity and duration are introduced and evaluated on the data set. The results show that in summer the largest and long‐lived lightning outbreaks can be linked to three weather patterns. Two of these have strong southerly flow affecting the domain of study. The third is a low‐pressure system centred over the UK with weak pressure gradient, which also produces larger lightning outbreaks during the autumn. In the wintertime, lightning outbreaks appear associated with deeper low‐pressure systems, higher pressure gradients and strong winds in four dominant weather patterns. In spring, lightning occurs mostly in smaller systems and can be of multiple weather patterns. It is hypothesised that the weather pattern number is a good predictor of atmospheric instability. Cases where lightning activity does not match the expected behaviour for a given weather pattern are thought to be due to anomalies or variation in atmospheric instability. Several applications of this work are also discussed, including looking at thunder days over the study domain in past and future climate scenarios. [ABSTRACT FROM AUTHOR]

São Vicente Island is in the Republic of Cape Verde. It lies within the Sahelian zone and faces several natural risks, one of which is flash flooding. Based on a questionnaire entitled Flash Flood Risk Perception in Cape Verde, applied to 203 subjects, influence the behavioural decision making adopted by people when confronted with this natural hazard. A multivariate technique was conducted on the main dimensions (categorical principal components analysis, CATPCA) to identify the primary factors associated with the perception of flash flood risk. Results of this analysis suggest that education level and gender explain more than 50% of the observed variance. Respondents with higher education show greater knowledge about this type of natural risk, although they ascribe its occurrence to divine punishment. Men and women have different perceptions about flash flooding, its causal attributions, and support from public entities regarding planning policies. Prior experience mainly influences the perception of the personal consequences of flash flood events. The results show the relevance of introducing risk education into schools to raise awareness about the causes and consequences of flash floods and improve readiness and responses to this risk manifestation. [ABSTRACT FROM AUTHOR]

Extreme flood events are becoming more frequent and challenging due to climate change. Key objectives of this study are to evaluate different criteria used in assessing the hazard to people during flood events and, once determined, the most suitable method is then used to assess the hazard and the safest route(s) for evacuation during a flood event and for a particular case study. The results of the application of two criteria are analysed in terms of the flood hazard assessment with the two criteria being based on a widely used empirical approach and a mechanics based approach. Both criteria are used to assess the flood hazard to people during an extreme flash flood, which occurred on 16th August 2004 in Boscastle (UK). Results obtained for this study have highlighted that the mechanics based criteria are preferable in identifying the ideal escape routes, when considering the flood characteristics and the corresponding response of a human body. The main novelty of this study lies in linking the flood hazard rating with the human body characteristics, when determining the safest route and with a revised formula being developed, which includes the effects of ground slope in the application to a real case study. [ABSTRACT FROM AUTHOR]

Prolonged droughts can cause environmental stress, even in ecosystems that are already adapted to dry conditions. In these areas life greatly depends on rare, sporadic rainfall events that can produce flash floods. Today, urban expansion, population growth and development are occurring worldwide, including along hyperarid coastlines, an ecological zone on the extreme of habitability. In such places, the absence of the already limited precipitation can lead to long‐lasting damage, and recognizing drought conditions in the past is useful for planning. Recognizing droughts, however, is challenging because they are not known as depositional events, and therefore may be under‐recognized in the sedimentological record. Floods in these hyperarid deserts carry eroded sediments and deposit them in either terrestrial or marine terminal basins. In the hyperarid (ca 30 mm rain per year) desert surrounding the northern Gulf of Aqaba‐Eilat some flash floods reach the sea and deposit alluvial sediments (mostly silt and clay) on the shelf. Following a recent 17 year drought period (1995 to 2012) a coarser surface layer was recognized offshore the outlet of an ephemeral river, despite the lack of incoming flood material. This coarsened layer resulted from a prolonged period wherein mixing and winnowing of fine particles at the surface was not interrupted by recurrent flash floods. In the bottom of two sediment cores collected from the same shelf, a similarly coarsened layer was observed. The terminal level of this deep coarse layer was dated to ca 660 yr bp, corresponding with the end of the Medieval Warm Period (ca 1350 ad). These findings suggest that the medieval anomaly resulted in a prolonged drought period in this already arid region. These results present a new interpretation of grain‐size distribution records that provides a means to reconstruct drought histories in hyperarid regions globally. [ABSTRACT FROM AUTHOR]

In November 2017, a high‐intensity storm caused a catastrophic flash flood that devastated the city of Mandra, in Greece, and led to the tragic loss of 24 people. The storm caused flooding in the two main tributaries of the basin, creating two very similar hydrological responses with similar flood characteristics that hit two different environments: one within the city and one outside. This work examines the fatal incidents that occurred in relation to the characteristics of the surrounding environment, to investigate the role of the latter in flood mortality. Then, the analysis carried out for the 2017 flood is expanded to a broader database of flood deaths for Greece (1960–2018) for comparison. Results show that certain mortality characteristics differ substantially depending on the setting. Outdoor incidents are more abundant in nonurban environments, in which victims exhibit mostly an active or risk‐taking behaviour against the imminent risk. Urban environments are characterised by a greater diversity of victim activities, higher percentages of indoor deaths, especially for older victims and more passive behaviours. Overall, the study presents evidence on how risk situations develop differently between urban and nonurban settings. Findings are relevant in shaping policy and education programs aiming to mitigate risk. [ABSTRACT FROM AUTHOR]

Extreme high‐magnitude and low‐frequency storm events in arid zones provide the necessary runoff to entrain sediments from source areas and therefore dictate the linkages between hillslopes and channels. Nevertheless, the erosive impact of large storms remains difficult to predict. Most of the uncertainty lies in the lack of topographic change maps associated with single hydro‐meteorological events. Consequently, event‐based erosion models are poorly constrained and their extrapolation over long time periods remains uncertain. In this study, a 15‐month Sentinel‐1A coherence time series, optical and field data are used to map the spatial patterns of erosion after the 5‐day storm occurred on March 2015, in the Atacama Desert. The coherence change detection (CCD) analysis suggests that temporal loss of coherence is related to variations in soil moisture, while permanent loss of coherence is related to modification of soil texture by erosion and sedimentation. Importantly, permanent loss of coherence is more apparent on gentle rather than steeper slopes, likely reflecting differences in regolith cover and thickness. These findings can contradict the landscape models predicting higher erosion on steeper hillslopes. The CCD technique represents a promising tool for analysing and modelling sediment connectivity in arid areas, giving a clear picture of the relation between sediment sources and sink pathways. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The aim of this study is to provide an evaluation of the impact of two largely used data assimilation techniques, namely three‐ and four‐dimensional variational data assimilation systems (3D‐Var and 4D‐Var), on the forecasting of heavy precipitation events using the Weather Research and Forecasting (WRF) model. For this purpose, two flash flood events in central Italy are analysed. The first occurred on September 14, 2012 during an Intensive Observation Period of the Hydrological cycle in the Mediterranean experiment (HyMeX) campaign, while the other occurred on May 3, 2018. Radial velocity and reflectivity acquired by C‐band weather radars at Mt. Midia (central Italy) and San Pietro Capofiume (northern Italy), as well as conventional observations (SYNOP and TEMP), are assimilated into the WRF model to simulate these damaging flash flood events. In order to evaluate the impact of the 3D‐Var and 4D‐Var assimilation systems on the estimation of short‐term quantitative precipitation forecasts, several experiments are carried out using conventional observations with and without radar data. Rainfall evaluation is performed by means of point‐by‐point and filtering methodologies. The results point to a positive impact of the 4D‐Var technique compared to results without assimilation and with 3D‐Var experiments. More specifically, the 4D‐Var system produces an increase of up to 22% in terms of the Fractions Skill Score compared to 3D‐Var for the first flash flood event, while an increase of about 5% is achieved for the second event. The use of a warm start initialization results in a considerable reduction in the spin‐up time and a significant improvement in the rainfall forecast, suggesting that the initial precipitation spin‐up problem still occurs when using 4D‐Var. [ABSTRACT FROM AUTHOR]

Synchronously and accurately estimating the flood discharges and dynamic changes in the fluid density is essential for hydraulic analysis and forecasting of flash floods, as well as for risk assessment. However, such information is rare for steep mountain catchments, especially in regions that are hotspots for earthquakes. Therefore, six hydrological monitoring sites were established in the main stream and tributaries of the 78.3‐km2 Longxi River catchment, an affected region of the Wenchuan earthquake region in China. Direct real‐time monitoring equipment was installed to measure the flow depths, velocities, and fluid total pressures of the flood hydrographs. On the basis of field measurements, real‐time mean cross‐sectional velocities during the flood hydrographs could be derived from easily obtainable parameters: cross‐sectional maximum velocities and the calibrated dimensionless parameter Kh. Real‐time discharges were determined on the basis of a noncontact method to establish the effective rating curves of this mountainous stream, ranging from 1.46 to 386.34 m3/s with the root mean square errors of ≤10.22 m3/s. Compared with the traditional point‐velocity method and empirical Manning's formula, the proposed noncontact method was reliable and safe for monitoring whole flood hydrographs. Additionally, the real‐time fluid density during the flood hydrographs was calculated on the basis of the direct monitoring parameters for fluid total pressures and water depths. During the flood hydrograph, transient flow behaviour with higher fluid density generally occurred downstream during the flood peak periods when the flow was in the supercritical flow regime. The observed behaviour greatly increased the threat of damage to infrastructure and human life near the river. Thus, it is important to accurately estimate flood discharge and identify for fluid densities so that people at risk from an impending flash flood are given reliable, advanced warning. [ABSTRACT FROM AUTHOR]

Rising salinity is a severe problem associated with land degradation in arid and semiarid regions. This study seeks to understand the spatiotemporal patterns of water salinity and their implication on the downstream water availability in the marginal grabens of northern Ethiopia. We measured the electrical conductivity (EC) of flash floods, baseflows, springs, wells, and lakes. A total of 1,168 samples was collected from 177 spatially distributed water sources (2016–2017). This study shows that there was a significant difference in the EC values of the water points between rainy (1,403 ± 425 μS cm−1) and dry seasons (2,261 ± 258 μS cm−1). Besides, there is an inverse relationship between the river discharges and the runoff EC in the study area. The average value of the water EC was also higher in the limestone area than in the basalt sites. In addition, the water EC increased from the escarpments (396 ± 176 μS cm−1) towards the outlets of the graben basins (1,670 ± 942 μS cm−1). These water EC hotspots in the dry season imply that the river basins are in a closing status due to the erratic rainfall and water extraction by humans. However, the rising water EC and moisture stress endanger the existing agricultural development. Because of the extent and seriousness of salinity, integrated catchment management strategies are needed to tackle the degrading water quality and quantity in the marginal grabens along the northern Ethiopia Rift Valley. [ABSTRACT FROM AUTHOR]

Urban and land use planning is conditioned by flood risk analyses and is a great research challenge. While methods are available for quantitative hazard and risk modeling, there is still a major gap related to reliable rational approaches for vulnerability assessment. This is particularly true for assessing vulnerability of buildings to floods. This vulnerability, which is important for economic reasons as well as for the safety of individuals and communities, is usually conceptualized on an empirical basis. Such empirical approaches, however, are plagued by considerable uncertainties, mostly because the damage‐causing mechanisms on the building envelope are poorly represented. In order to advance this area, this paper focuses on the structural vulnerability of traditional masonry buildings in alpine areas exposed to flash floods. A simplified physically based conceptual model which requires a limited amount of input data is proposed. The resistance of a wall impacted by a flash flood is studied in a limit analysis framework, considering different geometric ratios and building configurations. The resulting stability thresholds, expressed in dimensionless form as a function of the water depth, were positively compared with the results provided by numerical finite element models of the investigated geometries. The application of the proposed vulnerability model in risk mitigation strategies provides reliable information, suitable for a first‐level risk assessment on large building stocks. Key Points: A conceptual model to assess buildings structural vulnerability to flash floods is providedThe model is conceived for application on large stocks of masonry buildingsThe model was positively compared with a finite element analysis [ABSTRACT FROM AUTHOR]

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The present study aims to investigate how large-scale atmospheric circulation may influence flash floods in Portugal. For this purpose, weather types (WTs) in a northeastern North Atlantic sector are used and flash floods between 1950 and 2003 were identified in six hydrographic basins: three in the north of Portugal and another three in the south of Portugal. An adapted methodology for flash flood detection on daily discharges is applied. A total of 131 flash floods were detected: 35 in the northern basins and 96 in the southern basins.WTs are identified using the National Oceanic and Atmospheric Administration 20th Century Reanalysis V2. The sea level pressure composites for flash flood days by WT reveal the large-scale atmospheric patterns underlying their occurrence. The results show that the cyclonic (C) type was strongly associated with flash flood days in the northern basins. The C type pattern features deep low pressure systems north of Iberia, much stronger than average for this WT. For the southern basins, the C type still shows the highest relevance for flood occurrence, but to a lesser extent, since the easterly wind (E) and dual anticyclonic (AA) types also acquire some importance. The flash flood-inducing AA-type events are characterized by atypical largely zonal troughs extending towards Portugal. The flash flood-inducing E type events hint at the occurrence of anomalously strong cut-off low pressure systems over southern Portugal. Although the significance of these systems to precipitation in Portugal was addressed in previous studies, a systematized assessment of their role on flash flood occurrences is herein provided for the first time. The presented flash flood detection methodology can be applied to other regions where hourly flow discharge data are not available, thus being an alternative to the more conventional detection algorithms. [ABSTRACT FROM AUTHOR]

ABSTRACT Sub-daily rainfall extremes may be associated with flash flooding, particularly in urban areas but, compared with extremes on daily timescales, have been relatively little studied in many regions. This paper describes a new, hourly rainfall dataset for the UK based on ∼1600 rain gauges from three different data sources. This includes tipping bucket rain gauge data from the UK Environment Agency (EA), which has been collected for operational purposes, principally flood forecasting. Significant problems in the use of such data for the analysis of extreme events include the recording of accumulated totals, high frequency bucket tips, rain gauge recording errors and the non-operation of gauges. Given the prospect of an intensification of short-duration rainfall in a warming climate, the identification of such errors is essential if sub-daily datasets are to be used to better understand extreme events. We therefore first describe a series of procedures developed to quality control this new dataset. We then analyse ∼380 gauges with near-complete hourly records for 1992-2011 and map the seasonal climatology of intense rainfall based on UK hourly extremes using annual maxima, n-largest events and fixed threshold approaches. We find that the highest frequencies and intensities of hourly extreme rainfall occur during summer when the usual orographically defined pattern of extreme rainfall is replaced by a weaker, north-south pattern. A strong diurnal cycle in hourly extremes, peaking in late afternoon to early evening, is also identified in summer and, for some areas, in spring. This likely reflects the different mechanisms that generate sub-daily rainfall, with convection dominating during summer. The resulting quality-controlled hourly rainfall dataset will provide considerable value in several contexts, including the development of standard, globally applicable quality-control procedures for sub-daily data, the validation of the new generation of very high-resolution climate models and improved understanding of the drivers of extreme rainfall. [ABSTRACT FROM AUTHOR]

ABSTRACT Understanding flash floods in watersheds of the New York City water supply system ( NYCWSS) is important, as turbidity associated with flooding degrades water quality in the unfiltered portions of this water supply system. We examined synoptic-scale atmospheric conditions most frequently associated with flash flooding in this region. Flash floods between 1987 and 2013 were identified in two small watersheds of the NYCWSS using USGS 15-min discharge data at the Esopus Creek near Allaben, NY and Neversink River at Claryville, NY gauges, both located in the Catskill Mountains. A total of 25 flash floods were detected in these watersheds and there were 17 separate flash flood days. The Spatial Synoptic Typer Tools 4.0 were used to characterize the synoptic-scale atmospheric patterns influencing the study area based on NCEP/ NCAR 500-mb geopotential height reanalysis data. Through this procedure, 17 unique synoptic patterns were identified. Three of these types were found to be strongly associated with flash flooding events. Composites of these types show southwesterly flow which suggests advection of moisture from the Gulf of Mexico and Atlantic Ocean. The flash flood days were compared to the National Weather Service flash flood warnings. The flash flood warnings issued for Ulster County compared to the flash floods in the study watersheds highlight the highly localized nature of flash flooding in this region. [ABSTRACT FROM AUTHOR]

Boulders moving in flash floods cause considerable damage and casualties. More and bigger boulders move in flash floods than predicted from published theory. The interpretation of flow conditions from the size of large particles within flash flood deposits has, until now, generally assumed that the velocity (or discharge) is unchanging in time (i.e. flow is steady), or changes instantaneously between periods of constant conditions. Standard practice is to apply theories developed for steady flow conditions to flash floods, which are however inherently very unsteady flows. This is likely to lead to overestimates of peak flow velocity (or discharge). Flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean-flow drag. These transient forces, generated by rapid velocity changes, are generally ignored in published theories, but they are briefly so large that they could initiate the motion of boulders. This paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow, and discusses the implications. [ABSTRACT FROM AUTHOR]

Extreme precipitation in the arid Middle East can cause flash floods with dramatic societal impacts. This study investigates the synoptic-scale dynamics of three extreme precipitation events that occurred in Saudi Arabia in autumn, winter and spring. Using ERA-Interim reanalysis, soundings and observational precipitation data, we study precipitation characteristics, the synoptic circulations, moisture transport pathways and forcing mechanisms for upward motion. All three cases involved strong tropical-extratropical interactions whereby midlatitude forcing instigated an incursion of tropical moisture over the Arabian Peninsula that fuelled the heavy rainfall. In each case, a midlatitude upper-level trough, associated with anticyclonic Rossby wave breaking, intruded into the subtropics. The phase relationship between this trough and the tropical low-level circulation was consistent with wave amplification through baroclinic growth. Eulerian and Lagrangian analyses reveal moisture transport from nearby and remote tropical regions, leading to above-normal tropospheric moisture content over Saudi Arabia. The autumn case (November 2009) showed a transient midlatitude upper-level trough that interacted with the climatological Red Sea Trough near the surface, being an 'Active Red Sea Trough' event. The winter case (January 2005) resembled tropical plume-like characteristics and demonstrated the coupling of a midlatitude cyclone and the equatorial low-pressure zone over Africa, an intensified subtropical jet stream, and pronounced moisture fluxes at middle and upper levels. The spring case (April-May 2013) involved a quasi-stationary cut-off low and persistent advection of low-level moist air masses, partly from the south Indian Ocean through cross-equatorial flow. The forcing of ascent was associated with low-level moisture convergence and decreased static stability (autumn case), dynamical lifting (winter case), strong surface sensible heating (spring case), and orographic lifting (all cases), favouring the build-up and release of potential instability. We discuss the three cases from a seasonal perspective and present a synthesis of their common key synoptic features. [ABSTRACT FROM AUTHOR]

Extreme weather events affect surface water supplies. Water quality measurements in Central Arizona were used to quantify changes in dissolved organic carbon (DOC) and turbidity following dust storms, floods, and drought-associated wildfires. This article describes specific extreme events, quantifies changes in DOC and turbidity, and discusses the effects of water quality changes on treatment processes and public perceptions of water quality. Study findings show a co-occurrence of some dust storm events (high particulate matter) and increased surface water turbidity, attenuation of high DOC loads following upper watershed storm events, significant increases in total organic carbon and turbidity following severe flooding in 2005, and differing effects of upland and lowland wildfires on water quality. Finally, a means to mitigate these effects on both perceived and actual water quality by integrating information from radar systems, stream gauge readings, and online water quality analyzers is suggested. [ABSTRACT FROM AUTHOR]

Understanding how people view flash flood risks can help improve risk communication, ultimately improving outcomes. This article analyzes data from 26 mental models interviews about flash floods with members of the public in Boulder, Colorado, to understand their perspectives on flash flood risks and mitigation. The analysis includes a comparison between public and professional perspectives by referencing a companion mental models study of Boulder-area professionals. A mental models approach can help to diagnose what people already know about flash flood risks and responses, as well as any critical gaps in their knowledge that might be addressed through improved risk communication. A few public interviewees mentioned most of the key concepts discussed by professionals as important for flash flood warning decision making. However, most interviewees exhibited some incomplete understandings and misconceptions about aspects of flash flood development and exposure, effects, or mitigation that may lead to ineffective warning decisions when a flash flood threatens. These include important misunderstandings about the rapid evolution of flash floods, the speed of water in flash floods, the locations and times that pose the greatest flash flood risk in Boulder, the value of situational awareness and environmental cues, and the most appropriate responses when a flash flood threatens. The findings point to recommendations for ways to improve risk communication, over the long term and when an event threatens, to help people quickly recognize and understand threats, obtain needed information, and make informed decisions in complex, rapidly evolving extreme weather events such as flash floods. [ABSTRACT FROM AUTHOR]

This paper presents an integrated methodology for sustainable development planning to conserve the excess water resources. This developed methodology takes advantage of geographic information systems software to integrate its strength to manage the flood impacts. This approach is of dual purpose: first, to reduce the flash flood hazard of hill torrents; and second, to make use of flood water as a source of irrigation by conserving flood water in an earthen reservoir. The developed integrated methodology works as a decision-making system for selection of best site selection for earthen reservoir. The results for three potential sites for earthen reservoir supports in a decision-making process for best site for reservoir by analysing calculations for storage capacity, earthen works, and lengths of canals from reservoirs to agricultural land. This study investigated that the tools of geoinformatics have the potential to resolve the engineering challenges linked with mitigation of flood hazards. It is evident from the study that practical considerations for future turn the study into an informational tool to mitigate flood. [ABSTRACT FROM AUTHOR]

Extreme floods are the most widespread and often the most fatal type of natural hazard experienced in Europe, particularly in upland and mountainous areas. These 'flash flood' type events are particularly dangerous because extreme rainfall totals in a short space of time can lead to very high flow velocities and little or no time for flood warning. Given the danger posed by extreme floods, there are concerns that catastrophic hydrometeorological events could become more frequent in a warming world. However, analysis of longer term flood frequency is often limited by the use of short instrumental flow records (last 30-40 years) that do not adequately cover alternating flood-rich and flood-poor periods over the last 2 to 3 centuries. In contrast, this research extends the upland flood series of South West England (Dartmoor) back to ca AD 1800 using lichenometry. Results show that the period 1820 to mid-1940s was characterized by widespread flooding, with particularly large and frequent events in the mid-to-late 19th and early 20th centuries. Since ca 1850 to 1900, there has been a general decline in flood magnitude that was particularly marked after the 1930s/mid-1940s. Local meteorological records show that: (1) historical flood-rich periods on Dartmoor were associated with high annual, seasonal and daily rainfall totals in the last quarter of the 19th century and between 1910 and 1946, related to sub-decadal variability of the North Atlantic Oscillation and receipt of cyclonic and southerly weather types over the southwest peninsula; and (2) the incidence of heavy daily rainfall declined notably after 1946, similar to sedimentary archives of flooding. The peak period of flooding on Dartmoor predates the beginning of gauged flow records, which has practical implications for understanding and managing flood risk on rivers that drain Dartmoor. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In a continuation of Part I on spatial integration, this paper presents research on the applicability of four satellite-based rainfall algorithms to derive hourly rain intensities in the mid-latitudes using highly resolved data from the first Meteosat Second Generation ( MSG-1) satellite. It focuses on the suitability of these algorithms to detect and monitor (flash) flood-related rain intensities for different temporal resolutions. The temporal integration is examined using six different integration steps ranging from 1 h to 24 h and three different data-filtering methods. The effects of this temporal integration on the quality of satellite-based rain intensities (amounts and locations) are examined with regard to heavy and possible flood-triggering rain intensities. Also, loss of temporal information and possible applications for flood risk management are considered. For this purpose, Saxony ( Germany) is used as an example, and 9 high-rainfall days between May and August 2006 were analysed by comparing satellite rain intensities to rain gauge-adjusted radar data. The results show that temporal integration leads to improvements in evaluation scores, especially for heavy rain intensities, by up to +62%. However, this integration leads to a decreased applicability for the detection of high-intensity rainfall events because they tend to be short in duration. [ABSTRACT FROM AUTHOR]

Flash floods represent one of the deadliest and costliest natural disasters worldwide. The hydrological analysis of a flash flood event contributes in the understanding of the runoff creation process. This study presents the analysis of some flash flood events that took place in a complex geomorphological Mediterranean River basin. The objective of the present work is to develop the thresholds for a real-time flash flood forecasting model in a complex geomorphological watershed, based on high-frequency data from strategically located hydrological and meteorological telemetric stations. These stations provide hourly real-time data which were used to determine hydrological and meteorological parameters. The main characteristics of various hydrographs specified in this study were the runoff coefficients, lag time, time to peak, and the maximum potential retention. The estimation of these hydrometeorological parameters provides the necessary information in order to successfully manage flash floods events. Especially, the time to peak is the most significant hydrological parameter that affects the response time of an oncoming flash flood event. A study of the above parameters is essential for the specification of thresholds which are related to the geomorphological characteristics of the river basin, the rainfall accumulation of an event, the rainfall intensity, the threshold runoff through karstic area, the season during which the rainfall takes place and the time intervals between the rainstorms that affect the soil moisture conditions. All these factors are combined into a real-time-threshold flash flood prediction model. Historical flash flood events at the downstream are also used for the validation of the model. An application of the proposed model is presented for the Koiliaris River basin in Crete, Greece. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

One of the most important goals of synoptic climatology is to analyse the relationships between atmospheric circulation and surface environmental conditions. Since manual (subjective) classification might reflect differently environmental phenomena as compared to computer assisted (semi-objective) classification, a comparison between both classifications was performed for the Eastern Mediterranean (EM) for 10 years. The overall frequencies of the 19 recurring synoptic types characterising the circulation regime over the EM are similar for both methods (correlation coefficient = 0.96, p-value < 0.0001). Nevertheless, comparing the classifications on a day-to-day basis for the three most common types showed a large degree of disagreement between the two. For synoptic types with a deep horizontal pressure gradient both classifications yielded the highest agreement (over 50%), improving the consistency between both classifications. Yarnal's (1993) 'environment-to-circulation' approach was tested on three surface environment processes: air pollutants, desert dust intrusions, and flash floods. Our results indicate that the weak pressure gradients associated with high ozone levels make the classification more difficult for both methods. Regarding dust outbreaks, classifications point on the importance of the cold cyclone location rather than its pressure gradient. As for flash floods, the flow pattern at the surface level is insufficient to predict atmospheric conditions prone for their occurrence, suggesting that upper air data is an essential factor for determining such highly convective events. Finally, two new approaches to evaluate the quality of classifications are presented, demonstrated on two frequent circulation systems persisting over the EM. A qualitative approach compares the composite mean sea level pressure maps from each classification, while a quantitative approach compares the resultant winds at a central site. This study demonstrates that comparing different synoptic classification methodologies and different environmental applications can lead to additional and valuable insights on the interaction between the environment and synoptic-scale circulation. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Flash flooding is one of the most hazardous natural events, and it is frequently responsible for loss of life and severe damage to infrastructure and the environment. Research into the use of new modelling techniques and data types in flash flood forecasting has increased over the past decade, and this paper presents a review of recent advances that have emerged from this research. In particular, we focus on the use of quantitative precipitation estimates and forecasts, the use of remotely sensed data in hydrological modelling, developments in forecasting models and techniques, and uncertainty estimates. Over the past decade flash flood forecast lead-time has expanded up to six hours due to improved rainfall forecasts. However the largest source of uncertainty of flash flood forecasts remains unknown future precipitation. An increased number of physically based hydrological models have been developed and used for flash flood forecasting and they have been found to give more plausible results when compared with the results of conceptual, statistical, and neural network models. Among the three methods for deciding flash flood occurrence discussed in this review, the rainfall comparison method (flash flood guidance) is most commonly used for flash flood forecasting as it is easily understood by the general public. Unfortunately, no existing model is capable of making reliable flash flood forecasts in urban watersheds even though the incidence of urban flash flooding is increasing due to increasing urbanisation. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article presents a study which cites a new way of modeling flash floods in dryland catchments in the southern Eastern Desert of Egypt. This is done by integrating remote sensing and digital elevation model (DEM) data in a geographical information system (GIS). It uses images of the Wadi Hudain catchment in southern Egypt to infer the hillslope areas contributing flow's flood events. Meanwhile, the flow direction, active channel cross-sectional areas and slope, and flow length were derived via the SRTM3 DEM. The channel flow velocities and the time-area zones of the catchment were estimated using the Manning Equation. Based on results, the marked increase in brightness of channel bed materials within the active channels implies the occurrence of runoff events in the past 20 years.

The article discusses the reach-scale biogeochemical model for temporary rivers. This study offers information on the reach-scale temporary flow biogeochemical model. It is a conceptual model that describes the hydrologic process and other chemical processes. The model discussed here stimulates sediment transport and nutrient fluxes in coastal areas caused by flash flood in extreme rains. The model is for assessment and quantification of hydrologic and geochemical processes in temporary river reach in Greece.

Semiarid flash floods pose a significant danger for life and property in many dry regions around the world. One effective way to mitigate flood risk lies in implementing a real-time forecast and warning system based on a rainfall-runoff model. This study used a semiarid, physics-based, and spatially distributed watershed model driven by high-resolution radar rainfall input to evaluate such a system. The predictive utility of the model and dominant sources of uncertainty were investigated for several runoff events within the U.S. Department of Agriculture Agricultural Research Service Walnut Gulch Experimental Watershed located in the southwestern United States. Sources of uncertainty considered were rainfall estimates, watershed model parameters, and initial soil moisture conditions. Results derived through a variance-based comprehensive global sensitivity analysis indicated that the high predictive uncertainty in the modeled response was heavily dominated by biases in the radar rainfall depth estimates. Key model parameters and initial model states were identified, and we generally found that modeled hillslope characteristics are more influential than channel characteristics in small semiarid basins. We also observed an inconsistency in the parameter sets identified as behavioral for different events, which suggests that model calibration to historical data is unlikely to consistently improve predictive performance for different events and that real-time parameter updating may be preferable. [ABSTRACT FROM AUTHOR]

Distributions of scour and fill depths recorded in three low-order sand bed dryland rivers were compared with the Weibull, gamma, exponential, and lognormal probability density functions to determine which model best describes the reach-scale variability in scour and fill. Goodness of fit tests confirm that the majority of scour distributions conform to the one-parameter exponential model at the 95% significance level. The positive relationship between exponential model parameters and flow strength provides a means to estimate streambed scour depths, at least to a first approximation, in comparable streams. In contrast, the majority of the fill distributions do not conform to the exponential model even though depths of scour and fill are broadly similar. The disparities between the distributions of scour and fill raise questions about notions of channel equilibrium and about the role of scour and fill in effecting channel change. [ABSTRACT FROM AUTHOR]

The provinces of northern Iran that border the Caspian Sea are forested and may be prone to increased risks of flooding due to deforestation and other land use changes, in addition to climate change effects. This research investigated changes in runoff from a small forested catchment in northern Iran for several land use change scenarios and the effects of higher rainfall and high antecedent soil moisture. Peak discharges and total runoff volumes from the catchment were estimated using the U.S. Soil Conservation Service "Curve Number" (SCS‐CN) method and the SCS dimensionless unit hydrograph. This method was selected for reasons of data availability and operational simplicity for flood managers. A geographical information system (GIS) was used to manipulate spatial data for use in the catchment runoff modelling. The results show that runoff is predicted to increase as a result of deforestation, which is dependent on the proportion of the catchment area affected. However, climate change presents a significant flood hazard even in the absence of deforestation. Other land use changes may reduce the peak discharges of all return period floods. Therefore a future ban on timber extraction, combined with agricultural utilisation of rangeland, could prove effective as "nature‐based" flood reduction measures throughout northern Iran. [ABSTRACT FROM AUTHOR]