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Global warming and climate change cause large temperature oscillations and uneven annual rainfall patterns. The rainy cycles characterized by frequent high-intensity rainfall in the area of the Stubo–Rovni water reservoir, which in 2014 peaked at 129 mm of water in 24 h (the City of Valjevo, the Republic of Serbia), caused major floods in the wider area. Such extremes negatively affect erosion processes, sediment production, and the occurrence of flash floods. The erosion coefficient before the construction of the water reservoir was Zm = 0.40, while the specific sediment production was about 916.49 m3∙km−2∙year−1. A hydrological study at the profile near the confluence of the Jadar and Obnica rivers, i.e., the beginning of the Kolubara river, the right tributary of the Sava (in the Danube river basin), indicates that the natural riverbed can accommodate flows with a 20% to 50% probability of occurrence (about 94 m3/s), while centennial flows of about 218 m3/s exceed the capacities of the natural riverbed of the Jadar river, causing flooding of the terrain and increasing risks to the safety of the population and property. The paper presents the impacts of the man-made Stubo–Rovni water reservoir on the catchment area and land use as the primary condition for preventing erosion processes (specific sediment production has decreased by about 20%, the forest cover increased by about 25%, and barren land decreased by 90%). Moreover, planned and controlled management of the Stubo–Rovni reservoir has significantly influenced the downstream flow, reducing the risks of flash floods. [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]

A flash flood is one of the hazardous phenomena, especially in dry regions. In Saudi Arabia, Jazan Province experiences data scarcity especially historical hydrological data and a lack of studies related to flash flood analysis and the rainfall and runoff interrelationship. Thus, this research aims to study the rainfall-runoff inter-relation, predict flash floods, and to map the risk areas in Jazan Province by the geological, geomorphological, and hydrogeological characteristics along with digital elevation model (DEM), watershed modeling system (WMS) and HEC-HMS models. Jazan Province encompasses 25 drainage basins, receiving a considerable amount of rainfall (ranging from 100 to 500 mm) (August, October-November, March) which intermittently cause strong and destructive flash floods. The DEM was used for delineating the catchment (drainage basins) parameters. Physiographic parameters of the catchments have been analyzed for mapping the hazard degree of the flash flood strength. Further, basins with high hazard degrees of flash floods were selected to assess rainfall-runoff inter-relation using the HEC-HMS models, GIS, and morphometric parameters. Forty %, 8%, 52% of the study area are high, medium, and low hazard degrees of flash floods, respectively. The groundwater recharge in the study area was calculated through the integration of satellite image analysis, SWAT and GIS techniques and it ranges from 0.002 mm/km2/year to 8 mm/km2/year with an average of 2.5 mm/km2/year. Rainfall-runoff inter-relation of study basins, assessed based on the integration of WMS and HEC-HMS models, indicates that the resulting runoff volume ranges from 18.5 × 106 m3 to 473.1 × 106 m3 at a recurrence period of 5 and 100 years at rainfall events of 65 mm and 116.8 mm, respectively. [ABSTRACT FROM AUTHOR]

Low-impact development practices (LIDs) have been used to reduce the impacts of flash floods in Brazil. However, there is a lack of investigations on their construction costs concerning the effects on the mitigation of flash floods. This paper addresses the construction costs and the runoff reduction by using seven scenarios of LIDs for a 32-km2 basin in the Midwest Brazil region. The LIDs considered were permeable pavement (PP), rainwater harvesting (RWH), and infiltration trenches (IT). The first three LID scenarios considered PP, RWH, and IT techniques, respectively, and the other four used different combinations of the three LIDs. Scenario 3 showed a decrease of 15% in the runoff, achieved at a lower cost using only IT. This result corresponds to a 2.6% runoff reduction per $1 million spent. These analyses may help in the identification of the balance between benefit and cost by local stakeholders for the use of LIDs. [ABSTRACT FROM AUTHOR]

The objective of this study is to simulate the rainfall-runoff process of mountain torrents in the Shou River Basin located in the southern part of Wenchuan County, Sichuan Province. The basin encountered a significant flood disaster on August 20, 2019, resulting in substantial human and economic losses, as well as limited hydrological data availability. This research employs a unidirectional coupling approach by integrating the Weather Research and Forecasting Model (WRF) with the Storm Water Management Model (SWMM) to accurately simulate rainfall-runoff processes. To account for the non-uniform distribution of precipitation within mountainous watersheds, high-precision rainfall data generated by WRF are extracted and utilized as input for the SWMM model. The quantitative comparison between the evolution of spatial distribution of rainstorms and hourly precipitation processes demonstrates that the WRF model effectively captures dynamic changes in rainstorms. Analysis results indicate a trend of rainfall center movement from midstream to downstream, with sub-watersheds' superimposed effect on rainfall response leading to increased flood peak response, duration, and destructive power. This study provides an effective method and reference for predicting and assessing mountain flood disasters in data-scarce areas such as the Shou River Basin. [ABSTRACT FROM AUTHOR]

This article introduces a groundwater vulnerability assessment model that utilises the fuzzy analytic hierarchy process (FAHP) in the Wadi AlHasa catchment, Jordan. The assessment takes into account both geomorphological and hydrogeological variables, employing a comprehensive methodology that integrates various parameters. To evaluate the catchment, the study employs remote sensing and Geographic Information System (GIS) techniques. The analysis of the digital elevation model enables the creation of a map illustrating the diverse geomorphology of the catchment. This geomorphology significantly influences drainage density, direction, and the spatial distribution and intensity of flash flood events. Moreover, the study develops and maps a fuzzy FAHP DRASTIC vulnerability index, which proves to be a valuable tool for assessing the susceptibility of groundwater resources to contamination. The unique feature of the index is its ability to incorporate uncertain or subjective data, providing a means to evaluate the significance of various influencing factors. This information serves as critical support for decision-making and management efforts geared towards safeguarding and enhancing groundwater resources. Within the study area, the DRASTIC vulnerability index values span from 0.08325 to 0.28409, with 18% of the site exhibiting a high vulnerability rate. Additionally, the article implements a managed aquifer recharge model (MAR), with 31% of the area falling into MAR classes. Among these, 22.1% are classified as a high MAR class, while 0.7% belong to a very high MAR class. These findings underscore the feasibility of MAR projects in regions with limited water resources. [ABSTRACT FROM AUTHOR]

Flash floods are sudden flood disasters that can be triggered by several factors, one of which is landslides that occur in the upper watershed. In Bogor Regency, there are 14 sub-districts located in the Upper Cisadane watershed area that are prone to flash flood disasters. This study aimed to determine the social, economic, physical, and environmental vulnerability assessment of the community in the Upper Cisadane watershed area based on the modification assessment from Regulation of The Head of National Disaster Management Authority Number 12 of 2012. This vulnerability assessment is part of disaster risk assessment, which is an approach to show the potential negative impacts of a disaster that occurs in an area. This potential negative impact can be seen in the potential number of lives exposed, property loss, and environmental damage. According to the vulnerability index, the Upper Cisadane watershed has high and very high classes of flash flood vulnerability, with ten (10) sub-districts having high vulnerability index classes, and fourteen (14) districts having very high vulnerability index classes. The level of vulnerability in the sub-districts is influenced by the level of social, physical, and economic vulnerability, which has high to very high classes compared to other sub-districts. The vulnerability index class maps from this study are expected to be used as references for local governments and related parties in regional spatial planning and flash flood disaster mitigation planning. [ABSTRACT FROM AUTHOR]

The study area is located in the Ulaanbaatar basin and is characterized by complicated environmental conditions determined by the peculiarities of an extremely continental climate with high rainfall intensity, a structural–tectonic pattern, the geological structure of the Ulaanbaatar intermountain depression, high seismic activity reaching 8 points, and the occurrence of catastrophic flash floods. In the conditions of rapid urban expansion, the high population density and housing development of Ulaanbaatar, and the implementation of large investment projects, it is an urgent task to assess the exogeodynamic hazard of the Ulaanbaatar catchment basin and develop the concept of geoecological safety of the territory. These methods and approaches consist of combining a complex of methods of engineering geology and geomorphology: monitoring the dynamics of exogenous processes; environmental assessment; the analysis of development factors (climatic, geomorphologic, tectonic, and lithological) and interaction of processes; laboratory methods of analyzing the composition, structure, and properties of soils; a detailed morphometric analysis of catchment areas; and the application of the basin approach. Based on the analysis of topographic maps on a scale of 1 : 50 000 and of digital relief models in GIS programs, the parameters of the drainage network of the catchment areas and of energy and basin geometry were calculated and reflected on map layers, and specialized maps were compiled. A detailed assessment of erosion-prone soils (composition, structure, and specific properties) revealed a moderate resistance of sediments to geodynamic impacts. As a result, maps of flood hazard of catchments under different rainfalls—short intensive heavy rainfalls and long-term rains—were compiled. The elaborated concept of geoecological safety of territories includes successive stages of works aimed at (a) identifying the exogeodynamic hazard, (b) the organization and implementation of monitoring works, (c) assessing geoecological safety with respect to hazardous exogenous geological processes, and (d) the development of recommendations for further use and specification of information. [ABSTRACT FROM AUTHOR]

Arid regions are prone to unprecedented extreme rainfall events that often result in severe flash floods. Using near-real-time precipitation data in hydrological modelling can aid in flood preparedness. This study analyzed rainfall data obtained from Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG V. 06) since 2001 to highlight recent trends of extreme rainfall indices for three selected watersheds in the UAE. Additionally, to validate the trends, the present study incorporated CHIRPS (Climate Hazards Group InfraRed Precipitation with Station data) into the analysis. Furthermore, for the first time, this study assessed the performance of the three products of IMERG in modelling flash flood events in the selected watersheds of UAE. A physical-based, fully distributed model was used to simulate the heaviest storm event. Also, a sensitivity analysis of the model's output to variations in the input parameters was conducted using the one-factor-at-a-time method. The result of the trend analysis indicated that IMERG and CHIRPS show similar trends in both datasets, indicating agreement and reliability in their observations. However, there are a few instances where IMERG and CHIRPS show slight discrepancies in the nature of the trend. In general, the trend analysis results indicated an increasing trend of total precipitation (mm) and consecutive wet days, which suggests a rise in the risk of flash floods. The simulation of the flash flood event showed that the IMERG final product outperformed the other two products, closely matching the model output of the gauge rainfall data with mean absolute error (MAE) of 1.5, 2.37, and 0.5 for Wadi Ham, Wadi Taween, and Wadi Maidaq, respectively. The model's performance was positively correlated with the size of the watershed. The sensitivity analysis results demonstrated that the model's output was most sensitive to infiltration parameters. The study's outcomes provide a good opportunity to improve near-real-time impact evaluation of flash flood events in the watersheds of the UAE. [ABSTRACT FROM AUTHOR]

Climate change has a direct impact on flash floods, and indirectly on the environment, society, and economy, due to the rapid development and difficulty of predicting this hydrological phenomenon. The main objective of this study is to assess the potential flash flood hazard areas in the Likodra watershed (218.62 km2), one of the most vulnerable parts to flash floods in Serbia, using the flash flood potential index (FFPI) and analytic hierarchy process (AHP) method. Recurring events from 1995 to this day and the devastating impact on settlements of the analyzed area show that this territory is extremely vulnerable. The data used include hydrological statistics (maximum daily rainfall) and spatial data on watershed geographical characteristics (slope, soils, land use, vegetation, drainage density) obtained or derived from various sources (maps, satellite images, digital databases) which were integrated into the GIS environment. The results indicate a severe flash flood hazard level, with high flash flood susceptibility classes occupying 76.20%, 87.78%, and 91.73% of the area, depending on the considered criteria and weights assigned to them. [ABSTRACT FROM AUTHOR]

River flooding has been triggering significant damage to lives and infrastructure and is a major worry all around the globe. To lessen these losses, proper planning and management methods need to be deployed. The purpose of this research is to fill a knowledge gap on the effects of reservoirs operation of the Idukki and Idamalyar to Periyar River Basin massive flooding. The proposed methodology is implemented on the Periyar River Basin located in Kerala, India, where severe flooding occurred during monsoon season in the year 2018. In this study, modelling technique has been used in two-step: (1) development of 1D physically based, distributed-parameter model (Soil and Water Assessment Tool, SWAT) to compute the stream flow and estimate the stream discharge at different outlet points; and (2) hybrid model is developed by linking SWAT with a well-known 2D hydrodynamic model (International River Interface Cooperative, iRIC) to display flood scenarios and to identify the flood-prone areas. The ArcSWAT user interface employed in the ArcGIS software was utilized to delineate the river basin. The SWAT model was calibrated and validated on daily and monthly basis at two gauge discharge stations, i.e., Neeleeswaram and Kalady. The statistical coefficients result obtained from SWAT model was in good agreement with the measured values for calibration and validation. The hybrid model simulation results compared with observed flood depth and remote sensing data demonstrated good capability of the model. Agreeable performances of computed results were observed in both flow fields and flood propagations. The result was compared with 2018 flood to check model accuracy and found to be satisfactory. The proposed framework can be utilized as an effective tool for efficient planning and management of natural disasters, such as flash floods. [ABSTRACT FROM AUTHOR]

The aim of this study is to identify and map the zones of different vulnerability to flash floods based on the geospatial analysis of natural conditions in the Porečka River basin (Republic of Serbia). The analysis covers the catchment area of the Porečka River (493.82 km²). Geospatial analysis was conducted using GIS software (QGIS 3.18). The Flash Flood Potential Index (FFPI) was used to determine the terrain's predisposition to flash floods, where the input data for determining the value of the index were the values of the following coefficients: the terrain slope, the type of geological substrate, the way of land use, and the bareness of the terrain. The analysis determined that 50.43% of the total territory of the basin belongs to the class of high and very high susceptibility to flash floods, and when looking at the length of watercourses in the basin, that percentage is 81.94%. The results of this study clearly indicate the advantages of using modern GIS technologies in the land use and risk management. Geospatial analysis is of particular importance in the field of managing regions that stand out as particularly vulnerable to some natural disasters. [ABSTRACT FROM AUTHOR]

This study addresses a crucial gap in our understanding of flooding in the predominantly agricultural Qaa'Jahran watershed in Yemen, where urbanization, though currently limited in extent, is rapidly expanding. We aim to discern the effects of earth fissures on flood dynamics, particularly focusing on their present significance. It is noteworthy that, in the long run, these fissures are anticipated to accumulate soil and sediment, potentially heightening the risk of future flooding. To achieve this, we developed a comprehensive floodplain delineation model, incorporating field observations and rainfall data from the Integrated Multi-satellitE Retrievals for GPM (IMERG) algorithm. Additionally, advanced hydrologic and hydraulic models were integrated to accurately simulate runoff hydrographs and create flood inundation maps. Our findings underscore the considerable influence of earth fissures, effectively mitigating runoff volume, peak discharge, flood depths and inundated areas during storm events. It is imperative to emphasize that specific areas remain persistently vulnerable, with water levels surpassing one meter during historical floods. This reinforces the ongoing risk of flooding in targeted zones and emphasizes the necessity of implementing tailored flood management strategies. In conclusion, this study makes a significant contribution to the field, providing a comprehensive analysis of the intricate factors influencing flooding in the Qaa'Jahran watershed. The integration of flood inundation mapping, advanced hydrologic modelling and meticulous consideration of earth fissures offers a distinctive perspective on flood dynamics in a rapidly evolving and urbanizing environment. Furthermore, it highlights the critical implications for effective flood risk management and urban planning, not only in the Qaa'Jahran watershed but also in similar urbanizing regions worldwide. [ABSTRACT FROM AUTHOR]

A new, fundamental catchment attribute called the hydrologic time scale τ governs the rate of delivery of runoff to a particular site, and is equal to ∫Qdt/∫∣dQ∣, where Q is discharge and t is time. The value of τ for any gauged site is readily calculated from tabulated discharge data by replacing the integrals with sums. This quantity, coupled with the square root of catchment area, A , form a coordinate pair that embodies the characteristic time and length scales for any catchment, which govern its flow dynamics. The fitting constants used in several unit hydrograph models are simple multiples of τ, so knowledge of τ allows rapid calibration of these models for the particular site, facilitating flow prediction from rainfall data. Values of τ reflect many different landscape attributes, but for multiple sub-basins in watersheds with homogeneous land use and lithologic conditions, they correlate linearly with A . The ratio A / τ provides a characteristic velocity that is high for channelized, flood-prone rivers, for flashy urban streams with high impervious cover, and for sites downstream of hydro-power dams. Sites with low velocities are resistant to flooding, as their landscapes have a greater ability to delay the delivery of runoff by retention, detention, and infiltration into the groundwater system. [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]

The hydrological processes of a catchment are the function of climate, land use and land cover. Changes in either climate or land use or land cover can result in alteration of the catchment's hydrological processes. In the recent past, Msimbazi catchment in Dar es Salaam has undergone drastic land cover changes mainly due to urbanisation. These land cover changes caused changes in the behaviour of river flow resulting in frequent floods. Therefore, this study analyses the impacts of the changes in land cover due to urbanisation specifically with the changes in river flow, surface run-off and base-flows. Previously generated land cover maps of Msimbazi catchment and a combination of spatial and meteorological climate datasets were used to parameterise the hydrological model (SWAT). The model was calibrated and validated using the Sequential Uncertainty Fitting algorithm (SUFI-2) on a monthly resolution. The results show that there is an increase in surface run-off, mean river flow and the reduction of base-flow with the increase in urbanisation within the catchment. These increase in river flows; surface runoff and reduction of base-flow indicates the likelihood of an increase in flooding events in the catchment. [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]

Flash flood disaster ranks top among all the natural hazards across the world due to its high frequency, severity and fatality. However, flash flood simulation is still challenging in small and medium-sized catchments with complex orography, flashy hydrological responses and poor observations. Three distributed hydrological models, i.e., TOPModel, HEC and CNFF, are selected to simulate flash floods in seven humid and six semi-humid catchments in China, with consideration of water balance (RER), peak flow rate (REQ) and its occurrence time (TP), hydrograph variation (SNSE) and model uncertainty. Influences of five catchment attributes are further investigated on individual model performances. All three models perform satisfactorily in humid catchments, but less satisfactorily in semi-humid catchments. Water balance is well obtained by CNFF, followed by HEC and TOPModel. Peak flow rate and its occurrence time are most accurately captured by CNFF and HEC, respectively. Hydrograph variations are well reproduced by HEC and CNFF. TOPModel performs well for picking peak flow and hydrograph variation in humid catchments. Uncertainty interval is narrowest for HEC with average relative interval length at 95% confidence level being 0.78 ~ 2.53. Most observations are bracketed by uncertainty intervals for TOPModel (64.79% ~ 91.91% of total). Three model performance indices (i.e., RER, REQ, and SNSE) are mainly affected by drainage area and forest ratio across humid and semi-humid catchments, while TP performance is mainly affected by mean slope in humid catchments. [ABSTRACT FROM AUTHOR]

The knowledge of spatio-temporal dynamics of soil moisture within the catchment is very important for rainfall–runoff modelling in flood forecasting. In this study the comparison between remotely sensed soil moisture and soil moisture estimated from the SHETRAN hydrological model was performed for small and flashy Jičinka River catchment (75.9 km2) in the Czech Republic. Due to a relatively coarse spatial resolution of satellite data, the satellite soil moisture data were downscaled, by applying the method developed by Qu et al. (2015). The sub-grid variability of soil moisture was estimated on the basis of the mean soil moisture for the grid cell and the known hydraulic soil properties. The SHETRAN model was calibrated and verified to the observed streamflow hydrographs at the catchment outlet. The good correlation between the two different soil moisture information was obtained according to the majority of applied criteria. The results of the evaluation criteria indicate that the downscaled remotely sensed soil moisture data can be used as additional criteria for the calibration and validation of hydrological models for small catchments and can contribute to a better estimation of parameters, to reduce uncertainties of hydrological models and improve runoff simulations. [ABSTRACT FROM AUTHOR]

• A categorisation method for detection of worst-case rain storms was derived. • A catalogue of 24 catastrophic rain series with 1-min resolution was presented. • Hydrodynamic simulations ranked the storms for different hazard attributes. • Hyetograph shape played an important role to understand flashiness of floods. In this study, a categorisation method was developed and applied to identify, select and rank reasonable worst-case rainfall storm events with time-series resolution of one minute and duration of up to one hour. The method yields a catalogue of 24 reasonable worst-case rainfall series, i.e. potentially catastrophic events that could be used for safe-to-fail flash flood model scenarios, thus encouraging the testing of critical infrastructure and pedestrian safety for people trapped in floods. The identified 24 events were shown to be considerably more extreme than corresponding design storms, with a return interval of 100 years. In the selection of the extremes, special care was taken to consider different hazard attributes, which are associated with intrinsically different observed hyetograph shapes, including rectangular, short and long triangle, and double peak shape categories. Hydrodynamic simulations were carried out using the 2D robust shallow water model hms++ to quantify the flooding impact of the 24 rainfall scenarios in a small urbanised catchment, which enabled the ranking of the scenarios based on different hazard attributes. Overall, the rectangular and long-triangle events showed the highest total discharge, and the rectangular events comprise both the longest duration for hazard velocity and deep flooding. The long-triangle events presented the highest peak discharge and peak water depths. The short-triangle ones showed the highest flashiness (shortest time to peak), whereas the double peak events yielded the longest time to peak. The ranking emphasised the importance of the hyetograph shape characteristics of extreme storms and which needs to be taken into account in flash flood management. The novel categorisation method is easily transferable to other regions, if high-resolution rain series are available. [ABSTRACT FROM AUTHOR]

Floods are considered one of the most severe natural disasters worldwide. They impact vast areas, particularly in arid/semi-arid regions, causing serious damages with thousands of human casualties and billions of Euros in economic losses. This study contributes to a comprehensive evaluation of flash flooding occurrences, impacts, and possible mitigation. In this study, The Dahab region in southern Egypt's Sinai Peninsula was selected for flash flooding vulnerability assessment. Although located in an arid region, it suffers from frequent and severe flash floods. Here, a straightforward workflow was applied to simulate the impact of flash flooding and assess the vulnerability of the Dahab area via consideration of a maximum storm event as a worst- case scenario. Originally, morphometric analysis was performed to determine the most hazardous sub-basins susceptible to flash flooding. The highest recorded storm event in the region was selected to calculate the maximum volume of surface runoff for the model simulation. Then, the hydrologic model and River Analysis System (HEC-RAS) software were used to calculate the inundation level across the entire city of Dahab. Despite some data limitations, this study shows clearly that the Dahab area would have problems incurring from flash flooding if no mitigation measures were to be considered. Results indicate that the area of Dahab is greatly vulnerable to flash flooding with approximately 72% of the total infrastructure being negatively impacted in the worst-case scenario. The adopted approach used in this study can be applied efficiently in similar regions in the Sinai Peninsula or elsewhere. [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]

The aim of this work is to confront the problem of the high rate of missed alarms for flash flood warnings and forecasts due to the random nature of rainfall. In order to improve the accuracy of flash flood warnings,considering the transmission and diffusion principle of probability distributions,and by taking the characteristic parameters of rainfall patterns as the control conditions,a random rainfall pattern method based on parameter control is proposed. In addition,a critical rainfall model of flash floods based on random rainfall patterns and an early warning model considering the risk appetite of decision makers are established. Taking the Peihe watershed in Henan Province,as an example,we performed a comparative analysis of the 6 h critical rainfall under different rain pattern sets,and determined the 6 h critical rainfall threshold space for the study area. The results are as follows:the influence ranges of the precipitation peak position coefficient (r),peak multiple ratio (bmax) and antecedent precipitation (Pa) on the critical rainfall are 32% to 34%,33% to 34%,and 15% to 22%,respectively;the rainfall pattern is found to have a greater impact on the critical rainfall than that of the antecedent precipitation;the 6 h critical rainfall threshold space determined by the double factors of r and bmax is 98 to 185 mm. Therefore,the critical rainfall warning model considering the different risk preferences of decision makers is effective,and has the potential for guiding the more reliable implementation of flash flood warning systems. [ABSTRACT FROM AUTHOR]

This paper concentrates information on the flash flood produced on 30 June 2018 in the Trotuş River basin. Flash floods are extreme phenomena, caused by torrential rainfall, occurring in a very short time in small hydrographical basins without being hydrometric monitored. These phenomena occur with increasing frequency and severity in the last decades both worldwide and in our country and can be manifested in almost any hydrographic area. Thus, in order to diminish the negative effects caused by floods, an important activity is forecasting and warning in case of dangerous hydrological events. Hydrological warning messages are issued with a certain amount of anticipation time, allowing the officials in the field of emergency management to take the necessary measures to protect people's lives and assets, as well as socio-economic objectives. In the operative procedure for elaborating flash floods warnings, the following informations are used from: national network of hydrometric stations, meteorological radar products, Romanian Flash-Flood Guidance System (ROFFG), South East European Flash-Flood Guidance System (SEEFFG), European Flood Awareness System (EFAS). The purpose of this paper is to present the mail informations used in operative procedure to elaborate flash flood warnings, analyzing the case study on the event produced on 30 June 2018 in the Trotuş River Basin. [ABSTRACT FROM AUTHOR]

Flash flood disaster is a prominent issue threatening public safety and social development throughout the world, especially in mountainous regions. Rainfall threshold is a widely accepted alternative to hydrological forecasting for flash flood warning due to the short response time and limited observations of flash flood events. However, determination of rainfall threshold is still very complicated due to multiple impact factors, particular for antecedent soil moisture and rainfall patterns. In this study, hydrological simulation approach (i.e., China Flash Flood-Hydrological Modeling System: CNFF-HMS) was adopted to capture the flash flood processes. Multiple scenarios were further designed with consideration of antecedent soil moisture and rainfall temporal patterns to determine the possible assemble of rainfall thresholds by driving the CNFF-HMS. Moreover, their effects on rainfall thresholds were investigated. Three mountainous catchments (Zhong, Balisi and Yu villages) in southern China were selected for case study. Results showed that the model performance of CNFF-HMS was very satisfactory for flash flood simulations in all these catchments, especially for multimodal flood events. Specifically, the relative errors of runoff and peak flow were within ± 20%, the error of time to peak flow was within ± 2 h and the Nash-Sutcliffe efficiency was greater than 0.90 for over 90% of the flash flood events. The rainfall thresholds varied between 93 and 334 mm at Zhong village, between 77 and 246 mm at Balisi village and between 111 and 420 mm at Yu village. Both antecedent soil moistures and rainfall temporal pattern significantly affected the variations of rainfall threshold. Rainfall threshold decreased by 8-38 and 0-42% as soil saturation increased from 0.20 to 0.50 and from 0.20 to 0.80, respectively. The effect of rainfall threshold was the minimum for the decreasing hyetograph (advanced pattern) and the maximum for the increasing hyetograph (delayed pattern), while it was similar for the design hyetograph and triangular hyetograph (intermediate patterns). Moreover, rainfall thresholds with short time spans were more suitable for early flood warning, especially in small rural catchments with humid climatic characteristics. This study was expected to provide insights into flash flood disaster forecasting and early warning in mountainous regions, and scientific references for the implementation of flash flood disaster prevention in China. [ABSTRACT FROM AUTHOR]

The article provides the result of analysis of the spatial and temporal distribution of flash floods caused by the breakthrough of high mountain lakes on the territory of Uzbekistan. [ABSTRACT FROM AUTHOR]

Flood hazard maps are one of the main components of any flood risk management strategy. It is predicted that the degree of flood risk is going to significantly increase in the future due to climatic and environmental changes, and hence it is increasingly important that state-of-the-art methods are implemented for assessing human stability in floodwaters. Therefore, this paper focuses on proposing more accurate and detailed guidelines for predicting flood hazard indices in small and steep river basins or catchments, prone to the occurrence of flash flooding. The results obtained in this study indicate that for river basins with an average bed gradient greater than 1% (i.e. torrential or flashy river basins or catchments), then the flood hazard indices should be predicted using criteria which are based on the physical interpretation of the processes that affect the human stability in floodwaters, i.e. mechanics based and experimentally calibrated flood hazard assessment methods. [ABSTRACT FROM AUTHOR]

[Display omitted] • Prediction of multi-temporal downscaled data at ungauged stations using adaptive emulator concepts. • Single interfusion algorithm for temporal downscaling of hydrological variables and calibration of physical models. • Hybridization of physical and emulator models to provide accurate daily to sub-daily scale outputs. Unprecedented flash floods (FFs) in urban regions have been rising due to the increasing intensity and magnitude of heavy rainfall resulting from human-induced climate and land-use changes. Modelling of FF along different spatiotemporal scales is extremely complex since FF models require multi-resolution forcing and observed information. In the absence of sub-daily, and multi-site streamflow data, multi-temporal downscaling (MTD) plays a crucial role for FF modeling. While a wide range of methods are available for the spatio-temporal downscaling of climate data, the applicability of the MTD strategy for streamflow has not been investigated yet. The current study proposed a MTD methodology for yielding the daily to sub-daily streamflow of gauged and ungauged stations using adaptive emulator modelling concepts. The proposed MTD framework for ungauged stations is a hybrid model that draws on conceptual and machine learning-based approaches to analyze catchment behavior. The study selected the Peachtree Creek watershed (United States) since it frequently experiences FFs. Results suggest that model-derived sub-daily streamflow had minimal uncertainty in capturing hydrological signatures and nearly 95% accuracy in predicting flow attributes over ungauged stations. The proposed framework can be useful for planning, mitigation, and management, where the fine resolution data is required. [ABSTRACT FROM AUTHOR]

The concurrence of flash floods and debris flows is of particular concern, because it may amplify the hazard corresponding to the individual generative processes. This paper presents a coupled modelling framework for the predictions of flash flood response and of the occurrence of debris flows initiated by channel bed mobilization. The framework combines a spatially distributed flash flood response model and a debris flow initiation model to define a threshold value for the peak flow which permits identification of channelized debris flow initiation. The threshold is defined over the channel network as a function of the upslope area and of the local channel bed slope, and it is based on assumptions concerning the properties of the channel bed material and of the morphology of the channel network. The model is validated using data from an extreme rainstorm that impacted the 140 km 2 Vizze basin in the Eastern Italian Alps on August 4–5, 2012. The results show that the proposed methodology has improved skill in identifying the catchments where debris-flows are triggered, compared to the use of simpler thresholds based on rainfall properties. [ABSTRACT FROM AUTHOR]

Within hydrological models, flow approximations are commonly used to reduce computation time. The validity of these approximations is strongly determined by flow height, flow velocity and the spatial resolution of the model. In this presentation, the validity and performance of the kinematic, diffusive and dynamic flow approximations are investigated for use in a catchment-based flood model. Particularly, the validity during flood events and for varying spatial resolutions is investigated. The OpenLISEM hydrological model is extended to implement both these flow approximations and channel flooding based on dynamic flow. The flow approximations are used to recreate measured discharge in three catchments, among which is the hydrograph of the 2003 flood event in the Fella river basin. Furthermore, spatial resolutions are varied for the flood simulation in order to investigate the influence of spatial resolution on these flow approximations. Results show that the kinematic, diffusive and dynamic flow approximation provide least to highest accuracy, respectively, in recreating measured discharge. Kinematic flow, which is commonly used in hydrological modelling, substantially over-estimates hydrological connectivity in the simulations with a spatial resolution of below 30 m. Since spatial resolutions of models have strongly increased over the past decades, usage of routed kinematic flow should be reconsidered. The combination of diffusive or dynamic overland flow and dynamic channel flooding provides high accuracy in recreating the 2003 Fella river flood event. Finally, in the case of flood events, spatial modelling of kinematic flow substantially over-estimates hydrological connectivity and flow concentration since pressure forces are removed, leading to significant errors. [ABSTRACT FROM AUTHOR]

In this study a hindcast analysis was performed of the 2015 Memorial Day flood for Brays Bayou in Harris County, Texas. Point rainfall totals as high as 27.84 cm (10.96 in.) occurred in this watershed in a 12-h period, causing severe flood damage to 1,185 residential properties. By validating hydrologic and hydraulic models representative of watershed conditions at the time of the storm, this study provides a comprehensive spatial perspective and understanding of the flows and stages, floodplain extents and depths, and residential damages that occurred. In addition, Project Brays, one of the largest urban watershed-scale flood reduction projects in the nation, was nearly halfway complete at the time of the 2015 Memorial Day storm. Subsequently, this study evaluates and compares the flood response to this rainfall event for different watershed conditions representing preproject conditions (2001), May 2015 conditions (current), and postproject conditions (2021 at the earliest). The flood reductions that were provided by Project Brays at the time of the storm and the flood impacts that would have occurred even if the project had been finalized prior to the storm are assessed in detail. [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]

Summary This paper analyzes the Girona River (Spain) flash flood, occurred on the 12th of October 2007, combining hydrological and hydraulic modeling with geomorphologic mapping and post-flood survey information. This research aims to reproduce the flood event in order to understand and decipher the flood processes and dynamics on a system of prograding alluvial fans. The hydrological model TETIS was used to characterize the shape and dimension of the October 2007 Girona River hydrograph. Subsequently, the flood event was reproduced using the free surface flow module of the model RiverFlow2D. The combination of hydrological and hydraulic models was evaluated using post-flood surveys defining maximum flooded area and flood depths. Then, simulations with different peak discharges were carried out to estimate the hydro-geomorphologic response of the Girona River floodplain, through the identification of the activation thresholds in different geomorphic elements. Results showed that the unit peak discharge of the October 2007 flood event (5 m 3 s −1 km −2 ) was among the largest ever recorded in the area, according to the existing literature. Likewise, the hydraulic model showed a good performance in reproducing the flood event (Fit A = 76%, RMSE = 0.65 m and NSE = 0.6), despite the complexity of the case, an ephemeral and ungauged river. The model simulation revealed the existence of an activation pattern of paleochannels and alluvial fans, which was altered by the presence of some anthropogenic disturbances. This multidisciplinary approach proved to be a useful strategy for understanding flash flood processes in ungauged catchments. It allowed understanding the mechanisms governing floods in alluvial fans systems and it represented a solid contribution for early warning plans and risk mitigation policies. [ABSTRACT FROM AUTHOR]

Summary Flash floods represent one of the most destructive natural hazards in the Mediterranean region. These floods result from very intense and spatially heterogeneous rainfall events. Distributed hydrological models are valuable tools to study these phenomena and increase our knowledge on the main processes governing the generation and propagation of floods over large spatial scales. They are generally built using a bottom–up approach that generalizes small-physics representations of processes. However, top–down or data-driven approach is increasingly shown to provide also valuable knowledge. A simplified semi-distributed continuous hydrological model, named SIMPLEFLOOD, was developed, based on the simple dynamical system approach (SDSA) proposed by Kirchner (WRR, 2009, 45, W02429), and applied to the Ardèche catchment in France (2388 km 2 ). This data-driven method assumes that discharge at the outlet of a given catchment can be expressed as a function only of catchment storage. It leads to a 3-parameter nonlinear model according to rainfall and runoff observations. This model was distributed over sub-catchments and coupled with a kinematic wave based flow propagation module. The parameters were estimated by discharge recession analyses at several gauged stations. Parameter regionalization was conducted using a Factorial Analysis of Mixed Data (FAMD) and Hierarchical Classification on Principal Component (HCPC) in order to find relationships between the SDSA approach and catchments characteristics. Geology was found to be the main predictor of hydrological response variability and model parameters were regionalized according to the dominant geology. The SIMPLEFLOOD model was applied for a 12-year continuous simulation over the Ardèche catchment. Four flash flood events were also selected for further analysis. The simulated hydrographs were compared with the observations at 11 gauging stations with catchment size ranging from 17 to 2300 km 2 . The results show a good performance of the model for the continuous and flash flood events occurring under wet conditions, whereas the model underestimates discharge for events occurring after a long dry period. The simple modeling approach provided interesting insight into the Ardèche catchment functioning and offers perspective for a better simulation of flash floods, mainly under wet conditions. [ABSTRACT FROM AUTHOR]

Summary The deadliest storm affecting Portugal since, at least, the early 19th century, took place on the 25 and 26 November 1967 causing more than 500 fatalities. This work aims to assess the most relevant aspects of this episode. This includes describing the associated meteorological conditions and key hydrological characterisation such as the level of exceptionality of the observed precipitation at different temporal scales, or the estimation of peak discharge values in 20 small river catchments affected. Additionally, from a human impact perspective we provide a full account of all the main socio-economic impacts, particularly the numbers and location of victims (dead, injured, homeless and evacuated). Based on the sub-daily time series of a representative station, and its Intensity–Duration–Frequency curves, we have found that the exceptionality of this rainfall event is particularly linked to rainfall intensities ranging in duration from 4 to 9 h compatible with return periods of 100-years or more. This range of time scale which are similar to the estimated concentration time values of the hydrographic basins affected by the flash flood event. From a meteorological perspective, this episode was characterised by strong convection at the regional scale, fuelled by high availability of moisture over the Lisbon region associated with a low pressure system centered near Lisbon that favoured the convective instability. Most victims were sleeping or were caught by surprise at home in the small river catchments around the main Lisbon metropolitan area. The majority of people who died or who were severely affected by the flood lived in degraded housing conditions often raised in a clandestine way, occupying flood plains near the stream beds. This level of destruction observed at the time is in stark contrast to what was observed in subsequent episodes of similar amplitude. In particular, since 1967 the Lisbon area, was struck by two comparable intense precipitation events in 1983 and 2008 but generating considerably fewer deaths and evacuated people. [ABSTRACT FROM AUTHOR]

Summary This paper analyses the meteorological conditions and the specific peak flows of 24 catastrophic floods that affected NE Iberian Peninsula in the period 1842–2000. We classify these floods according to the affected area, peak flow magnitude, and damages. Additionally, the NOAA 6 Hourly 20th Century V2 Reanalysis Data Composites have been used to analyze the synoptic conditions during each flood and to evaluate several stability indices, such as the convective available potential energy (CAPE), or the lift index. We found a good correlation between stability indices and the season when the flood occurred. For instance, if maximum CAPE is considered, larger values are found for summer floods, moderate for autumn, and low values during winter floods. We select 5 representative episodes occurred in different seasons and areas to describe in detail the synoptic conditions and to show the temporal evolution of the stability indices. In one the summer floods analyzed in detail, the largest instability, according to all the convective indices, is found. On the contrary, the winter case shows very low values of the convective indices, and autumn cases lay in between. During the other analyzed summer flood instability was low but snow thaw played an important role in producing the flood. Regarding hydrological variables, clear differences between floods occurred at the coast or at the Pyrenees are found. Coastal specific peak flows are larger than Pyrenean ones, especially for small catchment areas. We also combine meteorological (rainfall duration, CAPE), hydrological (specific peak flow) and geomorphological (catchment area) variables to show that for many of the analyzed floods these variables are related: the specific peak flow generally shows larger values when CAPE is also large. However some differences appear depending on the season and area. In those summer floods, where snow thaw played doesn’t played any role, either Pyrenean or coastal, specific peak flow seems to be correlated with CAPE. For autumn floods, depending on the area different correlations were found: Pyrenean floods seem to be a correlation between CAPE and specific peak flows, but not for coastal ones. For winter coastal floods we couldn’t find any correlation between CAPE and specific peak flows. [ABSTRACT FROM AUTHOR]

Summary The village of Pajares de Pedraza (Segovia, Spain) is located in the floodplain of the Cega River, a left bank tributary of the Douro River. Repeated flash flood events occur in this small village because of its upstream catchment area, mountainous character and impermeable lithology, which reduce concentration time to just a few hours. River overbank flow has frequently caused flooding and property damage to homes and rural properties, most notably in 1927, 1991, 1996, 2001, 2013 and 2014. Consequently, a detailed analysis was carried out to quantify the economic risk of flash floods in peri-urban and rural areas. Magnitudes and exceedance probabilities were obtained from a flood frequency analysis of maximum discharges. To determine the extent and characteristics of the flooded area, we performed 2D hydraulic modeling (Iber 2.0 software) based on LIDAR (1 m) topography and considering three different scenarios associated with the initial construction (1997) and subsequent extension (2013) of a linear defense structure (rockfill dike or levee) to protect the population. Specific stage–damage functions were expressly developed using in situ data collection for exposed elements, with special emphasis on urban-type categories. The average number of elements and their unit value were established. The relationship between water depth and the height at which electric outlets, furniture, household goods, etc. were located was analyzed; due to its effect on the form of the function. Other nonspecific magnitude–damage functions were used in order to compare both economic estimates. The results indicate that the use of non-specific magnitude–damage functions leads to a significant overestimation of economic losses, partly linked to the use of general economic cost data. Furthermore, a detailed classification and financial assessment of exposed assets is the most important step to ensure a correct estimate of financial losses. In both cases, this should include a consideration of the socio-economic and cultural conditions prevailing in the area, as well as the types of flood that affect it. [ABSTRACT FROM AUTHOR]

Flood risk assessment is generally studied using flood simulation models; however, flood risk managers often simplify the computational process; this is called a 'simplification strategy'. This study investigates the appropriateness of the 'simplification strategy' when used as a flood risk assessment tool for areas prone to flash flooding. The 2004 Boscastle, UK, flash flood was selected as a case study. Three different model structures were considered in this study, including: (1) a shock-capturing model, (2) a regular ADI-type flood model and (3) a diffusion wave model, i.e. a zero-inertia approach. The key findings from this paper strongly suggest that applying the 'simplification strategy' is only appropriate for flood simulations with a mild slope and over relatively smooth terrains, whereas in areas susceptible to flash flooding (i.e. steep catchments), following this strategy can lead to significantly erroneous predictions of the main parameters-particularly the peak water levels and the inundation extent. For flood risk assessment of urban areas, where the emergence of flash flooding is possible, it is shown to be necessary to incorporate shock-capturing algorithms in the solution procedure, since these algorithms prevent the formation of spurious oscillations and provide a more realistic simulation of the flood levels. [ABSTRACT FROM AUTHOR]

Flood protection is a society-wide task. The basic rules of prevention in flood protection are stipulated by the regulation of a secondary right of the European Union-Directive of the European Parliament and Council 2007/60/EC on assessment and management of flood risks. The paper is focused on preliminary flood risk assessment of flash floods. The task was to obtain knowledge on the spatial variability of flood risk from flash floods and in doing so supplement a preliminary flood risk assessment already conducted in 2011 for the purpose of proposing suitable flood mitigation measures for reducing the risk found. Flood risk in this study is understood as a combination of flood hazard and vulnerability. The main part of the work is devoted to the proposal of a methodological approach for preliminary flood risk assessment of flash floods. Application of the proposed approach in Bodva river basin, southern Slovakia, is described in the results section. [ABSTRACT FROM AUTHOR]

In recent years, flash flood disasters have occurred frequently in southwest China due to the increased frequency of extreme climate events. To solve this problem, great efforts have been made in studying the process of flash flood. However, little attention was paid on bearing body of hazard, the clusters of buildings. Thus the real disaster mechanism of flash flood remains unclear. Accordingly, based on the experiments of artificial flash floods in a conceptual solid model, this paper focuses on the flood-impacted inundation characteristics of the building clusters at different locations of the gully model, in order to obtain a better understanding of the disaster process and the interaction between the flash floods and building clusters. The results showed that, in a typical smallscale flash flood gully with hot and dry climate, 1) clusters of buildings on an alluvial fan could reduce about 35% of the flooding area by blocking the diffusion of the flood to the depression areas, and could also promote the deposition in lower reaches of the river channel by blocking the overbank flow from going back into the channel, making the width-depth ratio of the channel larger. 2) The flash flood rates of disaster and hazard on the alluvial fan are generally higher than that of the inner gully. For the inner gully, buildings located on the beaches along the lower river and the transitional areas of the straight channel and channel bends can easily be affected because of their lower elevations. For the alluvial fan, buildings nearby the meanders suffer the greatest impacts because of bank collapsing and flooding. 3) The safe vertical distance from a building to the river channel is 13 m for the buildings in the inner gully under extreme floods. Below this threshold, the smaller the vertical distance is, the greater the risk exposure is. For the buildings on the alluvial fan, especially for the buildings near the concave bank of the top rush point, the horizontal distance is more important, and the safe value is 80 m under extreme floods. [ABSTRACT FROM AUTHOR]

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Summary Rising atmospheric CO 2 and resulting warming are expected to impact freshwater resources in the tropics, but few studies have documented how natural stream flow regimes in tropical watersheds will respond to changing rainfall patterns. To address this data gap, we utilized a space-for-time substitution across a naturally occurring and highly constrained (i.e., similar geomorphic, abiotic, and biotic features) model hydrological system encompassing a 3000 mm mean annual rainfall (MAR) gradient on Hawai‘i Island. We monitored stream flow at 15 min intervals in 12 streams across these watersheds for two years (one normal and one dry) and calculated flow metrics describing the flow magnitude, flow variability (e.g., flow flashiness, zero flow days), and flow stability (e.g., deviations from Q 90 , daily flow range). A decrease in watershed MAR was associated with increased relative rainfall intensity, a greater number of days with zero rainfall resulting in more days with zero flow, and a decrease in Q 90 : Q 50 . Flow yield metrics increased with increasing MAR and correlations with MAR were generally stronger in the normal rainfall year compared to the dry year, suggesting that stream flow metrics are less predictable in drier conditions. Compared to the normal rainfall year, during the dry year, Q 50 declined and the number of zero flow days increased, while coefficient of variation increased in most streams despite a decrease in stream flashiness due to fewer high flow events. This suggests that if MAR changes, stream flow regimes in tropical watersheds will also shift, with implications for water supply to downstream users and in stream habitat quality for aquatic organisms. [ABSTRACT FROM AUTHOR]

Accurate flash flood prediction depends heavily on rainfall data quality and knowledge of catchment behaviour. A methodology based on global sensitivity analysis and hydrological similarity is proposed to analyse flash storm-flood events with a mechanistic model. The behaviour of medium-sized catchments is identified in terms of rainfall–runoff conservation. On the basis of this shared behaviour, rainfall products with questionable quantitative precipitation estimation (QPE) are excluded. This facilitates selection of rainfall inputs for calibration, whereas it can be difficult to choose between two rainfall products by direct comparison. A substantial database of 43 flood events on 11 catchment areas was studied. Nash-Sutcliffe efficiencies for this dataset are around 0.9 in calibration and 0.7 in validation for flash flood simulation in 250-km2catchments with selected QPE. The resulting calibration framework and qualification of possible losses for different bedrock types are also interesting bases for flash flood prediction at ungauged locations.Editor D. Koutsoyiannis [ABSTRACT FROM PUBLISHER]

The paper examines the flash floods that may appear in a representative river basin occupying the south-western Romania and also feature an example of the most recent flash flood from 2005-2006, more specifically, its causes and consequences. In order to accomplish the objectives, hydrological data were used to identify the characteristics of the floods. Finally, the case study of the flash flood was delivered through the field research, observational method, discussion with the authorities and investigation of the meteorological and hydrological available data. The research offers an insight on the dimension of damages triggered by a flash flood event, based on the statistical data provided by the village hall and the few remaining places preserving the traces of the floods (houses, bridges). Because we could not provide all the necessary data in order to determine the frequency and scale of such risk phenomena, the analysis is assessed on general hydrological statistics of flood events between 1964 to 2011. By leading the research, it resulted that the specific feature of the upper basin of Desnatui River is its temporary drainage and that in the periods of high flow, the capacity of the river channels is diminshed and the floods may occur. The paper succeeds to revive the insufficient scientific concerns on this kind of hydrological risks issued in the space occupied by the upper basin of Desnatui River and eventually, to supply the need for such study in the context of modern hydrological research preoccupations. [ABSTRACT FROM AUTHOR]

The program Simulation of Hydrological Extreme Events provides a set of functionalities that combined together allows constructing, manipulating, analyzing and comparing the hydrological processes involved in flash flood generation. The program makes use of existing databases of interest in hydrology and available in Spain, such as digital terrain models, coverage of rainfall or curve number. Two pilot watersheds from Spain were selected, Arás and Aguilón, where flash flood episodes have taken place. A sensitivity analysis of the flash flood episodes in response to changes in the main hydrological processes involved has been made, such as spatial and temporal distribution of rainfall, soil moisture status and water flow through channel network. In this work, we found that the antecedent moisture condition is the most influential factor in the magnitude of flash floods produced by the same amount of rain. The temporal distribution of the storm represents the second characteristic in order of relevance. In addition, terrain morphology (specially the slope) is found to be decisive in the results differences obtained. [ABSTRACT FROM AUTHOR]

Abstract: We propose an extended study of recent flood-triggering storms and resulting hydrological responses for catchments in the Pyrenean foothills up to the Aude region. For hydrometeorological sciences, it appears relevant to characterize flash floods and the storm that triggered them over various temporal and spatial scales. There are very few studies of extreme storm-caused floods in the literature covering the Mediterranean and highlighting, for example, the quickness and seasonality of this natural phenomenon. The present analysis is based on statistics that clarify the dependence between the spatial and temporal distributions of rainfall at catchment scale, catchment morphology and runoff response. Given the specific space and time scales of rainfall cell development, we show that the combined use of radar and a rain gauge network appears pertinent. Rainfall depth and intensity are found to be lower for catchments in the Pyrenean foothills than for the nearby Corbières or Montagne Noire regions. We highlight various hydrological behaviours and show that an increase in initial soil saturation tends to foster quicker catchment flood response times, of around 3 to 10h. The hydrometeorological data set characterized in this paper constitutes a wealth of information to constrain a physics-based distributed model for regionalization purposes in the case of flash floods. Moreover, the use of diagnostic indices for rainfall distribution over catchment drainage networks highlights a unimodal trend in spatial temporal storm distributions for the entire flood dataset. Finally, it appears that floods in mountainous Pyrenean catchments are generally triggered by rainfall near the catchment outlet, where the topography is lower. [Copyright &y& Elsevier]

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]