Hydrometeorological analysis of the 12 and 13 September 2019 widespread flash flooding in eastern Spain

On 12 and 13 September 2019, a long-lasting heavy precipitation episode (HPE) affected the València, Murcia and Almería regions in eastern Spain. Observed rainfall amounts were close to 500 mm in 48 h, being the highest cumulative precipitation registered in some rain-gauges for the last century. Subsequent widespread flash flooding caused seven fatalities and estimated economical losses above 425 million EUR. High-resolution precipitation estimates from weather radar observations and flood response from stream-gauges are used in combination with a fully-distributed hydrological model to examine the main hydrometeorological processes within the HyMeX program. This HPE was characterized by successive, well-organized convective structures that impacted a spatial extent of 7500 km2, with rainfall amounts equal or larger than 200 mm. The main factors driving the flood response were quasi-stationarity of heavy precipitation, very dry initial soil moisture conditions and large storage capacities. Most of the examined catchments exhibited a dampened and delayed hydrological response to cumulative precipitation: Until runoff thresholds were exceeded, infiltration-excess runoff generation did not start. This threshold-based hydrological behaviour may impact the shape of flood peak distributions, hindering strict flood frequency statistical analysis due to the generally limited lengths of data records in arid and semi-arid catchments. As an alternative, simple scaling theory between flood magnitude and total rainfall amount is explored.

A Temporally Varied Rainfall Simulator for Flash Flood Studies

Experimental studies of flash floods require rainfall simulations. For this reason, various rainfall simulators have been designed, built, and employed in previous studies. These previous rainfall simulators have provided good simulations of constant rainfall intensities; however, these simulators cannot generate temporally varied rainstorms. Thus, the effect of the temporal distribution of a rainstorm on flash flooding cannot be studied using current rainfall simulators. To achieve accurate and reliable results in flash flood studies, simulating rainstorms that are similar to natural precipitation events is essential, and natural rainfall varies temporally. Thus, a rainstorm simulator was designed and built using cascading tanks to generate rainstorm hyetographs that cannot be obtained using traditional rainfall simulators. The result of the rainstorm generated by the proposed instrument and its numerical model showed that the instrument can simulate the temporal distributions of rainstorms with an accuracy of 95 percent. Consequently, the proposed instrument and its numerical model can be applied for generating artificial rainstorm hyetographs in experimental and field studies of flash floods.

Assessment of Exposure to Flash Flooding in an Arid Environment: A Case Study of the Jeddah City Neighborhood Abruq Ar Rughamah, Saudi Arabia

In arid areas, flash floods represent one of the most severe hazards for people and infrastructure alike. The associated risks are compounded by increasing exposure and vulnerability through rapid and unregulated urbanization, poor infrastructure, and sociocultural factors, among other elements. This research explores the flash flooding risk in the Saudi Arabian city of Jeddah with a particular focus on the Abruq Ar Rughamah neighborhood, which experienced a destructive flood in November 2009 that resulted in 116 deaths and 1,200 families becoming homeless. The neighborhood is an interesting case study as it has two distinctive urban layouts representing planned and irregular settlement types. In this paper, the focus is placed on exposure to flash flood hazards using a geographic information system to study urban layouts, building types, and resident populations in conjunction with data from satellites, flood mapping studies, and topographic data. The results show that most of the study area is located along a natural flood path. The regions that were affected by the 2009 disaster received no comprehensive site rehabilitation. This paper concludes that it is important to develop a risk management strategy that includes limiting urban expansion in flood-prone areas and redesigning neighborhoods to increase flood resilience.

Dynamic Pluvial Flash Flooding Hazard Forecast Using Weather Radar Data

Pluvial flash floods are among the most dangerous weather-triggered disasters, usually affecting watersheds smaller than 100 km2, with a short time to peak discharge (from a few minutes to a few hours) after causative rainfall. Several warning systems in the world try to use this time lag to predict the location, extent, intensity, and time of flash flooding. They are based on numerical hydrological models processing data collected by on-ground monitoring networks, weather radars, and precipitation nowcasting. However, there may be areas covered by weather radar data, in which the network of ground-based precipitation stations is not sufficiently developed or does not even exist (e.g., in an area covered by portable weather radar). We developed a method usable for designing an early warning system based on a different philosophy for such a situation. This method uses weather radar data as a 2D signal carrying information on the current precipitation distribution over the monitored area, and data on the watershed and drainage network in the area. The method transforms (concentrates) the 2D signal on precipitation distribution into a 1D signal carrying information on potential runoff distribution along the drainage network. For sections of watercourses where a significant increase in potential runoff can be expected (i.e., a significant increase of the 1D signal strength is detected), a warning against imminent flash floods can be possibly issued. The whole curve of the potential runoff development is not essential for issuing the alarm, but only the significant leading edge of the 1D signal is important. The advantage of this procedure is that results are obtained quickly and independent of any on-ground monitoring system; the disadvantage is that it does not provide the exact time of the onset of a flash flooding or its extent and intensity. The generated alert only warns that there is a higher flash flooding hazard in a specific section of the watercourse in the coming hours. The forecast is presented as a dynamic map of the flash flooding hazard distribution along the segments of watercourses. Relaying this hazard to segments of watercourses permits a substantial reduction in false alarms issued to not-endangered municipalities, which lie in safe areas far away from the watercourses. The method was tested at the local level (pluvial flash floods in two small regions of the Czech Republic) and the national level for rainfall episodes covering large areas in the Czech Republic. The conclusion was that the method is applicable at both levels. The results were compared mainly with data related to the Fire and Rescue Service interventions during floods. Finally, the increase in the reliability of hazard prediction using the information on soil saturation is demonstrated. The method is applicable in any region covered by a weather radar (e.g., a portable one), even if there are undeveloped networks of rain and hydrometric gauge stations. Further improvement could be achieved by processing more extended time series and using computational intelligence methods for classifying the degree of flash flooding hazard on individual sections of the watercourse network.

A GIS Based Methodology to Obtain the Hydrological Response to Storm Events of Small Coastal Basins, Prone to Flash Flooding

The present study aims to investigate the hydrological response of small coastal watersheds to storm events. Areas around the Mediterranean Sea are usually characterized by streams with intermittent flows and flash floods are common. Firstly, we analyze the geomorphological, soil and land cover characteristics of the watershed in order to estimate their effect on surface runoff. Furthermore, the rainfall characteristics of an extreme event that caused flash flooding in the past are analyzed. By combining these factors, we are able to predict the response of this basin to severe storm events. The study area is located in the island of Samos, in Eastern Greece, where flash flood events are usual and pose a risk to areas located around rivers. In this area runoff is intermittent, occurring mainly during storm events and there is a lack of discharge or other instrumental measurements. By applying the SCS-CN method we estimate the response of two of the largest watersheds in Samos Island, through the construction of a Synthetic Unit Hydrograph (SUH). Firstly, we examined the record of historic floods in the area, selecting a large flash flood event (November 2001) and then obtained the daily rainfall data, which are used by the SCS method for the calculations. We applied the SCS methodology in order to estimate various parameters (e.g. lag time, time of concentration, maximum discharge), which also required the calculation of the Curve Number (CN) for each watershed. During this event (136 mm rainfall), we calculated a direct runoff (excess rainfall) of 44%-48% for these watersheds. This methodology can be particularly useful in simulating the hydrological response of small Mediterranean watersheds and to introduce better strategies for the management of the whole drainage basin.

The role of storm movement in controlling flash flood response: an analysis of the 28 September 2012 extreme event in Murcia, southeastern Spain

Flash flooding is strongly modulated by the spatial and temporal variability in heavy precipitation. Storm motion prompts a continuous change of rainfall space-time variability that interacts with the drainage river system, thus influencing the flood response. The impact of storm motion on hydrological response is assessed for the 28 September 2012 flash flood over the semi-arid and medium-sized Guadalentín catchment in Murcia, southeastern Spain. The influence of storm kinematics on flood response is examined through the concept of ‘catchment-scale storm velocity’. This variable quantifies the interaction between the storm system motion and the river drainage network, assessing its influence on the hydrograph peak. By comparing two hydrological simulations forced by rainfall scenarios of distinct spatial and temporal variability, the role of storm system movement on the flood response is effectively isolated. This case study is the first to: (i) show through the catchment-scale storm velocity how storm motion may strongly affect flood peak and timing; and (ii) assess the influence of storm kinematics on hydrological response at different basin scales. In the end, this extreme flash flooding provides a valuable case study of how the interaction between storm motion and drainage properties modulate hydrological response.

Analysis of Flood Fatalities in the United States, 1959–2019

Flooding is one of the main weather-related disasters that cause numerous fatalities every year across the globe. This study examines flood fatalities reported in the contiguous United States (US) from 1959 to 2019. The last two decades witnessed major flood events, changing the ranking of the top states compared to previous studies, with the exception of Texas, which had significantly higher flood-related fatalities than any other state. The rankings of counties within some states changed as well. The study aims to improve understanding of the situational conditions, demographics, and spatial and temporal characteristics associated with flood fatalities. The analysis reveals that flash flooding is associated with more fatalities than other flood types. In general, males are much more likely to be killed in floods than females. The analysis also suggests that people in the age groups of 10–19, 20–29, and 0–9 are the most vulnerable to flood hazard. Purposely driving or walking into floodwaters accounts for more than 86% of total flood fatalities. Thus, the vast majority of flood fatalities are preventable. The results will help identify the risk factors associated with different types of flooding and the vulnerability of the exposed communities.

The sequence of heavy precipitation and flash flooding of 12 and 13 September 2019 in eastern Spain. Part II: A hydro-meteorological predictability analysis based on convection-permitting ensemble strategies

On 12 and 13 September 2019, widespread flash flooding caused devastating effects across eastern Spain. Within the framework of the HyMeX program, this study examines predictability of the long-lasting heavy precipitation episode (HPE) conducive to flash flooding. A set of short-range, convection-permitting ensemble prediction systems (EPSs) is built to cope with different sources of meteorological uncertainty. Specifically, the performance of an Ensemble Kalman Filter, tailored bred vectors and stochastic model parameterizations is compared to more standard ensemble generation techniques such as dynamical downscaling and multiple physics. Results indicate EPS focusing on sampling model uncertainties related to parameterization of subgrid process are skillful, especially when deep convection and its interaction with complex orography are important. Furthermore, representation of small-scale thermodynamical aspects is improved through data assimilation, leading to an enhanced forecasting skill as well. Nevertheless, predictability remains relatively low at the catchment scale in terms of exceedance probabilities in cumulative precipitation and peak discharge. The analysis presented herein could serve as a basis for the future implementation of real-time flash flood warning systems based on skillful meteorological EPSs over small-to-medium, semi-arid watersheds in eastern Spain and, by extension, over the flood-prone Western Mediterranean region.

A Climatological Study of National Weat her Service Watches, Warnings, and Advisories and Landfalling Atmospheric Rivers in the Western U.S. 2006–2018

Atmospheric rivers (ARs) are synoptic-scale phenomena associated with long, narrow corridors of enhanced low-level water vapor transport. Landfalling ARs may produce numerous beneficial (e.g. drought amelioration and watershed recharge) and hazardous (e.g. flash flooding and heavy snow) impacts that may require the National Weather Service (NWS) to issue watches, warnings, and advisories (WWAs) for hazardous weather. Prior research on WWAs and ARs in California found that 50–70% of days with flood-related and 60–80% of days with winter weather-related WWAs occurred on days with landfalling ARs in California. The present study further investigates this relationship for landfalling ARs and WWAs during the cool seasons of 2006–2018 across the entire western U.S. and considers additional dimensions of AR intensity and duration. Across the western U.S., regional maxima of 70–90% of days with WWAs issued for any hazard type were associated with landfalling ARs. In the Pacific Northwest and Central regions, flood-related and wind-related WWAs were also more frequently associated with more intense and longer duration ARs. While a large majority of days with WWAs were associated with landfalling ARs, not all landfalling ARs were necessarily associated with WWAs (i.e., not all ARs are hazardous). For example, regional maxima of only 50–70% of AR days were associated with WWAs issued for any hazard type. However, as landfalling AR intensity and duration increased, the association with a WWA and the “hazard footprint” of WWAs increased quasi-exponentially across the western U.S.