ReAFFIRM: Real-time Assessment of Flash Flood Impacts: a Regional high-resolution Method

Flash floods evolve rapidly in time, which poses particular challenges to emergency managers. One way to support decision-making is to complement models that estimate the flash flood hazard (e.g. discharge or return period) with tools that directly translate the hazard into the expected socio-economic impacts. This paper presents a method named ReAFFIRM that uses gridded rainfall estimates to assess in real time the flash flood hazard and translate it into the corresponding impacts. In contrast to other studies that mainly focus on in- dividual river catchments, the approach allows for monitoring entire regions at high resolution. The method consists of the following three components: (i) an already existing hazard module that processes the rainfall into values of exceeded return period in the drainage network, (ii) a flood map module that employs the flood maps created within the EU Floods Directive to convert the return periods into the expected flooded areas and flood depths, and (iii) an impact assessment module that combines the flood depths with several layers of socio- economic exposure and vulnerability. Impacts are estimated in three quantitative categories: population in the flooded area, economic losses, and affected critical infrastructures. The performance of ReAFFIRM is shown by applying it in the region of Catalonia (NE Spain) for three significant flash flood events. The results show that the method is capable of identifying areas where the flash floods caused the highest impacts, while some locations affected by less significant impacts were missed. In the locations where the flood extent corresponded to flood observations, the assessments of the population in the flooded area and affected critical infrastructures seemed to perform reasonably well, whereas the economic losses were systematically overestimated. The effects of different sources of uncertainty have been discussed: from the estimation of the hazard to its translation into impacts, which highly depends on the quality of the employed datasets, and in particular on the quality of the rainfall inputs and the comprehensiveness of the flood maps.Peer ReviewedPostprint (published version

Validation of a nowcasting technique from a hydrological perspective

Nowcasting precipitation is a key point to anticipate risks in flood warning systems. In this environment, weather radars are very useful because of the high resolution of their measurements both in time and space. The aim of this study is to assess the performance of a recently proposed nowcasting technique (SPROG) from a hydrological point of view. This technique is based on the advection of radar precipitation fields and its main point is that the forecasted fields get smoothed as the forecasting time increases, to filter out the smallest scales of the field when they become unpredictable. The evaluation of the forecasted precipitation fields is done in two different ways: a) comparing them against the actually measured precipitation fields and b) according to the concept of “hydrological validation”, comparing the hydrographs calculated by a distributed rainfall-runoff model simulating operational conditions (using the forecasted precipitation fields) against the hydrographs calculated by the model with the entire series of radar measurements. This part of the study has been carried out in the framework of the Besos basin flood forecasting system.Postprint (published version

Hydrological evaluation of a nowcasting technique applied to flood forecasting

Postprint (published version

Comparison of two early warning systems for regional flash flood hazard forecasting

The anticipation of flash flood events is crucial to issue warnings to mitigate their impact. This work presents a comparison of two early warning systems for real-time flash flood hazard forecasting at regional scale. The two systems are based in a gridded drainage network and they use weather radar precipitation inputs to assess the hazard level in different points of the study area, considering the return period (in years) as the indicator of the flash flood hazard. The essential difference between the systems is that one is a rainfall-based system (ERICHA), using the upstream basin-aggregated rainfall as the variable to determine the hazard level, while the other (Flood-PROOFS) is a system based on a distributed rainfall-runoff model to compute the streamflows at pixel scale. The comparison has been done for three rainfall events in the autumn of 2014 that resulted in severe flooding in the Liguria region (Northwest of Italy). The results obtained by the two systems show many similarities, particularly for larger catchments and for large return periods (extreme floods).Peer ReviewedPostprint (author's final draft

El episodio pluviométrico del 10 de junio en Cataluña. Un primer estudio hidrometeorológico

[ES] En este artículo se estudia el episodio pluviométrico registrado el día 10 de junio de 2000 en Cataluña. En particular se analiza la lluvia caída en la cuenca de 97 km2 correspondiente a la riera de Magarola en su cruce con la autovía N-II en Esparreguera (Barcelona). Para ello se utiliza información de campo obtenida a partir de radar meteorológico, pluviómetros y pluviógrafos. Sobre la mayor parte de la cuenca se recogieron más de 150 mm de lluvia durante el episodio. También prácticamente sobre toda la cuenca cayeron más de 100 mm en dos horas. El cálculo del período de retorno asociado a la lluvia de 24 horas evidencia la naturaleza excepcional del fenómeno. No obstante la información histórica disponible es insuficiente y solo permite realizar estimaciones con una gran incertidumbre.Este trabajo se ha desarrollado dentro de la colaboración entre la UPC y el INM y específicamente en el marco del proyecto CICYT del Plan Nacional del Clima CLI99-1240-C02. Para realizarlo se han utilizado datos cedidos por el Servei de Metereologia de Catalunya, el SAIH y el INM. También se ha contado con la valiosa ayuda de Enric Velasco, de l’Agència Catalana de l’Aigua, del Padre Ramón Oranias de la Abadía de Montserrat,y especialmente con la de Ramón Pascual, Santi Valios y Sara Satué del Centre Meteorològic Territorial de Catalunya. Finalmente le debemos un especial reconocimiento a Miguel Lara, quién nunca ha dejado de confiar en nosotros desde que comenzamos a estudiar el radar meteorológico, hace ya diez años.Sempere Torres, D.; Corral Alexandri, C.; Berenguer, M.; Sánchez-Diezma Guijarro, R.; Dolz Ripollès, J. (2000). El episodio pluviométrico del 10 de junio en Cataluña. Un primer estudio hidrometeorológico. Ingeniería del Agua. 7(2):105-115. https://doi.org/10.4995/ia.2000.2838SWORD10511572Beven, K. y Kirkby, M. J. (1979) A physically-based variable-contributing area model of basin hydrology. Hydrol. Sci. Bull, 24 (27-53), 27-53.Chow, V. T. (1951) A general formula for hydrologic frequency analysis. Trans. Am. Geophysical union, 32 (2), 231-237.Chow, V. T. (1953) Frequency analysis of hydrologic data with spetial application to rainfall intensities. University of Illinois Eng. Expt. Station, USA.Creutin, J. D. y Obled, C. (1982) Objetive analysis and mapping techniques for rainfall fields: an objective comparison. Water Resour. Res., 18 (2), 413-431.Dubrule, O. (1984) Comparing splines and kriging. Comput. Geosci., 10 327-338.Duchon, J. (1976) Interpolation des fonctions de 2 variables suivant le principe de la flexion des plaques minces. Rev. Automat. Inf. Rech. Oper., 10 (12), 5-12.Elías Castillo, F. y Ruiz Beltrán, L. (1979) Precipitaciones máximas en España. Estimaciones basadas en métodos estadísticos. ICONA, Madrid.INM (1999a) Las precipitaciones máximas en 24 horas y sus periodos de retorno en España. Cataluña. Vol. 5. Ministerio de Medio Ambiente, Madrid.INM (1999b) Las precipitaciones máximas en 24 horas y sus periodos de retorno en España. Introducción y Metodología. Vol. 0. Ministerio de Medio Ambiente, Madrid.Kite, G. W. (1977) Frequency and risk analysis in hydrology. Water Resources Publications, Fort Collins, Co., USA.Martín, F. y De Esteban, L. (1994) Manual de interpretación radar. Publicación interna del INM. STAP (INM), Madrid.Rinehart, R. E. (1999) Radar for meteorologists. Rinehart Publishing, 428 pp.Rosengaus, M. (1995) Fundamentos de radares meteorológicos: aspectos clásicos. Ingeniería Hidráulica en México, X (1), 55-74.Sánchez-Diezma, R. (2000) Optimización de la medida de lluvia por radar meteorológico para su aplicación hidrológica. Universitat Politècnica de Catalunya, Proyecto de Tesis Doctoral, Informe EHMA/33051501-01/200001.Sempere Torres, D., Corral, C., Malgrat, P. y Raso, J. R. (1999) Use of weather radar for combined sewer overflows monitoring and control. Journal of Environmental Engineering, ASCE, 125 (4), 372-380.Zawadzki, I. (1984) Factors affecting the precision of radar measurement of rain. Proceedings of the 22 Conference on radar meteorology. American Met. Society, pp.251-256

Large wood debris that clogged bridges followed by a sudden release: the 2019 flash flood in Catalonia

The aim is the reconstruction of the October 2019 flash flood, that was documented through extensive field work: rainfall (300 mm in just a few hours), flood marks, times of flood passage and witnesses' snapshots and reports, channel changes, log drift (20,000 trees) and woody debris at bridges, as well as large damage and six fatalities. The methods are: hydrological model built for the rainfall-runoff in the basin and the flood routing in the river, use of hydraulic principles such as flow at waterfalls, flow against obstacles (trees), etc. and finally 1D/2D free surface numerical models. The uppermost 100 km2 produced discharges of 700 m3/s (up to 50 m3/s/km2, locally). Three bridges failed, but their cascading failure (when one failure triggers the next one downstream) was not proved. The main channel widened more than 10 times, dragging away soil and vegetation like a bulldozer. The resulting large wood debris that clogged two bridges worsened the inundation. An anomalous flow downstream, probably a surge of around 1090 m3/s, due to the failure of a woody jam at a narrow bridge, took two lives. Water Authority is now warning flood planners that vegetated, torrential basins may cause catastrophic floods in the valley towns, if their narrow bridges are sensitive to woody debris.Catalan Water Authority and its Tarragona officers, Meteoprades and Fons Signatus, Joan March and a number of witnesses. The UPC's contribution was funded by the contract CTN2000029 of the “Agència Catalana de l′Aigua”. The UB's contribution has been developed within the framework of the AGORA project, funded by the "Agència Catalana de l′Aigua". Our thanks to the "Museu de la Vida Rural", in l′Espluga de Francolí, and all the citizens who contributed to the return of experience. The UIB's research has been supported by the Ministerio de Ciencia, Innovación y Universidades (CGL2017-82868-R and PID2020-113036RB-I00/AEI/10.13039/501100011033 research projects, which are partially supported by the European Regional Development Funds).Peer ReviewedPostprint (published version

Validation of a nowcasting technique from a hydrological perspective

Nowcasting precipitation is a key point to anticipate risks in flood warning systems. In this environment, weather radars are very useful because of the high resolution of their measurements both in time and space. The aim of this study is to assess the performance of a recently proposed nowcasting technique (SPROG) from a hydrological point of view. This technique is based on the advection of radar precipitation fields and its main point is that the forecasted fields get smoothed as the forecasting time increases, to filter out the smallest scales of the field when they become unpredictable. The evaluation of the forecasted precipitation fields is done in two different ways: a) comparing them against the actually measured precipitation fields and b) according to the concept of “hydrological validation”, comparing the hydrographs calculated by a distributed rainfall-runoff model simulating operational conditions (using the forecasted precipitation fields) against the hydrographs calculated by the model with the entire series of radar measurements. This part of the study has been carried out in the framework of the Besos basin flood forecasting system

An operational flood warning system for poorly gauged basins: demonstration in the Guadalhorce basin (Spain)

This paper deals with the presentation of a flood warning system (GFWS) developed for the specific characteristics of the Guadalhorce basin (3,200 km2, SE of Spain), which is poorly gauged and often affected by flash and plain floods. Its complementarity with the European flood alert system (EFAS) has also been studied. At a lower resolution, EFAS is able to provide a flood forecast several days in advance. The GFWS is adapted to the use of distributed rainfall maps (such as radar rainfall estimates), and discharge forecasts are computed using a distributed rainfall–runoff model. Due to the lack of flow measurements, the model parameters calibrated on a small watershed have been transferred in most of the basin area. The system is oriented to provide distributed warnings and fulfills the requirements of ungauged basins. This work reports on the performance of the system on two recent rainfall events that caused several inundations. These results show how the GFWS performed well and was able to forecast the location and timing of flooding. It demonstrates that despite its limitations, a simple rainfall–runoff model and a relatively simple calibration could be useful for event risk management. Moreover, with low resolution and long anticipation, EFAS appears as a good complement tool to improve flood forecasting and compensate for the short lead times of the GFWS.Peer ReviewedPostprint (published version

A semi-distributed hydrological model using rainfall estimates by radar. Application to Mediterranean basins

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Improvements on flow forecasting using precipitation nowcasting based in radar advection techniques: assessment of predictability and uncertainty propagation

Peer ReviewedPostprint (published version