The intensity of rainfall in mediterranean environments. Extreme values according to the scale of observation

Mediterranean environments are dominated by episodes of torrential rain, whereby the critical parameter is not so much the amount of rain these episodes accumulate, but rather the intensity they can reach. The heavy intensities that extreme events can achieve are critical in the dynamics of soil erosion; those related to triggering of debris-flow and above all in hydrology, as they affect rainfall-runoff conversion processes, runoff and coefficient thresholds and flash floods generation

Contribution of hydrogeomorphology for mapping flood hazard in mediterranean ephemeral streams (ramblas)

Flood hazard mapping can be performed using different methods that, generally speaking, can be grouped into four main categories: historical and paleohydrological methods, hydrogeomorphological methods, hydrological-hydraulic methods, and the recently developed dendrogeomorphological methods. These groups of methods are not mutually exclusive and, in fact, they should be used complementarily. Unfortunately, in the case of ramblas, this combination is really difficult. On one hand, paleohydrological methods are not suitable for so small and torrential catchments as ramblas are and, on the other hand, hydrological-hydraulic methods demand a large amount of hydrologic data, non available in most of these systems. Indeed, many authors assert the inadequacy of these methods to predict extreme floods in Mediterranean small catchments. Anyway, hydrogeomorphological method is nowadays achieving more relevance. It is based on the location and typology of landforms and sediments generated during floods in order to delineate flooding areas and identify processes. It is a qualitative approach that gives a realistic image of the processes and it is enough to make decisions, with a minimum effort, in the 80% of the instances. Studies developed in the Mediterranean region of southern France and north-western Spain have demonstrated the effectiveness of this method in ephemeral streams, where channel and floodplain morphology are highly variable and changeable over time and, in addition, hydrological information is scarce or nearly inexistent. This work presents a method for mapping flood hazard in two Mediterranean small catchments -Barranc de Carraixet and Rambla de Poyo-, based on hydrogeomorphological interpretation. A synthetic hydrogeomorphological cartography was obtained supported by previous studies (carried out by Camarasa, Carmona and Ruiz) and taking into account the forms and processes developed during the great flood event of October 2000. Thirteen different landforms related to flooding processes where identified and valuated in terms of hazard, ranking from levels 1 to 8, in which level 1 represents the highest hazard (streams and critical points) and level 8 the safety areas (mountains and longshore bars). Hydrogeomorphological method has proved to be highly effective for mapping flood hazard in this kind of torrential ephemeral streams, where standard hydrological and hydraulic methods do not work properly. This method allows working with scarcity of hydrological data and, what is more, is quite adaptable to any change in flood area morphology (either natural or man made)

Criterios de selección y caracterización de episodios de lluvia. Aplicación a la Confederación Hidrográfica del Júcar (1989-2003)

El análisis espacio-temporal de la lluvia precisa de la identificación de episodios, en función del objetivo de la investigación y la escala de trabajo. En esta comunicación se proponen criterios de selección de episodios, con objetivos hidrológicos, y se aplican al territorio de la Confederación Hidrográfica del Júcar, en base a los datos diarios de 15 años (1989-2003), proporcionados por el Sistema Automático de Información Hidrológica. Se seleccionan 363 episodios y se caracterizan en base a indicadores de total, duración, intensidad e irregularidad, calculados en once unidades territoriales. El análisis de estos indicadores evidencia una dicotomía litoral-interior. El ámbito litoral (exceptuando Alicante) registra el mayor número de episodios, fundamentalmente en invierno, con un aporte más importante y mayor irregularidad. Por el contrario, en el ámbito de interior los episodios son de menor intensidad y mayor presencia en verano. El efecto orográfico es importante, no sólo por la altitud, sino por la orientación de las cadenas montañosas.Before analysing the variability of rainfall in space and time it is necessary to select rainfall events in relation to the objectives and scale of the study. This paper proposes criteria for selecting rainfall events, with hydrological objectives. These criteria are then applied to the territory of the River Jucar Water Authority and the daily data covering a 15 year period (1989-2003) recorded by the Automatic Hydrological Information System (SAIH). 363 events are chosen and characterised by indicators of total rainfall, duration, intensity and irregularity calculated over 11 regional units. The analysis of these indicators shows a littoral-interior dichotomy. The largest number of events were recorded near the coast (except in one region), mostly in winter, with greatest volumes and higher irregularity. In the interior the events were less intense and more frequent in summer. The orographic effect is important, not just in relation to altitude but also in relation to the orientation of the mountain ranges

La Creciente Ocupación Antrópica del Espacio Inundable y el Aumento de la Vulnerabilidad en las Poblaciones del Bajo Arga (Navarra)

Las inundaciones fluviales constituyen en los municipios de Falces, Peralta y Funes (Navarra) un problema histórico. Ubicados en el sector más meridional del amplio y fértil llano de inundación del río Arga, han sido los pueblos más afectados por los desbordamientos de este enérgico río. La lucha secular contra las inundaciones se concreta en múltiples actuaciones, especialmente agresivas a partir de 1960: corta artificial de meandros, canalización, motas longitudinales, escolleras, dragados¿Las obras han suprimido por completo el activo sistema natural de meandros libres y divagantes del bajo Arga, que en la actualidad, circula por un cauce artificial cuasi-recto hasta la desembocadura en el río Aragón. Ante un Arga debilitado y al amparo de las motas, el caserío, los servicios y las actividades industriales y agrarias van ocupando la llanura inundable. No obstante, las precipitaciones extraordinarias y el deshielo persisten, y con ellos los caudales extremos, capaces de superar las obras de defensa. La abierta exposición al riesgo de estas poblaciones multiplica las pérdidas económicas y vuelve ineficaz cualquier sistema de previsión y alarma de avenida. Flood are a historical problem for municipalities like Falces, Peralta y Funes (Navarra). These municipalities are situated at the southern part of the rich an wide floodplain of the Arga River. These villages were the most damaged when this energical river flooded. Since 1960 the ancient struggle against the floods have been more powerfull: artificial cutoff, channelization, levée construction, breakwater, dredgings Ingenerical works have abolited the meandering channel system of the lower Arga. Nowadays Arga River has an straigth artificial channel that flows into the Aragón River. Under the protection of the channelization, villages, services and industrial activities have taken up the floodplain. Nevertheless there is still the extraordinary rainfall and the meltwater, and with them the extreme floods. For these villages the increasing risk means bigger economical losses and turns inefficient all precaution systems and flood alarms

Relación entre el desarrollo económico y socio-urbano y el aumento del riesgo de inundación en la cuenca hidrográfica del Río Arga (Navarra)

El agua presenta para las sociedades humanas dos facetas opuestas e inseparables. Por un lado, es un recurso necesario para el desarrollo socio-económico de las poblaciones, y por otro, constituye el riesgo natural que mayores daños económicos y humanos ocasiona a nivel mundial (OLCINA, 1994). Dicho riesgo se deriva de la dinámica fluvial natural y es consecuencia de la ocupación indebida de los espacios inundables. Los ámbitos fluviales han sido desde la antiguedad espacios preferentes para la localización de asentamientos y actividades económicas. La cuenca del río Arga (Navarra) no es una excepción en este sentido. A lo largo del eje fluvial se ubican unas cincuenta poblaciones incluyendo Pamplona, la capital regional. Del proceso de expansión y desarrollo que se inicia en los años cincuenta-sesenta y que afecta, principalmente, a la comarca de Pamplona y al llano de inundación del Arga, derivan dos consecuencias hidrológicas claves: el aumento de la cantidad de agua a abastecer y la tendencia a ocupar zonas inundables, y por tanto, con riesgo para la población y sus bienes. En el presente trabajo se analiza la relación existente entre el desarrollo económico y socio-urbanístico de determinadas áreas de la cuenca hidrográfica del río Arga y el aumento del riesgo de inundación en estas mismas zonas

Rainfall-runoff modelling of ephemeral streams in the Valencia Region (Eastern Spain)

This paper presents preliminary results from the application of a transfer-function rainfallrunoff model to ephemeral streams in Mediterranean Spain. Flow simulations have been conducted for two small catchments (Carraixet and Poyo basins), located in close proximity to one another yet with significantly different geological characteristics. Analysis of flow simulations for a number of high-flow events has revealed the dominant influence of the rainfall on the catchment response, particularly for high-rainfall events. Particular success has been attained modelling the highest magnitude events in both catchments and for all events in the faster responding (Poyo) catchment. In order to investigate the viability of the model for forecasting floods in ungauged catchments, additional investigations have been conducted by calibrating the model for one catchment (donor catchment) and then applying it to another (receptor catchment). The results indicate that this can be successful when either the donor catchment is a fast response catchment or when the model is calibrated using a high-magnitude event in the donor catchment, providing that the modelled receptor catchment event is of a lower magnitude. Copyright 2002 John Wiley & Sons, Ltd

Peligro, vulnerabilidad y riesgo de inundación en ramblas mediterráneas: los llanos de Carraixet y Poyo

This paper proposes a methodology for mapping flood risk in ephemeral streams, based on the estimation of flood hazard and vulnerability, which has been applied to the floodplains of Barranc del Carraixet and Rambla de Poyo catchments. Hazard has been assessed using hydrogeomorphological methods. Vulnerability has been estimated as a combination between the economic value of land use and human exposure, for three models of intensity of use, related to different time periods: working days, nights, and weekends and holidays. The method has proved to be simple, effective and easily comparable. The results show different patterns of risk for each floodplain, even though both of them are part of the metropolitan area of Valencia.Este trabajo propone una metodología para la cartografía del riesgo de inundación en ramblas, basada en la estimación del peligro y de la vulnerabilidad, que ha sido aplicada a los llanos del Barranc del Carraixet y de la Rambla de Poyo. El peligro se ha evaluado a partir de métodos hidrogeomorfológicos. La vulnerabilidad, entendida como la combinación entre el valor económico de los usos del suelo y la exposición humana, se ha estimado para diferentes condiciones de ocupación del espacio, en función de la franja horaria: jornada laboral, noches y días festivos. El método se ha demostrado sencillo, eficaz y fácilmente extrapolable. Los resultados han puesto de manifiesto que pese a que los dos llanos de inundación forman parte del Área Metropolitana de Valencia y han sufrido una evolución parecida, presentan patrones de riesgo diferentes

Elaboración de cartografía de zonas inundables. Aplicación al llano de inundación del Río Arga (Navarra)

La inundación es el riesgo natural de mayor impacto social y económico, la persistencia del problema a pesar de los continuos avances tecnológicos pone de manifiesto la necesidad de estudiar más detalladamente el comportamiento de las áreas inundables y elaborar una cartografía de riesgo que contribuya a un uso más racional del espacio. Este trabajo presenta el proceso de elaboración del mapa de riesgo del llano de inundación del río Arga (Navarra) en los municipios de Falces, Peralta y Funes. La demarcación de áreas inundables para diferentes valores de caudal máximo se llevó a cabo mediante la combinación de métodos estadísticos, hidrológicos, hidráulicos y geomorfológicos. Algunos procesos metodológicos y análisis espaciales han sido automatizados mediante Sistemas de Información Geográfica. Floods are the most ImportEuit natural risk in the world. Frequerrtly engineering works are not enough to solve the problem. When hydraulic structures fail or they can not minimise negative impacts of floods, other measures are necessaries. Risk cartography is a powerful tool to environmental planning. This paper presents the methodology to elabórate the risk map of the floodplain of River Arga (Navarra, Spaín). Methodok)gy includes statistical, hydrok)gical, hydraulic, and geomorphological tools. Some spatial analyses have Iseen done using Geographic Information Systems

Identification of Mediterranean rainfall events for hydrological analysis. Criteria and characterization

Since Mediterranean environments are characterized by the irregularity of their climate and hydrological processes where, for instance, twice or even three times the average annual rainfall can be registered in a single torrential rainfall event, hydrological studies are increasing focussing on the analysis of actual rainfall events. Criteria to define what constitutes a rainfall event are not yet clearly established in literature. Although we can find detailed, a posteriori, studies about specific torrential rainfall events related to floods in a given basin, detection of rainfall events with a hydrological meaning from daily rainfall records remains difficult and normally has been done with reference to floods events at catchment level. This paper proposes criteria for selecting rainfall events with hydrological objectives. We consider not only events that can produce major floods but also those events around the runoff generation threshold which can also produce significant contributions to the subsurface flow. These criteria have been applied to the territory of the River Júcar Water Authority (43.000 km2), using daily data recorded by the Automatic Hydrological Information System (SAIH), and covering a 15 year period (1989-2003). A total of 347 events were identified and characterised by indicators of total rainfall, duration, intensity and irregularity calculated over 11 regional units. The synoptic atmospheric situation responsible for each episode has also been taken into account in the analysis. As a result, a classification of the episodes has been obtained. The analysis shows a littoral-interior dichotomy. The largest number of events were recorded near the coast (except in one region), mostly in winter, with greatest volumes and high irregularity. In the interior the events were less intense and more frequent in summer. The orographic effect is important, not just in relation to altitude but also in relation to the orientation of the mountain ranges with respect to the wind direction