Evaluation of extreme inundation levels in tidal river reaches based on a continuous reconstruction methodology

[EN] The hydrodynamics of coastal river reaches is complex due to the interaction of several sources of flooding as the astronomical tide, surge, wave set-up and river discharge. The tidal level might affect several km of river reach. In those reaches, the same inundation water level can result from different combinations of the sea level ad river discharge. At the present time, most flood inundation studies in tidal river reaches are based in the application of an arbitrary sea level at the downstream river boundary, without considering the interaction and possible correlations between sea level and river discharge. In Spain, the Guía Metodológica para el Desarrollo del Sistema Nacional de Cartografía de Zonas Inundables (Ministerio de Medio Ambiente y Medio Rural y Marino, 2011), recommends to consider the simultaneous occurrence of a sea level and river discharge associated to the same return period. This approach does not consider the probability of simultaneous occurrence of extreme sea level and river discharge. In order to improve this excessively simple and conservative approach, in this paper we propose a continuous simulation methodology that combines the generation of synthetic long-term time series of river discharge and sea level, 2D inundation modelling at a high spatial resolution and statistical sampling and interpolation techniques. The proposed methodology is applied to the coastal town of Betanzos (NW of Spain), which is classified as a high potential flood risk area by the regional authorities. The town is located at the confluence of two river reaches, in the inner part of Betanzos estuary. The inundation levels obtained with the proposed methodology are compared to those obtained with the standard methodology recommended by the Guía Metodológica para el Desarrollo del Sistema Nacional de Cartografía de Zonas Inundables (Ministerio de Medio Ambiente y Medio Rural y Marino, 2011). Results show that the standard methodology overestimates in a significant way the water levels for a given return period.[ES] Las desembocaduras, estuarios y tramos de ríos costeros son elementos hidrodinámicamente complejos debido a la interacción de varias dinámicas generadoras de inundación como la marea astronómica, la marea meteorológica, el oleaje y el caudal del río, entre otras. La marea astronómica es una onda larga que se propaga aguas arriba llegando a afectar a tramos de río situados a decenas de kilómetros de la costa. En estos tramos de río, el mismo nivel del agua puede ser el resultado de diferentes combinaciones de las dinámicas mencionadas. Los estudios de peligrosidad y riesgo de inundación en tramos de ríos afectados por la marea se han basado hasta ahora en la aplicación de una condición de contorno de nivel de marea fijada de forma más o menos arbitraria. En España, la Guía metodológica para el desarrollo del Sistema Nacional de Cartografía de Zonas Inundables (Ministerio de Medio Ambiente y Medio Rural y Marino, 2011), consciente de la complejidad de los procesos y en aras de simplificar, aconseja aplicar una condición de contorno de nivel correspondiente al mismo periodo de retorno que el de la avenida que se esté modelizando. Esta forma de proceder no tiene en cuenta la probabilidad de ocurrencia de varios eventos extremos al mismo tiempo. En este artículo se propone una metodología híbrida de simulación continua que combina la generación de series sintéticas a largo plazo de las variables implicadas, un modelado hidrodinámico bidimensional de alta resolución espacial de la zona de estudio, y técnicas estadísticas de muestreo de datos e interpolación de resultados. La metodología propuesta se ha aplicado a la ciudad costera de Betanzos (noroeste de España), localizada en la confluencia de dos ríos y en la cola de un estuario. Históricamente Betanzos ha tenido problemas de inundación por lo que está catalogada como un Área de Riesgo Potencial Significativo de Inundación (ARPSI) fluvial y costera. Los resultados se han comparado con los obtenidos aplicando la propuesta de la Guía Metodológica para el Desarrollo del Sistema Nacional de Cartografía de Zonas Inundables (Ministerio de Medio Ambiente y Medio Rural y Marino, 2011), observándose que estos últimos sobreestiman de forma considerable los niveles de inundación asociados a un determinado período de retorno.Los autores quieren expresar su gratitud al departamento de Planificación Hidráulica de Aguas de Galicia, perteneciente a la Consejería de Medio Ambiente y Ordenación del Territorio de la Xunta de Galicia, por los datos suministrados de los aforos, MDT y así como la serie sintética de caudales de la ARPSI fluvial ES014-CO-12-01. Además, agradecer al área de medio físico del Ente Público de Puertos del Estado, por los datos del mareógrafo del puerto de A Coruña.Sopelana, J.; Cea, L.; Ruano, S. (2017). Determinación de la inundación en tramos de ríos afectados por marea basada en la simulación continua de nivel. 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