Geomorfología como factor de riesgo a inundaciones en la cuenca hidrográfica de Mulegé, Baja California Sur, México

Abstract

Flooding records in the town of Mulege have indicated since its founding date that meteorological phenomena, such as intense regional rainfall events, have been the main flood threat. In diagnostic flooding assessments, the geomorphological aspects of a watershed and increases in the urban area have been highly relevant. Considering these factors, this study implemented conventional field geology techniques and validated fluvial flooding through modeling scenarios according to the methodology of the Centro Nacional de Prevencion de Desastres (National Center for Disaster Prevention). In Mulege watershed, three geomorphological units were identified: sierras, alluvial plain, and coastal belt. These units are crossed by a dense network of dendritic drainage channels characterized by an intermittent regime, which as a whole forms a seventh-order exoreic watershed. The funnel-shaped watershed and the high density of drainage channels are factors that naturally increase flooding frequency and intensity. Specifically, its wide pluvial catchment area within the watershed decreases in size at its lower portion, forming a small valley where the town is located. Thus, the lower watershed is a highly flood-prone area because of its low topographic relief. In addition to its intrinsic characteristics (form, topographic relief, slopes, lithology, fluvial network), the establishment and increase of the urban area in the geomorphological environment of this lower portion has significantly reduced the runoff area, representing a human induced risk that increases flooding vulnerability. The inhabitants in Mulege perceive flooding to be increasing in frequency and intensity, as those that occurred in 2009 during Hurricane Jimena and in 2014 during Hurricane Odile, in which flood depths exceeded two meters in height. These flooding scenarios were validated by the fluvial flood model: Rainfall recorded at the Mulege meteorological station during Hurricane Jimena (305 mm in 24 h) was equivalent to that calculated for a 100-year return period and during Hurricane Odile (184 mm in 24 h) to a 25-year return period. Therefore, we confirm that the town of Mulege is exposed to a continuous risk of sudden flooding that is defned by the sum and interaction of distinct processes of natural and anthropogenic origins.