Urbanizing alluvial fans as flood-conveying and flood-reducing systems: Lessons from the October 1997 Eilat flood

On many alluvial fans in the drier parts of the world, urban settings have replaced natural areas, which in the past had been the domain of stream channel wandering, sediment deposition, and attenuation of flow energy. Due to very long intervals between flood events, little attention had been paid to these natural processes, resulting in a fallacious sense of security from destructive floods and in widespread settlement before the newcomers to the desert were faced with the flooding problem. Each stage in the urban development and in the establishment of the infrastructure produced an irreversible situation, without leaving the necessary expanse required for the passage of floods and their sediments. Optimal planning for flood protection became increasingly difficult. The town of Eilat was established in 1949 on a series of small alluvial fans skirting the mountain front facing the Gulf of Eilat. Over the years the town expanded up to the mountain front, and rain events hit at varying intervals. The extremely arid climate and long floodless years "encourage" the locals to "forget" the flood damage shortly after its occurrence, leading to an overall neglect in the maintenance of drainageways which have not been active for years. The October 1997 rainstorm was the second most intense event since 1949. By then the town had doubled in area since the previous damaging storm (which occurred in 1982), and the floods could enter the town more or less directly, without any natural buffer zone. However, despite the extraordinary rainfall amounts of the October 1997 event, the combination of storm characteristics and urban setting created a situation wherein the damage to the town was minimal. The urban framework, which includes roads, streets, parking lots, playgrounds, parks etc., often without flow continuity between them, caused a divergence of the flows, attenuation of their velocity, and the capture of floodwaters in small local storages. The result was a fast deterioration in peak discharge, a longer flow duration, and little appreciable damage. The detailed analysis of the October 1997 rainstorm flood shows that a relatively minor shift in the priorities of urban design of alluvial fan developments towards a healthy balance between flood-conveying and flood-reducing systems may yield long-term benefits in the handling of moderate as well as high magnitude floods.