Reevaluating Flood Protection: Disaster Risk Reduction for Urbanized Alluvial Fans

Abstract. The deterioration of check dams and other flood prevention measures, combined with storms breaking historical records, has created an immediate risk of floods and debris flows breaching urbanized alluvial fans. In this study, we reevaluate these flood and sediment prevention measures and propose a different flood prevention paradigm.

Flood defense measures like check dams, terraces, and afforestation in steep mountain basins aim to retain sediments and prevent them from reaching the alluvial fan, ensuring the functionality of bypass canals and levees. However, this approach provides a false sense of security; natural and man-made weathering and erosion processes continue, causing sediments to accumulate in the control measures, gradually reducing their effectiveness and strength. Over decades, high-intensity rainstorms can trigger slope instability and flooding, leading to the collapse of these measures that carries the accumulated sediments into urban areas in the form of destructive debris flows. As the risk gradually increases over time, the long-term effectiveness of these measures is questionable.

Findings from disastrous events worldwide, together with 60 years of flood monitoring in the city of Eilat, highlight the potential for incorporating flood management within urbanized alluvial fans. It has been shown that, for long-term safety, the steep mountain basin should remain natural to allow the continuous evacuation of sediments. On the alluvial fan, the strategic placement of recreation areas, radial roads, and parks can effectively create space for incoming water and sediment. Our approach to disaster risk reduction proposes a shift in urban planning priorities to incorporate flood management by allocating 20–30 % of the alluvial fan—including the fan head and several wide radial road corridors down to the fan toe—for stream migration and sediment deposition. This concept was effectively tested using a physical analogue model in the laboratory.