Reconstructing palaeoflood magnitude and frequency for flood hazard assessment in arid regions: The northern Judea Desert Streams, Israel

Abstract

Ephemeral streams in desert regions exhibit a high interannual hydrological variability dominated by short-lived flows and isolated extreme floods. Increase in settlements, land use intensity and infrastructures during the past 70 years in arid regions in general and in the study area in particular requires a probabilistic approach to Flood Frequency Analysis (FFA). This analysis is hampered by the scarcity of rainfall and discharge gauging stations, and by the high temporal variability of extreme events. Here, we apply advanced palaeoflood hydrology techniques to estimate the magnitude and frequency of floods to improve the quality and reliability of flood risk analysis for infrastructure design. The study focusses on two basins in the Northern Judean Desert: Nahal (N.) Darga ephemeral stream (70 km²) and Nahal Arugot (215 km²). The sedimentological evidence of past large floods in Nahal Darga (70 km²) consists of 17 flood events deposited over the last 1000 years with a maximum palaeo-discharge of 300 m³ s⁻¹. In Nahal Arugot (215 km²), the palaeoflood record comprises 36 flood events that extend over the last 400 years, recording a maximum flood discharge of 830 m³ s⁻¹. In comparison, the palaeoflood discharges were 1.8 and 1.6 times larger than the maximum measured floods. Nevertheless, these discharges did not exceed the regional envelope curve of the Judean Desert. The improved FFA caused an increase in the estimated flood discharges for all return periods; in Nahal Darga, the 100-year flood increased from 110 m³ s⁻¹, for the gauged record only, to 160 m³ s⁻¹ (45%) for the combined records, and in Nahal Arugot from 660 m³ s⁻¹ to 980 m³ s⁻¹ (48%). The FFA for the systematic data only underestimated the frequency of the large floods because of their short and discontinuous records. The incorporation of the palaeoflood data into the dataset provided more solid values to the FFA, which significantly improved the reliability of risk assessment.