Reconstruction of the Holocene flood record for the Southern Judea Desert using Palaeoflood hydrology

The Judea Desert constitutes a distinctive hydrological region characterized by short and steep ephemeral streams draining eastward to the Dead Sea Valley. The climate is semi-arid at the upper reaches and arid to hyper-arid at the lower reaches of the basins. The hydrological data for these streams is scarce, which leads to poor estimation of the magnitude and frequency of floods. The lack of data is particularly significant when planning infrastructure such as roads, bridges, reservoirs, dams etc. Flood frequency analysis for risk assessment is therefore, based on various models such as rainfall-runoff, empirical, regional models etc.

The current study is based on Palaeoflood Hydrology which uses geomorphological evidence for real floods that accumulate in typical natural traps, along the course of the streams for hundreds and thousands of years. The collection of these data enables us to reconstruct the history of the floods in the streams during the last hundreds to thousands years. By combining these data with measured and historical data (if any), a long, solid database can be reconstructed. The applicability of the system in Israel has been proven in the larger streams in the Negev (Greenbaum, 2001) - a significantly different hydrological environment. The largest flood that occurred in the stream is important for regional envelope curves.

The method is based on field evidence in the form of slackwater deposits and other high water marks, which accumulate in typical natural sites and indicate on the minimum water elevation enabling discharge calculations using HECRAS hydraulic engineering software. The ages of the floods are determined by dating the flood deposits using radiocarbon and OSL.

In the Upper Nahal Rahaf stream (50 km 2 ), the palaeoflood record composed of 10 floods along a period of about 5000 years where three sites were located with 2-4 flood deposits at each site, including a rock shelter within which 2 flood remnants with reconstructed peak discharges of 1,200-1,300 m 3 /s. These flood sediments were found to overlie an Upper Paleolithic site dated to about 30 ka.

In Nahal Ze’elim stream (245 km 2 ) the record includes evidence of 21 floods with calculated peak discharges of 100-900 m 3 s -1 extending along a period of about 500 years. 5 sites were located - 4 of which close to the outlet. Each site recorded between 2-8 sedimentary units. The integration of the floods from all sites with their age revealed a vast information regarding major events. In further study this will also allow a renewed frequency analysis on the basis of wider knowledge.

The results also indicate that the flood frequency analysis for the gauged data only, in both streams, overestimates the frequencies of the larger floods, probably due to the short and discontinuous gauged records.