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.