MAGNITUDE, FREQUENCY AND HAZARD ASSESSMENT OF THE LARGEST FLOODS IN STEEP, MOUNTAINOUS BEDROCK CHANNELS OF THE SOUTHERN JUDEAN DESERT, ISRAEL
The present study applied palaeoflood hydrology method, which analyzes sedimentological evidence of past large floods in two southern Judean Desert streams - 250 and 55 km2, in size which yielded maximum palaeo discharges of 900 m3s-1 and 1250 m3s-1, respectively (1.3 and 2.3 times larger than the maximum measured floods, respectively), for records of 500 and 5000 years, respectively. Combining these data with measured and historical data produced a palaeo-hydrological data-base of several hundred to thousands years long.
These data updated the regional envelope curves for the maximum peak discharges; the combined systematic and palaeohydrological records for the last 500-700 years, updated flood frequency analysis (FFA). The 100-year flood in the larger basin decreased from 1750 m3s-1, for the systematic record only, to 760 m3s-1 (43%) for the combined records, and from 1260 m3s-1 to 980 m3s-1, in the other basin (77%).
Similar trends were observed in another four basins in the Judea desert ranging from 75-450 km2 in area. In all streams, the FFA for the systematic+historic data only, overestimated the frequency of the large floods due to their short and discontinuous records.
The palaeoflood data extends the record to several hundred years, reducing the peak discharge values for different return periods and significantly enhancing FFA reliability