GEOMORPHIC CHANGES ALONG THE BREACHED NAHAL OZ RESERVOIR FLOW PATH IN THE NW NEGEV DESERT, ISRAEL: A 20-YEAR PERSPECTIVE

Twenty years after the catastrophic dam-break flood of May 2001 at Nahal Oz Reservoir, we revisited Nahal Yare'akh, a small loessial gully, a tributary of Nahal Hanun sub-basin. In the 2.3 km long reach just below the reservoir, surrounded by agricultural fields and a JNF forest, the outflow of treated sewage water lasted for 12 hours, had a peak discharge of 1000 m³/s and a volume of 3.5*10⁶ m³. This catastrophic event is probably the largest anthropogenic-related flood in Israel and could not have occurred naturally. The goals of the research were to document the changes after 20 years that the gully and its surrounding valley went through in comparison to the morphology of pre- and post-flood morphology.

The results showed major changes were associated with anthropogenic activity. For example, a new bypass road was constructed along the valley as well as new dirt roads for agricultural and military activities. Also, some of the forest and seasonal shrubbery were burnt (by ignition balloons and rockets originating from Gaza). However, the natural morphology along the gully channel and the immediate area most affected by the flood did not recover and the remnants of the flood event are preserved despite the natural high erodibility of loess and the vegetation recovery of eucalyptus and pine trees.

The large canyon-like breach that formed during the flood has a form of misfit valley at present, with a small natural gully which runs through it. The most preserved prominent catastrophic flood features are vertical gully walls, several meters high. The repeat cross-sections show that the area turned to be topographically smoother, where most of the erosion along the walls occurs from gravitational mass wasting and hillslope processes whereas the fluvial nature was reduced. The channel of Nahal Yare’akh gully appears in a variety of morphologies, including segments where it completely disappears and does not maintain a consistent downstream hydraulic geometry. The typical morphology of the loess channels of the Negev with vertical loess banks and small waterfalls before the initiation of incision is completely missing. We conclude that the recovery process of the valley and the gully channel are slow and clearly preserve the catastrophic flood morphology. Without human intervention of moderating the vertical valley walls, the misfit breach morphology will probably last for a long period.