ABSTRACT
Rare events in Meditteranean environments are a major factor in channel and floodplain modifications. The catastrophic flood of 19–23 January 1969 in the Upper Jordan River was caused by a rare climatic event and intensified by the combined effect of human intervention. The season rainfall amount reached 180% to 250% of the annual average and was the highest amount measured on the whole region for a 150 year period. Daily maximum precipitation measured values were 170 mm and 190 mm. The flood was 108 m3 in volume with peak discharge and velocity of 215 m3s−1 and 6 ms−1 respectively. Lake Kinneret-base level of the Upper Jordan River basin reached the highest level in a 100-year period.
Suspended sediment from the river basin and the removal of the lower flood-plain soil cover – about 2 × 106 m3 – were transported to the lake and formed a new delta at the Jordan’s outlet; since its inception there has been a permanent growth in its size by consolidation of sediments, vegetation and a continuous supply of sediment of the river.
Bedload is composed of basaltic boulders. D50 values are between 100 and 500 mm with a maximum size of 2000 mm b-axis. The boulders are derived from rotational bank failures and landslides along the main river basaltic canyon and during the flood were deposited in a braided 0.02 slope floodplain.
The flood resulted in a sporadic influx of large quantities of eroded material, about 20 times the annual discharge values. The annual average bedload discharge is about 0.1% of the total floodplain sediment storage.
334The movement of coarse material in the braided system correlates with power expenditure of the flowing water as suggested by Bagnold. Morphological structures in the channel can be explained by changes in the unit stream power values. The flood generated four major large scale elements of bed relief; megabars, channels, erosional scarps and terracetes, and rectilinear harrows. The distribution of these macroforms, and their alignement and orientation with respect to the mean flood propagation axis allow a tentative reconstruction of the kinematic structure of the flood, in terms of secondary currents operating within the main flow. Megabars did not change since deposition during the flood.
The high magnitude-low frequency flood created a new adjusted morphology of channel and floodplain. The established relationship is reflected in planimetric braided system values, channel macroforms and channel hydraulic geometry.
