SEDIMENT TRANSPORT AND CHANNEL MORPHOLOGY DURING AN EXTREME FLOOD EVENT IN NAHAL MESHUSHIM, CENTRAL GOLAN HEIGHTS AND NE SEA OF GALILEE, ISRAEL

During January 2020, an unusual rainstorm occurred in northern and central Israel. The storm generated large floods, inundation, causalities and massive damage to property and infrastructure. In the central Golan Heights in NE Israel, the rain amounts exceeded 100 mm for 24 hours and generated an extreme flood event in the basaltic boulder-bedded Nahal Meshushim (draining 160 km²) with a 1:20 recurrence interval. The middle peak of the 3-peak flood hydrograph was 3.5-4.0 m high and had a discharge of 203 m³/sec, shear stress of 405 N/m² and maximum velocity of 6 m/s. This peak discharge was the third largest flood on record since 1969, and almost 4 times the bankfull discharge (53 m³/sec) for this stream.

Using three acoustic Japanese impact plates embedded in the concrete weir of the official IHS hydrometric station, intense sediment transport was recorded of all grain size fractions available in the bed, including mass entrainment of boulders. The movement of the boulders (> 1 m), which control the channel structure and morphology suggested breakup of the armor layer and existence of equal mobility transport (EMT) conditions that are rarely measured and described in the field, as they often destroy the measuring equipment and pose a hazard to personnel present on-site.

One of the most prominent results of the acoustic bedload system is regarding the existence of spatial EMT conditions: The momentary hydrologic conditions do not necessarily overlap with the concurrent sedimentologic conditions. Two main reasons can explain this unusual sedimentary pattern: 1. Boulders are not as abundant in the channel bed as smaller grain fractions and cannot satisfy the “demand”. 2. The bed transport responds relatively slowly to the quickly changing discharge due to particle roughness and grain interactions and thus sediment motion of the coarse fraction is discontinuous. EMT conditions occurred at 2.3-4.5τ_c. Pre- and post- flood measurements demonstrated the extremity of the flood. These included substantial decrease of the armor ratio in the pool upstream of the measurement site by almost half from 19.1 to 10.5, removal and stripping of bank and floodplain riparian vegetation, exposure of bedrock along the banks and channel and sediment transport of gravel up to cobbles over the floodplain - 2.0-2.5 m above the channel bed.