Frequency of boulders transport during large floods in hyperarid areas using paleoflood analysis – An example from the Negev Desert, Israel

Abstract

Direct measurements of boulder entrainment in desert wadis are not available. Also, estimating the hydraulic conditions and frequency of natural boulders transport is difficult because their presence cannot be related to their arrival date, discharge or any other hydraulic parameter. The 2004 flood (peak discharge of 470 m3 s-1; recurrence interval >100 years) in the ungauged Nahal Hatzera ephemeral stream (45 km2), transported and deposited concrete boulders and slabs detached from infrastructure upstream as well as natural boulders, 0.85-1.5 m in size, at the outlet of the Makhtesh Hatzera Erosion Cirque (MHEC). Geometrical survey and a drone air-photographic survey documented the geometry of the study reach and the location of boulders. Accumulations and analyses of flood slackwater deposits along the study reach established a paleoflood record of 23 floods with peak discharges of 200-760 m3 s-1 during the last 600 years (radiocarbon ages). Step back-water hydraulic analysis using the 1-D HEC-RAS program, provided water surface profiles to show the hydraulics along the study reach and assigned a peak discharge to the 2004 flood using the clear high water marks of the flood, as well as to the paleoflood-related sedimentary units. The hydraulic program also provided velocity, shear stress and stream power for each boulder at its specific location along the study reach and within the cross section. The Max program was used for flood frequency analysis (FFA) of the paleoflood record. The results indicate that most of the concrete boulders were deposited in the sub-critical backwater of channel constrictions where velocities were 1.5-2.1 m s-1. The largest boulders, were deposited in super-critical flow where velocity was 8-9.2 m s-1. For the sandy alluvial bed of the Nahal Hatzera channel, maximum shear stresses of 250-900 N m-2 are available to transport a range of concrete boulders sizes (0.85-1.5 m), reflecting the unstable, active sandy layer of the channel bed over which the boulders moved. The mobile boulders probably were deposited on the falling limb of the flood and actually represent conditions of dis-entrainment. The FFA analysis and resulting peak discharges provide maximum depositional hydraulic values for return periods of 10-1000 years: velocities 7.1-9.8 m s-1, shear stress - 310-507 N m-2 and stream power - 2500-4972 N m-1 s-1. The frequency of the 2004 flood, based on the FFA, indicates that boulders up to 2.1 m and weighing up to 15 tonnes can be transported at least once in 120 years. The geomorphic effectivity of this typical desert flash flood was small based on it's qualitative properties - high peak, short duration and the small visible changes along the course of the channel and banks indicating that surface and channel resistance thresholds were not exceeded.

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