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

Noam Greenbaum, Uri Schwartz, Paul Carling, Nathaniel Bergman, Amit Mushkin, Rami Zituni, Rafi Halevi, Gerardo Benito, Naomi Porat

Research output: Contribution to journalReview articlepeer-review

23 Scopus citations

Abstract

Direct measurements of boulder entrainment in desert wadis are not available. The 2004 flood (peak discharge – 470 m3 s−1; recurrence interval – 120 years) in the hyperarid, ungauged Nahal Hatzera ephemeral stream (45 km2), transported and deposited 0.85–2.1 m concrete boulders and slabs detached from infrastructure upstream and natural boulders. EDM and drone air-photographic surveys documented the geometry of the study reach and the location of boulders. Analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200–760 m3 s−1, during the last 600 years. 1-D HEC-RAS hydraulic analysis provided water surface profiles, discharges and hydraulics, along the study reach and velocity, shear stress and stream power for each boulder. MAX program and Pearson 3 distribution were used for flood frequency analysis. 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. The alluvial channel enabled to transport these concrete boulders, reflecting the unstable, active sandy layer of the channel bed over which the boulders moved. The maximum flood shear stress and stream power characterize medium-large floods with return period of 20–120 years and not for the largest floods, as expected. Boulders about 2.1 m and weighing about 15 t can be transported at least once in 120 years. The shear stress and stream power indicate that the moderate-large floods are the most geomorphically effective floods rather than the largest floods in Nahal Hatzera basin. Nevertheless, the ‘geomorphic effectiveness’ of the 2004 flood – a typical desert flash flood with high peak and short duration, was small based on the minor changes along the channel and banks indicating that their resistance thresholds were not exceeded and energy expenditure was mainly on boulders entrainment and transport.

Original languageEnglish
Article number103086
JournalEarth-Science Reviews
Volume202
DOIs
StatePublished - Mar 2020
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2020 Elsevier B.V.

Funding

The authors wish to thank the Royal Geographical Society Thesiger-Oman International Fellowship UK, for fund no. THES03/16 , to P.A. Carling, awarded in 2016, and N. Yoselevich of the Cartography laboratory, the Geography Department, University of Haifa for drawing the figures.

FundersFunder number
Royal Geographical Society Thesiger-Oman International Fellowship UKTHES03/16
University of Haifa

    Keywords

    • Boulders transport
    • Desert flood
    • Flood frequency analysis
    • Geomorphic effectiveness
    • Paleoflood hydrology
    • Shear stress
    • Stream power
    • Ungauged catchments

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