Geomorphology in Arid Regions

Binghamton Geomorphology Symposium 8

Donald O. Doehring

• Publisher: Routledge
• Year: 2020
• Print ISBN: 9780367280208
• eBook ISBN: 9781000046281
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Geography
• Index: Biological Sciences

This book, first published in 1980, collects together thirteen articles on 'Geomorphology in Arid Regions'. It uses the term 'arid' loosely to include studies from climes which might otherwise be considered semi-arid, in order to provide a diversity of papers dealing with important problems of interest to geomorphologists today.

A TENTATIVE SEDIMENT BUDGET FOR AN EXTREMELY ARID WATERSHED IN THE SOUTHERN NEGEV

Asher P. Schick

Department of Geography
The Hebrew University of Jerusalem

ABSTRACT

A sediment budget for the 0.5 km² watershed of Nahal Yael is attempted on the basis of detailed observations of rainfall, streamflow, sediment transport and deposition, and changes in geomorphic features. For the ten year period considered, the mean annual rainfall was 31.6 mm, 99 percent of the geomorphic work was accomplished during 5 days by 7 discrete events.

Sediment in suspension, computed from data obtained through an automatic sampling program, accounted for a mean annual yield of 127 tons. Bedload yield is estimated on the basis of, (1) distance of transport determined from traceable particles, (2) the area and depth of the scour layer for the inner channel bed and for the gravel bars, and (3) a comparison of grain sizes on the bed and the bars with sediment in transport as sampled by liquid samplers and bedload traps. A mean annual yield of 66 tons was found. The dissolved load is about one percent of the total load. The resulting mean annual sediment yield of 388 tons/km² considerably exceeds the accepted norm for arid environments. It also exceeds by a factor of 3 the estimated sediment yield, corrected for drainage area, for the 3,100 km² watershed of Wadi Watir, located in a similar environment in eastern Sinai.

While the internal sediment delivery ratios of the Nahal Yael drainage system are reasonably consistent, the aggradation rates as measured directly on the alluvial fan over a period of six years are one fifth of what they should be on the basis of the sediment transport computation. Although inadequacies of the sampling and measurement program may explain a part of the discrepancy, the main reasons are an insufficient understanding of the transport mechanism during violent desert floods, exchanges between suspended and bedload transport modes, and the importance of transient alluvial storage and its intense localization.

INTRODUCTION

The purpose of this paper is to construct a sediment budget for a small arid watershed and to use the quantitative relationships of this balance as a vehicle for describing and enhancing the understanding of the dominant geomorphic processes in extreme deserts.

Only few studies provide us with a fairly complete watershed sediment budget. Among these, the best known ones represent a large watershed in a mid-latitude highland area (Jaeckli, 1957), a small watershed in a high-latitude mountain area (Rapp, 1960), and a small watershed in a semi-arid hilly area (Leopold and others, 1966). All these studies encountered difficulties in sustaining the balance by equalizing denudation from upstream source areas with larger watersheds of which they form a part. The downstream diminution of denudation per unit area, long ago recognized by engineers under the term "sediment delivery ratio", is in reality a problem of magnitude-frequency relationships which affect the alluvial storage (Trimble, 1976).

A quantitative process study in an extremely arid environment cannot hope to duplicate the standards of accuracy of similar studies in more benign climatic zones. The important geomorphic events are rare, violent, and unpredictable, while logistics is very complex. There are also some minor advantages. The lack of soil, vegetation, and interflow simplify the water and erosion mechanism. Population sparseness results in less vandalism. Events are highly discrete, with long intervening periods of geomorphic stagnation which enable undisturbed documentation.

On the whole, however, sediment budget studies in the desert must be satisfied with less direct and dependable measurements and with more assumptions than would be the norm in more humid environments. The rarity of the data is the main reason for the presentation here of such a budget.

NAHAL YAEL

The venue of this study is the Nahal Yael watershed in the southern Negev, Israel. The site as well as its characteristics in terms of physiography, hydrometeorology, slope and fluvial processes have been described in some detail (Gerson and Inbar, 1974; p. 16–18; Schick, 1970, 1974, Schick and Sharon, 1974; Sharon, 1970; Yair and Klein, 1973). Briefly, Nahal Yael is a 2 km long ephemeral stream draining 0.6 km² of bare rocky desert into the major wadi of Nahal Roded, which debouches into the Arava rift valley near the northern end of the Gulf of Aqaba (Figure 1). Midday summer temperatures appro...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• Acknowledgments
• Intermontane Basins of the Arid Western United States
• Geomorphic Controls of Alluvial Fan Deposition in the Sonoran Desert, Southwestern Arizona
• The Formation of Pediments: Scarp Back-Wearing or Surface Downwasting?
• Origin of Segmented Cliffs in Massive Sandstones of Southeastern Utah
• Tectonic Geomorphology North and South of the Garlock Fault, California
• A Tentative Sediment Budget for an Extremely Arid Watershed in the Southern Negev
• Sediment Origin and Sediment Load in a Semi-Arid Drainage Basin
• A Comparison of Observed Sediment-Transport Rates with Rates Computed Using Existing Formulas
• Geomorphic Response of Central Texas Stream Channels to Catastrophic Rainfall and Runoff
• Vigil Network Establishment for Montana's Coal Region
• Weathering of Caliche in Southern Nevada
• Yardangs
• The Maria Effect: Equilibrium and Activation of Aeolian Processes in the Great Basin of Nevada