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.140 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.
