CHANNEL BED-STEPS ALONG NAHAL-YAEL, NEGEV DESERT, ISRAEL

The height and spacing of channel bed-steps formed in boulder and bedrock in the ephemeral channels of the Nahal Yael watershed show a strong correlation with the channel slope. The distance between steps is inversely proportional to slope at slopes of 0 to 20%, but then remains fairly constant at higher slopes. Step height increases proportionally with channel slope. These bed-steps are similar to those described on perennial channels elsewhere. where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps. However, the ephemeral flows in the tributary channels of the Nahal Yael system are not sufficient to submerge the clasts forming the boulder steps. Using Rouse's criteria, channel bed-steps are creating maximum flow resistance at slopes of 12 to 50% along the channels of Nahal Yael. Using Davies' criteria, bed-steps are creating maximum flow resistance at slopes of 3 to 10%. The bed-steps also relate to sediment transport; Nahal Yael has a low sediment yield by world standards, and the increase in flow resistance due to step formation may serve to increase sediment movement. Both the boulder and bedrock steps at Nahal Yael are hypothesized to reflect the scales of flow turbulence along the channels. We were unable to evaluate the role of flow volume or boundary roughness in controlling these scales, but channel slope appears to exert some control on the scale of turbulence, based on the correlation between slope and bed-steps. The hypotheses developed from field evidence at Nahal Yael should be testable in flume simulations.