FLASH-FLOOD AND BEDLOAD DYNAMICS OF DESERT GRAVEL-BED STREAMS

Comparatively little is known about the hydrology of desert flash-floo ds despite the extent of the world's drylands, There is even less know n about their sedimentary behaviour and particularly about the movemen t of coarse material as bedload. The results of an intense field monit oring programme carried out on an ephemeral gravel-bed stream in the n orthern Negev Desert are presented. In this semi-arid setting, flow du ration analysis indicates that the channel is hydrologically active fo r 2% of the time, or about seven days per year, and that overbank flow can be expected for only 0.03% of the time - about three hours per ye ar. Multipeaked flood hydrographs are the norm, reflecting many factor s including the arrival of separate slugs of discharge from contributi ng subcatchments, The passage of the initial flood bore is surprisingl y slow, but the rising limb of the flood hydrograph is rapid with a me dian time of rise of 10 minutes, in keeping with expected flash-hood b ehaviour. Bedload flux is high, averaging 2.67 kg s(-1) m(-1) during t he period that the channel carries flow. This gives very high bedload sediment yield despite the infrequent and short duration of flood flow s and matches the high yield of suspended sediment. The relationship b etween bedload flux and boundary shear stress is simple, in contrast w ith perennial gravel-bed streams, and the exponent of the log-log rela tionship is 1.52. Of great value is that the behaviour of the Nahal Es htemoa corroborates a pattern established by the authors previously in a smaller tributary stream. (C) 1998 John Wiley & Sons, Ltd.