THE NON-LAYERING OF GRAVEL STREAMBEDS UNDER EPHEMERAL FLOOD REGIMES

The two-layer format common to perennial streambeds, in which a relati vely coarse armour overlies a finer subarmour, develops as a function of both the ingress and subsequent near-surface winnowing of interstit ial material and the selective non-entrainment or slower transport vel ocity of coarse clasts. Ephemeral streams appear to lack such vertical layering or are characterized by weak layer development. Some of this may be due to the degree of mixing associated with the scour-and-fill process. However, continuous monitoring of bedload discharge in the N ahal Yatir in the northern Negev Desert reveals that sediment transpor t rates are extremely high so that the chance of armour layer developm ent through selective non-entrainment is much reduced. Indeed, a compa rison of the bedload and bed material size-distributions confirms a hi gh degree of similarity and hints at equal mobility regardless of clas t size. The monitoring programme also indicates that the bed becomes h ighly mobile at comparatively modest fluid shear, so that practically all floods are associated with high transport rates. Consequently, the winnowing that might be brought about by low transport-rate events do es not occur. Even within a single event, winnowing is precluded by th e rapid nature of flow recession that is so characteristic of flash-fl oods. The high degree of bed material mobility is attributable, in par t, to the lack of strength that would otherwise be a corollary of armo ur development. However, it also highlights the divergent nature of th e feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems.