Inbank and overbank velocity conditions in an arid zone anastomosing river

Relative to those at sub-bankfull flow, hydraulic conditions at overbank flow, whether in the channel or on the floodplain, are poorly understood. Here, velocity conditions are analysed over an unusually wide range of flows in the and zone river of Cooper Creek with its complex system of anastomosing channels and large fluctuations in floodplain width. At-a-station hydraulic geometry relationships reveal sharp discontinuities in velocity at the inbank-overbank transition, the nature of the discontinuity varying with the degree of flow confinement and the level of channel-floodplain interaction. However, despite inter-sectional differences, velocities remain modest throughout the flow range in this low-gradient river, and the large increases in at-a-station discharge are principally accommodated by changes in cross-sectional area. Velocity distribution plots suggest that within-channel conditions during overbank flow are characterized by a central band of high velocity which penetrates far toward the bed, helping to maintain already deep cross-sections. Floodplain resistance along Cooper Creek is concentrated at channel bank tops where vegetation density is highest, and the subsequent flow retardation is transmitted across the surface of the channels over distances as large as 50-70 m. The rough floodplain surface affects flood wave transmission, producing significant decreases in wave speeds downstream. The character of the wave-speed-discharge relationship also changes longitudinally, from log-linear in the upper reaches to nonlinear where the floodplain broadens appreciably. The nonlinear form is similar in several respects to relationships proposed for more humid rivers, with flood wave speed reaching an intermediate maximum at about four-fifths bankfull discharge before decreasing to a minimum at approximately Q(2.33). It does not regain the value at the intermediate maximum until the 10 year flood, by which time floodplain depths have become relatively large and broad floodways more active. Copyright (C) 2002 John Wiley Sons, Ltd.