THE INFLUENCE OF MICROFORM BED ROUGHNESS ELEMENTS ON FLOW AND SEDIMENT TRANSPORT IN GRAVEL BED RIVERS - A REPLY

Clifford et al. (1992) argue that differences in hydraulic conditions at the moment of sediment entrainment in Turkey Brook, flood by flood, are attributable to variations in the form drag that arises from cluster microforms. While not denying the possibility of variable cluster concentration, we show that the time-dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold. Clifford et al. further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event. This merits further examination. However, we point out that clusters are not only observed at low flow and, therefore, already present and able to contribute significant form drag as the next bedload generating flood waxes, but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events. The revelation by Clifford et al. that flow structure in the River Quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements (perhaps clusters) is not surprising. There is, however, some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing. Nevertheless, the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport.