Field experiments to assess the links between bed-material entrainment and invertebrate drift in gravel-bed rivers during floods
Entrainment of sediment from small (m2) patches of fine material in gravel-bed rivers is poorly understood. Although morphological features such as these patches play an important role in sediment transport, bedload studies have largely neglected the incipient motion of sediment from within them. Here we present the details of a newly developed device (a portable flume) that allows controlled flow manipulation in natural river channels, producing low bedload transport rates from patches of stream bed. We have already demonstrated (Gibbins et al., 2007) that such low rates produce marked invertebrate drift from patches and hence have ecological relevance. He we show how the device can be used to address questions related to incipient bedload transport and its hydraulic controls, questions which conventional approaches are unable to tackle adequately. The flume is placed in the river-bed, isolating the target patch of fine material. When the hinged doors fitted to the upstream end of the flume are opened, more water is funnelled into the flume, increasing hydraulic forces acting on the target patch. To produce further increases, a Perspex sheet can be slid vertically in an out of the flume, leaving a gap between its base and the stream bed. Water is forced under pressure between the bottom of the sheet and the stream bed, increasing velocity and shear stress over the patch. Field experiments using the flume were carried out in the Ribera Salada River (NE Iberian Peninsula), an upland unmodified stream in the Ebro basin. These indicated that the increment in near-bed velocity over the patches averages 120%, with velocity >2 m/s in some cases. The maximum bedload transport rate created by the manipulations was close to 7 g/ms, a value typically attained during the early stages of floods in the study section of the Ribera Salada. Simultaneous measurements of invertebrate drift during the experiments indicated that a marked increase in the loss of animals from the bed occurs at the point when sediment becomes unstable and bedload transport is initiated. Calculations suggest that at the highest shear stresses created by the experiments (i.e. 40 N/m2), the river-bed may be denuded of animals within a 30 minute period. The preliminary experiments in the Ribera Salada demonstrate the potential of the flume to study patch entrainment and sediment transport in patchy rivers, as well as to provide insights into the ecological consequences of these physical processes.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1825 Geomorphology: fluvial (1625);
- 1862 Sediment transport (4558);
- 1895 Instruments and techniques: monitoring