HR: 18:00h
AN: H74B-06    [Abstracts]
TI: The Influence of Research Designs in Understanding the Control of Morphological Patterns on Bedload Path Lengths in gravel-bed rivers
AU: * Lamarre, H
EM: helene.lamarre@umontreal.ca
AF: Universite de Montreal, CP.6128, Succ. Centre-Ville, Mtl, Qc H3C3J7, Canada
AU: Roy, A
EM: andre.roy@umontreal.ca
AF: Universite de Montreal, CP.6128, Succ. Centre-Ville, Mtl, Qc H3C3J7, Canada
AB: There may be a strong association between particle path length and the morphologic scale of prevailing pool- bar channel patterns in gravel-bed rivers. It has been shown that tracers introduced in a pool during channel- forming discharges have downstream path length frequency distributions that are symmetrical, with modes coinciding with pool-bar spacing. Evidence to support this hypothesis comes mostly from flume studies and there is only a limited support based on field data. For the past seven years, we have conducted field experiments in streams with gradients ranging from constricted pool and pool-bar systems to step-pools and cascades in order to link displacement distances of tracing particles to the spacing between bed features. Results showed that clast movements could not be predicted from morphological length scales. The objective of this paper is to define why the control of morphological patterns on bedload path lengths could not be seen from our dataset. We have tested hypotheses for which the limited predictive capacity of the morphological length scale results from 1) the identification of some bed units that may have been ambiguous along the bed profiles or 2) the selection of sampling parameters. The experiment was carried out in seven reaches located in Quebec and in the French Alps. The slopes ranged between 0.011 and 0.43 and the representative particle size (d50) from 42 to 110 mm. Detailed topographic maps of the bed were produced in order to describe the morphological patterns. We used passive transponders inserted into clasts to measure displacement distances. Between 100 and 450 clasts of different sizes were tagged in each reach between 2003 and 2008. The tracking was carried out at low flow using a portable antenna. In identifying morphological entities that represent no ambiguity from both field observations and long profiles, we did not observe significant evidence of a morphological control on the path lengths. The results show that even when selecting well defined pool-bar or pool-step single entities, the morphological length scale is not associated with bedload path length in six of the seven reaches. However, in one of the reaches, a step-pool channel, the clasts did tend to move downstream from one pool to the next pool. The influence of sampling strategies was tested using the size and shape of the tagged clasts, the magnitude of the flood events, the channel characteristics, and the duration of the experiments. For some transport events, the size and the shape of the tagged clasts are both correlated with individual displacement distances. These events are not observed at specific discharge values. In general, however, the correlations are not significant. Moreover, the frequency distributions of path lengths are similar whatever the size of the tagged clasts. The percentage of mobile clasts during an initial event, as well as channel and average hydrodynamic characteristics of the streams do not affect the displacement distances or the mobility of the clasts. The duration of the experiment (from the initial seeding to the last recovery of the clasts) appears to be the only sampling factor to play a role on the significance of the relations between displacement distances and channel width, slope and bed material size. The channel morphology may affect the bedload path length but, to test this effect in the field and eventually generalize the results to different channel systems, the sampling protocol must include duration rules applied to all stream channels. This study outlines the importance standardizing research protocols for long term studies of particle path lengths.
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2009 Joint Assembly