HR: 1340h
AN: H53B-0465    [Abstracts]
TI: Experimental Analysis on Flow Expansion Over Fan
AU: * Sittoni, L
EM: sitt0014@umn.edu
AF: St Anthony Falls Lab, University of Minnesota, 2 3rd Ave SE, Minneapolis, MN 55414 United States
AU: Paola, C
EM: cpaola@umn.edu
AF: St Anthony Falls Lab, University of Minnesota, 2 3rd Ave SE, Minneapolis, MN 55414 United States
AB: We present experimental evidence on the occurrence of large angles of flow expansion with no flow separation over depositional fans. The evolution of a number of self-formed experimental fans was analyzed using overhead images and detailed topographic surveys. Angles of flow expansion up to 45 degrees were found in association with a characteristic bed curvature. Although precise measurements indicate that transverse curvature appeared to slightly decrease downstream over the fans, an approximately constant value of curvature of about 0.1 (r/W = 0.1, where r is the dimensional curvature and W is the maximum width of the fan) fits well all fan sections analyzed. In addition, we found that bed curvature shows a weak proportional dependence with fan expansion angles (alpha around 20 degrees, where alpha is the local plan angle). The curvature appears sufficient to explain the common occurrence of unchannelized, simple fans with opening angles, and hence rates of bedload divergence, much larger than would be predicted from jet theory. We have also analyzed fan development. In our experiments, an instability phenomenon causes a sudden increase in channel width, in association with the formation of a scour. A common development pattern was observed: the upstream-migrating scour initiates flow expansion, inducing in turn the formation of a transient concave heart-shaped fan shape that then slowly develops into a final, steady cone-shaped deposit. Most of the deposition appears to occur during the initial expansion phase. During final steady conditions, fans were also observed to reach values of L/0.5W (L is the final fan length and W is the fan width) approximately constant and in the range 2-4.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005