Mohrig, David¹, Douglas Jerolmack¹ 
(1) Massachusetts Institute of Technology, Cambridge, MA

ABSTRACT: Adjustments within Trains of Dunes Driving Bar Growth in a Sandy Braided Channel

A sampling scheme combining the direct measurement of water depths with aerial photography was used to capture spatial adjustments within trains of dunes moving over bars in the North Loup River, NE. This data set resolves local changes in dune height, wavelength, and migration rate and connects them to sediment deposition on these bar surfaces. Measured rates of bed aggradation were as high as 10mm/hour on the stoss side of a bar and 55mm/hour on a lee surface. These deposition rates were very low in comparison to migration rates for the dunes transferring sediment into the bars. The average migration rate for dunes on bars was 2.5m/hour. Rates of dune migrations were relatively constant as these bedforms moved up onto and across bar tops, with one important exception, the zones where new dunes were being created. On these sections of the bed, the migration rates were as much as 3 times greater than those measured both immediately upstream and downstream from these zones of high activity. Local heights of dunes were observed varying at rates up to 100mm/hour. All of the kinematic data from the study reach will be used to evaluate how the feedback between bar and dune topography sets the spatial structure of the sediment transport field and to investigate the production of cross beds by the interacting dune and bar forms.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.