Measuring the spatial change in dune fields that build river bars and its implications for producing sets of dune cross-strata: North Loup River, Nebraska, USA

It has long been recognized that production of sets of dune cross-strata that make up river bars requires a systematic transfer of sediment out of the migrating bedforms and into the bar form. Characterizing spatial changes... [ view full abstract ]

It has long been recognized that production of sets of dune cross-strata that make up river bars requires a systematic transfer of sediment out of the migrating bedforms and into the bar form. Characterizing spatial changes within dune fields where systematic differences in average dune height and length are often less than the spread in values for each parameter requires thousands of height and length measurements. These data were assembled from the North Loup River, Nebraska, USA, using a combined set of low-altitude aerial photos and direct measurements of the bed topography, sediment transport field, and flow field. I will present the results from stoss sides of four linguoid bars in the river near the town of Taylor. Average water depths ranged from 0.8 m at the upstream end to 0.1 m at the downstream end of surfaces extending 40 – 60 m in the transport direction. Trains of dunes within these zones typically consisted of 20 – 30 bedforms. Dunes migrating into continuously shallower water adjusted in size via numerous interactions amongst the dunes themselves that included splitting, defect creation, and calving (e.g., Kocurek, Ewing, and Mohrig, 2010). Through these interactions, local mean dune height (H) decreased almost as rapidly as local mean flow depth (D). From upstream to downstream ends of a bar surface H/D typically increased from 0.28 to 0.32. Relative adjustment in local mean dune length (L) was not as great and L/D typically increased from 4.5 to 6.7. The net result was ever smaller and less steep (H/L) dunes in the downstream shoaling direction. Field measurements confirm that this reduction in dune size was accomplished by a systematic transfer of sediment mass into the bar. The calculated dune-set thicknesses due to climb represented from 7 to 2 percent of the associated, mean dune height. Set production due to variable scouring depths of dune troughs is estimated using the theory of Paola and Borgman (1991). The measured local variability predicted a constant value for the (mean thickness of preserved sets)/(mean topographical dune height) of twelve percent. Results from the North Loup River will be placed into a generalized scheme summarizing contributions of both dune climb and variable scouring to the production of set stratigraphy due to spatially evolving dunes.