We report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea-breeze conditions on a 3-m-tall dune at Point Ax50 Nuevo, California, we measured simultaneously the near-brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 107-108 grains mThe lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non-recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows, rather than to increase their size, implying that very large, thick grainflows require a different mechanism. s ~ at shear velocities of 0.33-0.40 m s ~ I20 -. J 0 J 40

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[1] Many applications in river research and management rely upon two-dimensional (2D) numerical models to characterize flow fields, assess habitat conditions, and evaluate channel stability. Predictions from such models are potentially highly uncertain due to the uncertainty associated with the topographic data provided as input. This study used a spatial stochastic simulation strategy to examine the effects of topographic uncertainty on flow modeling. Many, equally likely bed elevation realizations for a simple meander bend were generated and propagated through a typical 2D model to produce distributions of watersurface elevation, depth, velocity, and boundary shear stress at each node of the model's computational grid. Ensemble summary statistics were used to characterize the uncertainty associated with these predictions and to examine the spatial structure of this uncertainty in relation to channel morphology. Simulations conditioned to different data configurations indicated that model predictions became increasingly uncertain as the spacing between surveyed cross sections increased. Model sensitivity to topographic uncertainty was greater for base flow conditions than for a higher, subbankfull flow (75% of bankfull discharge). The degree of sensitivity also varied spatially throughout the bend, with the greatest uncertainty occurring over the point bar where the flow field was influenced by topographic steering effects. Uncertain topography can therefore introduce significant uncertainty to analyses of habitat suitability and bed mobility based on flow model output. In the presence of such uncertainty, the results of these studies are most appropriately represented in probabilistic terms using distributions of model predictions derived from a series of topographic realizations.Citation: Legleiter, C. J., P. C. Kyriakidis, R. R. McDonald, and J. M. Nelson (2011), Effects of uncertain topographic input data on two-dimensional flow modeling in a gravel-bed river, Water Resour. Res., 47, W03518,