Multiscalar model for the determination of spatially explicit riparian vegetation roughness

Improved understanding of the connection between riparian vegetation and channel change requires evaluating how fine-scale interactions among stems, water, and sediment affect larger scale flow and sediment transport fields. We propose a spatially explicit model that resolves patch-scale (submeter) patterns of hydraulic roughness over the reach scale caused by stands of shrubby riparian vegetation. We worked in tamarisk-dominated stands on the Yampa and Green Rivers in Dinosaur National Monument, northwestern Colorado, USA, where questions remain regarding the role of vegetation in inducing or exacerbating documented channel changes. Hydraulic roughness patterns were derived from patch-scale measurements made with detailed terrestrial laser scan (TLS) data that were extrapolated to reach scales based on correlation with light detection and ranging (LiDAR) (ALS) data. Two-dimensional, patch-scale, hydraulic models were used to parameterize the stage dependence of hydraulic roughness of typical patch types (i.e., sparse, moderate, and dense patches). We illustrate the value of using this approach to characterize vegetation roughness by applying our results to a two-dimensional hydraulic model of flow for one of our study sites. Results from this work predict that the roughness of vegetated floodplains increases with flow depth and is dependent on patch-scale stem organization. Geomorphically relevant patterns (i.e., areas of low or high shear stress that are likely to scour or fill during high flows) become apparent with the detail introduced by spatially explicit, depth-dependent roughness. To our knowledge, the multiscalar analysis presented here is the first to mechanistically account for shrubby riparian vegetation stand structure, and associated hydraulic roughness of vegetation patches, at the reach scale.