Re-Envisioning Cross Sectional Hydraulic Geometry as Spatially Explicit Hydraulic Topography

Abstract:

Traditional transect-based methodology for determining hydraulic geometry relationships depends on a complex set of opaque fieldwork and computational decisions that sometimes go unexplained. The fields of river hydraulics and fluvial geomorphology are in the midst of a transformation from considering limited cross-sectional data to using an abundance of spatially explicit data. Hydraulic geometry is one of the classic tools of fluvial geomorphology that is ripe for re-envisioning so it can continue to be useful in the spatially explicit era. This study developed a new method for analyzing discharge-dependent hydraulics coined “hydraulic topography” that not only increases the accuracy of the tool, but also should eliminate several sample- and assumption-based inconsistencies from traditional hydraulic geometry analysis. Hydraulic topography relied on detailed, near-census river surveying and served as the standard by which to assess cross sectional methods. Both hydraulic topography and uniformly spaced cross sectional hydraulic geometry sample approaches were applied to a series of high resolution 2D hydrodynamic simulations of the gravel-cobble bed lower Yuba River- their associated results were analyzed. More specifically, the power functions fit to discharge-dependent average width, depth, and velocity for three spatial scales were visually inspected and their corresponding exponents and coefficients were compared. Average cross sectional hydraulics at the segment scale spanned up to 1.5 orders of magnitude for a given discharge. Transect-determined rates of reach scale depth and velocity increase with changing discharge were consistently over- and underestimated, respectively, relative to the near-census benchmark. Both methods showed that relative to riffles, pools had lower velocities at low discharges but a higher rate of velocity increase with increased flows. Overall, 73 percent of cross sectional power regression parameters assessed fell between 10 and 50 absolute percent error with respect to the spatially explicit hydraulic topography approach. Although traditional transect-based sampling may be viable for certain uses, percent errors of this magnitude could still compromise engineering applications in river management and training works.