Hydraulic Topography

Participants

Gregory B. Pasternack and Bobby L. Gonzalez

Background

Transect-based hydraulic geometry is well established but depends on a complex set of subjective fieldwork and  computational decisions that sometimes go unexplained. As a result, it is ripe for reenvisioning in the light of the  emergence of meter-scale, spatially explicit data and algorithmic geospatial analysis.

Purpose

This study developed and  evaluated a newspatially explicit method for analyzing discharge-dependent hydraulics coined ‘hydraulic topography’  that not only increases accuracy but also eliminates several sample- and assumption-based inconsistencies. 

Methods

Using data and hydrodynamic simulations from the regulated, gravel–cobble-bed lower Yuba River in  California, power functionswere fitted to discharge-dependent average width, depth, and depth-weighted velocity  for three spatial scales and then their corresponding exponents and coefficients were compared across scales  and against ones computed using traditional approaches. 

Results

Average hydraulic values from cross sections at the segment  scale spanned up to 1.5 orders of magnitude for a given discharge. Transect-determined exponents for  reach-scale depth and velocity relations were consistently over- and underestimated, respectively, relative to  the hydraulic topography benchmark. Overall, 73% of cross-sectional power regression parameters assessed  fell between 10 and 50 absolute percent errorwith respect to the spatially explicit hydraulic topography baseline.

Conclusion

Although traditional transect-based sampling may be viable for certain uses, percent errors of this magnitude  could compromise engineering applications in river management and training works. 

Publications

• Gonzalez, R. L., Pasternack, G. B. 2015. Reenvisioning cross-sectional hydraulic geometry as spatially explicit hydraulic topography. Geomorphology 246: 394–406. http://dx.doi.org/10.1016/j.geomorph.2015.06.024.