Robust prediction of hydraulic roughness

Abstract : OVERVIEW: The National River Restoration Science Synthesis (NRRSS) demonstrated that, in 2007, river and stream restoration projects and funding were at an all time high and increasing exponentially (Bernhardt et al. 2007). Increasingly, these restoration projects rely on soft engineering techniques involving planting riparian vegetation to alter channel and floodplain hydraulics or geomorphology. The ability to quantify the influence of vegetation on channel and floodplain hydraulics, in particular hydraulic roughness, is critical for flood control concerns; however, diversity of vegetation type and behavior makes this parameter very difficult to quantify repeatedly and accurately. A fundamental concept of hydraulic theory in the context of river engineering is the influence of boundary conditions on flow through natural environments. This technical note presents a tool for estimating hydraulic roughness from boundary conditions in rivers. Hydraulic roughness, or resistance, is herein defined as the primary factor influencing retarding or resisting forces exerted by channel boundaries on stream flow. Calculation of hydraulic resistance is not a trivial matter due to the multitude of factors influencing roughness (e.g., bed material, bed forms, crosssectional and planform variability, vegetation, etc.). This document will present a theory of hydraulic resistance estimation, a synthesis of many resistance estimation techniques into a spreadsheet model, and an application of said model to Ham Branch, a tributary of the Trinity River. The diversity of methods applied in the model will allow users to isolate effects of diverse contributions to roughness (e.g., grain v. vegetative) and create a weight-of-evidence for an estimation of hydraulic roughness.