KRISTIN E. KASPER

PLAN B REPORT TITLE: ACCURACY OF HEC-RAS TO CALCULATE FLOW DEPTHS AND TOTAL ENERGY LOSS WITH AND WITHOUT BENDWAY WEIRS IN A MEANDER BEND

ABSTRACT: River systems are interconnected waterways that often change to reach a state of dynamic equilibrium. Dynamic equilibrium is a fragile balance between flow conditions, sediment transport, and environmental habitat in a river system. To study river systems in detail, complex hydraulic models have been developed. Hydraulic models calculate flow depths and energy loss through a river system and are defined as 1-, 2-, or 3-dimensional models. Differences between each model type depend on assumptions used to build the model. A 1-dimensional model assumes the primary component of a 3-dimensional velocity profile is along the x-coordinate axis. Therefore the velocity components along the y- and z-coordinate axis are assumed insignificant. Positive velocity components along the y- and z-coordinate axis refer to lateral flow to the right bank looking downstream and the vertical direction while positive velocity component along the x-coordinate axis refers to the flow direction. In 2- and 3-dimensional hydraulic models, the 1-dimensional assumption for the y- and z-coordinate axis does not hold true.

In 1-dimensional analysis, HEC-RAS is a common hydraulic model to study flow depths and total energy loss along a study reach of a river system. HEC-RAS is a 1-dimensional model that performs calculations for steady or unsteady flow in gradually-varied or rapidly-varied flow analysis. Even though HEC-RAS is a 1-dimensional hydraulic model, it is commonly used to model flow patterns where the velocity along the y- or z-coordinate axis are significant. For instance, HEC-RAS is used to study meander bends. Meander bends are undulating segments in a river system where the dominant direction of velocity is not necessarily along the x-coordinate axis. An added level of complexity develops when bank stabilization features such as bendway weirs are added to a HEC-RAS model. Bendway weirs are bank stabilization features built of local rock material. Bendway weirs are constructed along the outer bank of a meander bend in order to reduce bank erosion by directing high velocities along the outer bank to the center of the channel. While protecting the stream bank, bendway weirs support viable aquatic habitats, and riparian vegetation along a meander bend.

Since HEC-RAS is often used to model 3-dimensional velocity profiles with and without bendway weirs, research needs to be completed to determine the accuracy of HEC-RAS. Included in this study was an analysis to determine the accuracy of HEC-RAS to model flow depths and total energy loss along a meander bend with or without bendway weirs and a methodology to best estimate total energy loss given HEC-RAS output.