Riprap design for flood control channels and diversion structures relies on established design equations to develop riprap size, thickness, and gradations. These design equations use modeled flow and depth to estimate the minimum riprap rock size and layer thickness, with recommended adjustments based on placement methods. Many of these design equations, however, fall short in predicting riprap stability under highly turbulent flow conditions, such as those encountered downstream of a gated diversion structure, partially due to the limited ability of numerical models to resolve time-dependent turbulent fluctuations at the flow-riprap interface. Physical hydraulic models are a complimentary design tool used to validate riprap sizing as predicted from numerical models, as properly scaled physical hydraulic models can resolve flow turbulence and flow-riprap dynamics. Physical hydraulic models scaled to simulate flow dynamics for large scale hydraulic structures require substantial cost and space to develop, and numerical models are still generally used to develop well defined test conditions. Despite their inter-dependence, there are currently no established or defined guidelines for developing a test plan or evaluation criteria using physical and numerical model applications for riprap design. This talk will outline the test plan, evaluation criteria, and numerical and physical model approaches used to establish riprap sizing for the Mid-Breton Sediment Diversion (MBrSD) project. The project is a proposed diversion structure in Louisiana intended to divert up to 50,000 cfs of sediment-laden water from the Mississippi River into Breton Sound to re-establish hydrologic flows, carry land-building sediments, nourish marshes, and sustain land. Initial riprap sizing was developed using design equations and hydraulic outputs from a computational fluid dynamics (CFD) model of the diversion structure. The riprap layout was evaluated under a range of flow conditions and gate operations using a scaled physical hydraulic model, and riprap stability was evaluated using measured flow data, photographs, and laser bed scans. The design team identified several limitations in established riprap design approaches and riprap stability evaluation criteria. Through an iterative process and establishment of new evaluation criteria, a stable riprap layout was identified.
