Calibrating a Sediment Transport Model through a Gravel-Sand Transition: Avoiding Equifinality Errors in HEC- RAS Models of the Puyallup and White Rivers

The Puyallup and White Rivers in western Washington are high gradient, high supply systems with downstream water surface elevation controls. Both rivers are actively aggrading and include gradual cobble-gravel and abrupt gravel-sand transitions. Aggradation increases flood risk along both rivers. Therefore any future flood risk projections or examination of flood mitigation alternatives requires morphological analysis. The US Army Corps of Engineers (USACE) developed a Hydrologic Engineering Center, River Analysis System (HEC-RAS) sediment models of the White and Puyallup rivers to assess non-stationary benefits of flood risk management alternatives in these morphologically dynamic systems. However, it was not sufficient to calibrate these models to standard volume change metrics. These models demonstrate sediment transport's propensity for equifinality errors and the importance of multiple calibration metrics when evaluating sediment transport calibrations. This paper will overview the development of the Puyallup and White River sediment models, describe the calibration parameter evaluation and selection process, and demonstrate how multiple calibration evaluation metrics (e. g. volume change and bed gradation) avoided potential equifinality errors (i.e. errors emerging from non-unique solutions in multiple parameter models).