2D Model Parameterization of Porcupine River Training Structures

Abstract

Flexible river training structures such as tetrahedron frames (`porcupines') can be attractive for control of braided river channel networks in regions where permanent control structures (e.g. groynes or dams) are too expensive or potentially inefficient, such as systems with highly dynamic flow regimes and morphology. Porcupines provide hydraulic resistance, generating energy loss and reducing velocities that may further encourage sediment deposition. Porcupine systems have seen increasing implementation, especially for bed or bank protection, but were also recently implemented in a channel-control pilot project on the Ayeyarwady River in Myanmar. Many porcupine systems to date are designed by trial and error due to lack of quantitative design criteria. The 2019 pilot project used large-scale numerical modelling of resistance areas to evaluate the potential impacts of porcupine structures; however, how to best incorporate the impacts of porcupine structures on flow and sediment transport in numerical models has not been systematically evaluated. Improved models or better estimates of uncertainties could facilitate future porcupine system design. Therefore, this study examines methods for incorporating porcupine fields into numerical models and interpreting those models to make informed design choices. Data and observations from the 2019 pilot project implementation, a 2018 porcupine flume experiment, and extensive literature review have been used to examine the expected hydrodynamic and morphodynamic impacts of porcupine systems that need to be accurately captured in large-scale numerical models. A 2DV model using two resistance formulations representing porcupines was elaborated and tested against experimental porcupine performance. The Uittenbogaard (2003) rigid cylinder model was found to replicate well porcupine behavior for less dense systems under limited flow conditions; however further verification is needed for more dense systems, variable flow regimes, wider boundary conditions and mobile beds. The 2DV model was collapsed to 1DH, using two different resistance formulations, to examine the strengths and weaknesses of large-scale models in predicting porcupine performance. The Baptist (2005) resistance model was found to best represent porcupines; however, parameterization was not straightforward from porcupine geometry, indicating that further studies will be needed to confidently translate porcupines into `rigid cylinders' for input into the model. In addition, neither the 2DV nor 1DH models were able to capture the hydrodynamics at the leading and trailing edges of porcupine fields, which were found to be critical for design considerations from the pilot study data. Therefore, these important limitations need to be taken into account when interpreting model results and examining how to improve them in the future.