COMPARISON OF UNSTEADY AND QUASI-UNSTEADY FLOW MODELS IN SIMULATING SEDIMENT TRANSPORT IN AN EPHEMERAL ARIZONA STREAM

Hydrodynamic and sediment transport models are useful engineering tools for predicting unsteady flood flow and sediment transport. Many models such as HEC-RAS, HEC-6, and IALLUVIAL apply quasi-unsteady flow model, whereas others apply the unsteady flow model. It remains unknown if a quasi-unsteady flow model is sufficiently accurate for simulating sediment transport in rapidly varied unsteady flood events, especially in ephemeral rivers in arid and semiarid regions. This study compared the quasi-unsteady HEC-RAS 4.1 model with one-dimensional (1D) Finite Volume Method (FVM) based model in simulating flood flow and sediment transport in the Pantano Wash, a dryland river in the state of Arizona. The objective is to determine which sediment transport method is appropriate in predicting bed elevation changes in an ephemeral stream, Pantano Wash, and if an unsteady model is more accurate than a quasi-unsteady flow model in predicting sediment transport. Results showed that the quasi-unsteady HEC-RAS model and the 1D FVM yielded similar results of bed degradation and aggradation for this dryland stream, although the FVM model predicted better flood hydrographs. Among the seven sediment transport formulas embedded in HEC-RAS, Yang's and Engelund-Hansen's equations gave the best matches with the field measurements for this particular case study.