Numerical Investigation of Hyporheic Flows Induced by Gravel Bars and Sand Dunes
Hyporheic Flows , Gravel Bars , Sand Dunes , Empirical equations , Nonlinear Multiregression Analysis
Existence of gravel bars and sand dunes in river streams results in exchange of flow across the interface of stream flow and porous medium known as hyporheic flow. This type of flow conveys solutes and nutrients to the riprapian organisms living in the hyporheic zone, and in case of river contamination they can be the main mechanism of contaminating the habitat of aquatic organisms. Therefore, it is critical to understand the mechanics of hyporheic flows induced by bed forms, and develop methods and tools for estimating their characteristics. This thesis is intended as a contribution to these ends. In this study, the main effort has been made to develop a 3-D fully coupled numerical model capable of simulating the hyporheic flows both in turbulent and laminar flow regimes, while treating the interface between streamflow and porous medium as a permeable wall. The work involves an extensive series of numerical simulations conducted for different ranges of bed form geometry, permeability of porous materials and depth of alluvium, which are the key parameters controlling the characteristics of hyporheic flows. Predictive equations are developed for estimating the discharge, average velocity and average residence time in the hyporheic zones induced by a single gravel bar, a sequence of bars, and a sequence of dunes. It is found that by applying a combination of Wall and Cyclic boundary conditions on the interface between streamflow and porous medium, the effects of the permeable wall on both surface and hyporheic flow fields is simulated very well. Comparison between the simulated data and their predicted counterparts by the empirical equations for the hyporheic flows induced by a single gravel bar revealed that in more than 80% of the cases, the error is within 10% error range. Due to the more complex nature of the hyporheic flows induced by a sequence of gravel bars and sand dunes the accuracy of the developed predictive equations is less than the ones developed for hyporheic flows induced by a single gravel bar. However, the equations are still very good in estimating the characteristics of hyporheic flows.