Gravel transport estimation and flow simulation over low-water stream crossings

Low-Water Stream Crossings (LWSCs) with or without culverts can be desirable alternatives to costly bridge construction on very low-volume roads and trails. LWSCs are overtopped by high flows or by debris- or ice-laden flows. LWSCs are commonly used in the hill country of central Texas and are currently facing problems, such as sediment transport and deposition and structure maintenance because of their low grade and frequent overtopping at high stream flows. This study, as one of the integrated components of the Texas Department of Transportation Research Project 0--4695 - "Guidance for Design in Areas of Extreme Bed-Load Mobility," focuses on the numerical estimation on bed-loaded transport and flow simulations in gravel-bedded river - Johnson Fork River, Texas. In this dissertation, I developed an Excel VBA model, "SurGTAM," to estimate the gravel transport rate based on Gary Parker's surface-based transport equation (1990a). SurGTAM is an integrated model allowing researchers to compute bed-load transport rate in gravel-bedded natural channels and compute channel flow parameters under known channel discharge and geometry. SurGTAM is a research-based model with an easy and friendly user interface with an open source and is available for further developing. The model has been validated and successfully applied to Johnson Fork River, Texas for estimation of gravel transport rate at different locations with different grain size distributions. The results include gravel transport rate of different grain size groups and sensitivity analysis of gravel transport rate on gravel specific gravity. It was found that natural channel has higher capacity to transport gravel from upstream towards the low-water stream crossing but the road surface has lower capacity to transport or carry gravel to further downstream, and this capacity difference will result deposition of gravels in the upstream size of the crossing. The second part of this study is on three-dimensional flow simulations. The study of flow simulations provides the flow characteristics under different flow boundary conditions when the flow passes various hydraulic structures and can be applied to the study of sediment transportation by revealing the flow field, like velocity and shear stress distributions. In this study, FLOW-3D is utilized to model unsteady free-surface flows for several case studies in civil engineering hydraulics. FLOW-3D, developed by Flow Science, Inc., is a three-dimensional computational fluid dynamics software that can be utilized as a "Virtual Lab" to study complex flow phenomena. In the flow simulation study, I simulated and analyzed the flow characteristics in open channels under different geometry (with weirs or tailwater effects) and flow conditions (subcritical and supercritical flows) and flow over low-water stream crossing with culverts. The studies focus on the effects of different boundary conditions on flow simulations. The analysis results show that FLOW-3D provides accurate solution for complex open channel flows. The numerical simulations for Johnson Fork River provide support in determining sediment transportation in natural rivers. The simulation results of Guzman and Paks crossing, include complex flow characteristics, such as velocity distributions, show the influence of LWSC on flow velocity distributions before, on, and after the LWSC. The results can be used to estimate the gravel transport rates in the future.