Yafei Jia

Nonuniform sediment transport in alluvial rivers

A correction factor has been developed in this paper to account for the hiding and exposure mechanism of nonuniform sediment transport. This factor is assumed to be a function of the hidden and exposed probabilities, which are stochastically related to the size and gradation of bed materials. Based on this concept, the formulas to calculate the critical shear stress of incipient motion and the fractional bed-load and suspended load transport rates of nonuniform sediment have been established. These formulas have been tested against a wide range of laboratory and field data and compared with several other existing empirical methods. The predictions by these newly proposed formulas are very good.

The applications of the enhanced CCHE2D model to study the alluvial channel migration processes

This paper is to report a newly developed numerical-empirical model, the Enhanced CCHE2D (EnCCHE2D), and its application to simulating the alluvial channel migration phenomena. EnCCHE2D model is capable of predicting quasi-three-dimensional (3D) flow field and shear stress distribution on the bed, because a set of empirical functions of 3D flow characteristics formulated by results of a 3D model, CCHE3D, was integrated with CCHE2D, a depth-averaged hydrodynamic model, the predecessor of EnCCHE2D. The processes of sediment transport and meander migration were predicted based on these quasi-3D flow solutions. The advance or retreat of bank is calculated by considering not only the hydraulic erosion of bank surface and toe, but also the mass balance of sediment flux in the near-bank zone. As a result, the simulation of bank erosion, bar/pool formation and shifting, bank advance and retreat, channel widening and migration and meander evolution phenomena agree well with the available measurements of physical experiments.

VALIDATION AND APPLICATION OF A 2D MODEL TO CHANNELS WITH COMPLEX GEOMETRY

A depth-integrated two-dimensional hydrodynamic and sediment transport model, CCHE2D, has been developed for analyzing free surface flows for general applications to hydraulic engineering problems. Mixed schemes of finite element method (Efficient Element Method) and finite volume method are adopted for solving momentum and continuity equations. A unique velocity correction method was developed to couple the governing equations. Pressure (free surface) field is discretized on a staggered grid. Two zero equation turbulence closure schemes: a depth averaged parabolic, a depth averaged mixing length eddy viscosity model, and a two-equation k-e model for depth-averaged flows are included for different application needs. Algebraic systems of equations are solved implicitly by using SIP method. This model can be used for simulating steady and unsteady near field and natural river flows with relatively shallow water depth. Validation tests for this model show good agreements between the model prediction and data of physical models and field measurements.

NUMERICAL MODELING OF SUSPENDED SEDIMENT TRANSPORT IN CHANNEL BENDS

An algorithm to compute three-dimensional sediment transport effect was proposed in this paper to enhance the capability of depth-averaged numerical models. This algorithm took into account of non-uniform distributions of flow velocities and suspended sediment concentrations along water depth, it significantly enhanced the applicability of 2D models in simulating open channel flows, especially in channel bends. Preliminary numerical experiments in a U-shaped and a sine-generated experimental channel indicate that the proposed method performs quite well in predicting the change of bed-deformation in channel bends due to suspended sediment transport. This method provides an effective alternative for the simulations of channel morphodynamic changes.

Three-dimensional numerical simulation of water quality and sediment-associated processes with application to a Mississippi Delta lake

A three-dimensional water quality model was developed for simulating temporal and spatial variations of phytoplankton, nutrients, and dissolved oxygen in freshwater bodies. Effects of suspended and bed sediment on the water quality processes were simulated. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of suspended sediment and chlorophyll. The processes of adsorption-desorption of nutrients by sediment were described using the Langmuir Equation. The release rates of nutrients from the bed were calculated based on the concentration gradient across the water-sediment interface and other variables including pH, temperature and dissolved oxygen concentration. The model was calibrated and validated by applying it to simulate the concentrations of chlorophyll and nutrients in a natural oxbow lake in Mississippi Delta. The simulated time series of phytoplankton (as chlorophyll) and nutrient concentrations were generally in agreement with field observations. Sensitivity analyses were conducted to demonstrate the impacts of varying suspended sediment concentration on lake chlorophyll levels.

Verification and Validation of 3D Free-Surface Flow Models

In the past several years, computational models for free surface flow simulations have been increasingly in demand for engineering, construction and design, legislation, land planning, and management decisions. Many computational models have been hastily developed and delivered to clients without the proper scientific confirmation and certification. In order to correct this, the ASCE/EWRI Task Committee developed a new rigorous and systematic verification and validation process, Verification and Validation of 3D Free-Surface Flow Models, which discusses this procedure in detail. The topics include terminology and basic methodology; analytical solutions for mathematical verification; mathematical verification using prescribed or manufactured solutions; physical process validation; application site validation; the systematic model verification and validation procedure; systems analysis considerations, findings, and conclusions; and recommendations. The appendixes contain input data for test cases and formulations and codes for manufactured solutions. This publication will be indispensable for students and professionals working on computational models and environmental engineering.

2D Nearly orthogonal mesh generation with controls on distortion function

A method to control the distortion function of the Ryskin and Leal (RL) orthogonal mesh generation system is presented. The proposed method considers the effects from not only the local orthogonal condition but also the local smoothness condition (the geometry and the mesh size) on the distortion function. The distortion function is determined by both the scale factors and the averaged scale factors of the constant mesh lines. Two adjustable parameters are used to control the local balance of the orthogonality and the smoothness. The proposed method is successfully applied to several benchmark examples and the natural river channels with complex geometries.

Three-Dimensional Numerical Modeling of Secondary Flows in a Wide Curved Channel

Most natural rivers are curved channels, where the turbulent flows have a complex helical pattern, as has been extensively studied both numerically and experimentally. The helical flow structure in curved channels has an important bearing on sediment transport, riverbed evolution, and pollutant transport study. In this article, different turbulence closure schemes i.e., the mixing-length model and the k - ε model with different pressure solution techniques i. e., hydrostatic assumptions and dynamic pressure treatments are applied to study the helical secondary flows in an experiment curved channel. The agreements of vertically-averaged velocities between the simulated results obtained by using different turbulence models with different pressure solution techniques and the measured data are satisfactory. Their discrepancies with respect to surface elevations, superelevations and secondary flow patterns are discussed.

Parallelizing Alternating Direction Implicit Solver on GPUs

We present a parallel Alternating Direction Implicit (ADI) solver on GPUs. Our implementation significantly improves existing implementations in two aspects. First, we address the scalability issue of existing Parallel Cyclic Reduction (PCR) implementations by eliminating their hardware resource constraints. As a result, our parallel ADI, which is based on PCR, no longer has the maximum domain size limitation. Second, we optimize inefficient data accesses of parallel ADI solver by leveraging hardware texture memory and matrix transpose techniques. These memory optimizations further make already parallelized ADI solver twice faster, achieving overall more than 100 times speedup over a highly optimized CPU version. We also present the analysis of numerical accuracy of the proposed parallel ADI solver.

Numerical modeling of surface flow and transport phenomena with applications to Lake Pontchartrain

Abstract This article presents the capabilities of a numerical model, CCHE2D, by using its application to study the response of a highly complex water system, Lake Pontchartrain, Louisiana, under extreme conditions of Hurricane Katrina in 2005 and the flood water release from Bonnet Carré Spillway in 1997. The numerical simulations were validated using the field data collected by the US Geological Survey and US Army Corps of Engineers, as well as satellite imagery obtained from National Oceanic and Atmospheric Administration. The close agreements obtained with technically acceptable accuracy in both field properties and trends of their spatial and temporal variations fully demonstrated this models usefulness in predicting the hydrodynamics, sediment transport, and salinity distribution of lakes under extreme flood forcing. This model provides a useful tool for lake water quality management.