Blair P. Greimann

Two-phase formulation of suspended sediment transport

Using a two-phase formulation, the vertical and horizontal momentum equations for sediment are used to obtain the concentration and velocity profiles of a dilute suspension of particles in a 2D uniform flow. Assuming the form of the vertical turbulent intensities and dilute concentrations of sediment, one can solve the equations analytically and compare them with experimental data. No empirical coefficients in the model are tuned to match individual experiments, for which the experimental data cover a large range of particle sizes and densities. The models are shown to accurately predict two experimentally observed but theoretically unexplained phenomena: the increased diffusive flux of large particles, and the measurable velocity lag of particles. The increased diffusion of large particles is shown to originate from the added diffusive nature of the sediments Reynolds stresses. The horizontal velocity lag of particles is due to an induced velocity, termed the drift velocity, resulting from the correlati...

Soft Bedrock Erosion Modeling with a Two-Dimensional Depth-Averaged Model

Many rivers in Taiwan have steep slopes, are subject to typhoon-induced flood flows, and contain soft bedrock that is exposed at many locations and easily erodible. The occurrence of extensive bedrock erosion has been a major threat to river infrastructure at many locations. Soft bedrock erosion, therefore, is an important process to consider for river projects in Taiwan. In this study, bedrock erosion models are reviewed. A specific model is proposed by combining two existing models incorporating both the hydraulic and abrasive scour mechanisms. The proposed bedrock erosion model is incorporated into a two-dimensional mobile-bed model, and the integrated model is tested by simulating bedrock erosion downstream of the Chi-Chi weir on the Choshui River in Taiwan. A calibration study is performed to determine appropriate values of the model parameters based on two and a half years of measured data. The model is then assessed based on a verification study that compares model predictions of bedrock erosion of...

Method for Assessing Impacts of Parameter Uncertainty in Sediment Transport Modeling Applications

The predictions from a numerical sediment transport model inevitably include uncertainty because of assumptions in the model’s mathematical structure, the values of parameters, and various other sources. In this paper, the writers aim to develop a method that quantifies the degree to which parameter values are constrained by calibration data and the impacts of the remaining parameter uncertainty on model forecasts. The method uses a new multiobjective version of generalized likelihood uncertainty estimation. The likelihoods of parameter values are assessed using a function that weights different output variables on the basis of their first-order global sensitivities, which are obtained from the Fourier amplitude sensitivity test. The method is applied to Sedimentation and River Hydraulics—One Dimension (SRH-1D) models of two flume experiments: an erosional case and a depositional case. Overall, the results suggest that the sensitivities of the model outputs to the parameters can be rather different for er...

Predicting contraction scour with a two-dimensional depth-averaged model

Contraction scour is often encountered in natural rivers due to natural geological controls, bridges, or river restoration structures. Such scour may be better predicted with multi-dimensional than one-dimensional models. The aim of this study is to investigate whether a two-dimensional depth-averaged model is adequate for modelling contraction scour. This study shows that improved predictions are obtained with the present model relative to previous model investigations. The study also shows that the current model is adequate for predicting contraction scour and model results are comparable with those of three-dimensional modelling except for the prediction of aggradation downstream of the contracted channel section.

Coupling a Two-Dimensional Model with a Deterministic Bank Stability Model

Stream bank erosion can be an important form of channel adjustment in unstable alluvial environments and hence should be accounted for in geomorphic studies, river restoration, dam removal, and channel maintenance projects. Recently, one-dimensional and two-dimensional simulation models have become useful tools for predicting channel responses; but most either ignore bank failure mechanisms or implement only simple ad hoc methods. In this study, a twodimensional model (SRH-2D) is coupled with a deterministic bank stability and toe erosion model (BSTEM) to predict channel adjustment and planform development. Herein, the proposed coupling approach is described, along with numerical aspects of the procedures. For test and verification purposes, the coupled model is used to predict bank retreat of Goodwin Creek in Mississippi. A comparison of the model results with the measured data is presented and discussed.

Analyses of the erosion of fine sediment deposit for a large dam-removal project: an empirical approach

ABSTRACT Large quantities of fine sediment can be accumulated in reservoirs, and the potential impact of their downstream release is often a great concern if the dams are to be removed. Currently, there are no reliable numerical models to simulate the dynamics of the release of these fine sediments, mostly because their release following dam removal is often driven by a rapid erosional process not addressed by traditional sediment transport theory. However, precise quantification of fine sediment transport is rarely necessary to evaluate potential environmental impacts of alternative scenarios. Using the removal of Matilija Dam in southern California, USA, as an example, we quantify the likely magnitude of suspended sediment concentration and the duration of associated downstream impacts, two necessary (and most likely adequate) parameters for assessing alternatives. The analyses first estimate the general magnitude of suspended sediment concentration and duration of impacts based on field and experimental data; they then quantify the duration of impacts under both worst-case and reasonable assumptions according to the underlying physics and common sense. For rapid sediment release with fine-grained impoundment deposits, initial suspended sediment concentrations are likely to approach 106 mg/L, persisting for a few hours to no more than a couple of days. Suspended sediment concentrations are expected to decline approximately exponentially after the initial peak, reaching background levels within a few hours to a few days, provided that sufficient flow is available. The general method presented in the paper should be useful for stakeholders choosing amongst dam-removal alternatives for implementation under similar conditions.

Predicting Contraction Scour with a 2D Model

Contraction scour is often encountered in natural rivers due to channel contraction or river restoration structures. Such scour may be predicted better with multi-dimensional models than 1D models. It has been reported that the 2D depth-averaged model is inadequate for modeling the contraction scour. This study aims to investigate whether other factors may play important roles in the poor performance of 2D models. In the process, an improved prediction using the 2D model is obtained and factors influencing the model prediction are identified. The motivation of the study stems from the fact that a 3D model is too complex and many engineering applications have to rely on 2D models. This study shows that the 2D model is still adequate for predicting the contraction scour.

Two-Phase Flow Analysis of Sediment Velocity

Computing sediment velocity is critical to predicting sediment transport rates and the effect of sediment on the environment. There has been a large amount of research looking at the velocity of bed-load particles. The research has suggested that the shear velocity governs the bed-load sediment velocity, whereas the flow velocity governs the suspended sediment velocity. However, a single method to compute the velocity of bed-load and suspended load has remained elusive. In this paper, two-phase flow analysis is used to compute the velocity of bed-load and suspended load. The two-phase approach has an advantage in that it can account for the inertia of particles and the effect of turbulence on their transport.

Modelling of meander migration in an incised channel

Abstract An updated linear computer model for meandering rivers with incision has been developed. The model simulates the bed topography, flow field, and bank erosion rate in an incised meandering channel. In a scenario where the upstream sediment load decreases (e.g., after dam closure or soil conservation), alluvial river experiences cross section deepening and slope flattening. The channel migration rate might be affected in two ways: decreased channel slope and steeped bank height. The proposed numerical model combines the traditional one-dimensional (1D) sediment transport model in simulating the channel erosion and the linear model for channel meandering. A non-equilibrium sediment transport model is used to update the channel bed elevation and gradations. A linear meandering model was used to calculate the channel alignment and bank erosion/accretion, which in turn was used by the 1D sediment transport model. In the 1D sediment transport model, the channel bed elevation and gradations are represented in each channel cross section. In the meandering model, the bed elevation and gradations are stored in two dimensional (2D) cells to represent the channel and terrain properties (elevation and gradation). A new method is proposed to exchange information regarding bed elevations and bed material fractions between 1D river geometry and 2D channel and terrain. The ability of the model is demonstrated using the simulation of the laboratory channel migration of Friedkin in which channel incision occurs at the upstream end.

Calibrating a Riparian Vegetation Model for Sacramento River Studies

A one-dimensional flow and sediment transport model (Sedimentation and River Hydraulics- one dimensional, SRH-1D), was expanded to incorporate and assess riparian vegetation in the flood plain. The combined flow, sediment transport, and vegetation model SRH-1DV includes a ground water and vegetation module linking ground water fluctuations (in response to river flow) and the growth and removal of vegetation to geomorphic processes of river hydraulics and sediment transport. The various components of SRH-1DV were calibrated to measured data on the Sacramento River. Modeled flow was calibrated to the water surface elevation between gaging stations, and ground water was calibrated to well data. Sediment transport was calibrated to gravel bedload measurements at the Hamilton Bridge gaging station on the Sacramento River, but ultimately sediment transport computations were not used in the simulations. Cottonwood germination, growth, and removal were simulated for the Sacramento River from RM 300 to RM 150, including Red Bluff to Colusa. Data from two field studies (2005 and 2006) of cottonwood growth on sand bars at three sites were used to calibrate the cottonwood model and improve cottonwood desiccation methods. In a second round of simulations, the Sacramento SRH-1DV model was expanded to multiple vegetation types. Cottonwood vegetation was validated using repeat vegetation mapping from 1999 and 2007, and four additional vegetation types: mixed forest, Gooding’s black willow, narrow leaf willow, and the invasive plant, arundo, were calibrated with the 1999 and 2007 vegetation mapping of the Red Bluff to Colusa Reach (RM 250 to RM 143). Based on results, the arundo vegetation type was eliminated. Narrow leaf willow and Gooding’s black willow proved to be successful indicators of riparian lands in model simulations. Model simulations of cottonwood forest and mixed forests also compared well with mapped results when combined.