Flood Plain Filling by a Monoclinal Flood Wave

Theoretical equations are derived to estimate the flood wave celerity and the volume of lateral flow leaving the main channel and entering the flood plain for a two-dimensional monoclinal wave. The equations are based on conservation of water mass assuming an idealized compound channel. The parameters that affect the lateral flow volume are the ratio of flood plain depth to the depth in the main channel at the peak discharge, the ratio of the flood plain width to the main channel width, and the ratio of the flood plain roughness to the main channel roughness. The percentage of flood plain volume filled up by lateral flow from the main channel increases as the width ratio decreases, the roughness ratio increases, or as the depth ratio decreases.     

Functional Relationships of Resistance in Wide Flood Plains with Rigid Unsubmerged Vegetation

The effect of rigid, unsubmerged vegetation on flow resistance in wide flood plains was examined in this study. A series of experiments was run using rigid rods inserted into the flow field of a wide shallow flume. Several configurations of rod diameters with various lateral and longitudinal spacing were tested over a range of discharge values. Head losses across the rods were measured and a resistance-to-momentum-absorbing area concept was employed to relate density parameters to the resistance coefficient. The effects of flow depth, velocity, rod diameter (d), lateral spacing (r), and longitudinal spacing (l) were estimated. The results demonstrated that flow resistance is greatly impacted by both depth and velocity but that the effects are opposite in sign. The friction factor increased in only a slightly nonlinear fashion with depth, but it decreased in a highly nonlinear manner with increasing velocity, and the velocity effects were very sensitive to vegetation density parameters. Of the density parameters tested, the effects of diameter and lateral spacing were far more significant than are those of longitudinal spacing. The results also demonstrated that the effects of all parameters (density and depth) could be effectively combined into one relative density ratio (total vegetation stomatal area/bed cross sectional area) that was linearly related to the Darcy f and the Manning n.     

Momentum Exchange in Straight Uniform Compound Channel Flow

Transverse exchange of momentum between the channel and the floodplain in straight uniform compound channel flow is considered in this paper. This process results in the so-called “kinematic effect,” a lowering of the total discharge capacity of a compound channel compared to the case where the channel and the floodplain are considered separately. The mechanisms responsible for the momentum exchange are considered. The transverse shear stress in the mixing region is modeled using a newly developed effective eddy viscosity concept, that contains: (1) the effects of horizontal coherent structures moving on an uneven bottom, taking compression and stretching of the vortices into account and (2) the effects of the three-dimensional bottom turbulence. The model gives a good prediction of the transverse profiles of the streamwise velocity and the transverse shear stress of the flood channel facility experiments. Characteristic features of the lateral profile of the eddy viscosity are also well predicted qualitatively, but in a quantitative sense there is room for improvement. Secondary circulations are shown to be of minor importance in straight uniform compound channel flows.     

Effect of Riparian Vegetation on Flow Resistance and Flood Potential

An existing hydraulic model is modified to predict stage-discharge curves for channels with nonuniform cross sections, sand and gravel-bed materials, and flexible or nonflexible riparian vegetation. The model is based on a version of the flow momentum and continuity equations that account for lateral shear. The model accounts for the effects of vegetation using empirically calibrated flow resistance equations that incorporate measurable physical properties of vegetation. Separate flow resistance equations are used for flexible and nonflexible vegetation types. Simulated stage-discharge curves are compared with data obtained from three natural river channels. Discrepancies between simulated and observed data range between 2 and 45%, but most (～70%) discrepancies were <15%. Sensitivity tests are performed to determine the effects of different types of riparian vegetation on friction factor and flood elevation. Surfaces covered by nonflexible vegetation are rougher than those covered with flexible riparian vegetation. Based on simulations at the three study sites, operational maintenance regimes are proposed that minimize flood risk, while maximizing the environmental benefits of a well-developed riparian vegetation cover.     

Fluid-Particle Interactions and Resuspension in Simple Shear Flow

The lattice Boltzmann method (LBM) is used to simulate the particle resuspension process in a two-dimensional simple shear flow. At first, the lift force on a single disk-shaped particle attached at the channel wall is computed. It is found that when the particle is allowed to freely move in the viscous fluid, the resulting lift force is smaller than that when the particle is constrained to be stationary. The bulk properties of fluids with particles suspended under various concentrations are numerically calculated. The results agree reasonably well with analytical results by Batchelor when the volume fraction is lower than 50%. The resuspension process of a group of particles (up to 500) is simulated at different particle-to-fluid density ratios. It is found that the height and shape of particle bed depend on the particle density ratio and flow conditions. Interactions of groups of particles as well as the final shape of the bedform of the particles were studied during this resuspension process. Finally, the pressure distribution and flow above the bedform of particles was examined. The results obtained agree well with those observed in naturally occurring bedforms of sediments.     

Flow Patterns in Compound Channels with Vegetated Floodplains

Understanding the hydraulics of flow in a compound channel with vegetated floodplains is very important for determining the stage-discharge curve and for supporting the management of fluvial processes. In this paper, the flow patterns over different types of vegetation, such as tree, shrub, and grass, are described, based on an experimental study. For vegetation on the floodplain, the authors choose plastic grass, duck feathers, and plastic straws as model grass, shrubs, and trees, respectively. A 3D acoustic Doppler velocimeter was used to measure the local flow velocities for different types of vegetation on the floodplain, and the total discharge and flume slope were measured independently. In the cases of nonvegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an S-shaped profile, exhibiting three zones. For all cases, the fluctuating velocity followed a normal distribution. The influence of different types of vegetation on the distributions of the secondary currents, turbulence intensities, and Reynolds shear stresses were also analyzed.     

Flow Resistance and Momentum Flux in Compound Open Channels

New formulations are presented for flow resistance and momentum flux in compound open channels. As implemented in the St. Venant equations, these formulations facilitate a physically enhanced approach to evaluating conveyance, roughness, stage-discharge relationship, and unsteady flood routing in compound open channels. An analysis using steady flow data from the well-controlled experiments at the large-scale Flood Channel Facility, HR Wallingford, demonstrates the ability of the present approach to properly resolve the discontinuity of overall roughness across the main-channel bankfull level. Also, the proposed formulations are shown to be conducive to obviating the long-standing computational difficulty in unsteady flood routing due to small flow depths over flat and wide floodplains. The present work should find general applications in one-dimensional computation of river flows.     

Treatment of Natural Geometry in Finite Volume River Flow Computations

A method is proposed for the treatment of irregular bathymetry in one-dimensional finite volume computations of open-channel flow. The strategy adopted is based on a reformulation of the Saint-Venant equations. In contrast with the usual treatment of topography effects as source terms, the method accounts for slope and nonprismaticity by modifying the momentum flux. This makes it possible to precisely balance the hydrostatic pressure contributions associated with variations in valley geometry. The characteristic method is applied to the revised equations, yielding topographic corrections to the numerical fluxes of an upwind scheme. Further adaptations endow the scheme with an ability to capture transcritical sections and wetting fronts in channels of abrupt topography. To test the approach, the scheme is first applied to idealized benchmark problems. The method is then used to route a severe flood through a complex river system: the Tanshui in Northern Taiwan. Computational results compare favorably with gauge records. Discrepancies in water stage represent no more than a fraction of the magnitude of typical bathymetry variations.     

Vertical Mixing in the Fully Developed Turbulent Layer of Sediment-Laden Open-Channel Flow

Eulerian equations for the vertical flux and momentum of suspended particles in dilute sediment-laden open-channel flow in equilibrium have been derived using the two-fluid approach. Reynolds averaging has been applied in order to allow validation of individual terms with experimental data. Consideration of the various terms of the vertical momentum balance with experimental flume data indicates that the drag force has to be separated into a mean and a turbulent contribution with different timescales, respectively, the (gravitational) particle timescale and the integral turbulence timescale. The resulting formulation further provides a new theoretical closure for the turbulent Schmidt number.     

Channel Shape and Turbulence Issues in Flood Flow Hydraulics

This paper reports the results of some further reflections by the writer on the nature of the flow physics in flooded channels where the floodplain flow overtops that in the main channel. A lot of the early conclusions drawn on this issue come from results of the Flood Channel Facility (FCF) program carried in the United Kingdom in the 1990s, but also from numerical work such as that of the writer. This paper takes this work further and reports that the FCF geometry may have led to conclusions that are dependent on a channel layout that is not fully representative of nature. It indicates that the flow structure evolves as a function of the channel width-to-depth ratio, and requires different turbulence model approaches to be computed accurately as this happens. In particular the standard k–ε is shown to become less adequate. It is also shown that the bank slope is influential in determining the flow structure, and that the flatter the slope the more likely it is to present increasing difficulties for modeling.     

Unobstructed and Obstructed Turbulent Flow in Gravel Bed Rivers

An acoustic Doppler velocimeter was used to characterize turbulence in two gravel bed rivers. Data were collected in unobstructed flow and compared to recent investigations. Additional data collected in the wake of emergent boulders indicate that mean flow velocity, turbulent kinetic energy, gradients in the streamwise velocity, and Reynolds stress downstream from large rocks deviate from unobstructed flow results, but similar turbulence patterns are found behind each boulder. Results of this study are discussed with regard to natural channel design and fish habitat.     

Influence of Coherent Flow Structures on the Dynamics of Suspended Sediment Transport in Open-Channel Flow

The influence of suspended sediments on coherent flow structures has been studied by simultaneously measuring the longitudinal and vertical components of the instantaneous velocity vector and the instantaneous suspended particle concentration with an acoustic particle flux profiler. The measurements were carried out in clear water and in particle-laden open-channel flows. In both cases, they clearly show the predominance of ejection and sweep phases that are part of a burst cycle. The analysis further demonstrates the importance of the ejection and sweep phases in sediment resuspension and transport. Ejections pick up the sediment at the bed and carry it up through the water column close to the surface. It is shown that ejections and sweeps are in near equality in the near-bottom layer, whereas ejections clearly dominate in the remaining water column. The implications of these results for sediment transport dynamics are discussed.     

Effects of Added Vegetation on Sand Bar Stability and Stream Hydrodynamics

Vegetation was added to a fully developed sandy point bar in the meander of a constructed stream. Significant changes in the flow structure and bed topography were observed. As expected, the addition of vegetative resistance decreased the depth-averaged streamwise velocity over the bar and increased it in the open region. In addition, the secondary circulation increased in strength but became confined to the deepest section of the channel. Over the point bar, the secondary flow was entirely outward, i.e., toward the outer bank. The changes in flow led to changes in bar shape. Although the region of the bar closest to the inner bank accumulated sediment, erosion of the bar and the removal of plants by scouring were observed at the interface between the planted bar and the open channel.     

Case Study of the Use of Remotely Sensed Data for Modeling Flood Inundation on the River Severn, U.K.

A methodology for using remotely sensed data to both generate and evaluate a hydraulic model of floodplain inundation is presented for a rural case study in the United Kingdom: Upton-upon-Severn. Remotely sensed data have been processed and assembled to provide an excellent test data set for both model construction and validation. In order to assess the usefulness of the data and the issues encountered in its use, two models for floodplain inundation were constructed: one based on an industry standard one-dimensional approach and the other based on a simple two-dimensional approach. The results and their implications for the future use of remotely sensed data for predicting flood inundation are discussed. Key conclusions for the use of remotely sensed data are that care must be taken to integrate different data sources for both model construction and validation and that improvements in ground height data shift the focus in terms of model uncertainties to other sources such as boundary conditions. The differences between the two models are found to be of minor significance.     

Fluctuations of Suspended Sediment Concentration and Turbulent Sediment Fluxes in an Open-Channel Flow

A complex problem of turbulent-sediment interactions in an open-channel flow is approached experimentally, using specially designed field experiments in an irrigation canal. The experimental design included synchronous measurements of instantaneous three-dimensional (3D) velocities and suspended sediment concentration using acoustic Doppler velocimeters (ADV) and a water sampling system. Various statistical measures of sediment concentration fluctuations, turbulent sediment fluxes, and diffusion coefficients for fluid momentum and sediment are considered. Statistics, fractal behavior, and contributions of bursting events to vertical fluxes of fluid momentum and sediment are evaluated using quadrant analysis. It has been found that both turbulence and sediment events are organized in fractal clusters which introduce additional characteristic time and spatial scales into the problem and should be further explored. It is also shown that Barenblatt’s theory of sediment-laden flows appears to be a good approximation of experimental data.     

Case Study: Design of Flood Control Systems on the Vara River by Numerical and Physical Modeling

In the present paper, we investigate the effectiveness of a flood defense project based on storage reservoirs, presently under study for the Magra River and Vara River (Italy). We have focused the analysis on two detention reservoirs and studied their response to different hydrological scenarios mostly in terms of flood mitigation efficiency, leaving aside sediment transport issues. The analysis has been carried out with the aid of a physical model and one-dimensional numerical simulations. Experimental and numerical simulations have been performed spanning a wide range of hydrological conditions. Some of the results can be generalized for different applications where similar flood control systems are employed.     

Bridge Blockage and Overbank Flow Simulations Using HEC–RAS in the Keelung River during the 2001 Nari Typhoon

On September 16, 2001, Typhoon Nari resulted in severe flooding in the Keelung River basin. More than 1,000 shipping containers were swept by the rising water from the floodplain into the river, blocking 14 bridges. A severe overbank flow due to the blockage occurred at the Ba-Tu Railway Bridge. The overbank flow then passed through a railway tunnel and inundated Keelung City, resulting in significant damage. The objective of this study was to determine the influence of the bridge blockage and the Ba-Tu overbank flow on the water stages in the Keelung River during Typhoon Nari. The floating-pier-debris module and the lateral-weir module in the Hydrologic Engineering Center–River Analysis System (HEC-RAS) unsteady-flow routing model were applied to investigate water stage variation due to the bridge blockage and overbank flow. The numerical simulation results provided by this study served as an important reference for authorities who needed to clarify the responsibility of the containers’ owners for the loss of lives and property during this typhoon.     

Dynamic Wave Study of Flow in Tidal Channel System of San Juan River

In this work the complete equations of one-dimensional unsteady flow in open channels in integral form, and compatibility equations at the junctions of a channel network, are solved numerically. Analytical integration in space is used between each pair of consecutive irregular sections of a channel, and the nonprismatic term is expressed in terms of uncoupled functions of the geometry at the sections. The linearized system of equations for each time interval is solved by an elimination method based on a double-sweep algorithm. The model is applied to the estuary of the San Juan River in Venezuela, where oscillating currents by effect of semidiurnal tides take place and the amplitude of the wave at the mouth is amplified toward the inland direction. Alternating drying and filling is simulated by means of slight modifications in the bed geometry of upper river sections. Measured water elevation and flow rates available at two stations are used to calibrate the model, and a very accurate adjustment of the tidal levels observed in the river is obtained.     

Experiments on Selective Withdrawal of a Codirectional Two-Layer Flow through a Line Sink

For investigating selective withdrawal problems, laboratory experiments were conducted in a horizontal flume with a co-directional two-layer flow of different density into a line sink. Saline water was used as the fluid of the lower layer instead of sediment-laden turbid water for achieving a steady state. In this study, a trend curve was produced using the data of many runs which were taken under varying conditions from aspiration of both layers to only lower-layer aspiration. An adequate parameter for determining the critical condition was obtained from this trend curve. The critical condition is defined as the beginning or ending of aspiration for the upper layer. Suitable variables for the parameter are also discussed. A theoretical formula is suggested and verified, which is better than a traditional empirical formula for calculating the withdrawal concentration. Effects of slot elevation on the lower layer flow and the thickness of the mixing layer are also discussed.