Compound Channel Flow Research Articles

Application of the Shiono and Knight Method in compound channels with non-prismatic floodplains

The Shiono and Knight Method (SKM) is known as a capable tool to model flow in prismatic compound channels. This research explores whether SKM can be used to predict depth-averaged velocity and boundary shear stress in compound channels with non-prismatic floodplains. For this reason SKM was modified and used in combination with an extensive experimental data set of depth-averaged velocity and boundary shear stress measurement to investigate the issue over three various floodplain converging angles and relative depths. The modeling results indicate a good agreement between the experimental data and the modified SKM. Also, based on the momentum balance and using the experimental data on compound channel with non-prismatic floodplains lateral variations of the depth-averaged apparent shear stress, the Reynolds shear stress, and the secondary flow were investigated. The results reveal that the Reynolds shear stress is significant at the interface between the main channel and floodplains and secondary flow has linear variation.

Closure to “Interacting Divided Channel Method for Compound Channel Flow” by Fredrik Huthoff, Pieter C. Roos, Denie C. M. Augustijn, and Suzanne J. M. H. Hulscher

When floodplains become overflown, the difference between flow velocities in the main channel and the in floodplain cause large-scale mixing patterns in the river bank zone. To calculate the discharge capacity of a river, it is important to accurately estimate the momentum transfer that is associated with this flow mixing. This discussion adresses the issue of including topographical characteristics of the river to estimate the momentum transfer between flow in the main channel and the floodplain. 1020-1022 / JOURNAL OF HYDRAULIC ENGINEERING © ASCE / NOVEMBER 2009.

Discussion of “Interacting Divided Channel Method for Compound Channel Flow” by Fredrik Huthoff, Pieter C. Roos, Denie C. M. Augustijn, and Suzanne J. M. H. Hulscher

Discussion of “Interacting Divided Channel Method for Compound Channel Flow” by Fredrik Huthoff, Pieter C. Roos, Denie C. M. Augustijn, and Suzanne J. M. H. Hulscher

Two-dimensional analytical solution for compound channel flows with vegetated floodplains

This paper presents a two-dimensional analytical solution for compound channel flows with vegetated floodplains. The depth-integrated N-S equation is used for analyzing the steady uniform flow. The effects of the vegetation are considered as the drag force item. The secondary currents are also taken into account in the governing equations, and the preliminary estimation of the secondary current intensity coefficient K is discussed. The predicted results for the straight channels and the apex cross-section of meandering channels agree well with experimental data, which shows that the analytical model presented here can be applied to predict the flow in compound channels with vegetated floodplains.

Identification of compound channel flow parameters

Identification of compound channel flow parametersOptimization methods are used to estimate parameters required for routing floods through open compound channels. Besides initial and boundary flow conditions, data required especially include, cross-sectional area (A) of flow and conveyance (K) as functions of flow depth (y) for a representative cross-section of the study reach. Thus, instead of optimizing upon channel's geometric and hydraulic parameters, optimization is performed upon non-physical parameters in assumedA(y) andK(y) relationships. The optimization method selected for this application is the Nelder and Mead Simplex Algorithm. The objective function is expressed in terms of the relative differences between observed and simulated stages and discharges, which are evaluated based on the complete numerical solution of St Venant equations. This approach to formulating the optimization problem was applied to unsteady flow data sets for an experimental reach of the River Main in Northern Ireland. Based on statistical analysis, simulated and observed stages were found to be in good agreement.

Development of a three-dimensional numerical model to solve shallow-water equations in compound channels

Velocity gradient between main channel and flood plains in compound channels leads to the formation of a large shear layer and secondary currents between these two subsections. These phenomena in the interaction region bring about a complex three-dimensional nature of the flow in compound channels. To cope with these flows, many numerical investigations have utilized three-dimensional formulations with advanced turbulence models. However, the free surface in many of these models is fixed and rigid-lid assumption has been used. In the present research, three-dimensional shallow water equations were used to calculate the flow field in compound channels. Three-dimensional equations were integrated in layers and were combined with the continuity equation. In this formulation, free-surface elevation was calculated without the need to solve any additional equations. Velocity and bed shear stress distribution and the stage–discharge relationship in compound channels with smooth and rough beds and with different relative depths were analyzed to verify this model, and satisfactory results were obtained.

A study of hydraulic characteristics for flow in equatorial rivers

This paper presents the results obtained from field measurements taken in several frequently flooded equitorial rivers, including velocity distributions, stage discharge relationships, roughness behaviours and discharge estimation. These have illustrated the large difference in velocity between the main channel and floodplain under flood conditions, and the effects of momentum transfer between deep and shallow flow, which include reduction in main channel velocity and discharge capacity, leading to a reduction in compound section capacity at depth above bankfull. Another significant characteristic that has been found is that the floodplain regions behave as storage reservoirs (V = 0 m/s) in most cases due to high resistance of long and thick grasses along the flood plains (n = 0.07–0.1). Flow resistance relationships have been presented in terms of Manning's coefficient and Darcy‐Weisbach friction factor, showing the complex nature of flow resistance in the rivers and further explaining the danger inherent in the conventional practices of extrapolating inbank data for the analysis of overbank flows. Results for discharge estimation have been shown for comparison with actual data, the errors incurred by applying empirical methods to compound channel flows have been quantified and found to depend on the particular method used.

Interacting Divided Channel Method for Compound Channel Flow

A new method to calculate flow in compound channels is proposed: the interacting divided channel method (IDCM), based on a new parametrization of the interface stress between adjacent flow compartments, typically between the main channel and floodplain of a two-stage channel. This expression is motivated by scaling arguments and allows for a simple analytical solution of the average flow velocities in different compartments. Good agreement is found between the analytical model results and laboratory data from the literature.

Two dimensional analytical solution for a partially vegetated compound channel flow

The theory of an eddy viscosity model is applied to the study of the flow in a compound channel which is partially vegetated. The governing equation is constituted by analyzing the longitudinal forces acting on the unit volume where the effect of the vegetation on the flow is considered as a drag force item. The compound channel is divided into 3 sub-regions in the transverse direction, and the coefficients in every region’s differential equations were solved simultaneously. Thus, the analytical solution of the transverse distribution of the depth-averaged velocity for uniform flow in a partially vegetated compound channel was obtained. The results can be used to predict the transverse distribution of bed shear stress, which has an important effect on the transportation of sediment. By comparing the analytical results with the measured data, the analytical solution in this paper is shown to be sufficiently accurate to predict most hydraulic features for engineering design purposes.

Numerical modeling of turbulent compound channel flow using the lattice Boltzmann method

AbstractThe flow of water in a straight compound channel with prismatic cross section is investigated with a relatively new tool, the lattice Boltzmann method. The large eddy simulation model is added in the lattice Boltzmann model for nonlinear shallow water equations (LABSWETM) so that the turbulence, caused by lateral exchange of momentum in the shear layer between the main channel and floodplain, can be taken into account and modeled efficiently. To validate the numerical model, a symmetrical compound channel with trapezoidal main channel and flat floodplain is tested. Similar to most natural watercourses, the floodplain has higher roughness values than the main channel. Different relative depths, Dr (the ratio of the depth of flow on the floodplain to that in the main channel), are considered. The Reynolds number is set at 30 000 in the main channel. The lateral distributions of the longitudinal velocity, the boundary shear stress, the Reynolds stress and the apparent shear stress across the channel are obtained after the large eddy simulation is performed. The results of numerical simulations are compared with the available experiment data, which show that the LABSWETM is capable of modeling the features of flow turbulence in compound channels and is sufficiently accurate for practical applications in engineering. Copyright © 2008 John Wiley & Sons, Ltd.

The effect of skewness on bridge backwater prediction

Experiments are performed concerning backwater at bridge constrictions placed in a compound-channel flume with both normal and skewed crossings. The flume had smooth or rough main channel and flood plains. Five different types of bridge models were used in the experiments, and two different skew angles (ϕ = 30° and ϕ = 45°) were tested for each type of bridge compared with that for a normal crossing (ϕ = 0°). A new design chart is proposed yielding a quadratic equation for estimating backwater.Key words: bridge piers, backwater, flood control, compound-channel flow, skewed crossing.

PROPAGATION MECHANISM OF FLOOD FLOW IN COMPOUND CHANNELS

It is very important for the river management to grasp the hydraulic characteristics of flood flow in compound channels. The propagation speed of flood flow in compound channels has not yet been known, although that in simple cross-section channels was showed by Kleitz-Seddon. This study aims to clarify the propagation mechanism of flood flow and compare propagation speed from laboratory experiments with those of the theoretical expression derived for compound channels. As a result, theoretical formula provides a good expression for propagation speed of compound channels. Furthermore, we explain the effect of change in the channel cross-section on the flood propagation speed in rivers. Finally, we investigate propagation characteristics of various flood flows in the Tone River and Edo River.

Flow in Compound Channel with Abrupt Floodplain Contraction

Flooding rivers usually present transition reaches where the floodplain width can significantly vary. The present study focuses on an abrupt floodplain contraction (mean angle 22°) in order to determine whether one-dimensional (1D) models, developed for straight and slightly converging geometry, are equally valid for such a geometry. Experiments on a contraction model were carried out in an asymmetric compound channel flume. Severe mass and momentum transfers from the floodplain towards the main channel were observed, giving rise to a noteworthy transverse slope of the water surface and different head loss gradients in the two subsections. Three 1D models and one 2D simulation were compared to experimental measurements. Each 1D model incorporates a specific approach for the modeling of the momentum exchange at the interface boundary between the main channel and the floodplain. The increase of the lateral mass transfer generates moderate errors on the water level values but significant errors on the discharge distribution. Erroneous results arise because of incorrect estimations of both momentum exchange due to lateral mass transfers and boundary conditions which are imposed by the tested 1D models.

Sediment transport formulae for compound channel flows

Nine sediment transport formulae were reviewed and applied to both inbank and overbank flows in straight and meandering compound channels. The predictive capabilities of the formulae were evaluated using experimental data obtained from the large-scale UK Flood Channel Facility and the small-scale Ulster Channel. The Yang formula (1979) was found to give the best prediction for sediment discharge, performing well for both straight and meandering channels. The Schoklitch (1962) and Yang and Lim (2003) formulae also gave very good predictions for most of the data sets studied. The Ackers and White formula (1973) gave very good prediction for the large-scale facility but overpredicted the sediment discharge for the small-scale channel. The Karim and Kennedy (1981) formula could also be a good sediment discharge predictor, for the studied flow conditions, but the coefficient of proportionality used in the formula needs to be calibrated.

Sediment transport formulae for compound channel flows

Nine sediment transport formulae were reviewed and applied to both inbank and overbank flows in straight and meandering compound channels. The predictive capabilities of the formulae were evaluated using experimental data obtained from the large-scale UK Flood Channel Facility and the small-scale Ulster Channel. The Yang formula (1979) was found to give the best prediction for sediment discharge, performing well for both straight and meandering channels. The Schoklitch (1962) and Yang and Lim (2003) formulae also gave very good predictions for most of the data sets studied. The Ackers and White formula (1973) gave very good prediction for the large-scale facility but overpredicted the sediment discharge for the small-scale channel. The Karim and Kennedy (1981) formula could also be a good sediment discharge predictor, for the studied flow conditions, but the coefficient of proportionality used in the formula needs to be calibrated.

Turbulence modeling of compound open-channel flows with and without vegetation on the floodplain using the Reynolds stress model

A Reynolds stress model for the numerical simulation of compound open-channel flows with vegetation on the floodplain is described. The Reynolds stress model consists of various sub-models such as Speziale et al.’s model, Mellor and Herring’s model, and Rotta’s model for the pressure–strain correlation term, the turbulent diffusion term, and the dissipation term, respectively. For validation of the model, plain compound open-channel flows are simulated. The computed results were compared with measured data by [Tominaga A, Nezu I. Turbulent structure in compound open-channel flows. J Hydraul Eng, ASCE 1991;117(1):21–41] and the results show that the Reynolds stress model successfully simulates the mean flow and turbulence structure of plain compound channel flows. The model was then applied to compound open-channel flows with vegetated floodplains. Good agreement between the simulated results and data from an algebraic stress model by [Naot D, Nezu I, Nakagawa H. Hydrodynamic behavior of partly vegetated open channels. J Hydraul Eng, ASCE 1996;122(11):625–33] was found. However, it was shown that the RSM is capable of predicting the velocity dip and lateral shift in the maximum streamwise velocity, which were not observed in the data from algebraic stress modeling. Finally, a depth-averaged analysis of the streamwise momentum equation was performed to investigate the lateral momentum transfer in compound channel flows with vegetated floodplains. Compared with components by the secondary currents and Reynolds stress, the drag force due to the presence of vegetation appears to be a factor in reducing the bottom shear stress in both main channel and floodplain.

Experimental backwater analysis around bridge waterways

A series of five experiments was performed in a two-stage compound channel including various roughness conditions and different types of bridge models, namely, single-opening semi-circular arch bridge model (ASOSC), multiple-opening semi-circular arch bridge model (AMOSC), single-opening elliptic arch bridge model (ASOE), and single-opening straight-deck bridge model with and without piers (DECK) including different span widths. The performances of six different methods for computing backwater around bridge waterways were compared using the experimental data carefully taken on many combinations of cases. The results of the energy method, momentum method, WSPRO method, Yarnell's method, USBPR method, and arch bridge method were compared with experimental results. The results showed that energy method was able to simulate more accurately the measured backwater values than the other methods. The backwater differences between the experimental values and computed values by the energy method are generally within –3.2% and 0.8% in terms of flow depth. A simple generalized function for estimating bridge backwater is also proposed.Key words: bridge backwater, hydraulic model testing, flood control, compound channel flow, one-dimensional methods.

Flood routing of regulated flows in Medjerda River, Tunisia

Released flows from the Sidi Salem Dam Reservoir on the Medjerda River (Northern Tunisia) were routed downstream along the river lower water course using both hydrologic and hydraulic flood routing techniques. The hydrologic flood routing method used is that of Muskingum while the hydraulic flood routing procedure used a numerical model RUFICC (Routing Unsteady Flows In Compound Channels). The model is based on the complete numerical solution of St. Venant equations using a four-point implicit finite difference scheme. Compared to observed hydrographs at downstream sections, a better agreement was achieved using the hydraulic flood routing technique. Statistical parameters and scattergrams were used to test and confirm this agreement.

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 qualita...

A comparison of one-dimensional methods for estimating discharge capacity of straight compound channels

A series of experiments was carried out in a two-stage flume having a smooth main channel and smooth or rough floodplains to investigate the reliability and performance of four different one-dimensional methods for computing the discharge capacity of compound channels, namely, the single-channel method (SCM), the divided-channel method (DCM), the exchange discharge method (EDM), and the Ackers method (AM). Additional data from fixed- and mobile-bed compound laboratory channels with smooth and roughened floodplains and of a prototype compound river channel were also used in the computations. The boundary roughness and scale effects associated with the performance of the four methods are also examined. The results show that the EDM and the AM are able to simulate the measured discharge values more accurately than those of the traditional methods, namely, the DCM and the SCM. Although the error in discharge estimation produced by both the AM and the EDM was generally lower than 10% for both smooth and fixed boundaries, it increased up to 20% for mobile boundaries. Overall, the average relative error in discharge estimations using the AM and the EDM was about 5.4% and 7.1%, respectively, with a standard deviation of 6.7% and 6.8%, respectively. Key words: compound channel flow, stage-discharge relationship, one-dimensional methods.