Modelling of Overbank Flow in Two-stage Meandering Channels

Abstract

This research examines the flow in meandering compound channels through numerical and physical modelling which are significant for understanding the non-uniform flow and its behaviour. Accounting the momentum exchange at the junctions of a compound channel is a complex task in order to develop an improved model for predicting stage-discharge relationship, distribution of the flow in subsections, resistance, and distribution of boundary shear force.
In literature, much of experimental research works are focussed on simple meandering channels and less for meandering compound channel, while almost nil works has been carried out for doubly meandering compound channel (where both main channel and floodplain levees are meander). Experiments have been conducted on two stage meandering channels with different sinuosity constructed at the hydraulics-engineering laboratory of the department of civil engineering, National Institute of Technology Rourkela (NITR), India. The effectiveness of Manning’s n is analysed for the different flow configurations of meandering compound channels. A model is developed for determining Manning’s roughness coefficient which depends on the non-dimensional parameters like width ratio, relative flow depth, sinuosity ratio, meander belt width ratio, and bed slope. Various data driven models such as multivariate adaptive regression spline (MARS), group method of data handling (GMDH), gene-expression programming (GEP) and support vector regression approaches have been used to develop a model for predicting the Manning’s roughness coefficient of meandering compound channels by taking care of the aforementioned geometric and hydraulic parameters. These developed model equations through MARS, GMDH, GEP, and SVR approaches can be useful as a practical tool for the prediction of Manning’s roughness coefficient in a natural channel.
Moreover, the model is further used for estimating conveyance for large-scale as well as small-scale channels. Results of the developed model is compared with established approaches for calculating roughness coefficient which leads to the assessment of conveyance capacity of the meandering compound channels. The performance of all the developed models are evaluated by means of various statistical measures and uncertainty analysis to determine the best alternate. The models are developed using relevant experimental data obtained from laboratory experiments and the data from other researchers on the meandering compound channels. The results are found to be in agreement with experimental as well as river discharge data.
The flow structure in a compound channels becomes complicated due to the transfer of momentum between the deep main channel and the adjoining floodplains; which affects the shear stress distribution across the perimeter. Accurate prediction of shear stress distribution along the boundary in an open channel is the key to the solution of numerous critical engineering problems such as flood control, sediment transport, river bank protection and others. Therefore, an investigation concerning the distribution of bed shear stress in the main channel and the floodplains of meandering compound channels are presented. Models for predicting the percentage sharing of shear force for floodplain are developed using multivariate adaptive regression spline (MARS), group method of data handling (GMDH) and gene-expression programming (GEP) by taking five dimensionless parameters as the inputs. The width ratio, relative depth, sinuosity, bed slope, and meander belt width ratio of the channel are taken as input parameters. Influence of each parameter on predicting the percentage of shear force at floodplain by the developed models is also analyzed by adopting a sensitivity analysis. The predictive results are compared with results based on in situ measurement using other data driven approaches like support vector regression (SVR), and K-nearest neighbors (KNN). A comparative analysis of the developed MARS, GMDH and GEP model and previously developed analytical models are presented. (% ) fp S
The conventional channel division methods assume zero apparent shear forces on the respective vertical, diagonal, horizontal and variable-inclined interfaces. A modified variable inclined interface (MVI) is proposed by using GEP for which apparent shear force is calculated as zero. Modified variable inclined interfaces are also used to calculate discharge in meandering compound channels. Performance of the developed models of shear force percentage is evaluated with previously developed analytical methods through different statistical measures. Using the modified-inclined interface, the error between the measured and calculated discharges for the meandering compound channel is found to be the minimum when compared with that using other interfaces. Moreover, the equations agree well for predicting discharge for large-scale as well as small-scale channels besides the natural river data.
In this research work, an attempt is made to improve a discharge estimation method where the modified variable inclined interface divides the main channel as well as outer and inner
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