Flow Resistance In Alluvial Channel Research Articles

Machine learning approaches for adequate prediction of flow resistance in alluvial channels with bedforms.

In natural rivers, flow conditions are mainly dependent on flow resistance and type of roughness. The interactions among flow and bedforms are complex in nature as bedform dynamics primarily regulate the flow resistance. Manning's equation is the most frequently used equation for this purpose. Therefore, there is a need to develop alternate reliable techniques for adequate prediction of Manning's roughness coefficient (n) in alluvial channels with bedforms. Thus, the main objective of this study is to utilize machine learning (ML) models for predicting 'n' based on the six input features. The performance of ML models was assessed using Pearson's coefficient (R2), sensitivity analysis, Taylor's diagram, box plots, and K-fold method has been used for the cross-validation. Based on the output of the current work, models such as random forest, extra trees regression, and extreme gradient boosting performed extremely well (R2 ≥ 0.99), whereas, Lasso Regression models showed moderate efficiency in predicting roughness. The sensitivity analysis indicated that the energy grade line has a significant impact in predicting the roughness as compared to the other parameters. The alternate approach utilized in the present study provides insights into riverbed characteristics, enhancing the understanding of the complex relationship between roughness and other independent parameters.

Bed Load Pickup Rate and Flow Resistance for Turbid Flow on a Movable Plane Bed

This study investigates bed load transport and flow resistance in alluvial channels that contain fine suspended sediment. In all, 123 samples were run in a laboratory flume over uniform bed materials. Different hydraulic conditions were considered in an 8.0 m-long, 0.255 m-wide, and 0.6 m-deep tilting flume. Unit flow discharge ranged from 3.176 × 10−3 to 18.640 × 10−3 m2 s−1, and the slope gradient varied from 2% to 5%. Two sediment-size classes (median diameter, d50, of 1.7 and 3.29 mm) and five suspended load concentrations (10, 20, 30, 40, and 50 g/L) were used. A preliminary analysis showed that the bed load pickup rate increases with an increase in the concentration of suspended particles. Dimensional analysis, which was performed satisfactorily on the bed load transport data, indicated that shear stress is the dominant parameter in bed load pickup. Moreover, the resistance of turbid flow in a channel with an erodible bed was studied. The net results were found to be an increase in the flow resistance over a movable plain bed by increasing suspended load concentration. In other words, increasing suspended particles increased the mobility of bed material and led to a higher friction factor. Special attention was paid to the separation of the friction coefficient between boundary resistance and mobile-bed resistance caused by the mobility of bed materials. The results showed that the damping effect of suspended sediment caused lower boundary resistance with increasing suspended load concentration. On the other hand, suspended particles increased the mobility of bed materials and led to a higher mobile-bed resistance. Analysis indicates that the relative importance of mobile-bed resistance is more than the boundary friction for the high bed load transport rate. Therefore, the damping effect of the suspended sediment on boundary friction can only be observed for low sediment transport or a fixed bed.

Flow resistance in alluvial channel

In the present work, a resistance equation has been developed empirically for the alluvial channel by using a large database of flume and field experimental observations. Advantage of the present model is that it includes the effect of Froude number, Reynolds number and particle relative roughness parameter on friction factor. Developed model predicts well the friction factor in the alluvial channel. The validity of the equation is tested with most recent experimental and field observations and is found in good agreement. Curves of friction factor at different relative roughness have been developed for the field application.

Flow resistance in alluvial channels

There have been numerous fluvial studies of flow resistance in alluvial channels during the last few decades. Significant progress has been made towards predicting flow resistance (and therefore velocity) for a given discharge. These past applications rely heavily on the characterization of particle sizes and the effects of changing relative submergence on flow resistance estimates. Different types of equations have been shown to provide reasonably good estimates in specific environments. Major difficulties arise from characterizing mobile beds, bed topography and its evolution and how these factors control rates of change of average velocity as discharge rises along a given river reach. Different issues can be recognized as a function of the spatial and temporal scales of investigation. A case can made that more emphasis should be placed upon reach-scale investigations. Detailed studies of bed topography, its maintenance, its evolution (at the reach scale) and its interactions with macroturbulence structure and sediment transport would ultimately provide valuable information and improved knowledge on both flow resistance processes and applications (predictions). Moreover, technological means now allow detailed characterization of bed topography and flow fields of large river systems. Such promising avenues should be further pursued with the goal of providing not only a better understanding of flow-bed-sediment transport interactions in large river systems but also a better understanding of flow stage variations, flood hazards, flow resistance estimates and therefore partitioning of depth and velocity as discharge rises along major river systems.

Flow resistance and bed form geometry in a wide alluvial channel

This paper explores the underlying mechanism of flow resistance in a wide alluvial channel with bed forms. On the basis of published data, it is shown that the grain roughness can be taken as equal to 2 times the median diameter of the bed sediment. An empirical equation for the bed form roughness has been proposed, and it depends on the bed form height and bed form steepness. The influence of the bed form length and height on the total bed shear stress and energy slope is deliberated, and empirical expressions for the length of the separation zone behind the bed forms are also proposed. The study proposed an equation to compute the total bed shear stress as a function of the grain and bed form roughness as well as the important role of the bed form geometry in the overall flow resistance in alluvial channels. The model is tested and verified against 670 flume measurements and 1540 field observations. The computed and measured energy slopes are in good agreement with close to 71% of all data sets within the ±20% error band.

Flow Resistance in Ice-Covered Alluvial Channels

This paper describes the influences of floating level ice covers on flow and bed-load transport in alluvial channels and proposes a semi-empirical procedure for calculating flow resistance in such channels. The procedure can be applied in conjunction with several existing methods for estimating flow resistance in alluvial channels under open-water conditions. Previously, cover influence was poorly understood, with the consequence that flow-resistance calculations for ice-covered alluvial channels entailed an inordinate amount of guesswork. The cover influences described herein were determined from the results of experiments conducted with a laboratory flume. The main findings of the flume experiments are that ice covers increase flow depth, decrease bulk flow velocity, increase dune length, and significantly decrease bed-load transport rate. Additionally, it was found that sediment transport relationships developed for open-water flow are applicable to ice-covered flow provided they are used in conjunctio...