Comparison of Empirical and Analytical Solutions for Open-Channel Flow Velocity with Common Grass Species in Taiwan

Ping-Cheng Hsieh,Yi-Cheng Lin,Yung-Chieh Wang

doi:10.3390/w13131839

Ping-Cheng Hsieh, Yi-Cheng Lin + Show 1 more

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https://doi.org/10.3390/w13131839

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Abstract

Grassed channels utilize the soil stabilization and water infiltration enhancement functions of grass in order to conserve soil and water in drainage systems. The construction processes and hydraulic mechanisms of grassed channels are more complicated, depending on the conditions of both soil and grass. As flow resistance is affected by grass characteristics, giving a single value of Manning’s n for a grass type under different flow conditions may lead to over-conservative designs or safety concerns. In this study, grassed flow experiments were carried out in a flume, with a bed of red soil covered by three grass species and with the flow conditions of three bed slopes. Average flow velocities were evaluated using five methods, including Manning’s equation and an analytical method. Comparison between the methods showed that Manning’s equation was unable to properly reflect the grass characteristic effects on the flow, but the analytical method performed better in estimating the average velocity and velocity profiles. The experimental results will be useful for the verification of mathematical methods, including analytical solutions and numerical models of grassed flow. For application, the relationships of average flow velocity against the grass layer relative height were proposed based on the analytical method as a reference for a hillslope drainage system design in Taiwan.

Highlights

Published: 1 July 2021The purpose of soil and water conservation is to conserve soil and water resources, maintain land stability, and restore ecosystem during land development
The comparison among the results of different evaluating methods shows that V Manning > V ultra > V anal > V elect > V bucket ; the variation trends of V anal, V elect, and V bucket with the relative height were similar and close to one another
In order to show the fluctuation of average velocity measurements varied with the relative height of each method, we calculated the “total increment of average velocity of each method”, which was the difference between the largest and the smallest average velocity divided by the smallest average velocity

Summary

Introduction

The purpose of soil and water conservation is to conserve soil and water resources, maintain land stability, and restore ecosystem during land development. Reasonable land utilization with appropriate conservation measures can prevent or mitigate soil and water hazards, and move towards the goal of sustainable land development and environmental conservation. Taiwan is frequently struck by typhoons and heavy rains, and the concentrated flows often lead to serious soil and water hazards, such as floods, debris/mudflows, and large-scale landslides [1,2]. Conventional drainage structures are commonly made of concreate or reinforced concrete, which has impermeable surfaces with a low roughness. The hydraulic computation in conventional concrete channels is straightforward and the average flow velocity and flowrate can be estimated using Manning’s formula and continuity equation.

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