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

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.

Analysis of the Flow Performance of the Complex Cross-Section Module to Reduce the Sedimentation in a Combined Sewer Pipe

The difference in the amount of stormwater and sewage in a combined sewer system is significantly large in areas where heavy rainfall is concentrated. This leads to a low water level and slow flow velocity inside the pipes, which causes sedimentation and odor on non-rainy days. A complex cross-section module increases the flow velocity by creating a small waterway inside the pipe for sewage to flow on non-rainy days. While considering Manning’s equation, we applied the principle where the flow velocity is proportional to two-thirds of the power of the hydraulic radius. The flow velocity of a circular pipe with a diameter of 450 mm and the corresponding complex cross-section module was analyzed by applying Manning’s equation and numerical modeling to show the effects of the complex cross-section module. The tractive force was compared based on a lab-scale experiment. When all conditions were identical except for the cross-sectional shape, the average flow velocity of the complex cross-section module was 14% higher while the size of the transported sand grains was up to 0.5 mm larger. This increase in flow velocity can be even higher if the roughness coefficient of aging pipes can be decreased.

The Study on Effectiveness and Flow Characteristic of Grassed Swale Drainage System in UTHM

The drainage system is an infrastructure that requires systematic planning of construction which can function properly to reduce the risk of flooding. Flooding occurs due to the rapid development resulting in lack of permeable surfaces. This will lead to increase the surface runoff, where the flow velocity and flow discharge also will be increased. Therefore, grassed swale is one of the sustainable drainage systems that can be applied to solve this problem. This study aims to identify the effectiveness of the grassed swale drainage system at Universiti Tun Hussein Onn Malaysia (UTHM), to determine the hydraulic characteristics and the effectiveness of vegetation used in the swale drainage system. Through this approach, the solution of past studies, related journals, and Manual Saliran Mesra Alam (MSMA) [1] are used as a reference for this study. Data collection was conducted on grassed swale with total length 60 meters. To obtain an accurate data, measurements of flow velocity have been taken three times, and for three days after raining. Data was observed by 81 times and analyzed using the Manning’s equation. Manning’s equation was adopted to determine the value of hydraulic coefficients for the grass swale channel. The results obtained, demonstrates that the value of Manning’s n for all sections is stated in a range of 0.015 to 0.030. The results also showed a difference in the size of the design, the flow velocity, the water depth, and the flow discharge of the grassed swale drainage system.

Field Estimates of Hydraulic Resistance of Ice-Covered Rivers

A large volume of velocity profiles measured from ice-covered rivers is used to estimate the composite Manning’s coefficient no associated with the ice and riverbed. Estimates are obtained using four methods, based on (I) depth-averaged and maximum velocities, (II) logarithmic velocity distributions fitted to field data, (III) energy slope combined with the logarithmic velocity distributions, and (IV) the Manning’s equation. Estimates of the composite coefficient range from 0.012 to 0.040. The coefficient can increase up to seven-fold through the winter. Method (II) has shown advantages. The energy slope appears to be between 25% and 50% of the water slope.