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Yes / The present study investigates the turbulent hydrodynamics in an open channel flow with an emergent and sparse vegetation patch placed in the middle of the channel. The dimensions of the rigid vegetation patch are 81 cm long and 24 cm wide and it is prepared by a 7× 10 array of uniform acrylic cylinders by maintaining 9 cm and 4 cm spacing between centers of two consecutive cylinders along streamwise and lateral directions respectively. From the leading edge of the patch, the observed nature of time averaged flow velocities along streamwise, lateral and vertical directions is not consistent up to half length of the patch; however the velocity profiles develop a uniform behavior after that length. In the interior of the patch, the magnitude of vertical normal stress is small in comparison to the magnitudes of streamwise and lateral normal stresses. The magnitude of Reynolds shear stress profiles decreases with increasing downstream length from the leading edge of the vegetation patch and the trend continues even in the wake region downstream of the trailing edge. The increased magnitude of turbulent kinetic energy profiles is noticed from leading edge up to a certain length inside the patch; however its value decreases with further increasing downstream distance. A new mathematical model is proposed to predict time averaged streamwise velocity inside the sparse vegetation patch and the proposed model shows good agreement with the experimental data. / Debasish Pal received financial assistance from SRIC Project of IIT Kharagpur (Project code: FVP)

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science in Engineering Johannesburg, 2017 / The development of methods which are better able to predict the effect of large scale emergent roughness elements on the flow characteristics requires a better understanding of the drag coefficient under conditions likely to occur in the field. A laboratory investigation was carried out with newly developed equipment to quantify the drag force on various shaped cylinders, as well as the drag on an individual cylinder surrounded by an array of cylinders. The relationship between the drag coefficient and cylinder Reynolds number for a single circular cylinder was found to be of similar form but larger in magnitude than the established relationship for an infinitely long cylinder; the relationship departs from the infinite cylinder relationship for low cylinder Reynolds numbers. Contrary to previous research, the results for the multiple cylinder investigation did not reveal a clear relationship between the cylinder density and drag coefficient. Equations were developed and verified with existing laboratory data. These should be improved and extended by further research for field use. / MT2017

Flow-vegetation interactions are critically important for most hydraulic and sediment processes in streams and rivers and thus need to be accounted for in their management. The central goal of this project therefore was to improve the understanding of flow-vegetation interactions in patchy-vegetated river beds, which are typical in rivers. Based on laboratory experiments covering a range of selected hydraulic and patch mosaic scenarios, the hydraulic resistance mechanisms, turbulence structure, and transport mechanisms were studied. The effects of regular patch mosaic patterns (aligned and staggered) on the bulk hydraulic resistance were investigated first. For the cases in which the relative vegetation coverage BSA in respect to the total flume bed is low (BSA = 0.1), the patches mutual positions do not affect values of the friction factor. When the parameter BSA increases to intermediate values (BSA = 0.3), the spatial distribution of the vegetation patches and their interactions become crucial and lead to a significant increase in the bulk hydraulic resistance. When further increase of the vegetation cover occurs (BSA = 0.6), the effects on hydraulic resistance of patch patterns vanish. To clarify the mechanisms of the revealed patch effects on the overall hydraulic resistance, flow structure was assessed at both scales: individual patch and patch mosaic. The presence of a submerged isolated vegetation patch on the bed introduces a flow diversion which strongly alters the velocity field and turbulence parameters around the patch. Coherent structures, generated at the canopy top due to velocity shear, control the mass and momentum transfer between the layers below and above the vegetation patch. At the patch mosaic scale, a complex three-dimensional flow structure is formed around the patches which depends on the patch spacing and spatial arrangements. For the low surface area blockage factor (BSA = 0.1), the patches are sparsely distributed and the wakes are (nearly) fully developed before they are interrupted by the effects of the downstream patches. At the intermediate surface area blockage factor (BSA = 0.3), significant differences in flow structure between the aligned and staggered patches were observed. For the highest surface area blockage factor investigated (BSA = 0.6) both aligned and staggered patch mosaic configurations showed a similar behaviour. The results on the flow structure are used to provide mechanistic explanation of the observed patch mosaic effects on the bulk hydraulic resistance.

The Santa Ynez Mountains of Santa Barbara County, California, have seen many major storm events during the past century. San Jose Creek, which runs out of these mountains, through the town of Goleta, and into the Pacific Ocean, has experienced several intense flood events as a result. The lower portion of the creek was diverted in 1960 to alleviate flooding through Old Town Goleta. However, flooding still occurred in the storms of 1995 and 1998. This study incorporates a hydraulic analysis component of a project aimed at re-designing this diverted portion of the channel. It presents an analysis of modifications to this reach in order to improve its capacity and reduce flooding during a 100-year event. As one of the most prominent software for hydraulic modeling for steady and unsteady state open channel flow, HEC-RAS is used to analyze multiple variations in channel geometry and combinations of lining materials. Of these modifications, the best configuration is suggested.

Incipient motion criterion in sediment transport is very important, as it defines the flow condition that initiates sediment motion, and is also frequently employed in models to predict the sediment transport at higher flow conditions as well. In turbulent flows, even a reasonably accurate definition of incipient motion condition becomes very difficult due to the random nature of the turbulent process, which is responsible for sediment motion under incipient conditions. This work investigates two aspects, both of which apply to incipient sediment transport conditions. The first one deals with the role of turbulence in initiating sediment motion. The second part deals with the nature of sediment-fluid interaction for more general and complex flows where the number of sediment particles that form the rough surface is varied. The first part of this work that investigates the role of turbulence in initiating sediment motion, uses a video camera to simultaneously monitor and record the sediment (glass ball) motion and corresponding fluid velocity events measured by a three-component laser Doppler Velocimeter (LDV). The results of the single ball experiment revealed that the number of LDV flow measurements increase dramatically (more than four folds) just prior to the ball motion. The fluid mean velocity and its root-mean-square (rms) values also are significantly higher than the values that correspond to the flow conditions that yield no ball motion. The second part of the work, investigation of the fluid-sediment interaction, includes five tests with varying number of sediment particles. In order to understand the nature and extent of fluid-solid interaction, velocity profile measurements using the 3-D laser system were carried out at three locations for each of these five cases. Plots of mean velocities, rms quantities located the universal layer at about 1.5 ball diameters above the porous bed. However, at higher sediment particle concentrations, this distance reduced and the beginning of the universal layer approached the top of the porous bed. / Ph. D.

Yes / There has been little study on the microscopic bubble entrainment and diffusion process on the high-speed self-aerated flows although the problem under investigation is theoretically important and has important engineering application. This study presents an experimental investigation on visual processes of free surface air entrainment and single bubble diffusion in supercritical open channel flows. The typical surface deformation, single air bubble rising and penetration are recorded using a high-speed camera system. Results show that for a single bubble formation process, surface entrapment development and bubble entrainment through a deformation evolution underneath the free surface are the two main features. The shape variation of local surface deformation with time follows an identical power law for different bubble size generations. The entrained bubble size depends on both size scale and shape of entrapped free surface. As the single bubble moves downstream, its longitudinal velocity is approximately the same as that of water flow surrounded it, while its vertical velocity for rising and penetration increases with the increase of the water flow velocity. An empirical-linear relationship for the bubble rising and penetration velocity with water flow velocity is obtained. This study demonstrates that the microscopic bubble movement can improve the self-aeration prediction in the open channel flow and advance the knowledge of our understanding of the macroscopic and microscopic air–water properties in hydraulic engineering. / National Natural Science Foundation of China (Grant number 51609162), Sichuan Science and Technology Program (Grant number 2019JDTD0007) and the Open funding of the State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University (Project No: Skhl1809).

Yes / Manning’s roughness coefficient (n) has been widely used to estimate flood discharges and flow depths in natural channels. Therefore, although extensive guidelines are available, the selection of the appropriate n value is of great importance to hydraulic engineers and hydrologists. Generally, the largest source of error in post-flood estimates is caused by the estimation of n values, particularly when there has been minimal field verification of flow resistance. This emphasizes the need to improve methods for evaluating the roughness coefficients. Trinidad and Tobago currently does not have any set method or standardised procedure that they use to determine the n value. Therefore, the objective of this study was to develop a soft computing model in the calculation of the roughness coefficient values using low flow discharge measurements for a stream. This study presents Gene-Expression Programming (GEP), as an improved approach to compute Manning’s Roughness Coefficient. The GEP model was found to be accurate, producing a coefficient of determination (R2) of 0.94 and Root Mean Square Error (RSME) of 0.0024.