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Title: Modeling of vegetated flows with uncertainty estimation
Authors: Busari, Afis Olumide
Advisors: Li, Chi-wai (CEE)
Keywords: Turbulence.
Channels (Hydraulic engineering)
Vegetation mapping.
Issue Date: 2016
Publisher: The Hong Kong Polytechnic University
Abstract: Vegetation growth in natural and artificial waterways (open channels) is desirable due to ecological and environmental concern. It can be actively used as a flood management tool, and to enhance the sustainability and restoration of ecosystem. Its growth in channels, however, increases the hydraulic resistance and leads to energy loss, which can be problematic. The consequence is the decrease in mean velocity, thereby reducing the channel conveyance capacity. The hydraulic resistance produced by sparsely distributed vegetation stems has been well studied while the effect of dense vegetation on hydraulic resistance has not been thoroughly investigated. Numerous studies of flow through vegetation in open channels are often based on cylindrical shape elements. Scarcity of data through review scrutiny reveals there is a need for more laboratory studies on flows over blade-type vegetation, and investigation of the effect of different vegetation distribution pattern on hydraulic roughness parameter. In addition, the field conditions are always with uncertainty. There is a practical need to investigate the propagation of uncertainty in the flow and vegetation parameters towards the uncertainty in hydraulic roughness parameter. The objectives of this work are: (i) to develop a hydraulic roughness model for submerged flexible vegetation; (ii) to acquire experimental data of flows through blade-type vegetation under emergent and submerged conditions; (iii) to clarify the inconsistent findings of previous research by investigating the dependency of the bulk drag coefficient (Cd) on plant distribution pattern instead of the solid volume fraction (φ), through the experimental study of the sheltering and channeling effect of vegetation stems on the behaviour of flow through emergent and submerged vegetation; (iv) to propose empirical equations relating Cd to the lateral and longitudinal spacing of stems for both submerged and emergent conditions; and (v) to assess the propagation of uncertainties in the modelling of vegetated flow. To achieve the above objectives, this study is subdivided into four parts. Firstly, this study used Spalart-Allmaras model to generate synthetic velocity profile data for hydraulic roughness determination. In the model, turbulence is simulated by the closure with a modified length scale which is dependent on the vegetation density and vegetation height to water depth ratio. Flexibility of vegetation is accounted for by using a large deflection analysis. The model has been verified against available experiments. Based on the synthetic data an inducing equation is derived for submerged flexible vegetation, which relates the Manning roughness coefficient to the vegetation parameters, flow depth and a zero-plane displacement parameter. The derived equation has been verified using well-documented experimental data as well as field data. The equation performed better than the existing equations especially for submerged flexible vegetation. Generally, the predictive capability of these equations depend largely on the Cd values. Secondly, a systematic laboratory study has been carried out to investigate the effect of the distribution pattern of vegetation stems on the hydrodynamics of gradually varied flow (GVF) through emergent blade-type vegetation. The drag induced by flow through vegetation is affected by the velocity, shape of vegetation stems and wake interference among stems. Previous studies have accounted for the interference effects generally by relating Cd of vegetation to the solid volume fraction φ of the vegetated zone and the results were found to be inconsistent. The Cd values are well documented for cylindrical shape while that for blade type vegetation is less reported, and the drag characteristics for the two shapes can be different. In this work, a blade-type finite artificial vegetation patches of solid volume fractions ranging from 0.005 to 0.121 have been used and the stem Reynolds number tested ranges from 500 - 2600. The longitudinal water surface profiles have been measured and the effect of increasing areal density of vegetation with respect to varying longitudinal and lateral spacing under the flow conditions is examined. The momentum equation that relates the vegetation resistant force and water surface profile has been used to obtain the value of Cd. The results shows that Cd decreases with increasing stem Reynolds number, decreases with increasing φ at fixed lateral spacing due to sheltering effect, and increases with φ at fixed longitudinal spacing due to channeling effect. The inertial contribution due to pressure loss in the stem wake decreases with increase in transverse and longitudinal spacing, while the effects of viscous shear stress, vortex shedding and jet spreading increases with the increase in longitudinal spacing over the experimental range. An empirical equation relating Cd to the lateral and longitudinal spacing instead of φ has been obtained and validated.
Thirdly, the hydrodynamic behaviour of GVF through submerged blade-type vegetation is investigated. The distribution layouts of stems are similar to those cases of emergent vegetation. The blade Reynolds number Re ranges from 670 to 1150 and six flow rates are used in each set of the experiments. The vertical profiles of stream-wise velocity have been measured using Acoustic Doppler Velocimeter (ADV). The theoretical longitudinal momentum equation relating the vegetation resistant force, water surface slope and mean velocity in the vegetation layer is used to determine the Cd value. Using a regular-array pattern of vegetation elements, the interference effects have been studied independently by varying the longitudinal element spacing (Sx) and lateral element spacing (Sy) respectively. The results showed that the distribution pattern of vegetation elements can exert significant effect on the Cd values, and the associated flow characteristics, including flow adjustment length, peak Reynolds stress and flow division in the clear water zone and vegetation zone. An empirical equation relating Cd to the lateral and longitudinal spacing is proposed for submerged flexible vegetation. For the same vegetation distribution pattern, the Cd values obtained for submerged and emergent conditions are similar for cases with high areal density of vegetation. Lastly, numerical simulation of flows over dense vegetation has been carried out using the experimentally determined Cd as input. The flow evolution within the vegetation patches and the clear water zone due to sheltering and channeling effects among vegetation stems have been successfully replicated. For practical applications, the accuracy of the prediction by the proposed equations and numerical model is further assessed. The sources of uncertainty are due to the limitations of the equations/model and the variability of the input vegetation and flow parameters. The uncertainty of the inducing equations and numerical model in the estimation of the roughness coefficients is expressed by the Normalized Root Mean Square Deviation (NRMSD) and the propagation of the uncertainty due to the variability of the vegetation and flow parameters existing in nature is investigated by using the method of Unscented Transformation (UT). The method is found efficient and gives a more accurate estimation of the mean roughness (or drag) coefficients. By measuring the vegetation and flow parameters with uncertainty ranges, the inducing equations together with the UT method can be used to compute the mean and covariance of the Manning roughness (or drag) coefficient. In summary, this study contributes to the knowledge and understanding of vegetated flows. It advances the previous studies in that the dependence of the hydraulic roughness parameter (drag coefficient) on the distribution pattern of vegetation has been investigated and the propagation of the uncertainty in the parameter estimation has been quantified. The proposed equations and numerical model have been verified against experiments and can be applied to conditions where laboratory and field study have not been performed. The model thus will be useful for river/wetland restoration and vegetation management projects.
Description: PolyU Library Call No.: [THS] LG51 .H577P CEE 2016 Busari
xxix , 254 pages :color illustrations
Rights: All rights reserved.
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