Flow Modelling of Mahaweli River Reach with the Riverbed Intake of Udunuwara-Yatinuwara Water Supply Scheme K

ENGINEER - Vol. XXXX, No. 04, pp. 129-133, 2007 © The Institution of Engineers, Sri Lanka Flow Modelling of Mahaweli River Reach with the Riverbed Intake of Udunuwara-Yatinuwara Water Supply Scheme K. H. S. S. Karunanayake and S. B. Weerakoon Abstract: The Udunuwara-Yatinuwara water supply scheme of the National Water Supply and Drainage Board abstracts water through a riverbed filtration system located in a bifurcated channel of the Mahaweli River at Peradeniya. The diverted flow through the channel has become inadequate to supply the abstraction demand during the periods of low river flow. The flow pattern in the river and the discharge through the channel are investigated by the application of a two-dimensional depth- averaged flow computational model to a 450 m long river reach including the channel. The model was set up, calibrated and verified for the reach using measurements carried out at the site. The model was applied to ascertain the possibility of increasing the flow through the channel during low flow in the river by changing the river bed level in the reach. Keywords: Flow computation, modelling, water intake 1. Introduction to flow over the filters to enable extraction of the demand even during periods of low flow in the Udunuwara-Yatinuwara water supply scheme is river. There are various options available to one of the major water supply schemes of the solve this problem; some of them are changing National Water Supply and Drainage Board part of the river bed level by armouring and/or (NWS&DB) providing drinking water to desilting, construction of groyne, spur dyke, Udunuwara, Yatinuwara divisional secretarial compound weir, and weir. In this paper, only divisions. At present, it serves a population of the potential of diversion of adequate flow into about 90,000, various institutions and industries the bifurcated channel by changing the river bed in the area. The scheme consists of three water level is investigated using computational intakes located at Alpitiya, Nillambe and hydraulic modelling. However, detail Peradeniya. investigation of each option giving due The present study is focused on the intake at considerations to all issues is required to achieve Peradeniya, sited on the right bank of the an optimum solution for implementation Mahaweli River near the Sarasaviuyana railway purpose. station. The intake has been constructed to Computational models for rivers are essentially 3 extract 4600 m of water daily into two collecting based on the theoretical formulations relating wells on the right bank of the river through two the flow, fluid and geometric parameters river bed rapid sand filters. River flow is derived by the application of conservation laws bifurcated by an artificial island and one of the and some constitutive relationships. It is the bifurcated flow is arranged to flow over the usual practice that varying approximations, river bed sand filters. Filtered water is collected justifiable to the river flow situation in hand, are through perforated pipes 'and arranged to flow made to the model equations in order to obtain into the collecting wells. Though the intake a solution with acceptable level of accuracy. functioned satisfactorily at the initial period Among the computational models available, after construction in 1991, the bifuicated flow one-dimensional river models based on the over the filters is not sufficient to supply the demand during low flow period of every year which is about 30% of the time at present. K.H.S.S.Karunanayake, B.Sc.Eng. (Peradeniya), M.Sc.Eng. (Peradeniya), Engineer, National Water Supply and Drainage Board, Therefore, it is required to find a long term Geteniabe, Peradeniya. Eng. (Dr.) S.B.Weemkoon, B.Sc.Eng. (Peradeniya), M.Eng. (Tokyo), engineering solution which is economical and D.Eng. (Tokyo), C.Eng., MIE(SL), MSLAAS, Senior Lecturer in Civil environmentally sound to divert adequate water Engineering, University of Peradeniya. Peradeniya 129 ENGINEER Saint Venant equations have been applied to water is assumed to be negligible compared river systems for gross prediction of flood water with the gravitational acceleration yields the levels, discharge distributions and sediment depth-averaged equations applicable to shallow transport along rivers. The applications of three• free surface flows (Vreguendhil and Wijbenga, dimensional computational studies have been [5]). The effective stress tensor components in limited, mostly, to laboratory channel flows with depth-averaged equations are modelled by a rectangular cross-sections, mainly due to the gradient-diffusion model (Molls and Chaudhry numerical instabilities encountered in the [4]). The shear stress on the free surface is computations when the irregular boundary neglected and the bottom shear stress is topography inherent to natural rivers is taken modelled by relating it to the local free surface into account. On the other hand, the depth• slope. Then, the local free surface slope is averaged models, which are economical, are computed from Manning's formula. The steady applied to shallow water rivers. The state depth integrated two-dimensional applications of depth-averaged models in momentum equations and the continuity Cartesian coordinates to natural rivers have equation are given in Eqns. (1) to (3). been presented by ASCE [1]. Also, Wijbenga [9], Wenka et al. [8], Ye and McCorquodale, +hv°u 'u o'u gun'(u' hu OU + 017, .i: o +E + +v') -0 ... (1) gh 2 OX OY OX p [eD 0X ,Y i)y' l hy, Weerakoon [6], Weerakoon et.al [7] presented applications in -curvilinear coordinates which hu +hv +gho17,_!!._[E o'v]+gvn'(u'+v') � � o'v +E 0 ... (2) are more suitable for applications to rivers. ax i)y i)y p ,. ox' ,,. i)y' hY, Objectives of this study are: ........... .......................... (3) • to calibrate and validate a depth-averaged computational model to the river reach of the Mahaweli River including the river bed Here, h = the water depth, z = the vertical water intake ofthe Udunuwara-Yatinuwara direction, zb = the bed elevation, z, = zb +h is the water supply scheme, which has been water surface elevation, g = the gravitational planned to abstract 4600 m3 / d, acceleration, fl,= the water surface elevation, p= the density of fluid, h = the local water depth, • predict flow pattern when the river bed level Eif= the eddy viscosity coefficient in direction j is changed, and l ' l ' /, and are the • to ascertain the changing of river bed level as on surface u=,;Judz v=,;Jvdz z• r• one of the potential solution to increase horizontal velocity in the x direction at a point diversion through the channel. along the vertical coordinate direction and the horizontal velocity in they direction at a point Accordingly, Surface Water Modelling System he vertical coordinate direction along t respectively. (weblink [11]) which is a two-dimensional hydrodynamic modelling system using finite• 2.2 Depth-averaged Model - Surface Water element analysis is set up to the river reach of Modelling System 450 m length. The validated computational model is then applied to obtain the velocity The Surface Water Modeling System (SMS), pattern and water depth under different bed which is a comprehensive environment for topography. hydrodynamic modelling includes an interface for the two-dimensional finite-element 2. Computational Model hydrodynamic model RMA2 (U.S. Army Corps of Engineers). The RMA2 hydrodynamic model 2.1 Governing equations is based on Eqns. (1) to (3) and iterative solution is computed for velocity and depth at discrete Equations that describe the flow in surface points in a river reach under given boundary water bodies are based on the classical concepts conditions. SMS mesh module is used to of conservation of mass and momentum. Depth• construct two-dimensional finite element mesh wise integration of the momentum equations of the river reach. The model parameters, and the with the continuity equation source/sink data, and boundary conditions are assumption that the vertical acceleration of assigned directly to the nodes, and elements of • ENGINEER 130 the mesh. With a mesh constructed, the RMA2 is the velocities at the grid points when the used to compute flow velocities and water- upstream discharge was 9.7 m3/s and urface elevations at each mesh node, and the abstraction rate from each filter was 0.027 m3 / s boundary between wet and dry regions in the (Case I). The boundary conditions for the model. SMS with RMA2 model was setup to computations were the upstream discharge and investigate the flow in a 450 m long reach of the the downstream water calibrated for the river Mahaweli River which includes the existing reach to agree the model predicted resultant river bed water intake of the Udunuwara• velocities with the depth-averaged values of the Yatinuwara water supply scheme. measured velocities across a river section. This was achieved by refining the Manning's coefficient and the 3. Model Calibration and Verification roughness (n) eddy viscosity () within the ranges typical to the given river reach comprising of highly rough river bed with 3.1 Field Measurements boulders and drops (Chow [2], Wijbenga [9]). The In order to calibrate and verify the model, computed results are found to be acceptable with stream flow measurements were carried out in the measured values for then value of 0.025 and 2 / the river reach using the survey instruments, eddy viscosity of 1.2 m s. Comparison of electromagnetic currentmeters fixed to the measured and computed velocities at the river wading rods, and depth poles while stationed sections A-A and C-C are shown in Figures 2 and 3. vectors for of the reach on a dingy/ boat by trained technical staff. Computed velocity part are shown in River morphology and the bed contours in 450 Figure 4. m long river reach which includes island and --+- observed -- computed the channel are shown in Fig.

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