Symposium on Infrastructure Development and the Environment 2006 7-8 December 2006, SEAMEO-INNOTECH University of the Philippines, Diliman, Quezon City, PHILIPPINES THREE-DIMENSIONAL HYDRODYNAMICS SIMULATION OF MANILA BAY 1Tanuspong POKAVANICH, 2Kazuo NADAOKA 1 Graduate Student, Dept of Mechanical and Environmental Informatics, Tokyo Institute of Technology 2 Professor, Dept of Mechanical and Environmental Informatics, Tokyo Institute of Technology Abstract: To reconstruct a complex circulation of Manila Bay, the study utilized a three- dimensional turbulent flow model. The model took into account effects from various external governing factors including tide, river discharge, solar radiation, and wind. The mathematical model was calibrated with field survey data obtained in 2001. The simulated bay current pattern showed a clear combination between barotropic flow driven by ocean tide and baroclinic flow from stratified system inside the bay. The temporal analysis showed strong linkage between bay’s hydrodynamics and semi-diurnal tidal component. The study conducted a lot of sensitivity tests to quantify the magnitude and how each governing factors contribute to the overall bay circulation. Results suggested that the strong stratification inside the bay was mainly created by freshwater loads from rivers. The wind stresses appeared to response for the near-surface freshwater distribution, discharged from the surrounded rivers. Key Words: Manila Bay, Hydrodynamics simulation, Three-dimensional model, Bay circulation, Combined effect flow 1. INTRODUCTION Manila Bay is a semi-enclosed bay located on the southwestern part of Luzon Island between latitude 14o15’- 14o85’ and longitude 120o35’- 121o00’ (Figure1). The bay width varies from 22km at its mouth to maximum of about 60km. Its length is about 53km with the average depth of 20m. Manila Bay is a very important water body of Philippines and extensively used for various purposes. The bay receives discharged water from numerous sources includes 26 river’s catchments (account for about 17,000 km2.), highly polluted domestic and industrial refused water drained from Metro Manila and Laguna De Bay. Although M. Prudente et al. (1997) inferred that fish of Manila Bay are still not adversely affected by heavy metal contamination (i.e. Hg, Pb, Cd), there are plenty of evidences on it already symptom from too much nutrition load which course euthophication problem. In the 90s, there used to be a regular occurrence of the planktonic bloom during the southwest monsoon (IMSWES workshop, 2000). Jacinto et al. (1998) noted that rough estimates of the discharge of inorganic nutrients into the bay is approximately 40x106 mol/yr of inorganic P and 600x106 mol/yr of inorganic N. With increasing population and still 1 ineffective waste management, values of waste loading rates are higher today. The coupled environmental pressure from increasing population and rapid developments of Metro Manila and the bay’s surrounding provinces urgently ask for a comprehensive study to better understand it’s hydrodynamic and water quality characteristics. Manila Bay owns a very narrow mouth so residence time of water inside the Bay is relatively longer. De Las Alas and Sodusta (1985) simulated the response of Manila Bay to the quasi-steady forcing by prevailing winds. They concluded that the individual average wind blow at specific period of the year controlled bay’s circulated gyres differently. There are Northeasterly winds, with speeds averaging about 5m/s from October to January (Figure2a); Southeasterly winds, with speeds ranging from 3 to 6m/s from February to May (Figure2b); and Southwesterly winds, with speeds of 5 to 7 m/s from June to September (Figure2c). Villanoy and Martin (1997) modeled the bay’s current from the combined effects of ocean tide and uniformed wind. They suggested a relative importance between tide and wind induced current to the overall bay circulation. Their tidal-driven 2- dimensional hydrodynamic indicated that the residual tidal velocities are strongest at the mouth where it enters the bay north of Corregidor and exits to the South. Moreover, their results of wind-driven circulation from Southwesterly wind (Figure2d), showed the existence of two asymmetrical counter-clockwise gyres similar to the works of De Las Alas and Sodusta (1985), except that the location of convergence a bit deviates to the West. The bay exhibits a strongest stratification during rainy season from the higher air temperature and outstanding discharge amount. In contrast, the bay shows a relatively uniform water column throughout the year (IMSWES workshop, 2000). The evidences on a temporal and spatial variation of its hydrodynamic governing parameters (e.g. salinity, temperature and wind) demonstrate a necessity of 3-dimensional modeling (IMSWES workshop, 2000; Tamura et al., 2003). The realistic simulation should, therefore, be able to reconstruct the bay’s circulation base on combined driven forces including a density driven. The present research aimed to study a complex circulation of Manila Bay by combining a tide-induced current, a density-driven current and a wind-driven current. The study conducted various numerical experiments to investigate the bay’s 3-dimensional hydrodynamic characteristics to quantify the roles of each current-induced factor. 2. SIMULATION MODEL 2.1 Governing Equations A well established three-dimensional turbulence flow model (Delft3D-Flow) from Delft Hydraulic-Netherlands, was used in this study. The governing equations written in orthogonal curvilinear co-ordinate (ξ,η) are as follows: Continuity equation ∂ζ 1 ∂((d +ζ )UG ) 1 ∂((d + ζ )VG ) + ηη + ξξ = Q (1) ∂t Gξξ Gηη ∂ξ Gξξ Gηη ∂η Equation of motions (in ξ -direction) 2 ∂u u ∂u v ∂u ω ∂u uv ∂ Gξξ v ∂ Gηη + + + + − − fv ∂t Gξξ ∂ξ Gηη ∂η d + ζ ∂σ Gξξ Gηη ∂η Gξξ Gηη ∂ξ 2 1 1 ∂ ∂u (2) = − Pξ + Fξ + 2 ν v ρ0 Gξξ (d + ζ ) ∂σ ∂σ Figure1. Manila Bay location, field survey points, Sangley meteorological station, simulated grid, locations of major ( ) and minor ( ) discharge point. Figure2. Wind driven circulation model of Manila Bay after De Las Alas and Sodusta 3 (1985;a-c), and Villanoy and Martin (1997;d). Large arrows indicate wind direction. (Picture from Fernando P. Siringan et al., 1998) 2 2 1 ∂ u 1 ∂ u Fξ =ν H + (3) G G ∂ξ 2 G G ∂η 2 ξξ ξξ ηη ηη 1 g ∂ζ d + ζ 0 ∂ρ ∂σ ∂ρ P = + g + dσ ′ (4) ξ ∫ ρ0 Gξξ Gξξ ∂ξ ρ0 Gξξ σ ∂ξ ∂ξ ∂σ f = 2Ωsinφ (5) Transport of matters (Sanility&Heat) ∂(d + ζ )c 1 ∂[ Gηη (d + ζ )uc] ∂[ Gξξ (d + ζ )vc] ∂ωc + + + + λd (d + ζ )c − S ∂t G G ∂ξ ∂η ∂σ ξξ ηη d + ζ ∂ Gηη ∂c ∂ Gξξ ∂c 1 ∂ ∂c = DH + DH + DV (6) G G ∂ξ G ∂ξ ∂η G ∂η d + ζ ∂σ ∂σ ξξ ηη ξξ ηη Equation of state (Eckart,1985) 1000× (5890 + 38t − 0.375t 2 + 3s) ρ = , (7) (1779.5 +11.25t − 0.0745t 2 − (3.80 + 0.01t)s) + 0.6980(5890 + 38t − 0.375t 2 + 3s) where u and v are the horizontal velocity, ω the vertical velocity, g the gravitational acceleration, Q the contribution per unit area from discharge and withdrawal of water, precipitation and evaporation, Pξ the pressure gradients, Fξ the unbalance of horizontal Raynold’s stresses, fv is the coriolis terms, ν H and ν V are the horizontal and vertical eddy viscosity, DH and DV are the horizontal and vertical eddy diffusivity, λd the first order decay process, S the source and sink terms per unit area due to the discharge, withdrawal of water and the exchange of heat through the free surface, ρ the water density, s the salinity, t temperature. The study assumed ν H and DH to be a constant. The ν V and DV are calculated from the second order k − ε turbulent closure model (Uittenbogaard et al., 1992). The boundary conditions (in ξ -direction) are written as follows. At the surface: ν H ∂u 1 r = τ s cos(θ ), (8) H ∂σ σ =0 ρo r 2 τ s = ρaCdU10 (9) At the bottom: ν H ∂u 1 = τ b , (10) H ∂σ σ =−1 ρo r r r gρoub ub τ b = 2 , (11) C3D 4 g ∆zb C3D = ln(1+15 ) (12) κ ks where τ s and τ b are the shear stress on the water surface and sea bottom, Cd the wind drag coefficient, H the total water depth, ρ a and ρ0 the air and water density, U10 the average wind speed at 10 meter above free surface, θ the angle between wind stress vector and local direction of the gird line, C3D the 3D Chezy coefficient, ks the Nikuradse roughness length. At the water surface, the heat exchange is modeled by taking into account the separate effects of solar (short wave) and atmospheric (long wave) radiation, and heat loss due to back radiation, evaporation and convection (Murakami et al., 1985). The bed stress formulation is related to the current just above the bed. There is no transported flux across the bottom. At the open and close boundaries, the salinity of seawater is assumed to be uniform. The water temperature was set to be stepwise that consisted by two different temperature layers, the warmer above (See table1). There is no flow through all close boundaries. 2.2 Model Setup and Calibration The simulation was carried out using a sigma coordinate system with horizontal orthogonal curvilinear grid. Table1 shows the computation conditions of the hydrodynamic model. Figure1 shows the horizontal grid system, the locations of river discharge surrounding the bay, and the field data stations. The meteorological concerned data (e.g. solar radiation, air temperature, relative humidity, wind) obtained from Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) (Figure1 and Figure7). The simulation continue for two weeks to cover a cycles of neap and spring tide, from 3rd -17th October, 2001.