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Numerical Computation of the Transient Salinity Distribution in a Macro-Tidal Estuary

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Numerical Computation of the Transient Salinity Distribution in a Macro-Tidal Estuary 666 Journal of Marine Science and Technology, Vol. 25, No. 6, pp. 666-670 (2017) DOI: 10.6119/JMST-017-1226-06 NUMERICAL COMPUTATION OF THE TRANSIENT SALINITY DISTRIBUTION IN A MACRO-TIDAL ESTUARY Seung Oh Lee1 and Chang Geun Song2 Key words: macro-tidal estuary, flow reversal, salinity distribution, and man-made structures such as bridges and submerged weirs. hydrodynamic model, advection-dispersion model, saline In addition, Pal-dang Dam and tidal water level are the two main length. factors controlling the flow behavior of water discharge. The analysis of salinity distributions in estuaries is a signi- ABSTRACT ficant issue in both marine science and estuary engineering. This is because salinity in water body exerts various influences on In this study, the flow reversal and salinity distribution asso- water movement, water quality and ecology. In hydrodynamic ciated with tidal effects were investigated at a macro-tidal es- aspect, salinity changes the density of water and the movement tuary located in tidal range of the Yellow Sea in South Korea. of water body. It affects the quality of drinking water and the A 2-D hydrodynamic model (RMA-2) and a 2D advection- production of crops. Regarding ecosystem concerns, it is closely dispersion model (RAM4) were employed to simulate flow be- associated with the ecological health, landscape, and biodiver- havior and salinity distribution, respectively. The results showed sity. Consequently, the analysis of salinity distribution is a signi- that reverse flow occurred five times over 3-day period and the ficant issue in terms of marine science and estuary engineering. maximum reverse flow extended over 32.9 km upstream. Sali- In this study, bathymetry of the lower Han River was produced nity distribution was obtained by estimating the salinity input based on the digital nautical charts. Salinity at the downstream with analytical solution of advection-dispersion equation. The boundary of the Han River was obtained by applying the ana- total length of the saline wedge calculated from numerical si- lytical solution of a one-dimensional (1-D) advection-dispersion mulation was in agreement with that obtained from an empirical equation to the measurements obtained at Incheon tide gauge formula. station. RMA-2, a two-dimensional (2-D) hydrodynamic model was used to analyse reverse flow under tidal effects in the Han I. INTRODUCTION River Estuary. The velocity and water depth outputs from RMA-2 The tidal range in the Yellow Sea is about 9 m, and this wide were used as inputs into RAM4, a 2-D advection-dispersion finite range enables the water from the Yellow Sea to intrude into the element model, to analyse the characteristics of the salinity dis- main channel of the Han River in South Korea. Consequently, tribution. fluvial geomorphology and the brackish ecosystem are well preserved in this region. According to the survey of the National II. DESCRIPTION OF NUMERICAL MODELS Institute of Environmental Research (2005), many endangered RMA-2 is a two-dimensional depth-averaged finite element species such as Platalea minor, Haliaeetus albicilla, Anser hydrodynamic model (Norton et al., 1973). It computes water fabalis and Rana plancyi chosenica Okada inhabit the estuary surface elevation and horizontal velocity components for sub- of Han River. Hydraulic analysis of the downstream of Han critical flow. It was originally developed by Norton, King and River is very important since it supplies the industrial and drink- Orlob of Water Resources Engineers in 1973 for the Walla Walla ing water to the citizens in Seoul and Gyeonggi Province. Ac- District of the U.S. Army Corps of Engineering. The governing cordingly, the Ministry of Environment of Korea declared the equations consist of one continuity and two momentum equa- estuary of Han River marsh to be protection zone in April, 2006. tions obtained by depth-averaging Navier-Stokes equations. The flow characteristic of the estuary of Han River is very com- RAM4 is a depth-averaged advection-dispersion model (Seo plex since there are many tributaries, including Im-jin River, et al., 2008). It can predict and estimate the transport of pollutant that has been injected continuously or instantaneously. The go- Paper submitted 06/14/17; revised 08/22/17; accepted 10/30/17. Author for verning equation of this model is as follows: correspondence: Chang Geun Song (e-mail: [email protected]). 1 ()hC School of Urban and Civil Engineering, Hongik University, Seoul, Korea. qhC D hC khCQxyt,, 0 (1) 2 c Department of Safety Engineering, Incheon National University, Incheon, Korea. t S. O. Lee and C. G. Song: Transient Salinity Distribution in Tidal Estuary 667 Im-jin River Q: 218 cm s 4.0 1.7 -0.6 -2.9 Yu-do -5.2 Transient tide level -7.5 Gok-reung Stream -9.8 Q: 1.83 cm s BOD: 8.9 ppm -12.1 -14.4 Shin-gok Gul-po Stream Subm erged weir Q: 1.07 cm s Transient discharge BOD: 11.4 ppm Fig. 1. Bottom elevation contour (left) and input conditions (right). where h is depth, C is depth-averaged concentration, {q} is depth- bank increases; and finally, below the confluence with the Gok- averaged velocity vector, k is first-order decay coefficient. reung Stream, the entire channel deepens. Q(x, y ,t) = M(x xs, y ys) (t) is source term to handle Input parameters used in the numerical simulations were pollutant injected instantaneously at the location (xs, ys). The determined from existing literature. We adopted the roughness dispersion coefficients [Dc] in the above equation is defined in coefficient of 0.025 as suggested by the Ministry of Maritime Seo et al. (2008). The horizontal 2-D finite element model is Affairs and Fisheries (2001); this value was proposed for the constructed by Streamline-Upwind/Petrov-Galerkin (SU/PG) Shin-gok-Wal-got section in the case of discharge below 2000 scheme. Crank-Nicolson finite difference is used as the temporal m3/s. Since the study area is a restricted military area near the discretization method. Numerical integration is carried out by Military Demarcation Line, it is impossible to obtain values for Gauss quadrature in natural coordinates. Boundary conditions, the dispersion coefficients from field measurements. Therefore, such as instantaneous mass injection or concentration injections we obtained estimates using the empirical formula of Elder and that are steady, pulsed or unsteady, can be assigned. More de- Fischer, which is accurate when applied to the estimation of tailed descriptions on the model performance and applicability longitudinal dispersion coefficients in 2-D advection-dispersion of RAM4 are available from (Lee and Seo, 2010; Park et al., models (Fischer et al., 1979; Seo and Cheong, 1998). Since we 2016). were simulating a meandering channel where secondary flow was dominant, we adopted the coefficient of 0.6 in the compu- tation of transverse dispersion. The estimated longitudinal (D ) III. NUMERICAL SIMULATIONS L and transverse (DT) dispersion coefficients were 7.47 and 0.75 2 1. Study Reach and Input Conditions m /s, respectively. Following Seo and Song (2007) and Seo et al. (2009), the Biochemical Oxygen Demand (BOD) decay From its confluence with the Gok-reung Stream, the Han River coefficient was chosen to be 0.1/day. flows 13.7 km northwestward to the sea. However, owing to A numerical simulation was carried out for a 3-day period, this area's proximity to the Military Demarcation Line, survey from 23 to 25 June 2006. This period was chosen because of fa- data, both along and across the river, are unavailable for this vourable hydrologic conditions, including large tidal ranges section of the river. Therefore, we constructed bathymetry of measured at Incheon tide gauge station (Fig. 2). The flow rate this area with the aid of digital nautical charts. Our study area from the Im-jin River into the main channel of the Han River spans from the Shin-gok submerged weir (representing the up- was assumed to be 218 m3/s, which was the average value for stream boundary of the Han River in the finite element models) this period. to Yu-do (representing the downstream boundary). It includes the Im-jin River and 36.8 km of the Han River. The finite ele- ment mesh of the study area contained 16,066 nodes and 5,920 2. Salinity Distribution elements. Fig. 1 shows the bathymetry, mesh layout and input Salinity distribution in the lower Han River is determined parameters used in the numerical simulations. From the Shin- by freshwater inflow discharge and tidal range. The Han River gok submerged weir downstream to the first meandering region, Development Project took place from 1982 to 1986. Since its water depth on the right bank is greater than that on the left bank; completion, water quality observations carried out for periods but, downstream from the first meander, water depth on the left of more than 13 hours regularly indicate the absence of salinity 668 Journal of Marine Science and Technology, Vol. 25, No. 6 (2017) 2500 Discharge of Shin-gok submerged weir Tide level of Yu-do 2.6 2000 1.8 1.0 Tide level (m) 1500 0.2 1000 -0.6 Discharge (cms) -1.4 500 -2.2 0 -3.0 0 6 12 18 24 30 36 42 48 54 60 66 72 hour 6/23 6/24 6/25 day Fig. 2. Boundary conditions. Velocity mag. (m/s) Velocity mag. (m/s) 1.00 1.00 0.90 0.90 0.80 0.80 0.70 0.70 480000 0.60 480000 0.60 0.50 0.50 0.40 0.40 0.30 0.30 0.20 0.20 0.10 0.10 0.00 0.00 Salinity (PSU) 475000 Salinity (PSU) 475000 11.10 11.10 9.99 9.99 8.88 8.88 7.77 7.77 6.66 6.66 Y 470000 Y 470000 5.55 5.55 4.44 4.44 3.33 3.33 Water depth (m) Water depth (m) 2.22 12.0 2.22 12.0 1.11 10.8 10.8 1.11 465000 0.00 9.6 465000 9.6 8.4 0.00 8.4 7.2 7.2 6.0 6.0 4.8 4.8 3.6 3.6 2.4 2.4 1.2 1.2 460000 0.0 460000 0.0 170000 48000 170000 48000 X X (a) t = 46.25 hr (b) t = 51 hr Velocity mag.
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