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Verification of a Numerical Model USING Field Monitoring Data fOR modelling Bohai Bay
Dekui Yuan(1), Jianhua Tao(1),
Binliang Lin(2), Roger A. Falconer(2)
Tianjin University, Tianjin, China
Cardiff University, Cardiff, UK
Address to which correspondence should be sent:
Binliang Lin, Division of Civil Engineering, Cardiff University, Queen's Buildings, PO Box 686, Cardiff CF2 3TB, UK
Telephone: +44 1222 874696, Fax: +44 1222 874597, Email: linbl@carsiff.ac.uk
Bohai Bay is a shallow semi-enclosed coastal basin located along the north-east coast of China. It covers a large area of about 7000 km2, with the depth of water being less than 10 metres for most part of the basin. Bohai Bay receives industrial and domestic wastewater from several large cities but its capability of tidal flushing is relatively weak. Several detailed studies including field measurement work and numerical modelling study have been undertaken to assess the water quality of Bohai Bay during the last few years. This paper presents some of the prediction results from the numerical modelling study.
Keywords: hydrodynamics, water quality, numerical model, field measurement, calibration, shallow water
Bohai Bay is a large semi-enclosed coastal basin located within the Bohai Sea along the north-east coast of China. It receives industrial and domestic wastewater discharges from a number of large cities, including the Capital Beijing and China's third largest city Tianjin. In recent years China's economy has developed rapidly and there has been a corresponding increase in industrial and domestic developments around Bohai Bay. The area around Bohai Bay is becoming north China's economic centre. However, Bohai Bay is a very shallow water basin, with the depth of water being generally less than 10 metres. The tidal mixing and exchange between Bohai Bay and Bohai Sea are weak, thus the hydro-environmental and ecological management issues of this coastal region is of a growing concern. Several detailed studies including field measurement work and numerical modelling study have been undertaken during the last few years to assess the water quality of Bohai Bay.
In order to assess the water quality and in the meantime to provide quality field data for calibrating the numerical models for Bohai Bay, a large scale field measurement program was carried out in August 1993. During this program flow velocity and water quality indicators were measured at nine sites and three tracking stations. These measurements were undertaken continuously for 72 hours and more than 20,000 data records were obtained.
A parallel numerical modelling study has also been undertaken to predict the flow and water quality of Bobai Bay. A two-dimensional depth integrated model has been used for this study with the field data measured being used to calibrate the model. This paper presents some of the prediction results from this numerical modelling study.
A water quality monitoring programme is managed by the Tianjin Environment Monitoring Centre which includes regular measuring of water quality constituents for 28 waste water discharging points along the coastline of Bohai Bay and 22 measurement stations within the Bohai Bay. Field observations are undertaken in May, August and October each year to monitor water quality in wet, average and dry seasons.
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In order to investigate the tidal flushing capability of Bohai Bay and to provide data for verifying numerical models for further investigation, a large scale continuous field measurement operation was undertaken from 1 August to 4 August, 1993. More than 30 scientists from the Bureau of Tianjin Environment Protection, Tianjin University, Beijing University and Qingdao Ocean University took part in this operation. Seven ships were deployed and measurements were undertaken at 9 current measurement locations and 3 tracking stations. The measurement operation was lasted for 72 hours and more than 20,000 data of velocity and water quality indicators were obtained. This is the most successful field measurement program that has been ever been carried out in Bohai Bay. Figure 1 shows some of measurement locations from which field data measured are to be used in this paper. |
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Fig.1 Locations of Field Measurement |
For shallow water flow such as Bohai Bay, the variation in the vertical plan is insignificant, it is acceptable to use depth-integrated two-dimensional models. The governing equations are as following[1]:
continuity equation:
(1)
momentum equation:
(2)
(3)
where x, y = co-ordinates; g = gravitational acceleration; 
, 
, U and V = depth average velocity components in the x and y
direction respectively; 
= source discharge
per unit horizontal area; b = momentum correction factor for a non-uniform vertical
velocity profile; f = Coriolis parameter due to the Earth's rotation; h = water
depth below datum; 
= total water depth; r = the
density of fluid; e
= depth averaged turbulent eddy viscosity; 
= tsurface wind shear
stress components in the x and y direction respectively; and 
= bed shear stress
components in the x, y direction.
For a horizontal or quasi-horizontal flow, the three-dimensional solute mass balance equation can be integrated over the water depth to give the following two-dimensional depth integrated advective-diffusion equation[2]:
(4)
where 
= depth averaged
solute concentration; 
,
,
,
= depth averaged dispersion-diffusion coefficients in 
directions respectively; 
= decay term; 
= concentration of
source flux from outfalls. The dispersion-diffusion coefficients were given by
Preston[3].
Water levels at locations W1 and W2 (see Figure 1) were used to specify the sea open boundary condition, with the values of water level at each grid point being inter- or extrapolated from these two points. There are two main rivers connected to the Bohai Bay, i.e, Haihe River and Beitang River. At these two river boundaries, discharges were described.
Solute concentration values at the sea open boundary and the two main river boundaries were described using field measurement data.
In this paper we will only concentrate on modelling COD. In order to determine the bio-chemical degradation coefficient, laboratory studies of decay rate for COD have been undertaken by using the water collected from Bohai Bay. Since the decay coefficient K varies with temperature, these experiments were undertaken for different temperatures. After a detailed statistical analysis, the following formula has been established,
(5)
where K is the decay coefficient for COD, C0 is the initial concentration of COD, t is the time, and:
(6)
where T is the absolute temperature.
The modelling area was represented with a mesh of 71´36 cells, equally spaced at 2000m intervals. Numerous runs have been undertaken to simulate water level, velocity and various water quality indicators within the Bohai Bay. In the following computer predicted velocities and COD concentrations will be compared with the field data.
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Fig.2 Comparison of the velocity at station E14 |
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Fig.3 Comparison of the velocity
at station F3 |
Figures 2 and 3 show comparisons of
the predicted and measured velocity magnitudes at stations E14 and F3 (see
Figure 1). It can be seen that the
computational results are very close to the filed data with the relative error
being generally less than 20%.
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Fig.4 Variation of the
concentration of COD at station F3 |
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Fig.5 Comparison of the
mean concentration of COD at station E14, F1, F2 and F3 |
Figure 4 shows the predicted COD concentrations at different stages of a tidal cycle and its mean value at station F3. Figure 5 shows a comparison of the predicted and measured mean COD concentrations at 4 locations, i.e, station E14, F1, F2 and F3. It can be seen that the numerical results are very close to the field measured data.
Numerical model has been used to predict flow and water quality for Bohai Bay. The model was verified by the field measured data, and the formulation of the decay of COD was successfully introduced into this model. It is reasonable to simulate the flow and water quality in the coastal and estuarine water in which the vertical variations are insignificant.
Falconer R. A. (1993). An Introduction to Nearly-Horizontal Flows, Coastal, Estuarial and Harbour Engineer's Reference Book, Eds. M. B. Abbott and W. A. Price, Chapman & Hall, London, England, pp. 27-36.
Falconer R. A. (1986). A Two-Dimensional Mathematical Model Study of the Nitrate Levels in an Inland natural Basin, Proceedings of the International Conference on Water Quality Modelling in the Inland Natural Environment, Bournemouth, England, pp. 325-344.
Preston R. W. (1985). The Representation of Dispersion in Two-Dimensional Shallow Water Flow, Central Electricity Research Laboratories, Leatherhead, England, Report No. TPRD/L/278333/N84, May, pp. 13.
Chen, Y. and Falconer, R.A. (1992). A Quick BASIC Graphics Software for Two-Dimensional Hydrodynamic Modelling. Hydraulic Engineering Software IV: Computer Techniques and Applications, eds. Blain, W.R. and Cabrera, E., Computational Mechanics Publications, Southampton, 433-444.
