HYDRODYNAMIC MODELLING AND ALGAL BLOOM TRACKING IN HONG KONG

HYDRODYNAMIC MODELLING AND ALGAL BLOOM TRACKING IN
HONG KONG’S COASTAL WATERS IN MARCH-APRIL 1998

Joseph H. W. LEE¹ and Bo QU²

¹ Department of Civil Engineering, The University of Hong Kong, Hong Kong, China.

Tel: (852) 2859-2672; Fax: (852) 2559-5337; E-mail: hreclhw@hku.hk

² Department of Civil Engineering, The University of Hong Kong, Hong Kong, China

Tel: (852) 2857-8470; Fax: (852) 2559-5337; E-mail: quba@hkucc.hku.hk

Abstract: Hong Kong lies on the coastal shelf of Southern China. Despite the frequent occurrence of red tides and associated fishkills over the past two decades, the relation between the water movement and algal blooms/red tides has not been studied. In this study, a calibrated three-dimensional hydrodynamic model, Delft3D, is used to study the tidal circulation in Hong Kong’s coastal waters. Particular emphasis is placed on understanding the possible cause of the April 1998 massive red tide, and to develop a predictive harmful algal bloom (HAB) tracking model.

Based on the tidal boundary conditions and the measured wind velocity during the period March – April 1998, the 3D flow field and surface drogue tracking for a release in different parts of the northeast coastal waters are computed. The results show that a bloom initiated in Mirs Bay would likely move along northeast to southwest direction and possibly be transported to the southeast coastal waters under the combined action of tidal current and wind. Computed drogue tracking patterns are generally consistent with the reported sequence of red tide events in Spring 1998. Extensive simulations show that the major cause of the bloom being transported into the East Lamma Channel (and causing severe fishkills) is the unusually strong wind in March 1998 coupled with the change of wind direction during almost diurnal tidal conditions at the beginning of April.

Keywords: red tide, algal bloom, drogue tracking, hydrodynamics, environmental hydraulics

1 INTRODUCTION

In sub-tropical coastal waters around Hong Kong and South China, algal blooms and red tides (due to the rapid growth of microscopic phytoplankton) are often observed (Hodgkiss 1991). Algal blooms often lead to discoloration of the marine water, which may lead to beach closures, severe dissolved oxygen depletion, fish kills, and shellfish poisoning. Over the past two decades, massive fish kills due to oxygen depletion have been observed in some of the marine fish culture zones in Hong Kong (Lee et al. 1991); a few toxic algal blooms have also been reported. In April 1998, a devastating red tide has resulted in the worst fish kill in Hong Kong’s history - it wiped out over 80 percent (3400 tonnes) of fish stocks, with estimated loss of more than HK$312 million (Dickman 1998). Whereas the general ecological response of phytoplankton to environmental conditions has been extensively studied, the causality and dynamics of algal blooms are extremely complicated and not well-understood. In particular, for the massive red tide of 1998, the press reports appear to suggest a migration of a red tide initiated in the northeastern waters of Hong Kong, in Mirs Bay and perhaps even Daya Bay, which drifted to the southern coastal waters by tidal currents. Alternatively the observations may also reflect merely a change of environmental conditions in the various bloom locations during March-April 1998. This paper reports a detailed study on the possible role of tidal hydrodynamics on the observed red tide event of 1998, and in particular the fishkill in the fishfarms adjacent to East Lamma Channel. A calibrated three-dimensional hydrodynamic model (Delft3D) is employed to study the tidal circulation during the dry and wet season. Extensive numerical drogue tracking experiments have been performed for a release in the northeastern waters of Hong Kong under a variety of hypothesized scenarios.

2 HYDRODYNAMIC MODELLING OF HONG KONG’S COASTAL WATERS

2.1 Pearl estuary model

The three-dimensional hydrodynamic model, Delft3D, is employed for this study. The model solves the shallow water equations with the usual hydrostatic approximation; a two-equation k- model is used for turbulent closure. Orthogonal curvilinear coordinates are used in plan while sigma-coordinates are used in the vertical direction. The equations are discretized on a staggered grid in the horizontal; an Alternating Direction Implicit (ADI) scheme is applied for time integration, while both central and upwind finite differences are used for spatial discretization (Delft Hydraulics 1997,1998). Fig.1 shows the Pearl River Estuary Model set up for studying the tidal circulation and water quality of Hong Kong’s coastal waters. A 152x121 boundary-fitted orthogonal grid is used, with ten uniformly distributed vertical layers. Tidal forcing is applied along the eastern, western, and southern boundaries; nine tidal constituents are used to generate a time history of predicted water levels at the open boundaries; measured salinities for both the dry and wet season are used. Seasonal mean freshwater flows from the eight outlets of the Pearl River are used for the inflow boundaries. A quadratic wind stress formulation is adopted; the wind drag coefficient is assumed to vary linearly with wind speed. The model has been calibrated and validated against extensive tidal level, velocity, and salinity measurements (Delft Hydraulics 1998).

2.2 Red tide tracking model

For the present study, the aim is to study the relation between tidal circulation and the reported sequence of events from March 19 to mid-April 1998 (Fig.2). The Pearl Estuary grid is used, with the computations performed using two, four, and ten layers. Extensive drogue tracking experiments are performed both for the typical dry and wet season, and also using the actual wind data during March-April 1998. The grid size varies from 70m to 7 km; a Courant number in the range of 0.24 to 1.8 is used. The model accuracy has been studied by comparing numerical results with analytic solutions for long wave propagation in a rectangular and an annular channel with either a constant depth or quadratic bottom bathymetry. For cases in which the linearized analytical model is valid, the computed velocities are in very good agreement with the analytical solutions.

In view of the extensive drogue tracking simulations that need to be made on the entire Pearl Estuary grid, the number of vertical layers has been suitably reduced. Computations have been made with a simplified 4-layer model (with 0.1:0.2:0.3:0.4 vertical distribution) and 2-layer model (0.5:0.5), and compared with results of the original 10-layer model. The results show that as far as algal bloom tracking is concerned, the two-layer model is efficient and adequate. The effect of the coarse grid in Mirs Bay has also been studied. Based on the above validation results, an intensive study in drogue tracking for 1998 March to April is undertaken using a 4- or 2-layer Pearl Estuary grid, with a Courant number of around 0.24. The differences in the hydrodynamic conditions between a typical dry season around the onset of Spring and during March-April 1998 are studied.

3 NUMERICAL MODEL RESULTS

3.1 Tidal current and salinity in hong kong waters

The hydrography of Hong Kong’s coastal waters is mainly influenced by three factors: tidal currents, monsoon-affected ocean currents, and Pearl River discharges. In the dry season, the salinity is approximately vertically homogenous and ranges from about 34 ppt at the open boundary to 15-20 ppt at the mouth of the estuary near Shenzhen (Fig.3a). In general, tidal currents flow from the northeast through Victoria Harbour and northwest waters towards the Pearl River Estuary during flood, and from W/NW to W/SW during ebb (Fig.3b). In the southern waters, the current flows from NE to SW. In the wet season, the flow (not shown) is mainly from west to east. The flow pattern is heavily influenced by the Pearl River and much more complicated, with significant vertical salinity/density stratification, and horizontal salinity gradients. This study mainly focuses on the effect of the tidal and wind on the 1998 red tide which occurred towards the end of the dry season.

3.2 Algal bloom tracking in spring 1998

The 1998 red tide was truly an unusual event compared with all past blooms in Hong Kong waters. It is recorded that 30 red tide incidents appeared in the first 6 months of 1998 with the most serious events during the period 10 March to 17 April (Anderson 1998). Most of Hong Kong’s mariculture farms were affected by a red tide that appeared first in the northeast Kat O area, then down south through Port Shelter, then westwards to Lamma Island (Fig.2). In order to unravel what really happened in Spring 1998, drogue tracking is performed for a release from different locations in the northeast waters, in particular Kat O and northeast Mirs Bay, and at different stages of the tidal cycle (Fig.4). The measured daily prevailing wind at Waglan Island in March and April 1998 is used as input (Fig.5a); it can be noted the wind speed in March 1998 is quite strong, even above 10m/s in the beginning of March and on 22^nd March. The tidal condition during early April is also shown in Fig.5b.

Based on detailed study of the numerical results, the following observations can be made. In general, the average measured wind direction in Hong Kong is (northeast direction) and wind speed is 5 m/s; these typical values are used for the dry season. Surprisingly, a bloom initiated in most of north or middle/midwest parts of Mirs Bay will not likely travel down south. Due to the relatively low wind velocity, the topography, and the direction of the tidal currents, the drogue would drift down south rather slowly. For a stronger NE wind speed (10 m/s), it is more likely that the drogue would flow from north Mirs Bay to the southern waters. However, it is not possible to drive the flow pass through the East Lamma Channel. For dry season in general, it usually takes about 12-27 days from the northern to southern part of Mirs Bay, and then about 2-5 days to Lamma Island.

On the other hand, for the actual wind conditions in March-April, and a drogue release from neap tide (on 20^th March), spring tide (on 27^th March) and in between (on 23^rd March), the drogue would most possibly move from most parts of Mirs Bay, Tolo Harbour and even from Daya Bay down southwest direction to Lamma Island or Victoria Harbour (Fig.4). A bloom located in northeast and southwest of Mirs Bay would eventually arrive at the East Lamma Channel in 12 days. This means the massive fishkill in Lo Dick Wan Fish Culture Zone, which occurred around April 7, can be caused by a red tide initiated in northeast Mirs Bay or Kat O around March 20-27. The travel time required from northern end to entrance of Mirs Bay differs significantly for neap tide (9-27 days) and spring tide (6-15 days) conditions. The computed drogue tracks are generally consistent with the sequence of red tide events as reported by the press.

3.3 Role of hydrodynamic transport

The unusually strong wind in March 1998 and early April, and the change in wind direction from NE to SE around 1^st April (Fig.5a) appear to play a major role in the massive red tide in April 1998. Fig.6 shows drogue tracks for two cases: a) when the wind speed is reduced to 5 m/s while keeping the wind direction unchanged in March-April 1998. The drogue cannot possibly enter the East Lamma Channel; drogues released either from Tolo Harbour or the middle and west part of Mirs Bay would be grounded (encounter the shoreline) rather than travelling further down south; similar situation applies for the dry season in general. b) On the other hand, if we use the actual wind speeds in the wind forcing, but imposing a constant NE direction after 1^st April, the red tide would likely be transported to the south of Lamma Island without passing into the East Lamma Channel (Fig.6b).

Fig.7a shows the computed surface current field in East Lamma Channel. Based on the drogue tracking patterns for different release time and location, the area where an initiated bloom could possibly be transported to the East Lamma Channel is indicated in gray. It seems highly unlikely that the fishfarms in the east Lamma area would be affected by a bloom originating outside of this “strip of influence”. The interesting water movement in Spring 1998 appears to be related to a combination of the change in wind direction, strong wind, and the almost diurnal tides occurring in early April. Computed velocities in the area show that phytoplankton at the surface can be transported towards the NW direction along the East Lamma Channel during a flood tide for an extended period of almost 10 hours. Compared to the normal dry season, the surface current near Lamma Island (Fig.7b) has a much increased current speed towards the NW direction (more negative velocities) up the East Lamma Channel.

4 CONCLUDING REMARKS

Extensive numerical drogue tracking experiments using a calibrated 3D hydrodynamic model have been performed to study the role of hydrodynamics in the massive red tide and fishkill in Hong Kong waters in March-April 1998. The results suggest that a red tide which originated in Kat O or Mirs Bay in the Spring can possibly be transported to the southern coastal waters of Hong Kong. In contrast, red tides originated in these locations in the summer (wet season) would likely be transported away from Hong Kong (Fig.8a). The massive red tide and fishkill in east Lamma Island is mainly caused by the change of wind direction, strong wind, and strongly diurnal tidal conditions in early April. Extensive studies performed using 10-layer and 4-layer models, as well as a fine grid in Mirs Bay (Fig.8b), have shown the conclusions are generally valid. While useful insights have been gained, many important factors such as turbulent diffusion, algal growth and sinking, and vertical migration need to be studied in future work.

Acknowledgements

This study was supported by a Hong Kong Research Grants Council Central Allocation Group Research Project. The assistance of Dr..Anthony Lee of the Hong Kong Environmental Protection Department in Delft3D and the Pearl Estuary Model is deeply appreciated. Related discussions with the Hong Kong Agriculture, Fisheries and Conservation Department have been most helpful.

References

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[2] Literature review and Background Information, Technical Report No.1, Agriculture and Fisheries Department, Hong Kong Government.

[3] Delft Hydraulics (1997). Upgrading of the Water Quality and Hydraulic Mathematical Models: Draft Model Validation Report, Technical Services Division, Civil Engineering Office, Hong Kong Government.

[4] Delft Hydraulics (1998). Upgrading of the Water Quality and Hydraulic Mathematical Models: Final Model Calibration and Validation Report, Technical Services Division, Civil Engineering Office, Hong Kong Government.

[5] Dickman, M.D. (1998). Hong Kong’s worst red tide, Proc. Int. Symp. Env. Hydraulics, (Ed. Lee J.H.W. et al.), Balkema, pp.641-645.

[6] Hodgkiss, I.J. and Ho, K.C. (1991). Red tides in sub-tropical waters: an overview of their occurrence, Asian marine biology, 8, 5-23.

[7] Hong Kong Observatory (1999) Summary of Meteorological Observations in Hong Kong (1998).

[8] Lee,J.H.W., Wu, R.S.S., and Cheung, Y.K. (1991). Forecasting of dissolved oxygen in marine fish culture zone. J. of Env. Engr. ASCE, 117, 816-833.