Numerical Study of Hydrodynamic Effect of Oyster Cultivation in Shenzhen Bay, China

Abstract:

Shenzhen Bay lies on the southwestern corner of Shenzhen, geographically in the eastern side of the Pearl River Estuary in South China(Fig. a). Water environment of Shenzhen Bay is facing serious ecological risks from multiple pressures coming along with the development of Shenzhen City. About one-fourth of the bay is occupied by oyster cultivation, which affected both the ecologic and hydrodynamic characteristics of the bay.

In the cultivation area, the oyster strings hung on the floating rafts were conceptualized as rigid cylinders. We parameterized the resistance losses of the oyster strings according to the number and dimension of oyster strings in each grid in Delft3D hydrodynamic model. The significant parameter energy loss was defined as the difference of the energy without and with oyster strings. Total energy loss (Eloss), including kinetic energy loss (Ekloss) and potential energy loss (Eploss), was examined using empirical orthogonal function (EOF) analysis. In addition, hourly data of 10 typical stations were selected to make a comparison during ebb and flood periods in June.

The main conclusions of this paper are as follows:

• The first mode of the Eloss explained 88.96% of the spatial Eloss variability (Fig. b). Positive value represents the decrease of the total energy due to the oyster cultivation in Shenzhen Bay. Furthermore, a lot of energy was dissipated at the entrance of the cultivation area, then the energy loss decreased gradually along the bay.
• The first mode of the Ekloss explained 81% of the spatial distribution of kinetic energy loss. Positive value of the Ekloss means kinetic energy decrease. As shown in Fig. c, the kinetic energy decreased greatly in the cultivation area, while it increased in parts of the passageway outside the cultivation area. The results indicated that the cultivation lowered the flow in the cultivation area.
• Potential energy loss fluctuated periodically with the tidal wave, and varied in proportion to the flow velocity (Fig. d&e). Eploss increased during the flood tide and decreased during the ebb tide, which implied that the flood tide was blocked into the Bay and the ebb tide was blocked out of the Bay.

To sum up, the main hydrodynamic characteristic of oyster cultivation was kinetic energy decreasing. Mean Ekloss accounts for 6.89% of the mean kinetic energy for oyster cultivation.