Determination of Flow Characteristics of Ohashi River through 3-D Hydrodynamic Model under Simplified and Detailed Bathymetric Conditions

The Ohashi River is a narrow water stream that connects two brackish lakes in Japan. Intermittent saline water intrusion often occurs in Lake Shinji from Lake Nakaumi through Ohashi River. In this study, two approaches were discussed to reproduce the hydrodynamic conditions of a morphologically complex river. In the first approach, the river sinuosity was straightened. The straightening of the river resulted in a higher flow velocity and water flux coefficient due to the reduction in the flow path and the resistance, and this approach was found to be appropriate for the reproduction of the flow velocity. However, the river shape was visually quite different from the actual river morphology. In the second approach, the prime focus was given to the shape and bathymetry to quantitively reproduce the flowrate of the saline water intrusion. This approach resulted in an underestimation of the flow velocity, which was compensated by increasing the cross-sectional area of the river. A slower flow velocity causes up to a 3-h time lag for the water mass to pass through the Ohashi River, which in principle should affect the temporal variations of the water temperature and salinity. Fortunately, as the typical time scale for water temperature and salinity fluctuations in the Ohashi River is a few days, a 3-h time lag did not cause any problems.

Responses of Hydrodynamics and Saline Water Intrusion to Typhoon Fongwong in the North Branch of the Yangtze River Estuary

The typhoon impact on an estuarine environment is complex and systematic. A three-dimensional hydrodynamic and salinity transport model with a high-resolution, unstructured mesh and a spatially varying bottom roughness, is applied to investigate the effects of a historical typhoon, Fongwong, which affected Shanghai, on the hydrodynamics and saline water intrusion in the North Branch (NB) of the Yangtze River Estuary (YRE). The model is well validated through observation data of the tidal level, current velocity and direction, and salinity. The numerical results of this typhoon event show that: (1) the tidal level and its range increase toward the upstream part of the NB due to the combined effects of the funnel-shaped plane geometry of the NB and the typhoon; (2) the current velocity and the flow spilt ratio of the NB varies with the tides, with a maximum increase by 0.13 m/s and 26.61% during the flood tide and a maximum decrease by 0.12 m/s and 83.33% during the ebb tide, i.e., the typhoon enhances the flood current and weakens the ebb current; (3) the salinity value increases in the NB to a maximum of 1.40 psu and water is well-mixed in the vertical direction in the typhoon’s stable and falling period. The salinity distribution gradually recovered to the normal salt wedge pattern in 3 days following the typhoon. Although this study is based on a site-specific model, the findings will provide valuable insights into saline water intrusion under typhoon events, and thus assist in implementing more efficient estuarine management strategies for drinking water safety.

Penaeid shrimp (Penaeus monodon) culture in Bangladesh.

The shrimp (Penaeus monodon), locally known as bagda, is known as white gold for its high economic value in Bangladesh. The shrimp sector of Bangladesh is seen as having become important in economic terms, contributing significantly to foreign exchange earnings and employment generation in rural areas. P. monodon culture is practiced in the Bagerhat, Chokoria, Cox's Bazar, Khulna, Moheshkhali, Shatkhira and Teknaf regions and recently has been introduced in some coastal areas of Noakhali district. Three culture systems are found in Bangladesh, i.e. traditional, improved traditional and semi-intensive with production rates of 150-200, 250-400 and 2,000-2500 kg ha-1, respectively. Seed is collected from natural sources and is produced in hatcheries. There are 44 penaeid shrimp hatcheries in Bangladesh. The present shrimp fry production is 3.05 billion. More than 200,000 people are engaged in shrimp fry collection and marketing and more than 350,000 people are engaged in shrimp farming. Ancillary industries like shrimp hatcheries, feed industries, ice factories, processing factories, net industries and shrimp culture apparatus industries provide direct employment for 25,000-30,000 people. Adverse environmental impacts reported are reduction of grazing land and saline water intrusion due to mismanagement but these could be overcome by introducing environmentally friendly shrimp culture. As the climate of Bangladesh is favourable for P. monodon culture, it is necessary to give more attention to improve it as it will open a new horizon in the economy of the country.