Technical Studies

The morphology of Touchien and Fengshan Rivers’ estuary changed rapidly in the past years. In 2005, the Water Resources Planning Institute (WRPI) evaluated the hydraulic characteristics and the flood-prevention capability on Touchien and Fengshan Rivers’ estuary. In 2007, WRPI proposed six engineering plans to carry out the countermeasures, based on the short-term and long-term simulations using the CCHE2D-Coast model. In 2008, WRPI decided that Case 6 was the best plan, in which both the land utilization and flood-prevention capability of Touchien River estuary could be considered in the meantime. In 2009, the Second River Bureau carried out Case 6 and proposed the engineering detail design project.

Due to the complicated natures of flow and sediment transport pattern, the engineering layout of Touchien and Fengshan Rivers’ estuary needs some laboratory and numerical study to insure the optimum layout proposed by WRPI. In this study, CCHE2D model was used to simulate the estuarine morphological changes under design flow for the existing layout and several proposed layouts with diversion dikes having different angles and lengths. The upstream boundary conditions (discharge and sediment transport rate) and the downstream boundary conditions (tide stage), together with the estuarine topography are necessary for predicting the estuarine morphological changes.

According to the simulation results, the best layout can be obtained. In the existing layout with Q100 flood occurred, the discharges of left and right channels divided by the Jiugang Island were 6,864 cms and 1,536 cms, respectively. In the proposed layout, the associated discharges for the left and right channels could be 5,532 cms and 2,868 cms, respectively. The flood-prevention capability was increased due to the engineering layout was put into practice, and the water stage around the Jiugang Island had a decreasing trend. Meanwhile, the physical experiment results, including the water stage, discharges of left and right channels through Jiugang Island, bed changes, had the same trends with those of the numerical model. This engineering layout has been finalized based on the integration of physical and numerical models’ results in this year. In the future, more in-situ data obtained in the field is needed to improve the long-term stability of the estuary and the associated hydraulic structures.