Technical Studies


Impact Assessment Of Sediment Transport At Tansui Estuary On Surrounding Coast (2/2)

Tamsui river is the largest river in northern Taiwan. Spatio-temporal variation of morphology in the Tamsui River estuary and its adjacent coast is highly dynamic and therefore complex. Primary reason of the variations is the complicity of the hydrodynamic and morphodynamic processes in the area. Morphological changes often occur in a tropical storm or typhoon. Sediment releases from reservoirs at upstream can also contribute to the morphological changes in the estuary.

To better understand hydrodynamic and morphodynamic processes in the estuary driven by river flood flows, waves, tides, and winds, an integrated coastal process model, CCHE2D-Coast, is used to simulate flows and morphological changes due to multiple typhoons ( Aere(2004), Fong Wong(2008), and Saola(2012) ) and monsoon.

The river flood flows during a typhoon convey sediments to the estuary. Sediments deposit in the Danshui River Estuary. The deposition can raise the bed levels near the entrance of the Tamsui Fishing Port. The deposition will increase up to 2.8m. It may hinder the navigation to the fishing port. Inside the port, a relatively small amount of bed change occurs. The results show that erosions occur at the west coast between the Taipei Harbor and the Lin-Kou Fishing Harbor. Certain depositions occur at the lee side of the newly-planned breakwater in the Lin-Kou Port. Shoreline erosions may happen to the further west coast of the Lin-Kou Port. And the results in the west coasts including the Taipei Harbor and the Lin-Kou Fishing Ports show that during the short-term typhoon events the river flood and sediment releases from the river won’t influence very much the bed changes in the west coasts.

In the monsoon (2009-2010), various sedimentation processes ended up with complex patterns of erosion and deposition.During northeast monsoon, river flows are lower than those in summer; tidal flows dominate over the Danshui River Estuary. Meanwhile, longshore currents become stronger along the coasts due to high waves in winter. In the north of the Tamsui Fishing Port, because of the combined effect of tidal currents and wave-driven longshore currents, the southwestward longshore sediment transport tend to be much greater than other coasts. This alongshore sediment transport helps the river mouth sand bars growing in winter. Multiple circulations are formed and maintained in the river mouth and around coastal structures. Those circulations trap sediments from adjacent coasts and the estuary to alter the local morphology.

The long-term bed changes can identify several coastlines being eroded: (1) the west coast between the Taipei Harbor and the Lin-Kou Port; (2) the west coast of the Lin-Kou Port; and (3) the Wazhiwei Coast and its adjacent south river bank. Based on the key features of sediment transport and morphological changes in the coasts and estuary, for erosion protection and sedimentation prevention, some preliminary ideas can be provided according to the particular functions of the water areas.

As a major issue in the Tamsui Fishing Port is deposition of sediments in the port and the refilling problem in the navigation channel, it is found that dredging is necessary, especially when a great amount of sediment comes from upstream, bringing down by river floods in a typhoon season. Installing a coastal structure can be also an engineering solution to preventing the port from deposition. In the Wazhiwei Coast, due to complex circulations in different seasons driven by river flood, tides, and wave action, maintenance of the coast is a difficult task, because we don’t know if the river sediment supply every year is sufficient to refill the eroded bed during typhoon seasons. Again, a structural engineering approach can also be a solution.