Morphodynamics and Evolution of a Coastal Sand Dune in Northwestern Taiwan

Topographic change, its dynamic mechanism, and the long-term evolution of a coastal sand dune in northwestern Taiwan was discussed through the monitoring of the seasonal and interannual topographic changes and sedimentological studies. The strong northeast monsoon in winter often blows up the dry sand on the back beach, and transports the sand landward  along the coast. The sedimentary structure analysis of the foredunes also shows that different types of parallel and cross laminations are dominant at different dune locations.

In summer, the foredune is susceptible to the influence of typhoon waves and storm surges, and often erodes the fore slope to form dune scarp. However, in the following winter, the scarp can gradually return to the dune slope through the accumulation of the dune ramp and the slope slumping. Overall, the foredune ridge has been moving inland toward southeast over the decades. Several sites of sand encroachment onto the windbreak forests are identified. The artificial sand fences on the fore slope make the surrounding sand surface piled up, and the fore slope becomes steeper that more likely to cause large-scale slumping.

The results of the ground penetrating radar survey showed that the surface sediments of the foredunes were about 5-10 meters thick, showing low-angle parallel bedding. Below the existing dune sediments, the distribution of the strata under the dunes (i.e. algal reef layer, old dune sediments, and salt marsh mud) can be observed. Vibration sediment core samples also show that there is an algal reef platform below the beach and dune deposits in this area, which is exposed at the lower fore beach and could extend at least few hundred meters to the inland side. The sea level at the time of the formation of the algal reef platform (about 3,000-4,000 years ago) may have been higher, and the secondary sand dunes on the current inland side may be the foredunes at that time.