Morphological variation of the beach under the construction of submerged breakwaters-a case study in Pintung coastal area of Taiwan

The need to stabilise and protect beaches and coastlines is important due to the increasing recreational activities in the coastal zone. In Taiwan, the coastal area is hit by about four typhoons a year, and the large waves caused by the typhoons often lead to coastal disasters and beach erosion. The beach could also be the buffer zones against the large wave attack and coastal flooding and erosion. Although the gravel beaches have a great advantage over the sandy beaches in terms of energy absorption capacity, this advantage disappears under the strong wave attack during the typhoon season in Taiwan. The gravel is carried offshore, and it is difficult to return to the beach under the swell wave. During the extreme wave induced in typhoon conditions associated with storm surge, it causes gravel beach closer to the steep nearshore zone, which may then be irreversibly evacuated downslope. The extreme waves bring breaking waves high up on the beach, creating the step-reflecting berms. Energy reflection can thus contribute to further downslope transport of gravel to depths from which it can no longer be recovered by fair-weather waves. The gravel beach can be very steep, accompanied by a typically narrow surf zone and an energetic shore break.

The FuAng coast in the south of Taiwan has suffered severe beach erosion since the construction of breakwaters in 2015. The strong reflection creates partial standing waves that scour the sediment at the toe of the beach. In addition, the mixed sand and gravel beach is washed offshore during storms and cannot be brought back to the beach by the weak swell wave. The aim of this paper is to analyse the erosion problem using the shoreline evolution from the satellite images, the variation of the beach profile in several sections and the volumetric variation of the bathymetry. An integrated coastal protection countermeasure using submerged detached breakwaters with artificial beach nourishment is proposed to mitigate the beach erosion problem. Physical experiment and numerical simulation are used to verify the proposed countermeasure. The results show that the proposed method can effectively protect the coast, prevent the beach erosion and form a salient to be a good buffer zone to prevent the wave impact.