Impact of Anthropological Pressure on the Coastal Area: Cross-Shore Beach Analysis. A Case Study

Filipa S. B. F. Oliveira

Laboratório Nacional de Engenharia Civil, Lisboa (PT)

The objective of the present study is the assessment of Hac-Sá beach profile response to a typical local storm event, prior and after the advance of constructions and infrastructures on the backshore zone of the beach. The present analysis allows the interpretation and understanding of recent erosion episodes (Figure 1) occurred in the narrowest sector of the beach, where the beach width has been reduced due to the advance of solid constructions built on the upper limit of the beach profile.

Figure 1 - Photos a) and b): Impact of the erosion episode occurred in October 2000 at the southern sector of Hac-Sá beach

Hac-Sá beach (in Figure 2) is a narrow pocket beach located in Hac-Sá bay, in the east coast of Coloane, Macau (a special administrative region located in the south coast of People's Republic of China). It is a beach of 1200 m long, exposed to ESE.

Figure 2 - Location of the area of study (based on chart "Macau, China. Portos de Macau, Taipa e Coloane.", 2001, by Capitania dos Portos)

The short-term morphological changes in Hac-Sá beach are the result of a fast interaction between the hydrodynamics associated to a storm event (more energetic waves and sea level rise in the surf zone) and the beach morphology, during a short period of time. The result is a sudden increase of the offshore sediment transport due to the increase of the seaward gravity driven current (the undertow) and consequent development or reinforcement of the offshore bar.

In order to assess Hac-Sá beach cross-shore dynamics prior and after the advance of the constructions in the backshore zone of the southern sector of the beach, a numerical model (DHI 2000) was applied to simulate the beach profile response to a maritime storm event. Cross-shore profiles were used to represent the northern and southern sectors of the beach. The evolution of each profile was simulated for two situations, prior to the advance of the upper limit of the beach (conditions of 1985) and presently (conditions of 2002), at mean sea level. The results show that presently, the profile in the northern sector has a beach retreat that is about 63% of the retreat occurred in 1985, and the profile in the southern sector, which in 1985 was not significantly affected, has now a retreat about 2.2 times higher than the one observed in the northern sector of the beach. Such results reveal the increase, from 1985 to 2002, of the retreat of the southern sector relatively to the retreat of the northern sector in the presence of a typical local storm event.

These results illustrate the vulnerability of the southern sector of the beach, where the presence of advanced solid infrastructures (vertical seawall of concrete along the upper limit of the beach) can limit the natural expansion of the beach profile in the presence of a more severe maritime storm event. In fact, the presence of a highly reflective structure like a vertical seawall (absence of dissipative conditions) can even worsen the erosive process by increasing the offshore sediment transport due to the increase of the energy locally involved. Despite the permanence of the sand eroded from the emerged part of the beach inside the bay, forming an horizontal platform with development at 0.0m chart datum, it cannot be transferred back to the emerged part of the beach and re-establish the previous equilibrium profile. Such impossibility is due to the present insufficient beach width that generates frequent situations of direct wave action over the solid structures existent in the upper limit of the beach and subsequent increase of turbulence and reflected energy.

References: