RIASSUNTO
Abstract
Coastal erosion caused by structures due to exploitation is a serious problem for coastal management in Taiwan. These structures disturb the continuity of the original littoral sediment transports, resulting in retreats of the shorelines in the downstream areas. This paper tries to assess the responsibilities of stakeholders regarding coastal defense and attempts to propose adequate remedies. Trend and mechanism analyses were used to assess the effects the coastal exploitation made on the coastal defenses. In the former, historical satellite images, maps, and bathymetric survey data were used to analyze the evolution of the coastlines. In the latter, we used numerical models to estimate the area where the marine mechanisms were changed by the constructions. Taoyuan, Yunlin and Chiayi coastal sectors were used as case studies. Based on the results, both engineering and non-engineering measures, such as sand bypass and land use control, respectively, were proposed for a sustainable usage of the coast. Using these results, the relative role of the governmental officials and land-users in the area was also investigated.
Introduction
The total length of the coastal line around Taiwan is about 1,100 kilometers. According to their geotectonic structures, the coasts can roughly be categorized as rocky, sandy, and coral-reef coasts. In general, it could be said that, the morphology of a coast is governed by both natural and anthropogenic factors. The former includes sediment source, tidal and wave forcing; the latter can be activities such as channel dredging, land reclamation, seawall, or riverbank protection. Different geologic features cause the coasts in Taiwan to react differently to the impacts of these forcing.
In the years of 2005 - 2008, a project named "Dynamic monitoring of geological environment and resources - application of the FORMOSAT-2 Images" had been carried out by the ‘Central Geological Survey, MOEA’. The project analyzed the satellite images of FORMOSAT-2 for the developments of the coasts of northern and eastern Taiwan. According to a report of the project, in the preceeding three decades, the coasts of Sanchih (New Taipei City, NTC), Fulong (NTC), and Gichi (Hualien) have been retreating at rates of 1.77~2.86 m/year. In fact, almost all the coastlines in the northern part of Taiwan suffer from erosion. The report pointed out that, diminishing sediment sources in the upstream, erosion by marine forcing at the seaward side, and structures protruding coast lines are the main reasons for the receding coastlines in northern Taiwan.