圖書典藏及影音數位平台

台南雙春海岸，海岸範圍北起八掌溪出海口南岸，南至急水溪出海口南岸，海岸線總長約3.8公里。雙春海岸位於八掌溪口與急水溪口間，早期因河川輸砂砂源豐富，沙灘寬闊。依本局歷年調查研究結果顯示：於1992年前海岸線多為往海側前進狀，其中八掌溪左岸海岸線突出外海相當明顯，僅近雙春濱海遊憩區呈現往內陸凹陷狀；1992年後海岸線逐漸轉為侵蝕狀況，至2003年雙春海岸整體海岸線已轉為平直狀；依據海岸線變遷分析數據顯示，2002~2007年間整段海岸線年平均後退約有4~5公尺左右。依據海岸環境營造計畫(104~109年)執行策略二「加強工法研發，提升海岸防護技術能力」，推動本計畫「雙春砂質海岸防治新工法研究」，徵求兼顧海岸自然景觀與砂灘防護功效之柔性新工法，於雙春海岸進行現地試驗。目的藉由現地試驗之過程，評估海岸防治新工法之適用性與應用性。

A two years site test project is being carried out on the Shuangchun Coast. The purpose of the project is to evaluate the applicability of the innovative method for coastal defense. The proposed soft construction work, the “artificial marine forest”, does not influence tidal currents and coastal sediment transport, meanwhile, it does not affect the natural coast landscape. The site test location is at 7 m deep off the Shuangchuan Coast. The “artificial marine forest” is composed of 870 sets of single cable tensioned floating bodies that extends 100 m long and 10 m wide at the test site. The wave damping ratio and the damage rate of the “artificial marine forest” are the main concerns. A summary of this year's achievements is as follows: I. Background data collection and analysis 1. Nearshore climate The tidal type of the Shuangchun Coast is semi-diurnal, with an average tidal level of EL + 0.31 m and an average tidal range of 1.304 m. According to the field observation in 2002, the wave height in the winter monsoon (from February to March) is about 0.25 m to 0.95 m and the period is between 5.5 sec. to 9.0 sec. While during the summer monsoon (July to August), the wave height is 0.75 m~1.75 m and the period is between 6.5 sec. to 10.0 sec. The maximum significant wave height during typhoon events are about 2.75 m ~ 3.30 m, and the corresponding periods are 9.5 sec. ~ 11.5 sec. The current velocity are from 4 cm/sec to 42 cm/sec during the winter monsoon (Feburary to March) and from 4 cm/sec to 49 cm/sec during the summer monsoon (July to August). During the flooding tide period of the winter monsoon, longshore sediment transport dominates. While during the flooding tide period of summer monsoon, offshore sediment transport dominates. 2. River hydrology The main stream of Bajan River is about 80.86 km, with an average gradient of 1/42. The catchment area is about 474.74 km2. The wet season is mainly between May and September. The annual runoff is about 744 million m3. The flooding flow rate is 4,000 CMS, the annual sand transport capacity of 2.128 million tons. The main stream of Qishui River is about 65 km long with an average slope of 1/118. The drainage area is about 379 km2 and the annual runoff is 536 million m3. The planned flood flow rate is 2,920 CMS, the annual sand transport capacity of 1.58 million tons. II. The Shuangchun Coast morphological Changes 1. Long term coastal morphological changes of Shuangchun Coast Based on five available bathymetric surveys in the past 11 years (2005, 2011, 2012, 2013, 2014), it indicates that the zone EL -2.0 m~ EL -5.0 m southwest of the Bajan River mouth and the Qishui River mouth is deposited during 2005 to 2011. While it eroseded along the Shuangchun Coast in the same zone level during 2011 to 2014. Satellite image analysis shows that the shoreline of the south bank of Bajan River mouth retreated about 361 m during 1993 to 2002, and it was about 383 m during 1993 to 2015. It represents an average of 22 m per year retreat. 2. The river sediment discharge and the coastal sediment drift mechanism A 2-D hydrodynamic and morphological model was employed to investigate the sediment transport and the bathymetric changes of the Shuangchun Coast. The model was established and calibrated. Preliminary results show that this model is able to simulate the bathymetric changes around the Waisanding offshore sand barrier. III. The Shuangchun Coast morphological Changes 1. Soft coastal defense works around the world Some soft coastal defense works around the world are listed in Table 4-1. Nowadays, the new strategy on coastal prevention has gradually evolved into a plane defense concept rather than the traditional line defense concept. The Integrated Shore Protection System (ISPS), which was proposed by the Japanese, is the most representative. This system suggests that the coastal control should be composed of three rows of defense depth, namely (1) to reduce the wave energy in the offshore region (such as offshore or submerged breakwaters), (2) to reduce the wave energy on the beach, (3) to absorb wave energy on the shore (such as gentle slope or stepped seawall and other rigid works). 2. Comments on previous tested soft coastal defense works At least three experiments on the soft defense work in the Shuangchun Coast have been carried out in recent years. Firstly, a flexible offshore breakwater in combination with artificial beach nourishment was constructed in 2004. The second, a basket netting method was tested in 2007. The third, a flexible interception method was tested in 2011. Among these tests, only the first one can effectively reduce the wave energy, however, it was totally damaged in a typhoon event. Althought the rest two types of soft defense work can withstand for years, they do nothing on terms of wave energy reduction. The main reason is that these two types of work located inside the surf zone, where most waves have broken. The only role for these types of defense work is to redistribute the sand. 3. the “artificial marine forest” The basic concept of this method is to establish a buffer zone between the shoreline and the sea by constructing a “forest” to reduce the wave energy and thus to protect the coast. On the coast protection function classification, it can be the first line of the ISPS. The proposed new method utilizes the interaction between a single cable-tensioned cylindrical floating body and the wave to consum the wave energy. As there are a large number of cylindrical floating structures deployed in the sea, the scene is similar to a marine forest. According to literatures, it is known that the capacity of wave attenuation in such an "artificial marine forest " depends on the diameter of cylindrical floating body and the amount of its numbers. IV. The site test planning and execution 1. to decide the location and the size of the “artificial marine forest” The location was decided to be at 7 m deep area so that it is out of the surf zone of 10 years typhoon waves attack. A rectangular area of long side 100 m and short side 10 m was planned. The long side is parallel with the bathymetric contour that enables the “artificial marine forest” to face the usually incident wave direction. 2. the design and the installation of “artificial marine forest” The“artificial marine forest” is formed by a large number of single cable-tensioned cylindrical floating bodies. The wave attenuation by this innovative scheme depends on the number of the floating bodies. Therefore, the test area was divided into two zones, where there are 500 sets of floating bodies in Zone (I) and 350 sets in Zone (II). The anchorage system is made of cotton rope tied with sand bags. It can withstand at least 5 m typhoon waves, and the relevant material has a life span of at least two years in the sea. 3. the deployment of the “artificial marine forest” A well prepared deployment plan is of importance to the safety and smooth operation on site. The plan included pre-operation, test area lay off, maritime transport and delivery program. The first phase of offshore operations was completed on 6 September, and the second phase of offshore operations was delayed by a typhoon warning and completed on 6 November. V. The site monitering Survey results, including the bathymetry, the wave, the tidal current, the status of the soft work (the artificial marine forest) and the beach runup heights, are as follows: 1. the bathymetry survey The survey of the topography was completed in late June and mid-November respectively. During this period, three typhoons, Nibert, Moranti and Meiji had attacked the test site. Results show that the region shallower than 2.0 m were deposited, while the region deeper than 2.0 m were apparently eroseded. 2. short term wave, current and sediment survey The monitoring stations were set up on both sides of the long side of the test area, about 25m from the test area. The dates of the survey were September 22-23 and November 17-18. The results show that the average wave transmision through the “artificial marine forest” is about 0.905 for Zone (II) and 0.911 for Zone (II). And the average wave energy loss per unit width is about 17.8% in Zone (I) and 16.6% in Zone (II). The tidal current survey showed that the impact of the test area on the currents on both sides of the coast was not obvious, and the sand drift survey showed that the direction of the net sand transport near the test area was toward the southeast. 3. the status of the soft work after deployment A total of 520 sets of single cable-tensioned cylindrical floating body were deployed in Zone (I) in this year. And it experienced a strong typhoon Moranti and moderate typhoon Meiji, resulting in a loss of about 170 sets of the floating bodies, which is 32.7% to the deployed number in Zone (I) and 19.5% of the total (Zone (I)+ Zone(II)). The main reason for the loss of floating bodies is the friction between the cotton rope and the bamboo piles and the breakage of the cotton rope caused by the friction between the cotton rope and the anchor bar. 4. the beach runup height Two sets (the north and the south stations) of runup sensors were set up on the backshore of Shuangchun Coast beach. During typhoon Meiji, the maximum runup height (EL + 3.08m) was detected at the north station, and the maximum runup height EL + 2.37m and EL + 3.09 m were detected during Typhoon Moriente and Meiji respectively at the south station. VI. The performance evaluation The“artificial marine forest” was designed to attenuate the wave energy. The indices to evaluate the performance of this project are the wave attenuation capability and the damage rate of the floating bodies. The average wave energy attenuation per unit width is about 17% under the presented 100 m X 10 m layout. The damage rate of the floating bodies is about 32% in Zone (I) area that underwent typhoon events twice. To improve the performance of the“artificial marine forest”, it is suggested to extend the size of the testing area to 100 m X 20 m. And to add 600 more sets of the single cable-tensioned cylindrical floating bodies.