雲林海岸基本資料調查(1/2)(林厝寮海堤至台子村海堤) (附光碟)

摘要
一、計畫概述
(一)計畫緣起與目的
雲林海岸北起濁水溪出口，南至北港溪出口，海岸線總長約55公里，本段海岸近年來由於沿岸砂源減少，海岸呈現顯著侵蝕現象，沿岸海灘、防風林地急遽消退，嚴重者達百餘公尺。本計畫藉由辦理雲林海岸之基本資料蒐集分析、水深地形測量及海域調查分析等工作，建立與更新本段海岸基本資料，以作為日後海岸規劃與設計之應用參考。
(二)計畫工作範圍
本計畫將針對雲林海岸進行海岸基本資料調查，第一年度地形水深測量範圍為箔子寮漁港至台子村海堤海域，沿岸長度約7.8公里。
二、基本資料蒐集分析
(一)地理位置
雲林海岸線長達55公里，屬砂質海岸。本段海岸較具規模之砂洲包括自台西附近箔子寮砂洲以及雲林縣西南端之外傘頂砂洲。因有外海砂洲之阻隔，外海浪潮不易直接入侵，故早年沿海保護工雖甚簡陋，但仍勉強可以禦潮。近年來由於工業區開發、河川整治與地層下陷等之影響，致使海岸補充砂源減少。
(二)人文社經
沿海麥寮、台西、口湖及四湖四鄉戶數為36,052戶，約佔全縣人口15%。各鄉鎮中之耕地面積以古坑鄉最多(占全縣11.7%)，惟多為旱田。雲林縣的工廠總數為1,428家，主要以食品製造業為主，占全縣工廠登記數之30%。水產養殖概分為淺海養殖、鹹水養殖與淡水養殖3類，全縣以口湖鄉之養殖面積最大，約佔全縣養殖面積之42%。家禽主要以雞、鴨、鵝及火雞為主，其中又以雞為最大宗。
(三)氣象
蒐集計畫區內麥寮站資料統計結果，海岸風力以冬季的東北季風最為顯著，全年平均風速約5.97m/sec，冬季期間平均風速值7.31m/sec，夏季期間平均風速值4.65m/sec。月平均溫度大致介於16.1~28.3℃之間，全年平均溫度為攝氏23.1度。全年平均氣壓為1011.6百帕。全年平均相對溼度介於80.0%～87.8%之間，全年蒸發量介於29.5mm~94mm之間，其中以當年9月94mm為最高。各年每月降雨天數集中在每年4月及6-8月間，84年至101年間之年降雨平均值約為1,627公釐。颱風為威脅臺灣地區最嚴重之自然災害，對雲林海岸可能產生直接影響或危害的颱風侵台路徑為第三類、第七類及第九類侵台路徑颱風，過去117年(1897年～2013年) 共412個颱風，上述路徑之颱風共有110次，約佔侵臺颱風總數之26.70%，平均每年發生0.94次。
(四)海象
每年最高潮位都發生於夏秋季之大潮期間，每年最低潮位都發生於冬季之大潮期間。雲林近岸海域海流漲潮時往北~東北、退潮往南~東南，主流向受到風驅流與洋流影響東北季風往南、夏季往北。漲(退)潮期間流速較大，各月速度皆於62.5~75公分/秒；潮流為潮汐漲退所引起之沿岸半日週期流動。潮流以往復運動為主，淨流於夏季時期之軌跡較為平順，無東北季風期受風應力致軌跡往偏向南轉折之現象。海氣象條件主要隨季節轉換而呈現週期變化，冬季受東北季風影響，月平均示性波高介於1～2米，週期約5～7秒，波向以西北～北為主。夏季月平均波高僅約0.5米，週期約4～6秒，波向以西～南為主
(五)河川水文及主要河川輸砂量
雲林縣位於台灣中部，受季風、颱風、地理位置、中央山脈及天然地形之影響，境內河川7成以上的年逕流量集中於每年4~10月。在本計畫區位內最大河川輸砂砂源為北面之濁水溪及南面之北港溪。以平均含砂量、最大含砂量及平均輸砂量而言，濁水溪皆遠高於北港溪。
(六)海岸防護設施
雲林海岸長度約55公里，屬砂質海岸，一般性海岸防護設施則約有36公里，尚未有防護設施之地區包括麥寮鄉麥寮海埔新生地及口湖鄉開南島兩地，海岸防護措施計有後安寮海堤等13處海堤，海堤長度約36,000m 及另有防潮堤共16,716m，。沿岸海堤之沉陷速率約為每年5~10cm/年；其中海口海堤從83年整建後至目前下陷量約為1.535~1.730m，下陷速率約為10cm/年，其餘海堤下陷速率約每年下陷4~7cm。由於部分海堤構造物長期受到潮、浪侵襲與土體滲流產生淘空與表面老劣化現象，另加上地層下陷、海水位上升及海岸線變遷等各項不利因素，恐降低海堤之安全性與防護性。
(七)地層下陷
雲林地區為我國地層下陷相當嚴重之行政區，雲林地區的長期壓縮速率超過每年3cm之地陷監測井主要集中在內陸，包括龍岩國小、土庫國中、秀潭國小、元長國小、客厝國小及內寮派駐站；而沿海一帶地層下陷速率約小於每年2cm。依據經濟部水利署雲林地區水準樁點檢測資料顯示，民國81年至88年主要累積下陷量乃集中在沿海鄉鎮，民國85年以後雲林地區地層下陷中心主要位於內陸虎尾、土庫、元長及褒忠一帶，民國92年至95年間均逾10cm/年以上。而95-102年間最大沉陷速度約為6.7cm/年。而沿海一帶地層下陷速率約小於每年2公分。
(八)海岸地貌變遷
雲林縣海岸河川輸砂之多寡為當地海岸變遷主要因素之一；濁水溪為主要之砂源。影響沿岸漂砂之因素主要有二項因素:
1.人為活動
約1980年雲林縣海岸線地形即略呈灘地縮減之情況，顯示此一時期起海埔地圍墾開發等人為活動略已影響沿岸漂砂活動。港口與防波堤的興建，亦會對海岸漂砂特性造成某一程度的影響。
2.自然營力作用
波浪、潮流作用產生南向優勢漂砂活動導致本計畫區海岸沿岸砂洲持續向南遷徙，以及湧浪與風砂運動產生砂洲向內陸側後退。主要受冬季東北季風波浪及颱風所影響。
三、控制測量
在執行本計畫海岸地形測量作業之前，需先完成陸上平面及高程控制測量工作，以確保所引用之控制基準無誤，相關作業包含海岸控制樁埋設及航標點布設、已知平面及高程控制點檢測、新設控制點(海岸控制樁及航標點)平面及高程控制測量、臨時驗潮站高程聯測及測量平差計算等工作。為考量後續海岸地形監測之需求，在本計畫區海堤以平均1公里之間距，共佈設7座海岸控制樁(點號為「YL01~YL07」)，以作為後續海岸地形監測之控制基礎。
四、海岸地形測量
(一)陸域地形測量
本次作業以航空攝影同機搭配空載光達LiDAR(空載雷射掃瞄)方式測繪，施測範圍擴大至二年度計畫區(包含第一年度箔子寮漁港至台子村海堤、第二年度林厝寮海堤至箔子寮漁港)，以利更完整掌握二年度計畫區之海岸變遷情資。
(二)海域地形測量
本年(103年)度地形水深測量範圍為箔子寮漁港至台子村海堤段海域，於年度冬季與夏季波浪作用後(2014年4月及10月)各進行一次。並利用兩次測量成果進行海岸地形侵淤分析。經由本年度兩次詳細之海域地形調查分析顯示，計畫範圍內仍持續發生侵蝕，砂洲仍持續南移。
五、水深地形侵淤分析
(一)全域地形水深侵淤分析
比較本年度冬季及夏季季風作用過後之地形差異，0m以下之體積在夏季波浪作用過後約減少3,324,485立方公尺，較4月份減少1.09%，近岸0 ~ -1m水深處流失約4.55%(約21萬立方公尺)，而水深1 ~ 3m處合計體積增加約9.1%(約74萬立方公尺)；而水深6m以深之外海則全部呈現侵蝕；無論就體積變化及平均刷深深度而言，冬季波浪作用所造成之地形變化遠大於夏季波浪之作用，且自102/10~103/09間整體是呈現侵蝕行為，淨侵淤量約為侵蝕3,450萬立方公尺，約較102/10之體積短少10.3%，外海侵蝕情況仍然持續中；侵蝕的主因以短期行為而言，應是與當地自然營力(東北季風所引起風浪流交互作用)及補充砂源短缺；長期行為除了上述兩項因素外，長期人為活動及海岸構造物之影響也是主因之一。
(二)海岸斷面特性分析
本團隊於本區執行前期計劃時已在本區(箔子村漁港~台子村海堤段)規畫10條測線進行海岸斷面特性分析，如概略地將Sec.38~40、Sec.41~43、Sec.44~47分別代表計畫範圍內北段、中段及南段，觀察後即可發現本區之南段為侵蝕較為嚴重之區域；補充砂源短少及強烈之冬季季風作用均是可能之原因；藉由地形剖面分析也顯示，大部分之侵蝕行為發生於冬季季風作用期間。
(三)長期砂洲變遷分析 
根據資料年份及砂洲輪廓相似性可以將箔子寮砂洲之變化概分為三個時期；1993年~2003年(10年間)、2003年~2010年(7年間)及2011年~2014年。在1993年~2003年間，砂洲之形狀逐漸向南移動外，外海之小砂洲也逐漸南移。自2003年後，砂洲外形逐漸成形，最內側與最外側之灘線已然固定；2011年之後，外灘線均無太大變外，砂洲外形逐年變長。可以很明顯觀察出箔子寮砂洲往南遷移及向內陸靠攏之趨勢，同時外海之小砂洲在2001年已不復見。
長期分析砂洲變遷可發現，其最南端位置N座標位移(或偏差)與年代之關聯性為一簡單之線性關係，由此可推估年平均移動速率約為321公尺/年；長期變遷趨勢為砂洲往南遷移，以及往東向陸地靠攏，其中砂洲向南遷移的情況相當明顯，與時間呈現高度負相關性，而水平位移與時間之相關性較不明顯。砂洲之南移反映出南北向漂砂行為甚為明顯，也成為外傘頂洲之砂源之一，箔子寮砂洲持續南移，未來有可能與外傘頂砂洲合併，而將箔子寮至台子村之間形成一個淺內海。砂洲體積流失範圍集中在箔子寮漁港外及箔子寮砂洲之南段(左右兩側，左側較右側流失嚴重)，而箔子寮砂洲中段及箔子寮漁港北側則呈現堆積。從侵淤分佈可以觀察侵蝕最為嚴重之區域是砂洲西南側；而從平面面積及外觀變化可知，砂洲北端左右兩側灘線仍維持不變，但砂洲南端之外形略為頸縮，兩側均有侵蝕現象發生，砂洲持續向南增長。故可以得知，近一年來因砂洲南端在季風作用後侵蝕現象較為強烈，故砂洲體積及面積減少。
冬季季風及夏季季風(含西南氣流及颱風事件)為主要之漂砂活動時期；大部分之侵蝕現象均在冬季波浪作用期間發生。颱風否影響本區之地形變化則是與颱風路徑及規模有關
六、漂沙調查分析
本年度計畫需於箔子寮漁港至台子村海堤段海域，在冬季與夏季波浪作用後各進行乙次海岸漂砂調查分析工作。
(一)底床質採樣調查分析
本年度海域底床質砂樣，需於箔子寮漁港至台子村海堤海域範圍內至少每500m一個斷面，採樣水深預定分別為-2m、-5m、-8m及-12m，以及高、低潮灘線砂樣。砂洲靠海岸內側有以及外海12m水深處有較細，除砂洲內側區域外，計畫區粒徑大小空間分布與水深相當一致，兩季空間分布差異不大。兩季底質比較可見，砂洲內側之細底質區域有向南移動趨勢。
(二)底床懸浮載採樣調查分析
底床懸浮載採樣點位需配合最近之地形水深資料，規劃選取4條海岸漂砂調查斷面，每一斷面依水深設置3處採樣點位。冬季季風作用觀測期間，優勢漂砂方向主要為向岸方向及南向漂砂為主。夏季季風作用後全方位輸砂率觀測結果主要以南向為主，於砂洲前後側位則為離岸優勢方向。利用實測之潮位及海流資料，模擬當地波流場之變化。
冬季季風作用後，計算結果顯示箔子寮南側砂洲後方屬於良好之遮蔽區域，於漲退潮時段都獲得良好之遮蔽效果，在季風波浪作用下在近岸地區有持續近岸流作用，而近岸流之產生與波高(波能)有關，計算結果顯示波高大於2m時，箔子寮外側近岸地區始有明顯近岸流活動。由於計畫區與南側外傘頂洲南向波浪皆不大；根據實測結果，大於2m之波浪皆為北向波浪，在碎波造成南向沿岸流，進而造成近岸漂砂往南。夏季季風作用後由季風波浪造成之水動力結果觀之，夏季季風時計畫區沿岸砂洲地帶地形變化不會太大。

Abstract
1. The project overview
(1) Introduction of research
This project works on the part of Yunlin coast, 55 km long extended from Chuo-shui River north to Beigang River south. In the recent years, due to deduction of sand support from rivers, it suffers serious coastal problems, such as retreats of coastline and windbreaks. Therefore, we expect the achievement of this research project that can be established and updated the basic information of this coastal region for future application of coastal management and design, by way of fundamental data collection and analysis.
(2) Research scope
The measurement range of water bathymetry for the first year is 7.8 km long extended from Bozihliao Fishing Port to Taizih Seawall along the Yunlin coast.
2. Fundamental data collection and analysis
(1) Geographical characteristics
In this Coastal and near shore morphology was providing protection from waves and surges by offshore sandbar, such as Bozihliao sandbar and Wai-Shan-Din sandbar. But in recent years, the sandbar area was shown substantial loss due to Industrial zone development, river regulation and land subsidence.
(2) General social and economic background
The administrative region along coastal includes four townships, Mailiao, Taihsi, Kouhu and Sihhu. And there were 36,052 households living in coastal townships, accounting for around 15% of the total county population.
The total numbers of factories in Yunlin County are 1,428 homes that are mainly centered on the food industry, accounting for 30% of all the registered factories. In the aquaculture, there are three classifications of aquaculture form include marine waters, brackish water and freshwater. And the largest aquaculture area in Yunlin County is Kouhu township, accounting for around 42% of all the aquaculture. In Yunlin County also has the poultry farming such as chickens, ducks, geese and turkeys.
(3) Meteorology
The meteorological data are collected from Mailiao meteorological station. The annual average wind speed is about 5.97 m/sec, the average wind speed during the winter months is 7.31 m/sec, and during the summer is 4.65 m/sec. The average monthly temperature is between 16.1℃ to 28.3℃,and the annual average temperature is 23.1℃. The annual average pressure is about 1011.6 hPa. The average annual relative humidity is around 80.0% to 87.8%, and the annual evaporation is between 29.5mm to 94mm, which the highest evaporation is 94mm in September. The rainy seasons during every year are in April and June to August, and the annual rainfall is about 1,627 mm during 1995 to 2012.
The most threatening for invasion routes of typhoon to Yunlin coastal are routes 3, 7 and 9. In 412 typhoon events over the past 117 years (1897~2013), there were 110 typhoon events classified routes 3, 7 and 9, accounting for around 26.70% of all the typhoon events.
(4) Oceanography
A) Current: The main transport direction of offshore current is controlled by wind-driven flow and ocean current. There is considerable seasonal variation of the offshore current. It is from north to south in winter, and conversely in summer, it is from south to north.
B) Tide: The high tide direction is north (N) to northeast (NE). The low tide direction is south (S) to the southeast (SE), with a velocity size of about 62.5 to 75 centimeters per second, being a half-day flow.
C) Wave: The wave directions in summer are mainly west (W) to south (S), with a wave height below 0.5 meters and 4~6 seconds in the period. And the mainly wave directions in winter are northwest (NW) to north (N), with a wave height between 1~2 meters and 5~7 seconds in the period.
(5) River hydrology
There are more than 7 percent rivers with annual river runoff concentrated in April to October .The largest sources of river sand transport in the research area are Jhuoshuei River and Beigang River. Jhuoshuei River is much higher than the Beigang River with average sand content, maximum sand content, and the amount of average sand transport.
(6) Coast protection structures
Yunlin coast owns 13 regular coastal protection sea dikes (around 36,000 meters), and 16,716 meters of sea dikes to against tidal waves. There are still two areas with no protective equipment including Coastal Reclaimed Land-Mailiao and Kainan Dao in Kouhu Township.
The seawall has become emptied and surface deterioration due to the long-term invasion by tidal-currents and waves. The subsidence rate of the seawalls are about 5 ~ 10cm per year.
(7) Land subsidence
Yunlin is the most serious subsidence area of entire country .The annual land subsidence rate along the coast is less than 2cm. The maximum subsidence rate is about 6.7cm per year during 2006~2013.
(8)Coastal morphological changes
There are two main factor items of sediment transport changes, as described below.
A) Anthropogenic activities: including channel dredging, land reclamation, sea wall or riverbank protection
B) Natural forces: transport of sediment around an estuary is governed by many natural factors such as sediment source, tidal flow and waves.
3. Control measure
In this project, there are seven coastal controlling piles (point numbers are between YL01 to YL07) with average spacing of 1 km, as a control base of monitoring coastal terrain.
4. Topographic Survey of Coastal Zone
(1) Land topography
The land topography survey is working with aerial photography, and pairing with a LiDAR (laser scanning).
(2) Ocean bathymetry
The ocean bathymetry measuring times are action after winter and summer wave season (April 2014 and October). And use the two measurement results for analysis of coastal morphological changes.
5. Investigation of coastal morphological changes
(1) Analysis on the areal extent of coastal erosion and deposition
The net changes in coastal terrain invasion deposition is -34.5 million cubic meters during 102/10~103/09, which is showing an erosion trend. The result showed that coastal morphological is continued erosion, due to natural forces (such as wind, wave, and current induced from winter monsoon), the shortage of sand source, and anthropogenic activities.
(2) Cross-section profile analysis of coastal zone
In this project, we use of 10 survey lines for cross-section profile analysis of coastal zone, such as Sec.38~40 (Northern Section)、Sec.41~43 (Middle Section)、Sec.44~47 (Southern Section). The more serious erosion area is Southern Section. Most of the erosion behavior is occurred in the winter.
(3) Long-term Changes in sandbar
The long-term change for sandbar migration is from north to south, and turned landward. The annual average moving rate is about 321 meters per year. The main erosion areas of sandbar are in the offshore and southern of Bozihliao Fishing Port (the erosion area on the left side is more than the right side). And the main deposit areas are found in the middle of Bozihliao Sandbar and northern of Bozihliao Fishing Port.
The main activities periods of sediment transport are during the wave actions in winter and summer (including typhoon season).
6. Investigation of fluvial sediment
The fluvial sediment survey analysis is once per year after the winter and summer wave action.
(1) Sediment grain size measurement
Compare results from two surveys, the sediment particle size distribution is insignificant; and the sediment transporting is tend to south in the fine sediment area of sandbar landside.
(2) Sampling survey of suspended-sediment loads
During the winter wave action (wave height greater than 2m), the main advantage direction of sediment transport is onshore and southward. According to the observational results, the wave greater than 2m is northward, causing southward alongshore current and southward alongshore sediment transport. On the contrary, during the summer wave action (wave height less than 0.5m), the sandbar topography changes will not be too significant.