宜蘭海岸防護基本資料調查(2/3)

本計畫分為三年度執行，本年度 (102) 為第二年。主要研究範圍，自梗枋溪出海口以南至澳仔角以北沙岸。工作項目包括：(1) 基本資料蒐集分析、(2) 海岸水深地形測量與地形變化分析、(3) 近岸漂沙特性調查分析、(4) 海岸潮波流調查、(5) 海岸結構物影響與防護成效檢討、(6) 研提海岸防護對策及環境營造改善初步構想、(7) 海岸防護改善方案評估比較。
歷年海象資料分析顯示：蘇澳港潮位站1981~2013年間，最高高潮位1.44m，最大潮差2.65m，大潮平均潮差1.78m。烏石港潮位站2006~2013年間，最高高潮位1.37m，最大潮差2.56m，大潮平均潮差1.66m。2002~2009年間蘇澳港外海流資料，主要方向N~NNE與SSW~SSE向，平均流速夏季為14~29 cm/sec、冬季為14~45 cm/sec。蘇澳浮標(1999~2013年)夏季波浪主要方向為東南東方位，示性波高（H1/3）平均1.49 m，示性週期（T1/3）平均約 6.4 sec。2010年9月19日凡那比(FANAPI)颱風來襲時，記錄最大波高 15.05 m、週期為 13.1 sec。冬季波浪以東北方向最多，示性波高平均約 2.01m，示性週期平均約 6.6 sec。
海岸地形變化分析顯示：2001年到2012年第二次測量結果之間的海域地形侵淤比較，總體呈現侵蝕的情況。平均侵蝕高程在-1~-2m。侵蝕情形在頭城海水浴場0~-5m等深線海域與公館-10~-15m等深線範圍最為嚴重，頭城海水浴場掏蝕的深度平均在-3~-4m間，公館外海則接近-5m。在蘭陽溪出海口與烏石港附近海域，有3~4m淤積高程。蘭陽溪口淤積推估受河川輸砂豐沛所致，而烏石港淤積多發生於海堤北側。長期海岸線變遷方面，1898~1921年間，海岸線平行後退最大可達-200m。1921~1978年，蘭陽溪北岸淤積約400m。1978~2001年間，頭城海水浴場後退-48m，上大福則後退-31m。2001~2006年間，主要侵蝕區域在上大福，退縮-19m，烏石港南堤則淤積超過150m。2006~2012年4月期間，大福海岸退縮-16m，其餘海岸多呈淤積狀態。另以衛星影像分析短期海岸線變化，3~6月灘線向海側推移，7~9月則是海岸線後退。逐月變異量與整體變遷量統計中，最北邊的外澳北堤後變化最劇烈，其次為最南邊的新城溪沙堤。2010年9月凡那比颱風期前後海岸變化，呈現大幅度侵蝕情況，海岸線退縮量在-33.0m~-11.3m之間。
計畫區潮波流現場調查結果：潮汐型態屬於混合潮型，以當地中潮為基準，平均高潮位0.494m，平均低潮位-1.187m，平均潮差0.988m，最大潮差2.074m。第一次調查結果 (2012/04)，示性波高為1.21m、週期6.23sec，主要波向NE到E方。海流方向以平行海岸線為主，底層平均流速為15.4cm/sec，主要流向SE向，最大流速為48.7cm/sec，流向為ESE；表層平均流速為22.6cm/sec，主要流向ESE，最大流速為74.4cm/s，流向為SSE。第二次調查結果 (2012/09)，示性波高1.52m，週期7.45sec，主要波向亦為NE到E方。海流底層平均流速為21.4cm/sec，流向集中於ESE、SE與WNW，最大流速為76.0cm/s，流向為ESE；表層平均流速為31.4cm/sec，集中於SE與NW向，最大流速為121.6cm/s，方向是SE。
計畫區海岸底質調查結果：第一次調查 (2012/04)，海域中值粒徑分佈為97.5~ 580.9 μm，高、低潮線中值粒徑分佈為228.9 ~ 498.9 μm；蘭陽溪口以北底質粒徑分佈趨勢為往北遞增，以南則呈向南遞減。第二次調查 (2012/09)，海域中值粒徑分佈為7.8~285.9 μm，高、低潮線中值粒徑分佈為211.9 ~ 695.1 μm；蘭陽溪口以北底質以社頭粒徑最大，向北向南遞減，蘭陽溪口以南呈向南遞減。計畫執行期間，兩次使用補砂器調查懸浮漂砂，成果顯示海域淨漂砂方向約略垂直海岸線指向外海，少部分漂砂方向平行岸線。第一次調查 (2012/04)，碎波帶內淨傳輸率在 10.12 ~ 32.56 cm3/min 之間，碎波帶外淨傳輸率則在4.49 ~ 50.56 cm3/min 之間。第二次調查 (2012/09)，碎波帶內淨傳輸率在 12.97 ~ 109.93 cm3/min 之間，碎波帶外淨傳輸率則在3.14 ~ 49.14 cm3/min 之間。
在海岸與海堤的溯升安全評估上，以50年重現期颱風波浪為設計條件，斷面PL5、PL13、PL14、PL19及PL20等有發生溢淹的可能，其原因為此五個斷面位置均位於河口處，最大高程明顯低於附近海岸，故易造成越堤溢淹的情形。
頭城海水浴場防護成效綜合評估：在經費充裕情形下，離岸潛堤配合人工養灘防護方案，對於頭城海水浴場區域可達到最大防護的功效。壯圍鄉、季新村與港邊里防護對策改善建議：(1) ?方案：維持現狀。(2) 人工養灘：定期抽取蘭陽溪出海口附近淤沙，回填海岸沖蝕區段。(3) 海岸植生：建議透過定沙防護工程與複層植生，提供防風林較佳之長環境，提供防風林群聚及完整之效果。(4) 地工砂管：設置於植被區前增加高度以消減波浪能量而自然回沙。

The main work (102) of this project includes the collection of background data, scope of the study between South Fang-Xi to North Ao-Zai-Jiao sandy in Yi-Lan area. The following tasks have been carried out for the second year period: (1) Data’s collection and analysis (2) Topographic survey and topography in Coastal water depth. (3) Drift littoral characteristics survey (4) Situ Investigation on Tide, Wave, Current and Sediment (5) Review protection and the impact of coastal structures (6) Proposed coastal protection measures and to improve the environment and create an initial idea (7)Comparative assessment of coastal protection improvement plan.
The calendar of walrus data analysis : Between 1981 and 2013 the tide station in Su-Ao harbor, the highest tide is 1.44 meter, the maximum tidal range is 2.65 meter, tide average tidal range 1.78m. Between 2006 and 2013 the tide station in Wu-Shi harbor, the highest tide is 1.37 meter, the maximum tidal range is 2.56 meter, tide average tidal range 1.66m. At 2006 ~ 2009 , The ocean current data of Su-ao harbor, the ocean current main direction of N~NNE and SSW~SSE, the summer average velocity is 14~29 cm/sec , the winter is 14~45 cm/sec . the buoy of Su-Ao(1999-2013) summer waves direction of east-southeast, the average height of significant wave (H1/3) is 1.49 meter, the average significant wave period（T1/3）is 6.4 sec. When September 19, 2010 FANAPI Typhoon , the maximum wave height is 45.05 meter, period is 13.1 sec. almost the winter waves in the northeast , the average height of significant wave is2.01 meter, the average significant wave period is 6.6 sec.
The topography of coastal illustrate: sea erosion and deposition terrain between the results of the second measurement, the overall situation showed erosion. The average erosion of elevation between -1 to 2 meters. The most serious erosion situation at Tau-Cheng the isobath waters between 0 to 5 meter and Gong-Guan 10~15 meter, Tau-Cheng average of erosion depth between -3~-4 meters. The Gong-Guan has erosion depth near the -5 meter. Near Lan-Yang rivulet and Wu-Shi harbor have accretions 3~4 meters. Lan-Yang rivulet has accretions caused by river sediment. The Wu-Shi harbor has accretions occurred in the north seawall. Between 1898 and 2001, the maximum parallel to the coastline retreat can be -200 meters .Between 1921 to 1978, The north Lan-Yang rivulet has accretion about 400 meter. The Tau-Cheng coastline retreat about -48 meters between 1978 to 1978 , the Da-Fu coastline retreat -31 meter. Between 2001 and 2006 , almost has erosions at Da-Fu, the other coastline has accretions. Another satellite image analysis of short-term shoreline change, the beach line move to seaward side between March and June, the coastline back on July to September. Monthly variance with the overall changes in the amount of statistic, the north Wai-Ao embankment most dramatic change, followed by most south Xin-Cheng rivulet. The coastal change at September 19, 2010 FANAPI Typhoon, has substantial erosion, the Coastline back between -33.0 meter and -11.3meter.
Planning district field investigations tidal, wave and current: the tidal model belong mixed type of tide. The basis of moderate tide, high tide is 0.494 meter, average low tide is -1.187 meter, the average tidal range is 0.988 meter, the maximum tidal range is 2.074 meter. The results of the first survey (2012/04), the height of significant wave is 1.21 meters, period is 6.23 sec, the main wave direction NE to E. The main current direction parallel to the coastline, The underlying average velocity is 15.4 cm/sec, the main wave direction SE, the maximum velocity is 48.7 cm/sec, wave direction ESE, The surface layer velocity is 22.6 cm/sec, the main wave direction ESE, the maximum velocity is 74.4 cm/sec, wave direction SSE. The results of the second survey (2012/09), the height of significant wave is 1.52 meter, period is 7.54 sec, the main wave direction NE to E. the underlying currents velocity is 21.4 cm/sec, concentrated flow to ESE 、SE and WNW, the maximum velocity is 76.0 cm/sec, wave direction ESE ; The surface layer velocity is 31.4 cm/sec, concentrated flow to SE and NW, the maximum velocity is 121.6 cm/sec, wave direction SE.
Planning District coastal sediment survey results: The first survey (2012/04), The D50 between 97.5 and 580.9μm, the high、low tide line’s D50 distributed between 228.9 and 498.9μm, The North Lan-Yang rivulet Sediment particle size distribution trends north increments, diminishing of South to south . The second (2012/09), the D50 distributed for the sea area between 7.8 and 285.9μm, the D50 distributed of the high and low tide line between 211.9 and 695.1μm;, the biggest sediment particle at She-Da north in Lan-Yang rivulet delivery to north and to south ,but south Lan-Yang rivulet delivery to south. During the project execution using the sand is catching to Survey Suspended drift twice, The results display direction of drift sea area perpendicular to the shoreline off point, another direction to parallel to the shoreline. The first survey (2012/04), in surf zone’s transfer rates between 10.12 and 32.56 cm3/min . The second survey (2012/09), in surf zone’s transfer rates between 12.97 and 109.93 cm3/min, out of surf zone’s transfer rates between 3.14 and 49.14 cm3/min.
The run-up security assessment of coastal and seawall, 50-year return period for the design typhoon wave conditions, the case PL5 、PL13、PL14、PL19 and PL20 has flooding over may occur, The reason for this purpose five cases are located at the mouth , the high elevation has significantly lower than near the coast ,so easy lead to flooding over embankment situation.
The comprehensive assessment of Tau-Cheng Protective effect, for the Tau-Cheng can be achieve maximum effectiveness of protection. Protection measures to improve the Zhuang-Wei village、Ji-Xin village and Gang-Bain ' s proposal: (1) Maintain the status quo. (2) Artificial beach nourishment: Regularly collect silt at Lan-Yang rivulet refilling the coastal erosion area. (3) Coastal planting: Though planting on sand fixation of engineering protections , complete effect to provide windbreaks clusters .(4) Geotextile tube works: Set in front of coastal planting increase the height can be wave energy to abatement and sand back.