圖書典藏及影音數位平台

摘要 經濟部水利署第四河川局為執行彰化海岸基本監測調查，以持續建立較完整之海岸防護基本資料，提供彰化海岸後續防護工程設計、規劃與維護管理之應用。舉辦本兩年度海岸監測調查計畫(102、103年)。第一年度(102年)工作項目包含(1)背景資料蒐集整理；(2)控制與地形水深測量；(3)海氣象觀測調查；(4)漂沙觀測調查；(5)水深地形侵淤分析與(6)潮間帶治理防護建議方案，並綜整調查工作結果提出海岸防護因應措施與建議。第一年度(102年)工作項目(2)、(3)與(4)三項現場調查皆於颱風前(3~5月)與颱風後(9~11月)完成。 依地形測量成果可知，地形等深線走向約為北北東-南南西，往南方向等深線有向外海偏離之趨勢。施測海域平均低潮線至離堤岸約2km~3.4km，海灘坡度約為1/1500，平均水位線至低潮位之坡度約1/700~1/1200，平均低潮位線至-10m水深之坡度較陡約為1/130~1/180，-10m水深至-20m水深之坡度約1/320~1/420，全斷面(岸邊至-20m水深)坡度約1/320~1/400。102年颱風前後地形顯示，近岸0m線以內侵淤變化不明顯；0m~-5m間則為侵淤互現；-5m線以外，北部海域則明顯侵蝕、南半部則為淤積現象。 海象觀測調查部分，由於施測期間多為東北季風期，因此測得海域波浪成分主要為風浪，且線西與芳苑外海所觀測到的波浪特性一致。而芳苑外海流速尺度大於彰濱線西外海，潮流所引起之流動為漲潮沿岸往東北，退潮往西南，週期半日。 由底質調查結果，漢寶海堤以及王功新生地海堤處高潮線底質粒徑明顯比其他區域大許多，顯示兩斷面高潮線處波浪能量較其他地區大，研判該區可能為海岸侵蝕潛在區域。捕沙桶調查結果顯示3m水深漂砂活動與波浪大小有明顯直接關係，於10m水深時漂砂活動則與波浪大小、流速流向無明顯之關係。由海岸斷面漂沙特性知海岸夏季呈現淤積，冬季則呈現侵蝕，全年則為侵蝕。海岸斷面侵淤特性分析與本年度漂砂調查結果可互為印證。 本計畫進一步以數值方式評估計畫區現有海堤防護能力，就大潮平均高潮位下50年颱風重現期波浪計算而言，彰濱工業區線西至鹿港西海堤與大城海堤，溯升高程超過現有堤頂高程，有越波現象，但溢流量尚在建議規範0.005 m3/m/sec之內。 針對彰化海域80年至101年地形資料進行侵淤分析，可知彰濱工業區崙尾海堤東北角外海及芳苑鄉北側近海於-5m~-15m間、芳苑鄉漢寶海堤外海於-5m~-15m間與濁水溪口南北兩岸為淤積分布；鹿港西海堤外海水深-5m~-10m間與王功新生地海堤及永興海埔地海堤近岸-10m內呈現侵蝕分布。土方侵蝕較嚴重區域有彰濱工業區線西海堤、漢寶海堤北段與王功海堤北段，單位面積土方侵蝕速率介於每月0.5~1.0 cm；外海區(-5m~-15m)除彰濱工業區線西海堤、漢寶段海堤與大城海堤呈淤積趨勢，其餘皆呈現侵蝕狀況，侵蝕速率介於每月0.2~1.1 cm。 計畫區整體潮間帶近21年(80~101年)來的規模變化為縮小的情形，由98年至101年低潮位線(-1.5m)至岸側距離變化顯示漢寶海堤外海低潮線向內縮退縮仍有約100m、王功新生地海堤北段退縮約200m，顯示該區位低潮線持續退縮中。王功新生地海堤南段外海潮間帶僅約1.2km，為計畫區潮間帶最窄處。 本計畫透過現況問題蒐集、數值模擬與本計畫兩季調查結果，歸納計畫區潛在問題並建議因應措施如下： 1. 由數值模擬結果與近3年(98~101年)地形侵淤分析顯示彰濱工業園區崙尾西海堤至鹿港西海堤、永興海埔地海堤與大城鄉海側承受波浪作用力有變大趨勢，若堤前水深持續侵蝕恐發生掏空現象影響海堤防護能力。建議優先對彰濱工業園區崙尾西海堤至鹿港西海堤、永興海埔地海堤與大城鄉海段陸側至-15m等深線範圍定期舉辦海域地形量測。 2. 由民國90~101年間海堤高程歷年變化，知彰化沿海堤頂確受地層下陷所影響，然近3年地陷速率皆趨於穩定，高程變動值約為每年0.07cm，顯示地層下陷現象在計畫區並未加劇。 3. 海堤溯升分析顯示彰濱工業區崙尾海堤至鹿港西海堤波浪溯上高度最大達5.4~7.6m，餘裕不足0.4~0.72m，有越波情形發生但溢流量並未超過安全建議量(0.005m3/m/s)。而全興海堤、寓埔新生地海堤、永興海埔地海堤與大城海堤溯升高程已相當接近堤頂高程，需持續觀測與進一步分析。建議優先對彰濱工業區崙尾海堤至鹿港西海堤海段、全興海堤、寓埔新生地海堤、永興海埔地海堤與大城海堤舉辦海堤堤頂高程測量。 4. 計畫區整體潮間帶近21年(80~101年)來的規模變化呈現小幅度向岸側退縮趨勢。全海岸現有潮間帶規模平均尚保持2km以上寬度。潮間帶規模縮小的現況並無造成立即性明顯負面影響。建議持續進行2~3年一次的地形監測，以掌握最新現象。

Abstract To provide the reference basis for the design and the management of engineering work, the 4th River Management Bureau of the Water Resources Agency planned to establish a long-term fundamental background for the Chang-Hua coast. Therefore, a 2-year (2013 & 2014) project was arranged to conduct the field investigations study. The main works in the first year (2013) included: (1) the gathers of the basic coastal information; (2) the measurements of coastal topography; (3) the observations of current, wave and tide; (4) the investigations of coastal sediment transport; (5) the analyses of the topography migration during past decades; and (6) propose the concepture idea for coastal management and protection of the intertidal zones. The aforementioned field investigations (item 2, 3& 4) were performed during Mar~May and Sep~Nov, respectively. The measured coastal bathymetry roughly showed the isobaths oriented in NNE–SSW. The seabed slope is milder at the southern part of Chang-Hwa County that that at the northern part. For the measured area, the distances from the seawall to mean low water level is about 2km~3.4km. Generally speaking, the slope of the region from the shoreline to 20m isobath is 1/320~1/400. The discrepancy of seabeds around typhoon seasons shows that the morphologic change from shoreline to 0 m isobath isn’t obvious. Also, it is observed that the mixture of erosion and deposition pattern are found within 0m isobath to 5m isobath. For the northern and southern areas farther from 5 m isobath, the bathymetric changes are erosional and depositional, respectively. Basically, wind wave dominates in the study area according to the measurements conducted in this report. The wave properties of the sea areas around Hsien-Hsi township and Fang-Yuan township are consistent. The semi-diurnal tidal flow induced by the variation of tidal elevation at Chang-Hua coast showed that the flood flow and ebb flow are in the direction of NE and SW, respectively. The survey of coastal sediment transport pointed out that both the particle sizes of -IIbed material and wave energy at high water level around Han-Bo village/Wang-Gong reclaimed land are much bigger than that at the other locations. The sediment transport at 3m isobath depends on wave forcing, while the sediment transport at 10m isobath isn’t related to wave motion and current. The analyses on near-shore morphologic change show the depositional and erosional characters during summer and winter, respectively. And, the morphologic change shows the erosional characters all year round. The consistency is found between the survey of the sediment transport and the analyses of near-shore morphologic change. To evaluate the performance of the seawalls for coastal protection around Chang-Hua County, the numerical simulations were conducted based on the typhoon wave condition correspond to 50-year return period. Based on the analysis results, the wave overtopping was expected along the seawalls from Hsien-Hsi to Lu-Kang around Chung-Hua coastal industrial park and the average overtopping discharge is under the safety standard being 0.005 m3/m/sec. The analysis of the morphology changes from 1991 to 2012 remarked that the areas bounded by 5m isobath to 15m isobath at the north-eastern corner nearby Lane seawall, and at the northern of Fang-Yuan township were depositional character. The erosion was observed at the region between 5m isobath to 10 m isobath around Lu-Kang, area within shoreline to 10m isobath around the seawalls of Wang-Gong/Yong-Sing reclaimed lands. For more detail, the sediment volume changes rate within shoreline to 5 m isobaths per area is about 0.5~1.0 cm per month nearby the seawalls around Hsien-His, Han-Bo, and northern of Wang-Gong. For the regions bounded by 5 m and 15 m isobaths, the seabed volume changes rate is about -1.1~-0.2 cm per month around the whole near-shore except the regions outside Hsien-His seawall, southern of Han-Bo seawall and Da-Cheng township. The width of intertidal zone in Chang-Hua coast was decreased during past 21 years (1991~2012). In recent three years (2009~2012), the distance from shoreline to the low water level (1.5 m isobath) decreased about 100 m off Han-Bo seawall and decreased roughly 200 m at northern of the seawall around Wang-Gong township. The -IIInarrowest intertidal zone, 1.2 km, located at southern of the seawall around Wang-Gong reclaimed lands. According to the investigations and analyses, some potentially coastal hazard locations were proposed and suggestions were made as follows, 1. According to the numerical simulations and the morphologic change during recent three years (2009~2012), the tendency of wave forces on the seawall from Lane to Lu-Kang around Chung-Hua coastal industrial park, Hsien-His and Da-Cheng township is increased. If the erosion in front of the levee side of the seawall is aggravated, the toe scour at seawalls may be occurred and imperiled on the seawalls. Therefore, it is suggested that the regular measurement of seabed between shoreline to 15m isobath outside of the locations: (1)areas from Lane seawall to Lu-Kang seawall around Chung-Hua coastal industrial park; (2)Hsien-His township; and (3)Da-Cheng township. 2. According to the past records of the seawall crest elevation from 1991 to 2012, the seawall crest elevations are influenced by the land subsidence around Chang-Hua coast. However, the elevation change rate approached to stable state with the value being 0.07 cm per year. It showed that the situation didn’t become severe. 3. The runup analysis over the seawall showed that the runup over the Lane seawall to Lu-Kang seawall around Chung-Hua coastal industrial park reached to 5.4 to 7.6 m which are over the seawall crest elevation about 0.4 to 0.72 m. In this situations, the overtopping will be occurred with the average overtopping discharge which is under the safety standard being 0.005 m3/m/sec. The runups over the seawalls at Quan-Xing village, Yu-Pu/ Hsien-His reclaimed lands and Da-Cheng township are quite close to those seawall crest elevations. Therefore, the regular measurements of the above-mentioned seawalls will need to make future. 4. The widths of intertidal zone in Chang-Hua coast are slightly decreased during past 21 years (1991~2012). The narrowest intertidal zone with the width being -IV- 1.2 km is at southern of the seawall around Wang-Gong reclaimed lands. From the point view of coastal protection, the decreased situation doesn’t have current negative effect on the coast at Chang-Hua. It is suggested that the biennial or triennial measurements of coastal bathymetry should be held.