淡水河口南側海岸地形變遷之研究

Abstract

河口海岸地形非常容易受到環境營力的影響而產生變化，除了受到自然因子，如河川流量、輸沙及沿岸流等影響之外，也深受人為活動的影響，包括上游地區的人為工程與下游海岸的港口建設等。相對地，河口海岸地形的變化，亦會影響到人類的土地利用型態，故此區域的相關研究，顯得相當重要。 數十年以來，淡水河口南側海岸為台灣地區海岸侵蝕嚴重的區域之一。此區的地形變化，主要受到淡水河流量的影響。於1993年台北國際商港開始興建後，其北防波堤對於地形變化的影響，非常值得探討。本研究的目的在於探討河口海岸地形變化的形貌特性及人工構造物對其之影響，藉此瞭解地形變遷的特性與趨勢。 1987年至1998年間，淡水河口南側的地形變化，大致呈現侵蝕為主的趨勢；1998年之後，則以堆積為主要趨勢，堆積的位置主要分布於防波堤之北側，此區域的海岸線，由平直轉變為弧型，並且向海堆積擴張。 研究結果係分析淡水河口河道至北防波堤間的地形變化，1998年之前，影響地形變化的主要原因，應為淡水河流量之變化。當淡水河流量增大時，研究區呈現侵蝕的狀態；淡水河流量減少時，研究區呈現堆積的狀態，此時期的地形變化，與流量之間呈現負相關，顯示出流量中的輸沙並無沈積於研究區中，反而流量大則造成侵蝕。1998年北防波堤完工後，當淡水河流量增大時，研究區呈現堆積的狀態，顯示出防波堤攔阻大部分的沈積物，導致淡水河之沈積物堆積在防波堤北側。 而季節性的變化在研究區中亦相當明顯，於東北季風盛行的季節，距離海岸較近區域，以侵蝕為主要的趨勢；距離海岸較遠的區域，以堆積為主要的趨勢。台灣北部地區，9月至隔年4月受到東北季風影響，使風速與波浪增大，造成冬季的海岸以侵蝕為主。反觀，夏季則傾向於堆積作用的盛行。本區海岸從1987年到2003年，全區年平均海床變化大致處於平衡狀態。但是，此期間季節性的地形變化非常的顯著。而在台北港防波堤建成之後的1998年至2002年期間，前述的平衡狀態因為突堤效應，轉為堆積狀態，並使季節性的侵蝕堆積循環也變弱，最終消滅了沙脊沙槽交替的海床地形。 而夏季的地形剖面，主要由狹窄且坡度較陡的前濱區與坡度較緩的階台所組成。依其夏季剖面變化的型態，可以分為兩類：邊坡變化型及平行變化型，其中以邊坡變化型佔多數，表示此區域主要的營力，應是來自垂直海岸方向的營力，但亦受到平行海岸方向營力的影響。但是，垂直海岸方向的營力影響較大，佔總值的57%，水平營力的影響只佔43%。 夏季地形不論是邊坡變化型或平行變化型，堆積現象佔總變化量的69%，而侵蝕現象僅佔31%。且兩種營力的分布範圍，呈現出區域的差異，越靠近淡水河口的範圍，主要是以平行海岸的流向為主，越往南側的區域，則是以垂直海岸的波浪為主。顯示出河口海岸地區所受的營力複雜，而且有相當明顯的區域差異。

The estuary is highly sensitive to environmental processes. It is simultaneously affected by natural factors and artificial factors. The change of coastal landform also influences land use, so coastal research is significant for human. In the Recent Decade, coast of southern Dansuie estuary was one of serious erosion part in Taiwan. Coastal landform change of southern Dansuie estuary is mainly influenced by river discharge, so the change of the discharge is important. The effects of Northern dike to landform change were worthy to research after port of Taipei built in 1993. The purpose of this research is to understand geomorphological changing and influence of artificiality at Dansui estuary. From 1987 to 1998, coastal landform change approximately trended to erosion. After 1998, Coastal erosion had been turned into accretion and deposition position appeared in northern dike. The coastline has changed from straight to arc shape, and furthermore expands to the seaward. The result of this study is to analyze coastal landform change from channel to northern dike. The main reason influencing the landform change should be discharge of Dansuie River before 1998. When the discharge increased, the coastal landform was eroded. Oppositely, when the discharge reduced, the coastal landform was accreted. Landform change is negative correlation with discharge in this period. When northern dike was completed in 1998, sediments were accreted in the study area as river discharge increasing. It's reveal that most sediments are blocked by northern dike, and deposited in north of dike. Seasonal dynamic change shows that there was sand ridge and trough phenomenon in nearshore and maintained dynamic equilibrium. The sediment budget maintained dynamic equilibrium, but it show seasonal replace. From September to next April, the coastal landform was usually eroded because northern monsoon increasing. After Port of Taipei was built in 1998, jetty produced to sand ridge and trough phenomenon were dissolved. The profile is composed of narrow foreshores which are steep and flat terrace in summer. There are two types of profile change processes, including cross shore and longshore sediment transport dominate. The process of cross shore dominant area is broader than longshore dominant area. Longshore dominant exists near Dansuie estuary, and cross shore dominant is very wide distributed in southern area.