桃園草漯沙丘-沙灘地形互動之研究

海岸沙丘與沙灘地形受到風、波浪與潮汐多種控制因子影響，沙灘與沙丘的沉積物系統依據風向、風速、波高與潮差會形塑不同的地形景觀。海岸沙丘與沙灘地形在不同觀測時間尺度下，因時距內平均的營力作用海岸沙丘與沙灘具有不同的地形變化特徵。本研究以草漯沙丘作為研究區，分別以短期及中期時間尺度探討草漯沙丘地形變化與控制因子的關係，短期聚焦在草漯沙丘的季節性變化，中期尺度則關注近20年的濱線與海岸沙丘變化。地形的季節性變化使用RTK-GPS測量2019年至2020年間的六座調查區的海岸沙丘與沙灘的地形高程，施測頻率為每4個月一次，分別代表冬季、春季與夏季的地形。近20年的地形變化使用2001年至2018年間的航空照片與衛星影像呈現濱線與海岸沙丘的變化。攔砂籬、風力發電機、風機維修道路與海堤等人為設施對草漯沙丘地形的影響亦為本研究探討之重點。季節性的監測結果顯示六座調查區的地形變化有所差異，主要差異在沙丘前坡坡腳的恢復與沙丘崖的侵蝕的時間分布不太一樣，沙丘崖的侵蝕頻率與空間分布可能與乾砂寬度相關，乾砂寬度較窄的區域波浪容易到達沙丘坡腳。乾砂寬度與波浪作用相關，波浪入射角、沙灘地形、波高會影響沿岸波能的分布，在波浪的攻擊點波能較大，導致乾砂寬度較窄，影響後方前列沙丘砂源之供給。大體上沙丘崖形成的季節以春夏兩季為主，其中又以夏季最多，冬季的沙丘崖消失恢復為沙丘的前坡面。氣象與海象資料顯示夏季的平均示性波高雖然較低，但夏季颱風的暴潮與湧浪在灘面上的波浪溯升可能刮蝕坡腳形成沙丘崖；冬季的平均示性波高較高，但強勁的東北風將後灘的乾砂吹往坡腳堆積，且冬季的水位沒有受暴潮影響而上升，海水較無機會抵達前坡坡腳。在沙丘前坡人工設置的攔砂籬因為積砂，造成坡腳與後灘之間的坡度變陡，亦干擾前坡上的飛砂移動。研究結果顯示近20年的濱線大致呈現後退，活動沙丘後緣則受到人工植林防風定砂的作用，大致穩定，整體活動沙丘的寬度縮減。風力發電機、風機維修道路與海堤會降低飛砂活動。沙丘上人為設施的設置位置需考慮地形與營力間的交互作用與沉積物動態，根據管理目的不同在適當的地形區實施相應的管理措施才能海岸沙丘帶的永續經營。前列沙丘上的攔砂籬方面，本研究建議後方較無人為設施的海岸沙丘段，可以嘗試將沙丘上的攔砂籬拆除，讓飛砂自由朝著內陸移動，讓整體的沙丘剖面形態與景觀恢復為較自然的樣貌。

關鍵字

沙灘-沙丘地形；時間尺度；沙丘崖；濱線變遷；人為設施

並列摘要

Coastal dune and beach morphology is controlled by a variety of processes and impact factors, with wind, waves, and tides. The sediment system of beach and dune will shape different landscapes according to wind direction, wind speed, wave height and tide range. Coastal dunes and beach morphology have different morphological characteristics at different time scales of observation due to the effect of time-averaged processes. Caota dunes is the study area, and investigate the relationship between topographic changes and control factors of Caota dunes at different time scales, focusing on the seasonal changes of Caota dunes in the short term and the recent 20 years changes of shoreline and coastal dunes in the medium term. Seasonal changes in topography were measured using RTK-GPS for coastal dunes and beaches in the six survey areas between 2019 and 2020, at a frequency of once every four months, representing winter, spring, and summer morphology, respectively. The topographic changes over the last 20 years are presented using aerial photographs and satellite images of the shoreline and coastal dunes from 2001 to 2018. The effects of anthropogenic facilities such as sand fences, wind turbines, wind turbine maintenance roads, and seawalls on the topography of Caota dunes are also the focus of this study. The seasonal monitoring results show that there are differences in topographic variation among the six survey areas, mainly in the recovery of dune stoss slopes and the erosion of dune scarps. Frequency and spatial distribution of the dune scarps may be related to the width of the dry sand, and the waves in the areas with narrower dry sand width can easily reach the dune foot. Dry sand width is related to wave action. The angles of incident wave, beach morphology, and wave height affect the distribution of wave energy along the shoreline, and a larger wave energy at the point of wave attack results in a narrower dry sand width, which affects the supply of sand to the foredunes behind. Generally, dune scarps are formed mainly in spring and summer, with summer being the most frequent season and dune scarps in winter disappearing and reverting to the foreslope of dunes. Meteorological and marine data show that the average significant wave height in summer is relatively low, but the storm surges of summer typhoons and swells run up on the beach may scrape and erode the dunefoot to form dune scarps; the average significant wave height in winter is relatively high, but the strong northeastern wind will blow the dry sand from the backshore to the dunefoot, and the water level in winter is not risen by storm surges, the seawater has no chance to reach the dunefoot of the foreslope. The artificial sand fences on the foreslope of the dune caused the steepening of the slope between the lower foreslope and the backshore due to sand accumulation, which also interfereed with the movement of sediments on the foreslope. The results of the study show that the shoreline has generally receded over the past 20 years, while the landward edge of the active dunes is generally stable due to the effect of forestation in preventing wind and sand fixation. Wind turbines, wind turbine maintenance roads, and seawalls will reduce the area of active dunes and reduce potential sand transporting. The location of artificial facilities on the dunes requires consideration of the interaction between geomorphology and processes as well as sediment dynamics, and to implement appropriate management measures in appropriate morphological areas according to management goals in order to sustain the coastal dunes. The study suggests that the sand fences on the dunes should be removed in the back of the dunes where there are no artificial facilities, so that the sand can move inland and the dune profile and dune landscape can be more natural.