圖書典藏及影音數位平台

計畫範圍為台子村(外汕頂洲北側3公里)至北門(急水溪口南側1公里)，沿岸長度約34公里。計畫區外海之外傘頂洲位於東石港西方海上約14公里，長約20公里，寬度1~3公里，面積約100餘公頃，呈東北、西南走向，為台灣最大沙洲，也是嘉義沿海地區之天然屏障。 由於外傘頂洲之侵蝕與變遷，嘉義沿海地區之天然屏障逐漸消失，必然衝擊嘉義沿海地區之防護作為，是故本局未雨綢繆，期藉由本計畫之基本資料蒐集分析、海岸波流與近岸地形變遷模擬，檢討外傘頂洲變遷對沿線海堤安全性影響及既有設施因應改善對策，以作為未來本段海岸防護規劃與設計之應用參考。

This project investigated the coastal area between Taizi Village (3km north of Wai-san-ding sandbar) and Beimen(1km south of the mouth of JiShui Stream), whose length is about 34km. For two-years, this project collected and analyzed coastal information, investigated the distribution of coastal protection engineering works, ran coastal wave field simulation, assessed the effect of changing Wai-san-ding sandbar on seawall safety and formed strategy of improving current seawalls to provide necessary information for planning and designing coastal protection in the future. In this year, the second year of this project, this project continued to collect and analyze coastal information, analyze topographic change of Wai-san-ding sandbar, evaluate the effect of changing Wai-san-ding sandbar on seawall safety, form strategy of improving current coastal protection and perform in-situ beachline survey. The works performed are described in the following sections: For analyzing topographic change of Wai-san-ding sandbar, this project explained the shoreline and area change of Wai-san-ding sandbar summarily and examined natural and human causes of Wai-san-ding sandbar change. The investigating results show that Wai-san-ding sandbar drifted continuously to south and east in the last twenty years. If the sandbar area kept decreasing, the level of the sandbar will become smaller than EL. 0m in 2028. For presenting the effect of changing Wai-san-ding sandbar on the oceanic wave field in the investigated ambit. In addition to the simulations of wave directions SW and SSW during typhoon, which were performed in year one of this project, the simulations of wave directions NNW, WNW, W and WSW were performed this year. The simulation results were used in drawing wave field distribution map, the map of wave level difference between current topography and abated Wai-san-ding sandbar and the cross-section map of wave level in front of seawall for every seawall part. The investigation results showed that since the northern part of seawall in the investigated ambit(from the seawall of Aogu tidal flat to the seawall of Dongshih tidal flat) were sheltered by Wai-san-ding sandbar, the influence of abating Wai-san-ding sandbar on its wave level in front of seawall was greater; the influence of abating of Wai-san-ding sandbar on the wave level in front of seawall of the southern part of seawall (from Wangliau seawall to the seawall of Hou Mei Li tidal flat) was so small that it can be ignored. With the corresponding hydrologic estimation method and empirical equations, the simulation results were used to evaluate seawall safety like wave runup level, wave overtopping rate and revetment block stable weight. In the condition of typhoon wave of 50 year recurrence year period, every seawall part of current topography showed no wave runup. In the case of risen water level and abating Wai-san-ding sandbar, wave runup occurs on the seawall of Aogu tidal flat and the seawall of Dongshih tidal flat. But the wave runup was still within safety margin. The stability and disaster prevention capability of current coastal structure in the case of risen water level and abating Wai-san-ding sandbar were estimated by analyze possible wave runup level and wave overtopping rate. For the investigated ambit, seawall improvement plans were proposed. The plans focused on improving front slope of seawall and included plans of duplex bands and duplex gradual slopes. In June 17, 2015, in-situ survey was performed by 34 experts including officials of Water Resource Agency and Fifth River Management Office, inspection committee members and members of other related agencies. The purpose of survey was to investigate the wave effect on sandbar low beach and overall beach distribution of Wai-san-ding sandbar on high water level.

本研究分析外傘頂洲的變遷對嘉義海岸防護的影響。研究範圍涵蓋太子村至急水溪口約34公里海岸線。研究發現，外傘頂洲近二十年來持續向南東漂移，若面積持續縮減，預計2028年將低於海拔0公尺。 外傘頂洲的變遷會影響海域波場，尤其對北部海岸（鰲鼓海埔地至東石漁港段）的防護影響較大，南部海岸（網寮至好美里段）影響則微乎其微。研究評估了海堤安全，發現現況下海堤安全無虞，但在水位抬升及外傘頂洲退縮情況下，北部海堤可能出現波浪越堤，但仍在安全範圍內。 為此，研究提出了海堤改善構想，包括複式平台、緩坡化、消波工調整等方案，並針對網寮、白水湖、布袋鹽場等海堤段提出環境營造對策，以提升嘉義海岸的防護能力。