外傘頂洲變遷對嘉義海岸防護之影響分析研究(2/2)

本計畫工作範圍為台子村(外傘頂洲北側3公里)至北門(急水溪口南側1公里)，沿岸長度約34公里。計畫分兩年度辦理，其目的希冀藉由海岸之基本資料蒐集分析、沿岸防護工分佈及海岸波場模擬，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。 本年度(第二年度)計畫繼續進行基本資料蒐集與分析、外傘頂洲地形變遷分析、外傘頂洲變遷對計畫區海域波場影響、外傘頂洲變遷對沿線海堤安全性影響、既有設施因應改善對策與外傘頂洲灘線現地踏勘等工作項目。其相關工作情形如下： 外傘頂洲地形變遷分析方面，歸納說明外傘頂洲海岸線及面積變化情形，再由自然力及人為因素兩方面分析外傘頂洲變遷的特性。分析結果顯示近二十年外傘頂洲仍持續向南及向東漂移。若砂洲面積縮小趨勢不變，於2028年以後砂洲高度將沒入EL.0m以下。 外傘頂洲變遷對計畫區海域波場影響方面，除第一年度計算之SW及SSW颱風波向外，增加NNW、WNW、W及WSW四組波向做模擬。以模擬結果繪製波場分佈圖、現況地形與外傘頂洲高度降低後之波高差異圖、各堤段堤前波高斷面圖。分析結果顯示計畫區北段海堤(鰲鼓海埔地至東石海埔地事業堤)位於外傘頂洲遮蔽區內，堤前波高受外傘頂洲高度降低之影響較大；計畫區南段海堤(網寮海堤至好美里海埔地事業堤)在外傘頂洲高度降低後影響較小或幾無影響。 外傘頂洲變遷對沿線海堤安全性影響方面，依據相關水理推估方法及經驗公式，由波場模擬結果分析海堤之安全性，如波浪溯上高、越波量、被覆塊安定重量等。各堤段之波浪溯上計算結果，在50年重現期颱風波浪條件作用下，現況地形時各堤段皆無堤後越波情形發生。若增加考量水位抬升、外傘頂洲高度降低、地層下陷等條件下，於鰲鼓海埔地事業堤及東石海埔地事業堤會發生越波情形，但其越波量尚在安全標準內。 既有設施因應改善對策方面，依海堤波浪溯上與越波量分析結果，探討目前計畫區海岸於外傘頂洲高程變化下是否影響現有海岸結構物之穩固與防災安全功能，先評估海岸結構物現況，再提出其具體之海堤改善方案。方案以改善堤前坡面為主，提出複式平台及複式緩坡兩種方案。 外傘頂洲灘線現地踏勘部分，於2015年6月17日，邀請水利署、審查委員、第五河川局、相關機關等專家共34人於高水位時段辦理現勘行程，瞭解外傘頂洲在高水位條件下，砂洲低灘受波浪作用之情形及整體砂灘分佈。

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.