台灣還需要消波塊嗎?

台灣還需要消波塊嗎?

撰文╱林宗儀

台灣不論是東部岩岸或西部砂岸,大量消波塊盤據海岸線,猶如城堡圍牆圈繞整座海島,展現「堡」島威力。這是什麼原因造成的?

台灣的海岸線究竟怎麼了?

全台海岸線在20世紀後半葉的侵蝕後退現象,多少受到氣候變遷的影響,但細究之下,有很大一部份和海岸工程結構體脫不了關係。只要增設硬體建築(港口設施、填海造陸的工業區、海岸防護設施等),就可能干擾原先達到地形平衡的海岸系統,海岸的各種地形營力(潮、波、流、風等)也就跟著變動,造成局部地區的侵淤變化。

防護結構為續命 消波塊越放越多

海堤、突堤、離岸堤等海岸防護結構體,基本上是由水泥結構體和大量消波塊組成。為了延長這些海岸防護結構體的壽命,政府單位通常會在堤腳處放置更多消波塊,以減少波浪直接撞擊破壞堤身,這樣的防護措施也就讓海岸線無可避免地佈滿消波塊。但是放置消波塊真能保護台灣的堤坊和海岸嗎?

消波塊非特效藥 浪太大會變殺手

其實消波塊並非海岸防護工程的特效藥,波浪較小時,消波塊尚能發揮功用,當波浪大到一定程度,消波塊可能會承受不了波浪的撞擊而破碎,甚至整塊被浪捲起、拋向岸邊並加速撞擊海堤堤面,反過來破壞水泥結構體。

此外,突堤、離岸堤的消波塊若排列堆置不當,也可能增強消波塊附近的渦流,剷挖侵蝕消波塊底部的海床,使其更快沒入海中,但政府單位卻常在沉陷的突堤、離岸堤上丟放更多消波塊因應。

在氣候變遷下面對海岸侵蝕問題,消波塊、水泥結構體並無法真正有效保護海岸,甚至結構體本身損壞維修的需求會更加頻繁,這是政府可以永久負擔的嗎?另一方面,這些防護工程會干擾鄰近地區波浪與海流,導致漂砂供輸失衡,進而演變成海岸侵蝕,因此海岸防護工程的傳統工法確實到了需要檢討並改變的時刻。

人工沙丘+撤離 主動遠離危險區

改變的方法可以選擇人工養灘、建造沙丘等軟性(或柔性)穩定工法,也可以選擇棄守、後撤搬遷等非工程的調適措施。

人工養灘(以外來砂源回填侵蝕區域)和建造沙丘(以設置砂籬、人工植生或人工堆砂方式形成沙丘)的設計思維,是希望藉由沙灘和沙丘堆積的鬆散砂體削減波浪、減輕災害。沙灘或沙丘平日積蓄的砂量越多,防護海岸的能力就越強。建造人工沙丘來防護海岸在歐美地區已行之有年,但全球海平面上升的趨勢,使人工沙丘的生命效期不如預期,導致維護成本經費不斷提高,最終可能超出政府可負擔的範圍。

棄守、後撤搬遷的選項是指海岸建築或公共設施受到波浪破壞後,居民可以決定不在原地修復而予以放棄,主動搬遷、離開危險海岸地區,或把重要且有價值的建築或設施往後搬遷到較安全的位置。但要往後搬遷多遠才安全?搬遷所需距離就是緩衝帶(或後退帶),政府可以透過土地規劃,訂出未來易受海岸侵蝕的區域做為緩衝帶,並以環境經營管理的角度規範區內的土地利用,允許低強度的使用方式(例如自然生態或環境教育場域),但不允許高強度的開發(例如旅館設施),以減輕未來發生海岸災害的損失。

事實上,沙丘防護工法的成功與否,在於人類是否能夠劃設適當的緩衝帶讓沙丘自由移動,使其不受人為干擾。運用人工沙丘等軟性工法搭配撤離措施,使人類主動遠離易致災地區,也就能夠減少災害損失。

但目前國內主流民意還是認為傳統硬體設施比較能帶來安全感,濱海居民也不願意放棄易致災的家園,對於後撤或劃設緩衝帶的觀念仍難接受,不過主管海岸防護的政府單位已經不再新增海堤,盡量朝海堤維修或撤退的方向來思考。

面對大自然強大力量,用堤防或消波塊來抵抗猶如螳臂擋車,並非解決海岸侵蝕後退問題的最佳藥方。了解自然的運作方式,繼而順應自然,才是海岸永續的正道。(摘錄)


本文取自《科學人》雜誌2019年9月號,更多的內容歡迎閱讀《科學人》雜誌。相關訊息網址:http://sa.ylib.com

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