八里海洋放流管管床形貌變化之研究

Abstract

八里海洋放流管位於淡水河口西南岸的八里外海，是大台北地區的家庭及工業廢水經處理以後，運送至河口外海排放的管道。放流管完工於西元1997年，總長約6,660 公尺。放流管在建置時，施工單位在八里外海的海床上浚挖出一條浚溝，放流管便被放置在這條浚溝內。此條浚溝雖然後來有利用級配石試圖將其填平，但在外海約三公里長的區域，因級配石拋置的位置錯誤，仍有相當大段的浚溝未被回填完整，並使得一部份的放流管還裸露在海床上。受到放流管周圍沉積物搬運的影響，未被填平的浚溝以及裸露的管線多年來有逐漸被沉積物掩埋消失的情況，並同時造成管床形貌的改變(宋國士，2013)。 本研究利用西元1997年至西元2013年放流管周圍的側掃聲納影像以及西元2008年至西元2013年放流管周圍的多音束地形資料，來分析自從放流管建置以來，浚溝回填的過程以及管線裸露情況的演變，並藉由多年來管床形貌的特徵及其變化的趨勢，來探討放流管周圍沉積物的來源、搬運以及回填機制。 結果顯示，多年來未被填平的浚溝以及裸露的管線逐漸被淡水河輸出的沉積物由首尾兩端向中央掩埋消失，又以靠岸端掩埋消失的速率較快。在西元2008年以前，海床上仍有較為明顯的浚溝形貌，當浚溝外的海床沉積物隨著底流流經未被填平的浚溝時，浚溝周圍產生的特殊流場除了會有助於沉積物於浚溝裡堆積，使得浚溝回填的速率相對較快以外，也會在浚溝周圍的海床上形成波痕；在西元2008年以後，隨著全段的浚溝接近被掩埋消失，浚溝回填的速率明顯趨緩，浚溝周圍波痕的發展則較不顯著。根據波痕的波形顯示，放流管周圍的底流流向和河口外海的潮流流向大致相符，且底流的優勢流向會隨著季節風場的轉變而有所不同，推測放流管周圍海床沉積物所受到的營力可能和潮流以及季節風所引起的風吹流有關。

Bali marine outfall pipe is located off the coast of Bali. It discharges household and industrial sewage of Taipei area to the sea. The outfall pipe was completed in 1997, with a total length of about 6,660m. Before the outfall pipe was built, the construction unit dug a ditch on the seabed in order to place the outfall pipe. Though there were attempt to use graded stones to fill the ditch, there was still a large segment of the ditch filled incomplete at the far shore, caused a part of the outfall pipe exposed on the seabed. Affected by later natural deposition, this segment of the ditch gradually backfilled by sediments, while changing geomorphology of the seabed around the outfall pipe. (Song, 2013) This study uses the data collected by side scan sonar from 1997 to 2013, and the data collected by multi-beam echo sounder from 2008 to 2013 to analysis the backfill process of the ditch and appearance changes of the surrounding seabed since the completion of the outfall pipe. Through the morphological change of seabed over the years, we wish to explore the Sediment transportation around the outfall pipe. The results show that the exposed outfall pipe and the ditch had been gradually backfilled by sediments from Tamsui River and disappeared from the head and the tail to the center, where the nearshore side disappeared faster. Before 2008, the ditch on the seabed was still obvious, when the bottom flow past through the ditch, the flow field would change under the influence of the undulating seabed, not only let sediments fill the ditch at a faster rate but also formed ripples around the ditch. After 2008, the whole segment of the ditch was almost backfilled complete, the rate of sediment backfill had been extremely slow, and the ripples around the ditch were relatively less obvious. According to the morphology of the ripples, the main direction of the bottom flow around the outfall pipe was coincide with the direction of tidal currents off the Tamsui estuary, and it would change with monsoons, which suggested that the submarine sediment transportation around the outfall pipe might be related to tidal currents and wind driven currents.