The Study of Morphological Change along Bali Marine Outfall Pipeline Trench

• Main Authors: Sung-En Hung, 洪崧恩
• Other Authors: 宋國士
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/qp9n78

碩士 === 國立臺灣大學 === 海洋研究所 === 106 === 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.