圖書典藏及影音數位平台

本計畫旨在探討曾文溪河口水理與輸砂機制、曾文溪下游河道疏濬改 善通洪斷面及上游曾文水庫放淤操作策略改變河川輸砂等行為對曾文溪河 口海域地形之影響，以及就海埔堤防前海灘侵蝕問題與人工養灘防治對策 進行水工模型試驗評估。本計畫全程為兩年(105~106年)，整體工作項目分 為基本資料蒐集與彙整分析、曾文溪河口與鄰近海域歷年地形變遷趨勢探 討、海岸漂砂觀測調查、曾文溪河口水理與輸砂機制初步探討、河口輸砂 對鄰近海域影響與評估、海埔堤防保護措施水工模型試驗及改善對策評估 等7項。

The is a two-years project (from 2016 to 2017), aiming to investigate the effects of Zengwen River downstream dredging and Zengwen Reservoir desilt operation on the topography changes around Zengwen River estuary, and to propose a beach erosion mitigation strategy in front of Qigu Seawall. There are 6 research items which include: 1.) background data collection and analysis；2.) the trend of topographic changes in Zengwen River estuary and adjacent waters；3.) investigation on the hydrodynamics and sediment transport mechanism around Zengwen River estuary; 4.) the effects of the estuary sediment transport on adjacent waters; 5.) hydraulic modeling test on the protection works of Qigu Seawall; and 6.) assessments on the countermeasures to the seawall protection . Results of are summarized as follows: In the reorganization of field data and the trend of topographic changes in Zengwen River estuary and adjacent waters, the topography and hydrological data around Zengwen River estuary were collected and analized. The topography changes in the past few centuries can be roughly divided into two periods. Before 1930, the natural balance between the river sediment supply and the sea-side dynamics donamited the Zengwen River estuary, resulting in the formation of many lagoons and offshore sandbars. While after around 1930, the systematic river management to prevent floods, the coastal reclamation and the construction of Zengwen reservoir are respondsible for the shoreline retreat. Based on satellite images from 1984, it shows that the shoreline between the the right bank of the Zengwen River to the junction of the Qigu River Revenment and the Qigu Seawall retreats year by year. In recent years (2012 ~), the erosion of the beach has become worse. And it not only results in a large amount of fish pond loss, but also endangers the safety of the seawall. In addition, the project uses ArcGIS 10.2.2 software, coupled with spatial interpolation method to analize the nearby bathymetric data. It again proves the erosion trend in this site. In the investigation on the hydrodynamics and sediment transport mechanism around Zengwen River estuary, based on the field observation of river flow, water level, suspended concentration and waves in 2013, three representative conditions, namely, a typhoon event condition, a summer monsoon condition and a winter monsoon condition were established. The results of numerical simulation show that the erosion features of Zengwen River estuary includes: (1) the waters infront the Qincunlun Seawall and the junction of the Qigu River Revenment and the Qigu Seawall were significantly affected by the waves; (2) under typhoon flooding condition, it overflows the right bank flooding plain and washes soils and even fish ponds away (3) sediment discharge during flood events has positive effect on the estuary and the adjcent coast in terms of beach protection; (4) in the winter monsoon wave conditions, significant topographic erosion occurred near the estuary and the adjcent coast. There are three sub-topics under the item of “the effects of the estuary sediment transport on adjacent waters”. The first one is “the effect of river flow and sediment transport on the recharge of coastal sediment sources. The second one is “the effect of dredging of Zengwen River and the Zengwen Reservoir desilt operation. The third one is “to propose a preliminary countermeasure and to evaluate its performance”. In the low flow period, only the summer monsoon wave condition simulation showed a small amount of siltation. On the other hand, in the high flow period all the three sea side conditions, the summer and winter monsoon waves and the 2016 recorded typhoon waves, result in siltation around the estuary. It indicates that the sediment laden high flows are helpful in terms of the coastal erosion mitigation in front of the Qigu Seawall. Results of the second topic show that in the summer monsoon wave condition also results in siltation in front of the Qigu Seawall. It means that the Zengwen reservoir desilt operation is positive in terms of the estuary siltation. We then proposed three artificial beach nourishment plans and tested by inputing the 2013 typhoon event data. Simulation results show that the larger the nourishment volumn, the more nourished soil washed away. As for the hydraulic modeling test on the protection works for the Qigu Seawall, a 1/100 verticlely and 1/200 horizontally scaled down model was constructed. The test conditions included 25 and 50 years return periods wave conditions and 10 years return periods river flow condition. Under the condition of flow-only (no waves) test, the erosion amount of the river channel will be deposited in the water between E.L. -3m ~ -2m. Erosions were found on the right bank flood plain and at the corner the Qigu Seawall. The results of wave-only (no river flow) test showed that there were obvious erosions in the waters near the seawall structures on the left and right bank of the estuary. The erosion was significantly enhanced in the 25 years of typhoon wave test. While in the wave and current interacted condition, results of the 10 years return period waves show that the amount of sand washed out from the river can still be deposited in the estuary between E.L. -3m ~ 2m, but the volume is obviously reduced. On the whole, typhoon waves usually significantly erode the adjacent shoreline at the junction of the Qigu River Revenment and the Qigu Seawall. However, in case the large waves and large flows happen at the same time, the discharged sediment would deposited in the shallower waters and thus mitigate the erosion potential. As for the beach nourishment cases, it shows that the nourishment material washed by typhoon waves would be transported to the north and the right bank of the estuary, resulting in the recharge to the adjacent coastal areas. Therefore, the beach nourishment plan did indeed have benefit for the seawall and the adjacent coasts. Under the action of typhoon waves (48hr), the nourishment material losing rates are case B-T25: 12.6%; case B-Q10T25: 10.0; case C1-T25: 8.0% and case C2-T25: 5.3%. Among them, the case C2-T25 with the protection of five geo-tubes in the has the best performance. It indicates that additional protection works can really reduce the loss of nourishment material. The assessment of the proposed countermeasures shows that the most important factor is the location of the nourishment material source. Among them, the case 4, which all the nourishment material was supplied from the nearest location (the right bank flood plain), has the lowest cost. As for the maintenance cost, if the nourishment material can be supplied from the nearest location, there is no need to build the five geo-tubes, since the recharge cost is much cheaper than the construction of the five geo-tuees. However, if the source of sand is to be transported from the Kuosheng Bridge to the Xikang Bridge section, the construction of the five geo-tubes will be considered as options. It is suggested to carry out the nourishment work, then keep monitoring and make further decision after this area was attacked by typhoon waves.