圖書典藏及影音數位平台

大甲溪上游發源於雪山山脈之次高山及中央山脈之南湖大山等群嶽，幹流由東往西橫貫大台中市，於大甲與清水間注入台灣海峽。本流域位處台灣島中西部，北與大安溪為鄰，南臨烏溪，溪幹全長124公里，平均坡度1/39，自民國82年迄今，大甲溪河道沖淤明顯受自然災害(九二一地震、桃芝颱風、敏督莉颱風)及辦理採砂作業等影響，河道呈現沖淤互見情況。近年本局為避免影響大甲溪防洪功能，並有效落實砂石開發供應，每年均需進行局部疏濬，為使大甲溪能持續辦理疏濬作業，本次爰依據100年2月22日，「大甲溪疏濬專案小組」第7次會議結論，擬辦理「大甲溪裡冷段開闊地區成為囚砂區可行性規劃」，作為爾後年度定期定點疏濬作業參考。 鑑於裡冷段大量土石淤積，影響通洪甚鉅，倘能適當深挖砂坑，可以利於防洪並提供砂源，惟其土砂量必需詳細依最新數值資料推估俾能辦理後續疏濬開採供應，除檢討上述採砂造成影響外，並評估該河段規劃囚砂區之可能性，爰提此委辦計畫。本計畫係以擇適當地點作為囚砂區提供建設所需砂源為主要目的，為考量大甲溪水系經常遭遇颱風或豪雨侵襲，預估上游河段挾帶之土石對中下游河段造成淤積影響防洪安全；評估現況調查水文條件、流域土地利用情形、河川排水特性、兩岸環境生態特質、河床穩定分析及輸砂能力分析，以永續經營管理之理念，提出有效且可行之具體管理策略，進而達到河道平衡及河川正常輸砂機能之目的。

A large amount of sediment deposits in the Li-Neng section of the Dajia River, which significantly affects flood capacity of the river. To dig some deep sand pools properly in the river, they are helpful for flood mitigation and they also can supply sands and stones for construction. The amount of sediment that could be removed has to be evaluated according to the updated numerical data. This project reviews influences of removing sediment and evaluates possibilities of installing sedimentation area. The Li-Neng section with wide breadth was chosen as the sedimentation area according to the 7th meeting of the “Special team for dredging operation of the Dajia River”, which was held on February 22, 2011. The sedimentation area is located in the main stream of the Dajia River, from 27K to 28K of the highway Tai-8, which length is about 900m. There are two main sources of sediment to the Dajia River. The one is from soil erosion of earth surface. The other is from debris of landside collapse. The yearly total amount of sediment to the Dajia River is about 11.68 million m3 according to the “Study on riverbed stabilities for the Dajia River downstream the Shi-Gang Dam (2/4), July 2009”, which assumed 40% of collapsed debris piled up on hill sides and the other 60% flowed to the river. The coverage of this project is between the Ma-An Dam and the Tian-Lun Dam. The yearly amount of sediment in this area is about 20.239 million m3. In addition, according to the “United and Integrated harness planning of the Dajia River basin, February, 2003”, the yearly averaged amount of sediment from sub-basins to the main stream is about 6.83 million m3. The Dajia River is a torrential river with cobble and gravel riverbed. Thus, this project applies the Schoklitsch equation to calculate sediment transportation of the river. Under high flow conditions, sediment transporting capacity upstream the Tian-Fu Bridge is less than that downstream the bridge. Therefore, for high sediment yield, sediment tends to pile up in the upstream river section. Sediment transporting capacity of downstream river section is large especially under high flow conditions. Scouring phenomenon happens easily if sediment yield from sub-basins is low and sediment transported from the upstream river section is not enough. This project adopts frequency analysis based on measured flow data to estimate peak flows. Downstream the section No. 68, the 100-year return period flood is used as the designed flood. Upstream the section No. 68, the 25-year return period flood is used as the designed flood. The HEC-RAS model is used for one-dimensional simulation and the CCGE2D model is used for two-dimensional simulation. One-dimensional simulation is applied to river section between the Shi-Gang Dam and the Tian-Lun Dam for long term simulation of scouring or depositing. Two-dimensional simulation is applied to the Li-Neng section of the Dajia River for discussing possibilities of sedimentation area. The results of long term one-dimensional simulation show that the trend of scouring or depositing does not change much. The reason is that the majority of long-term flow hydrographs are low flow conditions while the duration of high flow hydrographs takes only small percentage. Two-dimensional simulation is applied to river section between section No. 75 and section No. 79, the Li-Neng section upstream the Ma-an Dam. Under balance transportation and overload transportation, we review backfilling rates, amount of sedimentation and depths of averaged scouring or depositing for various sedimentation areas (3.5 hectares and 10 hectares) and various dredging depths (1m, 3m and 5m). Under overload transportation, twice amount of balance transportation, simulation results show that the amount of sediment in a 3m deep pool is about 472 thousand tons, which is 1.42 and 1.30 times of that of 1m deep pool and 5m deep pool respectively. Obviously, the amount of sediment does not increase with depth of the pool. In addition, through analysis of backfilling rates, amount of sedimentation and depths of averaged scouring or depositing, we find that 1m and 3m dredging depths will have better efficiency. Though a deeper pool will have a larger capacity, its cost increases at the same time and its efficiency is decreasing. Therefore, consideration on efficiency of dredging depth and sedimentation amount, we suggest adopting a sedimentation area with 10 hectares and 3m dredging depth. The benefit-cost ratio of the proposed sedimentation pool is 0.3. It is much low and the backfilling rate is only 11% of which the amount of sediment is about 65,264 m3 that is far less than the historical dredged amount, about 360,000 m3. At this stage, the riverbed is not yet stable, we suggest maintaining periodical riverbed surveying, channel dredging, floodplain management, alarm and safety measures, and flow capacity of 25-year return period flood.