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二仁溪流域地表起伏大，流域大都為台灣西南部泥岩分布。下游兩岸陸續開發，如高鐵台南站區。又因氣候變遷，颱風豪雨期間雨量動則大於200年頻率，超過二仁溪保護標準，且泥岩地形易沖刷進入河道，造成淤積之傾向。二仁溪河口至縱貫鐵路橋間之河段堤防完成整治，為評估河道防洪能力、河床泥砂平衡狀況及長期河床發展趨勢，以瞭解其沖刷成因，有必要針對流域之產砂、河道輸砂量及河床穩定趨勢，詳加調查並進行模擬評估，以研擬防治策略，作為未來河道治理與管理之參據。本計畫第一年主要工作目標為彙整相關模擬成果、河川特性及現地觀測，瞭解計畫區內之水工構造物及跨河構造物受災機制，作為初步研擬沖刷防治工法之參考；第二年則為透過水理輸砂模式模擬不同重現期距年流量及長期流量下對河道沖淤模擬，評估河床穩定性，並綜合兩年成果，提出整體沖淤防治策略。本流域於中下游地區主要問題為河道受到淤積，通水斷面減少，常有淹水情形發生，本河段歷年已多次進行疏濬；上游部分，二仁溪主流蜿蜒而行，部分凹岸地區受到沖刷導致邊坡崩落、土砂流失。經由歷年資料進行河床穩定性探討顯示，經由98年大斷面比較二仁溪寬深比(W/D)推估二仁溪河口至舊二層橋河段屬寬淺型河道，二仁溪其餘河段皆為窄深型。由深槽偏移變化率來看，二仁溪流路左右擺盪，為多砂河川常見類型，進入縱貫鐵路橋後，因受堤防束縮，因而流路較為穩定。比較民國90年及98年全斷面資料，二仁溪主流累積沖刷量約為43萬m3，河床平均高程略為下降0.04m。從橫向/縱向穩定指標得知，二仁溪河道由於流路蜿蜒河道窄深且河床質粒徑較細，河床變動較快，穩定性欠佳。經由二維輸砂模擬結果顯示，主要沖刷河段為斷面5至斷面6、斷面10至斷面12、斷面44、斷面63至斷面66、斷面81至斷面92、斷面99、斷面103至斷面104、跳橋上游(斷面119至120)等，究其原因可能為流速增大或地形變化所致。除此之外，整體河道大多屬淤積河段，主要受到上游泥岩地形沖刷至下游及輸砂量變大導致。整體而言，二仁溪為蜿蜒不定河川，易造成上游局部性的沖刷，導致下游淤積特性，而沖刷區域範圍多屬上游且鄰近地區較無保全對象並已施設相關保護工法。本計畫針對易淤積段布置三囚砂區，分別為斷面24至斷面25、斷面44至斷面45及斷面53至斷面54，疏濬量分別為20萬、10萬及15萬立方公尺，推估疏濬周期分別為2至3年、4至5年及8至10年。就防洪能力探討而言，疏濬可有效降低水位，提升防洪能力。經由減淤比及減淤效率之評估，歷年已疏濬區案例以中路橋上下游疏濬案例為較佳之疏濬河段，相對減淤效果較為顯著。未來可疏濬區段評估探討得知，斷面23至斷面26之方案一，疏濬型式及規模為較佳之疏濬案例。

The surface of the Erren River Basin varies greatly. Comprised of mudstone, the Erren River Basin primarily covers southwestern Taiwan. The downstream shores are gradually being developed, such as the Tainan High Speed Rail (HSR) Station area. In addition, because of climate change, and the amounts of rainfall from typhoons constantly exceeding the average rainfall in the past 200 years (and Erren River protection standards), the river easily washes the mudstone terrain into the river channel, leading to siltation. The river section between the Erren Estuary and the railway bridge has been completely dredged to assess flood control capacity, riverbed sediment balance, and long-term riverbed development. To understand the causes of erosion, it is necessary to perform detailed investigation and evaluative simulations regarding basin sediment yield, degree of river denudation, and riverbed stability trends. Thereafter, we can develop control strategies to serve as references for future river regulation and management.The primary objectives for Year 1 of this plan involved organizing relevant simulation results, river characteristics, and in-situ observations, in addition to understanding hydraulic structures and the mechanism that affect structures and span the river within the planning area. These objectives were the preliminary points of reference for developing erosion-prevention construction methods. In Year 2, we used hydrodynamic sediment transport models to perform river erosion and deposition simulations based on various return period lengths regarding annual and long-term flow, assessed riverbed stability, consolidated the results of both years, and proposed comprehensive erosion and deposition control strategies. Siltation, reduced cross-sectional flow, and frequent flooding are the primary problems for the mid and downstream areas of the basin. This section of the river has been dredged numerous times. In the upland areas, the main Erren River channel meanders; therefore, the curved river sections experience erosion, causing landslides and sediment loss.Riverbed stability studies, based on data from 2009 that have compared the width-to-depth ratio (W/D) estimates of large Erren River cross-sections, have classified the section from the Erren Estuary to the Old Erceng Bridge as a broad and shallow river. Conversely, the additional Erren River sections are narrow and deep. Judging from the deep channel offset’s rate of change, the Erren River’s flow path oscillates right and left, which is typical of sandy rivers. After the railway bridge, the embankment contracts and the flow path stabilizes. Based on comparisons of the cross-sectional data from 2001 and 2009, the cumulative erosion of the primary Erren channel is approximately 430,000 m3, and the average riverbed elevation slightly decreased by 0.04 m. The transverse/longitudinal stability indicators showed that, because the Erren River’s flow path is narrow, deep, and meanders, and the riverbed material features fine particles, the riverbed changes more rapidly and its stability is poor.The 2D sediment transport simulation results identified the most eroded cross-sections at Sections 5 to 6, 10 to 12, 44, 63 to 66, 81 to 92, 99, 103 to 104, and upstream from Tiao bridge (i.e., Sections 119 to 120), that were caused by flow velocity and topographical changes. In addition, the river path largely comprises silted river sections that typically are eroded from upstream mudstone and transport large amounts of sediment downstream. Overall, the Erren River Basin features a meandering and unstable river that can easily experience localized upstream erosion, producing downstream siltation. These eroded regions are chiefly located upstream and in adjacent areas that feature less protection. Relevant protection methods have subsequently been established. The proposed plan targets easily silted sections that are grouped into three sediment storage areas: Sections 24 to 25, 44 to 45, and 53 to 54. The dredging volume is separated into 200,000, 100,000, and 150,000 m3, and the estimated dredging cycles are divided into 2-3, 4-5, and 8-10 years.By examining flood control capacities, we observed that dredging can effectively decrease water levels and enhance flood control capacity. Based on the sedimentation reduction ratio and efficiency assessments, previous studies on dredging have established that the river cross-section near Jhonglu Bridge is the ideal section for dredging because of its greater potential for sediment reduction. Future studies can assess dredging areas to determine better methods of dredging and scales between Sections 23 to 26.