本文討論跨河構造物與洪氾之關係，係依據經濟部水利署頒布「跨河構造物設施設置審議規範試行要點」之相關條文，就通水遮斷面積率不得超過8%，及上下游兩座橋樑中心線距離小於兩倍既有橋樑之跨距者，其通水遮斷面積率須依各橋墩在水流方向投影面積之總和計算之規定；以基隆河為例，針對民國76年琳恩颱風及90年納莉颱風於基隆河流域造成之洪氾淹水情形，進行洪水面剖線模擬演算。 由本次進行之基隆河跨河構造物調查，民國58年基隆河從河口至五堵止，統計只有11座跨河構造物(10座公路橋樑，1座鐵路橋樑)，但到目前為止同一河段已增建到45座跨河構造物(公路橋樑29座、鐵路橋樑1座、水管橋1座、高速公路及匝道14座)，其中台北市圓山附近短短一公里內就有12座橋樑。另台北縣汐止高速公路系統交流道附近亦於1.3公里內建了九座橋樑，總計這兩處橋樑數就佔了基隆河口至五堵段(累距約35公里)橋樑總數的二分之一。 本文針對這兩處跨河構造物密集段加以處理，並分別以無橋樑及有橋樑狀況，以200年、100年、50年、20年四種不同頻率洪水進行模擬演算，並與水利署實測資料及兩岸地形相互比對檢討，找出洪氾原因或機制，以探討跨河構造物遮斷水流造成壅水之加成效應。 This study discusses the relations between the cross-river structure and the flood disaster according to the outlines of the deliberative code for cross-river structures, issued by Water Resources Agency, Ministry of Economic Affairs. The area percentage of blocking flow section should be not greater than 8%. In another case that, blocking area should be computed by the sum of projection area on flow direction if the distance between the upstream and downstream bridges was less than twice the distance between bridge piers. As an example, we will process the hydraulic analysis of the flood, resulted from Lien-An and Na-li Typhoons in 1987 and 2001, to simulate the inundation of the Kee-lung river basin. Based on the investigation, there were 11 cross-river structures from the first to Wu-du on Kee-lung river in 1969, but now it has increased to 45 cross-river structures including 29 road bridges, one rail bridge, one water pipe bridge, and 14 freeway bridges. To be specific there are 12 bridges within 1 km in Yuan-shan area and 9 bridges within 1.3 km in Hsi-chi system interchange area. The bridge number in these two areas is a half of all from first to Wu-du in the 35 kilometers. This research considered two cases, with and without bridges, in the aforementioned areas and performed numerical simulation analysis of 200, 100, 50 and 20-year frequency floods. After reviewed and discussed the results by comparing to the practical inundation area and topographic survey, we identify the mechanism of flood and the inundated reasons. This study provides the hydraulic agent and bureau as a reference to review and process the similar cases.