行政院國家科學委員會專題研究計畫成果報告:地震造成新山崩地質環境特性之研究─921大地震崩塌地對河川輸砂衝擊之研究

前兩年的研究發現，九九峰礫石層分 布區崩塌地所生產的淤砂可能以邊坡型和 山谷型土石流的型態進入河道。第三年度 的研究乃以九二一大地震前後的數值地形 資料，估算該區的地形變動量，並分析地 震前後河川輸砂量的變化。 數化民國87年1/5000相片基本圖的等 高線，以Arc/Info軟體將其轉成20×20公尺 的網格式地形高程模型（DEM），再利用 Imagine軟體校正為TWD97的投影系統，做 為九二一地震前的地形資料。然後與內政 部在九二一地震後所測繪製作的20×20公 尺DEM，以Arc/Info軟體進行高程比對，估 算研究區地形的變動量。結果面積4.35平方 公里的乾溝溪流域，侵蝕量達841,000立方 公尺，而堆積量達289,000立方公尺，估計 從流域移出的土方量約為552,000立方公 尺。另外，收集研究區下游及鄰近集水區 的烏溪橋、大肚橋與南岡大橋水文測站的 流量與懸移質輸砂量的資料，分析各站在 九二一地震前後，平時與豪大雨時的流量 與懸移質濃度的率定曲線，並以其推算各 年的總懸移質輸砂量。比較率定曲線，發 現南岡大橋及大肚橋地震後的懸移質淤砂 濃度有稍大於地震前的趨勢。不過以年輸 砂量而言，地震後卻因豪大雨的出現頻率 有限，因此年平均輸砂量還不及地震前。 可見得，豪大雨的發生頻率是本區淤砂輸 送的主要控制因子。Preliminary research in the previous two years concluded that the sediment derived from landslides in the conglomerate formations in the Ninety-Nine Peaks area, is transported to the nearby channels by two major forms of debris flows. One is the slope-typed debris flow, the other is the channelized debris flow. This study aims to estimate the topographic change and the fluvial sediment transport characteristics induced by the Chi-Chi Earthquake. The 1998 1: 5000 contour maps was first digitized and then converted into 20 x 20 meter DEM by Arc/Info software. This DEM represents the topography before the Chi-Chi Earthquake. The topography after the Chi-Chi Earthquake is represented by the 20 x 20 meter DEM produced by the Ministry of Interior after the event. The pre-event DEM was subsequently transferred into the TWD97 projection for consistency, and compared with the post-event DEM to estimate the topographic change induced by the Chi-Chi Earthquake. In the Gun-Chi watershed, the estimated quantities of erosion and deposition are about 841,000 cubic meters and 289,000 cubic meters in respective, which results in a sediment transport rate of 552,000 cubic meters for the watershed. Hydrological data from three nearby gauge stations were collected and analyzed. By fixing the coefficient of the regression lines, the intercepts derived from data collected before and after the Chi-Chi Earthquake were compared. It showed that the sediment concentration before the Chi-Chi Earthquake was slightly greater than that after the earthquake indicating that similar flow discharge carries more sediment in the post-event period. However, the estimated annual sediment transport rates after the Chi-Chi Earthquake did not exhibit significant change from pre-earthquake years. It is thus concluded that fluvial conditions may be more important in controlling the total sediment transport