花蓮溪縱谷段土石流扇判釋與流域特徵分析

Abstract

台灣為一個易發生土石流的環境，為有效減少土石流災害的發生，找出具有土石流發生體質的溪流為重要的第一步。水土保持局根據一套作業系統判釋全台的土石流潛勢溪流。而以地形的觀點出發，以土石堆積所造成的谷口土石流扇，為土石流發生過的地形證據，可以作為判別土石流是否發生的證據。而若能確定谷口的沖積扇是土石流作用造成的土石流扇，則可判定該溪流是具有土石流發生的體質。本文主要的目的為利用土石流扇尋找具有土石流發生體質的溪流，並用統計分析歸納出土石流扇的流域特徵，作為判釋土石流潛勢溪流的指標。 本研究以花蓮溪縱谷段為研究區，利用數值航測系統輔以等高線地形圖判釋研究區中38個沖積扇，再以沖積扇的形態特徵為主，土石流發生史為輔，初步分出其中4個洪水扇、4個土石洪水扇和30個土石流扇。後經流域計測分析修正為4個洪水扇、3個土石洪水扇和31個土石流扇。 本研究利用統計分析尋找不同主導營力之扇和有無土石流扇的判別因子。結果顯示流域長度、流域面積結合起伏比兩者為區分不同主導營力之沖積扇（包括洪水扇、土石流扇和土石洪水扇）的最佳因子，流域河床坡度和流域險峻值可區分洪水扇和土石流扇。另一方面，有效集水面積和形狀係數可區分無扇流域，但無法有效判別出有扇流域，因此人工判釋土石流扇仍有其必要性。歸納本研究區土石流扇的流域特性為流域長度＜4.2公里，流域面積＜1200公頃，起伏比＞0.4，流域河床坡度＞22％，流域險峻值＞0.5。 最後將本研究所判釋的土石流扇與水土保持局所判定的土石流潛勢溪流相比，對本區而言，水土保持局已判定出大多數可能再發生土石流的溪溝，但水土保持局可能因為使用圖資上的限制，或將「下游保全對象」當作必要條件，忽略一些具有土石流發生體質的溪流。本研究利用數值航測系統所成的立體像對判釋土石流扇，在使用的圖資方面較為精細。另本研究認為從預防不當開發的角度出發，溪流的下游即使現無保全對象，將來也可能因開發而造成災害，因此仍須加以注意。將土石流扇和土石流潛勢溪流相較結果顯示在本研究區中還有6條具有土石流發生體質的溪溝未被劃入土石流潛勢溪流中，而土石流扇可以作為一種指標地形，用以檢查或補充水土保持局劃定的潛勢溪流。For mitigating the debris flow hazards, 1420 streams around Taiwan island have been classified as the potential debris flow torrents, based on the channel gradient, effective basin size and the presence of protected targets. It is not certain, however, whether the identified streams are inclusive when only based on the criteria listed above. Thus, this research proposed to adopt the characteristic landform, i.e., debris flow fan as the supplementary indicator. The tributaries in the western side of the Hualien Stream in the Longitudinal Valley was chosen as the study area for their well-developed alluvial fans. Firstly, all tributary valley mouths were examined and 38 alluvial fans interpreted were digitized by using digital aerial photography. Among them, 30 debris flow fans were preliminarily classified based on their morphological parameters (fan area and fan channel slope) due to the lackness of fan deposit outcrops. Catchment morphological characteristics of all tributaries, then, were used to differentiate debris flow fans from the rest with the Discriminative Analysis. Among morphological parameters, catchment length, catchment area together with relief ratio, channel slope, basin roughness number justified 31 debris flow fans from 38 fans. Finally, the potential debris flow torrents without debris flow fans and the non- potential debris flow torrents with fans were discussed. It is suggested that six torrents should be added onto the list of the potential debris flow torrents. This research shows when properly examined with stereo pairs of aerial photographs, the debris flow fan could be a good supplementary tool to catchment morphological parameters on identifying potential debris flow torrents.