使用陸域雷射掃描監測以底岩為主的河道對於小至中等規模之洪水事件的反應

Monitoring channel responses to flood events of low to moderate magnitudes in a bedrock-dominated river using morphological budgeting by terrestrial laser scanning

不同規模與頻率的洪水事件對於河道形貌的改變程度有所不同。遙測技術以及數值地形資料處理的不斷進步，對於侵淤的時空變化分析十分有幫助。雖然基於數值高程模型（DEM）的形貌收支法（morphological budgeting approach）已被廣泛地應用於監測侵淤體積的時間變化，但是評估高能量的受限河道（confined channels）的地形有效性（geomorphic effectiveness）的研究仍然較為缺乏。本研究藉由陸域雷射掃描，監測立霧溪中三段河道從 2009 到 2012 年的變化，藉此探討河道對於小至中等規模洪水的反應，並評估其地形有效性。研究結果顯示，採用空間均一誤差（spatial uniform error）與空間差異誤差（spatial various error）所估計的侵淤體積大致相近，但後者較前者更能反映河床的變化以及資料密度。地形有效性的差異主要受控於河谷受限性以及河床中地形單元的組成與配置。小規模的年洪水可以移動礫石和砂，在受限與非受限河道都能創造出局部的侵淤（深度< 0.5 m）。在受限河道中，底岩的出露以及穩定的沉積單元限制了河道型態的改變。非受限河道由於有較低的單位河川功率，造成該河段中巨礫再次移動的時間間隔較受限河道為久。河床侵淤的空間型態的頻繁改變，顯示出立霧溪對於小至中等洪水事件十分敏感，此類地處旺盛造山帶的河流系統，具有陡急的河川、狹窄的谷床、豐富的沉積物來源但有限的儲存空間等特性。

Changes to channel morphology reflect geomorphic work by flood events of differing magnitude and frequency. Advances in remote sensing and digital terrain processing now allow for sophisticated analysis of spatial and temporal changes in erosion and deposition. Although the morphological budgeting approach using digital elevation models of difference has been widely applied to track volume estimation of changes to erosion and deposition over time, appraisals of geomorphic effectiveness in high-energy confined and partly confined channels are still lacking. This study applied terrestrial laser scanning to monitor three reaches of the Liwu River, a bedrock-dominated river in eastern Taiwan, from 2009 to 2012, to investigate channel responses to flood events of low to moderate magnitude and appraise their geomorphic effectiveness. Variability in geomorphic effectiveness reflected valley confinement and the composition/configuration of geomorphic units on the channel bed. Annual low magnitude flood events reworked gravel and sand deposits, creating local scour and fill (< 0.5 m in depth) in the confined and in the unconfined reaches. Lower unit stream power in the wider, less-confined reach resulted in longer intervals between phases of boulder reworking relative to the confined reach. Bedrock exposure and stable sediment storage units in the confined reach restricted changes to channel pattern. Successive moderate and low magnitude events in 2012 created an evident erosion of 7,556 m3 (~ 1 m in depth) and were able to modify channel configuration in partly confined and unconfined reaches. Frequent changes of patterns of scour and fill on the channel bed indicate that the Liwu River is highly sensitive to flood events of low to moderate magnitude. This reflects an active orogenic river system that is characterized by a steep channel and narrow valley floors with limited accommodation space but abundant sediment.