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民國88年發生921大地震以來，大甲溪石岡壩下游河段河床受到地層抬升影響造成有持續下降之趨勢，大甲溪下游段橋梁因沖刷恐將危及橋梁安全，可能導致橋梁部份損毀無法通行，另外取水構造物也因河床高程變動而需不斷改變取水位置，堤防護岸受到水流沖擊部份河段亦造成損壞。目前在石岡壩下游河段河床露出岩盤，顯見河道有持續刷深之趨勢，為減少下游橋梁、取水構造物及堤防等水中構造物因持續刷深，造成下游公共建設及居民財產損失。近年來許多不同單位於大甲溪石岡壩下游河段因使用目的不同而設置不同方式之固床工，本計畫配合中區水資源局提出之「大甲溪石岡壩下游河道穩定及消能改善工程計畫基本設計報告」，設計方案為於河道設置ㄧ座消能池及兩座固床工，並進行河道整理方案，進行水工模型試驗，透過不同重現期距洪峰流量下，量測不同斷面之水位、流速及福祿數等水理數據，探討方案建置對於河道穩定之效果及可能產生之影響，冀期試驗成果可提供規劃設計及其相關單位在此河段施設水中構造物等參考。

Since the river bed in the downstream of the Dajia River has a constant trend of scouring, it will endanger the bridges due to the aggravated scouring nearby piers. The position of water inlet structure probably has to be altered while the intake elevation changes tremendously. Moreover, the erosion speed of laccolith river bed below the Shigang Dam has to be investigated in order to predict its tendency. Therefore, the project of “Study on River Bed Stability for the Downstream Reach of Shigang Dam in Dajia River” was undertaken by the Water Resources Planning Institute, Water Resource Agency, MOEA.physical model results shows，riverflow in this project is no longer affected by the river island, and the energy dissipation effect by the energy dissipator, the overall flow field becomes wider and smooth, but it is still affected by the topography and the flow direction mainly to the left bank, in the section 12~14, the location of Shigang Water Recycling Center, the riverflow still impact the bank. Velocity measurement at the position of the energy dissipator. At 5-year frequency peak flow rate (Q5=3,800cms), the average velocity dropped from 11.1 m/s to 6.6 m/s, and the flow velocity decreased by 40.5%; At 100-year frequency peak flow rate ( Q100=8,800cms) is reduced from 13.3 m/s to 9.5 m/s, the flow rate is reduced by 28.6%.Velocity measurement in front of Pi-feng bridge, the average flow velocity dropped from 11.9 m/s to 8.1 m/s under the 5-year frequency peak flow rate (Q5=3,800cms), and the flow velocity decreased by 31.9%; At 100-year frequency peak flow rate (Q100=8,800cms) is reduced from 13.8 m/s to 12m/s, and the flow velocity is reduced by 13%.