在天然河道中由於輸砂不平衡的問題，造成底床逐年下降的現象，而橋基裸露也成為影響橋樑安全重要的問題之一。而一但發生橋基裸露現象將會改變橋墩之迎水條件，進而造成更嚴重之沖刷，因此不同形式之橋墩基礎的沖刷特性，需要進一步加以研究，以分析出適當且有效之橋基保護工法。 本研究利用水槽模型試驗分析在河道中不均勻橋墩（圓形橋墩在一較大之圓形橋基上）及群樁基礎之局部沖刷特性，分析其最大沖刷深度、沖刷坑長度寬度及堆積丘範圍，並比較在不同裸露高度下之沖刷變化。 實驗結果發現，不均勻橋墩在不同橋基位置時，隨著裸露高度增加而造成迎水面積增加，沖刷坑範圍及最大刷深增加。沖刷坑中心線剖面之墩前斜率，因為在重力作用下其斜率與底床質之水中安息角(θ=37.6°)相等，而與橋基裸露高度無關。而在群樁基礎時，當樁帽底部切齊底床時沖刷範圍最大，而無樁帽時沖刷範圍最小，最大沖刷深度及沖刷範圍與樁帽距底床高度程反比。 This study uses experimental model to examine the characteristics of local scour around both non-uniform piers (a cylindrical pier on a larger cylindrical foundation) and pile foundations in fluvial channels. The maximum scour depth、the length and the width of the scour hole and the range of pile-up mound is analyzed. Furthermore, the change of scour at different exposure elevation is compared by performing a series of case studies. The experiments of non-uniform piers in different exposure elevation showed that the increase of exposure elevation caused the increase of obstruction area. So, the range of scour hole and the depth of scour will increase. The upstream slope of the scour hole will equal to angle of repose of the bed sediment in water(θ=37.6°) because the effect of gravitation. In the presence of pile foundation, when the bottom of the cap is to fit with the bed, the range of scour hole will be the maximum. And when there is no cap, the range of the scour hole will be the minimum. The maximum scour depth and range of scour hole will decrease with increasing the elevation of cap relative to the bed.
