卵礫河床載衝擊破壞河道固床工之模擬分析

Abstract

固床工為保護河床之人工構造物，代替河床承受水流和河床載的衝擊。相對於水流作用力，被水流帶動的河床載對剛性固床工更具威脅性。來自上游的卵礫河床載硬度一般較混凝土高得多，當大顆粒卵礫河床載撞擊固床工，可能造成混凝土之破損，抵抗衝擊和磨蝕的能力減弱，進而破壞面積擴大為凹洞。 本研究運用數值方法模擬河川中不同尺寸河床載於不同流量下的運動行為，籍以得到河床載撞擊河床或固床工之位置、速度和角度。再經由Hertz(1884)的接觸力學理論，以及ACI544實驗之原理，計算不同尺寸的河床載對固床工床面所造成之損壞。混凝土受撞擊之破壞決定於撞擊產生正向動能與正向撞擊力。 不同大小的卵礫石會有不同的運動行為：尺寸小的顆粒易受水流影響，沿著流線方向為其主要之運動方向，撞擊床面時之垂直向動能與撞擊應力相對較小；尺寸大的顆粒主要受重力控制其運動行動，受水流影響相對小，其運動行為以彈跳為主，垂直方向的破壞力比較大，而其垂直方向的破壞取決於遷急點的高程差。 水文事件決定河川承受的流量，流量的增加則帶來更多上游的卵礫石，且尺寸會隨著流量增加而增大。雖然流量之提高不會增強顆粒的垂直向破壞力，但卻提昇經由水流帶往下游的大尺寸卵礫石數量，增加對河床或固床工的攻擊衝量與破壞力。 本研究進行多組模擬探討顆粒在不同條件下表現的撞擊行為，考慮不同之遷急點幾何形狀、顆粒直徑大小、反彈係數和流量，並繪出撞擊點分佈和計算破壞力大小，有助了解卵礫石河床載的運動機制，並能用以預測固床工可能受損的位置與程度。

The purpose of building groundsill-work (e.g., revetment and bed armor) is to protect the river from abrasion and impact caused by the bed load. Compared to the stress induced by water flow, the collision of bed load, especially some huge gravels, is more likely to harm the revetment and cause damage. The hardness of gravel is higher than the revetment material which is often concrete. As a result, when big gravel collide the revetment, the concrete may be damaged and may cause the failure of the whole revetment. These works first explores the gravel particle motion under several kinds of flow regime through computational fluid dynamics simulations, and then record each gravel particle condition when the particle hits the river bed or revetment. The recorded particle conditions include the particle coordinate, velocity and moving direction at the collision time. After that, it is possible to calculate the force induced by the gravel particle collision using the concept of Hertz theory(1884) of contact mechanics, then to can identify the damage caused by different size of gravel particle. The damage of concrete material is determined by normal force brought by the gravel particle collision. The motion behavior of gravel particle with different size behaves differently under water flow. The motion of small size particle is mostly controlled by the water flow, its moves following the streamline, the impact angle of its collision is comparatively small so that the normal force component is small. The motion of large particle is controlled by the gravity because the effect of water drag force is comparatively small to a large particle, it tends to move with saltation; therefore the normal component of impact force become larger relatively to small particle. The fall height determines the amount of normal impact force. The magnitude of hydraulic event controls the discharge of river; increasing discharge tends to bring more and larger gravel particle from the upper stream. Although the increase in discharge cannot enhance the impact force of large particle collision, its major role is to transport more and larger gravel which can damage the revetment more efficiency. A series of numerical simulations were conducted by considering different gravel particle size, discharge, shape of knickpoint and coefficient of restitution. Consequently, the collision distribution and impact force can be examined after data analysis. The findings from this study help to understand the motion of gravel particle traveling in river. It may also be adopted to predict the potential extent and locations of groundsill-work damage under a major hydraulic event.