拱型固床工之沖刷機制探討

Abstract

近年來因空拍機(UAV)的盛行，工程構造物也漸漸被要求應兼顧視覺上的美觀效果，因此各種創新型、拱型，或是複合式的固床工便應運而生，拱型固床工雖有許多優點，可導正流心、營造多元流況的棲地環境，及增加河川的溶氧量等，但其特殊形狀對下游產生的沖刷情形卻少有相關的研究探討，故本研究將針對不同的拱型固床工，深入探討其水理現象，及所造成之沖淤情形。 本研究將試驗渠槽佈置成上游坡度為2.7%，下游為水平渠道，並製作直線型、上拱型、下拱型、雙上拱型、雙下拱型，及上拱型且具低水河槽等六種固床工模型，變換0.0045cms~0.0178cms共七種流量，透過水槽試驗成果與SRH2D模擬結果，探討在河道中建置不同型式的固床工，其水理現象、沖刷深度、沖刷坑縱剖面之變化，及其下游的沖淤分布情況；此外，更進一步變換直線型固床工之高度，探討流量、堰高與最大沖刷深度之關係；另綜合固床工安定分析，以及受水流作用後，其應力、力矩、位移等分析成果，可得到結論茲分述如下： 1.水流流經直線型固床工，會呈均勻分布；流經上拱型會趨向渠道中心；流經下拱型會往兩岸偏折；流經上拱型且具低水河槽之固床工，其流量集中於渠道中心的現象會更明顯；流經雙上拱型會趨向兩個拱頂；流經雙下拱型，則會偏向兩岸及渠道中心。且沖刷坑之發展與上述水流之流向有相同的趨勢。 2.水流沖擊力與流量之關係有一頂峰值，此時水流沖擊力最大，然因本試驗之固床工較低矮，僅0.045m，且下游為水平渠道，故當流量再增加時，尾水深增加使上下游水位差減小，以致水流沖擊力亦逐漸減小。 3.直線型固床工的沖刷深度呈均勻分布，並以此作為對照組，進行各型式固床工之最大沖刷深度量化，得知上拱型在中心深了1.3%，兩岸淺了40%；下拱型在中心淺了66.9%，兩岸淺了26.7%，因頂峰值所對應之流量不同的緣故；上拱型且具低水河槽，中心深了27.2%，兩岸則淺了43.3%；雙上拱型在中心淺約27.2%，兩岸淺約33.3%，拱頂部分則深了8.6%；雙下拱型在中心淺了10.6%，兩岸深了10%，拱頂則是淺約39.7%。 4.試驗結果中，在堰高為0.045m時，最大沖刷深度與流量之關係亦有一頂峰值，當流量小於此峰值對應之流量，最大沖刷深度與流量呈正相關，此時堰前水深與固床工高度之比值小於等於1.73倍；反之，流量大於此峰值對應流量，當流量漸增時，因下游為水平渠道，且堰高較低矮，以致尾水深越深，跌水落差反而漸小，造成最大沖刷深度呈下降的趨勢，此時堰前水深與固床工高度之比值則大於1.73倍。在SRH2D模擬結果中，因堰高較高，故最大沖刷深度隨流量增加而增加。但整體來說，流量越大，沖刷量體仍會越多。 5.本研究認為上拱型與上拱型且具低水河槽之固床工較適合建置在直線河段，原因條列如下： (1)可有效導正流心，防止岸坡淘刷。 (2)可營造緩流、急流、淺灘、深潭等多樣化流況與環境，以提供生物棲息、運動、覓食，甚至上溯。 (3)可增加枯水期水位，兼顧水域生態。 (4)可利用拱的特性將水流作用力橫向傳遞至兩岸，結構較為穩固。 6.最大沖刷深度與固床工高度呈正比，且除了0.045m之堰高外，最大沖刷深度亦會與流量呈正比，因堰高越高，固床工受到下游尾水深的影響越小。此外，本研究經迴歸分析可求得直線型固床工之最大沖刷深度的經驗公式為： ε/H=5.174〖(q^2/(gH^3 ))〗^0.223 (R^2=0.98) 其中，ε為最大沖刷深度(m)，H為固床工高度(m)，q為單寬流量(cms/m)，g為重力加速度(m/s^2)。另，該公式的適用範圍為渠槽寬度1m，水深介於0.005m至0.07m間，上游坡度為0.027且下游為水平渠道，泥砂容重為2.65t/m^3，泥砂粒徑d50為1.15mm，單寬流量的範圍則介於0.0045cms/m至0.0178cms/m之間。 7.在規劃設計階段時，即應進行固床工安定分析之狀況為：河道發生枯水期或是大洪水之頻率十分頻繁；在亞臨界流況，上游未淤積時，固床工設計高度超過2.7m，而上游淤滿時，固床工設計高度超過2.6m；在超臨界流況，上游未淤積時，固床工設計高度超過3.2m，而上游淤滿時，固床工設計高度超過2.7m；固床工設計高度僅有1m，但基礎土壤較軟弱。 8.在地基穩固時，最大應力與力矩發生之位置，均位於固床工上游面之全段底部，位移量最大的部分在固床工之頂部；而地基淘空時，因失去基礎支撐，故最大應力、力矩會發生在固床工之兩側，位移量最大處則轉為渠道中心。

In recent years, due to the popularity of UAV, engineering structures are gradually required to take into account the visual aesthetic effect, so a variety of innovative, arched-type or composite groundsills emerge as the times require. Although arched-type groundsill have many advantages, they can guide the flow center, create a multi-flow habitat environment, and increase dissolved oxygen in rivers. However, its special shape has few related researches on the scouring, therefore, in this study, the hydraulic phenomenon and scouring caused by different arched groundsills will be discussed. In this study, the experimental channel is arranged into upstream slope of 2.7%, downstream is a horizontal channel, and six kinds of groundsill models are made, such as linear, upper arched, lower arched, double upper arched, double lower arched, and upper arched with low flume. Seven kinds of flow are transformed from 0.0045cms to 0.0178cms. Through results of experiment and SRH2D simulation, discuss different types of groundsills in the river, the changes in the hydraulic phenomenon, scour depth, longitudinal profile of scour hole, and the distribution of scour and sediment in downstream. Moreover, the height of linear groundsill was further changed to explore the relationship between flow, groundsill height, and maximum scour depth. In addition, based on the stability analysis of groundsill, and the analysis of stress, moment, and displacement after the action of flow, the conclusion can be obtained as follows: 1.The flow through the linear groundsill will be evenly distributed; through the upper arched groundsill will tend to channel center; through the lower arched groundsill will deflect to both sides; and it is more obvious that the flow is concentrated in channel center when the flow through the upper arched groundsill with low flume; the flow through the double upper arched groundsill will tend to two vaults; through the double lower arched groundsill will tend to both sides and channel center. And the development of scour hole has the same trend as the flow direction. 2.There is a peak value between the impact force of water and the flow, and the impact force of water is the largest at this time. However, because the groundsill in this test is relatively short, only 0.045m, and the downstream is a horizontal channel, thus, when the flow increases again, the increase of tailwater depth reduces the water level difference between the upstream and downstream, so that the impact force of water decrease gradually. 3.The scour depth of linear groundsill was evenly distributed, and as a control group, the maximum scour depth of each type of groundsill was quantified. It was learned that the upper arched type is 1.3% deeper in the center and 40% shallower on both sides; the lower arched type is 66.9% shallower in the center and 26.7% shallower on both sides, due to the different flows corresponding to the peak value. The upper arched type with low flume, the center is 27.2% deeper, and the two banks are 43.3% shallower; the double upper arched type is about 27.2% shallower in the center, 33.3% shallower on both sides, and the vault part is 8.6% deeper; the double lower arched type is 10.6% shallower in the center, 10% deeper on both sides, and the vault is about 39.7 % shallower. 4.In the experimental results, when the height of groundsill is 0.045m, there is a peak in the relationship between the maximum scour depth and flow. When the flow is less than the flow corresponding to this peak, the maximum scour depth is positively related to the flow, at this time, the ratio of the water depth in front of the weir to the height of groundsill is less than or equal to 1.73 times. Conversely, if the flow is greater than the flow corresponding to this peak, when flow gradually increases, because the downstream is a horizontal channel and groundsill height is lower, so the deeper the tailwater depth, the lower the drop, and maximum scour depth will decrease, at this time, the ratio of the water depth in front of the weir to the height of groundsill is greater than 1.73 times. In the SRH2D simulation results, because the groundsill height is higher, so the maximum scour depth increases with the increase of flow. But overall, the larger the flow, the more the scouring volume will be. 5.In this study, it is considered that upper arched groundsill and upper arched groundsill with low flume are more suitable for straight river. The reasons are listed below: (1)It can effectively guide the flow center and prevent the bank from scouring. (2)It can create a variety of flow conditions and environments, such as slow flow, rapids, shoals and deep pools, so as to provide habitat, movement, foraging and even migration. (3)It can increase the water level during the dry season and take into account the aquatic ecosystem. (4)The characteristics of the arch can be used to transmit the force of water to both sides, and the structure is relatively stable. 6.The maximum scour depth is directly proportional to the height of groundsill, and except for the groundsill height of 0.045m, the maximum scour depth will also be positively related to the flow. Because the higher the groundsill height is, the lower the influence of downstream tailwater depth on the groundsill is. In addition, through the multiple regression analysis, the empirical formula of the maximum scour depth of linear groundsill in this study is as follows: ε/H=5.174〖(q^2/(gH^3 ))〗^0.223 (R^2=0.98) Among them, ε is the maximum scour depth (m), H is the height of groundsill (m), q is the single-width flow (cms/m), and g is gravitational acceleration (m/s^2). In addition, the application scope of this formula is 1m channel width, water depth is between 0.005m and 0.07m, upstream slope is 0.027 and downstream is a horizontal channel, unit weight of mud is 2.65 t/m^3, silt size d_50 is 1.15mm, the range of single-width flow is between 0.0045 cms/m and 0.0178 cms/m. 7.During the planning and designing stage, the situation that stability analysis of groundsill should be performed is: frequent dry seasons or floods in rivers; In subcritical flow conditions, the design height of groundsill exceeds 2.7m when upstream is not silted, and when upstream is fully silted, the design height of groundsill exceeds 2.6m; In supercritical flow conditions, the design height of groundsill exceeds 3.2m when upstream is not silted, and when upstream is fully silted, the design height of groundsill exceeds 2.7m; The design height of groundsill is only 1m, but the foundation soil is week. 8.When the foundation is stable, the maximum stress and moment are located at the bottom of entire section in the upstream surface of groundsill, and the largest displacement is at the top of groundsill. When the foundation is emptied, due to the loss of foundation supporting, the maximum stress and moment will occur on both sides of groundsill, and the largest displacement will be transferred to the center of channel.