However, tandem piers of longitudinal bridges are distributed in the riverbed, which significantly disturbs river flow fields and causes soil erosion and scour. For example, the Xi’an-Hanzhong highway of China contained nearly 10 km of longitudinal bridges as shown in Figure 1. This eliminates building embankment and tunnels that cause significant effects on the local environment. Unlike the conventional bridges that are cross over the river, the longitudinal bridges are built in the river and extended along the river. To minimize the effects of highway infrastructures on the local environment and biological systems, longitudinal bridges are more frequently adopted. In the past two decades, many highways have been constructed in China, especially in the western mountainous areas. If the span is longer than 27.5D, the R MV of different piers are close to each other from the 2nd pier to the last one. The R MV becomes 0.82 at the second pier in each model when the span is shorter than 27.5D, and increases to 0.91 if the span is longer than 27.5D. The R MV (the ratio of the maximum x-velocity among piers in each row in different models to the maximum x-velocity of the two piers arranged side by side) of piers in the first row of different models is around 0.95. The largest R AVC is about −17.66% with a span of 0.52 m. R AVC (average velocity change ratio) of different spans increases gradually and tends to be stable with the increases of the span. For the area between the two columns of tandem piers, the profile of V Ri/V R 1 changes from a “∩-shape” to an “M-shape” in each model. ![]() ![]() For the area between the tandem piers and the wall, the V Ri/V R 1 (the ratio of the x-velocity at the i-th row to the x-velocity of the first row in each model) near the wall increases up to 1.26. With a span shorter than 27.5D (D is the diameter of piers), the shape and the lateral range of the x-velocity increases with the increase of distance downwards the x-direction. This research explores the effects of different spans of two columns of tandem piers on the characteristics of x-velocity near the river bed based on computational fluid dynamics (CFD) simulations.
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