Rationality in pier geometry of Kintaikyo Bridge from viewpoint of river engineering

Abstract

The Kintaikyo Bridge, with its elegant wooden arches, has a unique pier shape and continues to be loved by residents and visitors alike. Although this bridge is an active footbridge and an important landscape element along with the Nishikigawa River and its river beach, the rationality or irrationality of the shape of its piers remains unknown. This paper is intended to clarify the river engineering characteristics of the piers for the first time by conducting 1/129 scale hydraulic model experiments. The shapes tested were a perfect spindle shape (which has been adopted as a common theory for many years) and a reconstructed current shape based on the spindle shape, and for comparison, an oval and a non-regular hexagon shape with the same width and area. The current shape, along with the spindle shape, suppressed the water level rise around the pier more than the others. As for the riverbed fluctuation, the current shape slightly increased the scour more than the others, but it also maximized the sedimentation around the scoured part. In other words, the current shape has the potential to facilitate the procurement of sediment for post-flood restoration. In addition, the current shape overwhelmingly reduced the statistical dispersion associated with the experiment, suggesting that it stabilizes the trend of riverbed fluctuation even during actual floods. Based on the results, the future conservation of the Kintaikyo Bridge was also discussed.

Model experiments on hydraulic properties around multiple piers with reproduced 3D geometries

“…Classic hydraulic model experiments have been conducted on single piers with simple structural geometries. Cylindrical 2 , 3 , typical rectangular 4 , 5 , and specific bridge-pier geometries 6 have been examined. Jain et al 7 conducted experiments on the mixtures of riverbed materials at pier installation sites.…”

“…Through experiments on a thick rectangular pier, Kadono et al 20 showed that the scour area is enlarged when skew is toward the river flow, considering the collapse of a railroad bridge. Sato 6 experimentally investigated the effect of a skewed spindle shape on the scour and water level changes. Fael et al 21 tested rectangular and several other pier geometries, including group piles, and discussed the equilibrium scour depth.…”

“…Indeed, reviewed studies, whether experimental or simulation, have provided valuable academic insights into these individual conditions. For example, for a single pier, such as cylindrical 2 , 3 , 5 , 7 – 9 , rectangular 4 , 5 , 10 , or other special shapes 4 – 6 , the flow field, riverbed field, and bed material, among others are the main factor influencing scouring. Regarding multiple piers, however numerical results suggest that, in addition to these basic factors, the placement and number of piers may influence the scour characteristics by affecting the flow field 12 – 16 .…”

“…Notably, structures, such as piles, placed below the bottom of the pier may influence the scour depth 17 , 18 . In addition, the degree of scour is more severe when the pier is skewed relative to the center of flow, regardless of the installation or hydraulic conditions 6 , 20 , 21 . In practice, a combination of several conditions affect the pier structure rather than the independent action of single factors.…”