Research On Mechanism Of Flood Damage Of Bridge And Culvert In Qinba Mountain Area

There are lots of bridges and culverts along the express-way in the mountainous area in QinBa. The particularity of the engineering geology and the natural environment could easily damage and culverts in the expressway and pose serious threat to the normal operation of construction of the basic transportation such as express-ways, bridges and culverts, and tunnels, etc. Therefore, to adapt to the practical need to build expansively express-ways in the mountainous areas in the new situations, it is necessary to carry out the research of water-proof and crackup of bridge and culverts in the mountainous areas.This paper, based on the field survey of the bridges and culverts in advanced express-way and the analysis of the collected data, gives out natural and anthropogenic factors of the flood damage of bridges and culverts in mountainous areas. The natural factors includes stability of stream segment in bridge site, the damages caused by torrential flood to the bridges and culverts, and the washing action caused by the torrential flood to the bridge abutment, etc. The anthropogenic factors include the improper site search, the low flood frequency designed for the bridges, the bridge opening designed without meeting the requirements of draining flood and discharging sediment, the basement of the bridge abutment not buried deep enough, the bridges and culverts badly out of repair for years and without timely maintenance, and the flood damage to the bridges and culverts caused by the sand excavation and quarry, etc.Based on the analysis on the factors of the flood damage of bridge and culverts, simulating experiment on the backwater characteristics of the right and the skew bridge in mountainous areas in Qinba is carried out. According to the analysis, it finds out that the current velocity in openings of right bridge distributes evenly and the different running displays the same tendency among the different skew bridges, with the low current velocity in the upstream of the skew bridge opening and with the high current velocity in the downstream of the skew bridge opening, and that current velocity is gradually increasing from the upstream to the downstream along the bridge axis. The washing height is almost consistent with the backwater height in each abutment of the right bridge. The washing height and the backup water height vary with the different position of bridge abutment, with high backup water in the upstream and low backup water in the downstream. The washing water height in front of the abutment has the opposite tendency, with the high washing water height in the downstream and the low washing water height in the upstream. However, the washing water height is almost consistent with the backup water height in every abutment, which suggests that the whole energy of the washing action on every abutment is consistent. The washing water height and backup water height display normal regular pattern varying with the change of skew bridge angle, that is, the washing water height and the backup water height in front of abutment is increasing with the increasing angles of the skew bridge, and when the value of angle is 51.4°, the washing water height and backup water height reach the highest value, then decrease gradually with the increase of angles.Then, based on the analysis of the damage caused by torrential flood to the bridge, it derives the computational formula of the level force of torrential flood to bridge, the vertical force of bridge and the collision force caused by the flood debris to the bridge. Then it makes an evaluation of the computational formula, analyzes the damages caused by the block glacier to bridges. And it also makes a theoretical calculation and analysis of the three bridges in the express way from Shiyan to Tianshui in Ankang district.