Flood threshold value for bridge scour prediction and warning

In Taiwan, owing to deep slope, frequency of extreme rail fall, typhoon attacking and earthquake impacts, huge amount of sediment would generate from mountain area and flow with flood toward downstream river. Then, the bridge safety issue is serious discussed during flood event. Therefore, bridge-scour problems have attracted considerable attention in Taiwan, spatially in Zhuo-shui River. In this study, the effects of bend and contraction scours could be neglected because of the river reach near the bridge is roughly straight and the channel width are substantially larger than the pier diameters, respectively. In addition, according to the river bathymetry survey, the bed elevation is generally steady around the bridge area. Therefore, the total scour depth of monitoring bridge is dominated by the local scour. It indicates that a two-dimensional numerical model is adopted to simulate flow field and water depth without sediment transport calculation for collected local scour depth formulas. An appropriate turbulence model, K-epsilon (k-e) turbulence model, is the most common model used in Computational Fluid Dynamics (CFD) to simulate mean flow characteristics for turbulent flow conditions, spatially near bridge piers. Herein, the adapted empirical equations have been validated experimentally; using return-period hydrograph events, and they can give satisfactory simulation results. Then, flood threshold value for bridge scour prediction and warning can be established in the future. In the present study, based on the experimental results, those empirical equations will be suggested to compute the local scour depth evolution under unsteady flow caused by rapid changes of flow depth and velocity in field.