Blockage slope and ratio effects on box culvert hydraulics

Abstract

Culvert blockage at the inlet can dramatically alter the local hydraulic characteristics that often increase backwater rise, change the flow patterns, and thereby increase the risk of upstream flooding. The impact of culvert blockage is not usually included in the initial hydraulic design, although it can cause significant losses once the culvert becomes blocked. Culvert hydraulics including backwater rise with respect to blockage slope and ratio has been neglected in the studies conducted so far. In this study, we conducted a series of narrow flume tests to investigate the effects of four bottom blockage slope angles and five blockage ratios on the hydraulic characteristics of box culverts, including backwater rise, and upstream mean velocity at various Froude numbers. The results showed that Froude number, blockage ratio, and slope angle were regarded as the controlling variables for culvert backwater rise. Backwater rise increased dramatically with increasing blockage ratio and Froude number. Compared to blockage ratio B = 0.20, ΔH/hu increased by 2, 5, 9, and 13 times for B = 0.40, 0.60, 0.80, and 1, respectively. However, when the blockage slope angle decreased, the backwater rise reduced and upstream mean velocity increased in compared with vertical blockage. The relative backwater rise reduced from 0.32 to 0.29, when blockage slope angle decreased from 90° to 18.44° at B = 1 and Fo = 0.10. In addition, the length and maximum depth of water surface depression inside the culvert itself were larger with increasing blockage ratio and Froude number. For Fo = 0.10, d/ho = 0.13 and L/ho = 0.94 for B = 0.20 compared to d/ho = 0.32 and L/ho = 4.34 for B = 1. The results were also combined to develop an equation to predict backwater rise in the range 0.10 ≤ Fo ≤ 0.25, 0 ≤ B ≤ 1, and 18.44° ≤ α ≤ 90°. To enhance the accuracy, it is recommended to apply the equation for ΔH/hu ˃ 0.10. It is useful and practical in applications to add the probability of backwater rise or losses owing to culvert blockage within accounting of losses during the hydraulic design of culverts, especially in channels exposed to debris.