Whenever possible box culverts should be designed for free flow at design flow rate. A box culvert with surcharged flow is similar to inverted siphon which creates maintenance problem. There is a risk of blockage by silt and debris and the inspection and maintenance of submerged culvert is difficult when compared with free flow condition. For long box culverts (more than 20m) it is even more important to design for free flow to decrease the risk of blockage which results in an inaccessible path.
From hydraulic point of view, the change from free flow to submerged
condition would cause an increase in head loss leading to a rise in upstream water level.
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This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
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