DarthKermit: It's always a compromise between building structures that are so strong that no natural events like flooding or earthquakes are likely to damage them vs. keeping the cost to a reasonable level.
For example, every culvert could be built to be (say) 2 metres in diameter, but the cost would be huge.
We live rural and our road has many slips and diggers have been doing a great job clearing them away. The large volume of small rock and silt would overwhelm practical culvert designs or more likely the slip is going to move out from the bank away diverting the flow over the road. This was an extreme event (maybe slightly worse than 2005 here).
Where the inadequate designs are relevant is where the flow of water and small debris might be flushed away with an appropriate design. I continue to be frustrated with the local designers who are adding traversal covers (I think they are called this - covers with slots) at the entry to what were open throat designs. It takes very little debris to block these covers and water and small stones flow over the road.
There are designs for entries with sloped rails to control the size of the debris that work well but these cost a bit more so we get inferior solutions. Where I get angry is the local roading contractor then gets many callouts to "fix" blocked drains due to small amounts of rubbish getting trapped in the slots. This is stupid in a rural area and I suspect if the whole of life cost (captial and operational) were considered then different designs might be used.
We have crazy situations where large diameter culverts are installed under the roads, probably designed by a competent engineer sizing on the catachment area, and we cap the entry off with a flow limiter that takes a fraction of the culvert capacity.
Any drainage engineers out there who can help me understand how these design decisions are made?
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