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Should screens (trash or security) be always placed at culvert inlet?

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Screens are provided at culvert inlet owing to one of the following reasons:

(i) To trap trash or debris which might otherwise accumulate in the culvert and block the flow subsequently;

(ii) To prevent access into the culvert by children.

However, screens are not always placed at culvert inlet and they should be determined case by case. For trash or security screens installed in place, it would inevitably trap floating debris and rubbish. Unless they are regularly removed, it would eventually lead to rise in upstream water level leading to local flooding.

For new culvert, there is a trend that the use of trash screens is declining. Trash screens are only placed in culvert inlet where there is a high risk of blockage history. Instead the need of trash screen can be eliminated by adopting the following design features:

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(i) There are fewer changes of cross section, fewer bends and smooth transitions into the culvert. As such, it would provide fewer locations to trap debris and trash.

(ii) Provision of good access to the culvert to facilitate regular inspection.

(iii) The culvert should be not designed too long to enhance easy access to clear a blockage.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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