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Culvert under railway line at Twyn Road, Ystrad Mynach

Culvert Design and Analysis

Definition of 'culvert'

A culvert is a pipeline or covered channel used to carry a watercourse under an obstruction. Courts have established that it is the duty of a person who constructs a culvert to ensure the works which '...substitutes for the channel provided by nature are adequate to carry off the water brought down even by extraordinary rainfall...'.

Design Standards for New Culverts

Consent under the Land Drainage Act 1991 will be required to culvert a watercourse and depending on individual circumstances, the culvert will typically need to be able to convey the flow expected once every 100 years. It would also be necessary to consider the consequences when this flow is exceeded.

If the watercourse to be culverted is a 'main river' then approval will be required from the Environment Agency (in England) and Natural Resources Wales (in Wales). On the other hand if the watercourse is an 'ordinary watercourse' then approval would be required from the land drainage authority, usually the county or unitary council.

The design of a new culvert would be carried out in the light of CIRIA document C689, 'Culvert design and operation guide'. The two main elements of the hydraulic design work are likely to be the estimation of the flow in the existing watercourse and the hydraulic design of the culvert to convey that flow.

Flow Estimation and Hydraulic Design for New Culverts

For catchments of a mainly rural nature it is likely that the flow would be calculated using Flood Estimation Handbook methods. The design of the culvert itself, in addition to structural matters, is likely to include consideration of the horizontal alignment, level, size, inlet and outlet structures, the need for screens, etc. This would be carried out in conjunction with the hydraulic analysis of the proposals and this may require the use of computer 1D river modelling software such as ISIS or HEC-RAS to adequately size the culvert to convey the flow.

Existing Culverts

The analysis of the hydraulic performance of an existing culvert would largely also follow the methodology set out above.

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