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
If the watercourse to be culverted is a 'main river' then approval will be required from the Environment Agency (in England) or 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 C786, 'Culvert, screen and outfall manual', 2019. 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
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
software such as Flood Modeller, TUFLOW or HEC-RAS to adequately size the culvert to convey the flow.
The analysis of the hydraulic performance of an existing culvert would largely also follow the methodology set out above.
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