Althon offer a range of headwalls with channel extensions for pipework up to 750mm. Precast channel extensions are available in sections for you to build up to a required length and are joined using turnbuckles and mastic sealant.
Channel headwalls and extension pieces can be factory fitted with accessories such as Kee Klamp® handrails, walkways and a selection of gratings depending on the application. This also includes all manner of flow control devices.
Further information can be found here.