This activity is included as an alternative to wholly removing or opening a culvert where this is not possible for various reasons (cost or location constraints).
If a culvert can be removed then this technique is covered by the activity ‘Remove culvert’.
Image © Rivers Agency (NI) copyright and database rights 2013
Activities that deal with culverts need to be carefully designed so they do not affect the structural integrity of the culvert, increase flood risk, or increase the potential for blockages. The type of changes that can be made to a culvert will depend upon a number of factors including:
- Available space
- Access
- The gradient of the culvert bed
- Depth of water
- Flow velocities in the culvert
Techniques
Techniques that can be used to implement this environmental improvement are dependent upon the type and characteristics of the culvert:
- Cutting a two-stage channel in the centre of the culvert
- Laying the bed of the culvert with gravel (including within any new two-stage channel)
- The inclusion of shelves for mammal access
- Daylighting (creating holes for light to get into the culvert)
- Installation of baffles to maintain flow depths
- Removing or reducing drops at the inlet and / or outlet of the culvert
Benefits
Reducing erosion caused by land drainage can deliver a wide range of direct and indirect benefits, including:
- Direct benefits to salmonids, eels, coarse fish and other aquatic organisms by allowing free movement of populations and individuals, providing access to valuable habitat that may otherwise be isolated from migratory species
- Indirect benefits to plants and other aquatic organisms by naturalising in-channel habitats
- Improvements to flow patterns and a reduction in impoundment
- Improvements in the transport of sediment and a reduction in sediment accumulation upstream of the structure
- Restoration of natural processes, including erosion and deposition
- Reinstating a natural bed inside the culvert (where it was previously artificial)
Case Study Benefits
This diagram displays the benefit scores (using a high-level ecosystem service assessment methodology) associated with the technique used in the case study. More details on the methodology can be found here.