Culverts

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A culvert is used for crossing a watercourse or to allow water caught in a side ditch to across under the path for dispersal. In either case you need to make sure that the size of pipe will allow unrestricted flow of the water. Speak to local people, including land managers, about known stream water levels.

For culverts across watercourses, inspect any existing culverts and choose a size at least as big as those that are obviously working well. If there are none, you need to calculate the 'discharge area' of the watercourse in spate conditions (width of channel x maximum depth of water). You then need to specify a pipe, or pipes, with twice this area (the area of the pipe = radius2 x 3.14).

The most commonly used pipes are the 'twin wall' polypropylene type. They are lightweight, easy to handle, strong and relatively cheap. Concrete pipes can be used if deep cover is not possible and heavy vehicles require use of the path. Stone 'box' culverts may be suitable in some locations, e.g. if material is available on site and access is difficult. These are traditional culverts comprising a rectangular section stone channel, capped by flat stones under the path. The culvert may either be completely buried or else the capping stone can be incorporated into the path surface.

On small burns, place culverts on a straight section, preferably where water speed is likely to be low. If this is not possible, provide suitable rock armouring to prevent scouring and erosion.

Culvert

 

Specification
Gradients – Longitudinal run on pipes should be a minimum of 1:40 to prevent silting. Where possible, avoid pipe gradients greater than 1:15 to prevent potential erosion problems at the outlet
Sizing – Pipes should never run full. Ideally, they should be no more than two thirds full at the worst-case water flow. For deep, slow moving streams, use a pipe diameter which is twice that of the stream depth. For ditch system culverts use several 300mm diameter water pipes set at regular intervals – every 10 to 20m depending on amount of water flowing into ditches
Construction – Pipe bedding material should be smooth and free of large stones which could puncture pipe walls. Lay the pipe on a bed of graded aggregate (the top level of which must not be above the streambed level). Backfill the pipe with well compacted graded aggregate as used for path base. Cover depth should be 50-100% of pipe diameter, depending on expected loadings, with a minimum cover of 150mm
Headwall – Build a stone headwall around both ends of the pipe to retain pipe bedding and backfill materials and to hide the pipe ends. Headwalls may be dry or mortared. Walls should be 150-450mm thick depending on depth of backfill. All stones must fit together well and be firmly set into the stream or ditch sides. Install a large single mantle stone across top of pipe ends - take care not crush the outlet and inlet ends. Alternatively, arrange stones to provide an arch around pipe
Inlet – For multiple pipes provide flow 'splitters'– long thin stones wedged between pipes to direct water flow into pipes without washing out pipe backfill materials. Place a bed of flat stones around inlet to prevent scouring under or around the pipe
Outlet – Install a 'splash plate', which is a single flat stone immediately below the pipe outlet to prevent undercutting. Also provide a wider bed of flat stones, around the outfall, to prevent scour and erosion
Handrails – If culverts present a falling hazard to path users (more than 600mm drop), consider installing a suitable handrail of an approved design, depending on user types and the depth of the culvert

 

Maintenance
Annual inspections are necessary to ensure that culverts do not become blocked
More frequent inspections will be required in some situations - particularly after heavy rainfall which may have washed debris into the pipe
Inspect the culvert inlet and outlet to identify any scouring or erosion
Routine maintenance may involve removing any debris in the pipe or from the inlet or outlet ends

 

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