6.2 Alignment
Route alignment and the design of earthworks should respond to the broad scale of the topography as well as to small-scale landforms. The guiding principle is to work with the topography using engineering elements to minimise habitat fragmentation by maximising the opportunities for connectivity below and above the infrastructure. Earthworks should respond to even minor changes in the geological characteristics encountered along the route. The design of each scheme should take into consideration the full range of vertical and horizontal alignment standards available. These standards will vary considerably according to the type of infrastructure. A single carriageway road in mountainous country that is lightly trafficked may have tighter radius curves and steeper gradients than a dual three lane motorway which by virtue of speed and traffic volume will require gentle gradients and generous radius curves.
A relaxation of standards can be used to good effect to avoid a nature conservation site without compromising the safety of the traffic. The presence of protected or locally rare species or habitats may influence the choice of alignment and associated earthworks in the design solution. The need to limit incursions into valued habitat may require innovative design solutions such as the use of retaining structures where the road or railway is in a cutting, or the use of a shallow or low-level viaduct rather than an embankment.
Adopting an alignment following the natural contours is, in general, good practice. This helps to integrate a road within the landscape, reduces the need for earthworks and minimises disturbance to adjacent landuse.
6.2.1 Responding to ridges and valleys
An alignment that follows the foot of a major ridge will enable infrastructure to remain hidden from view. Even minor ridges offer opportunities for sensitive alignment; a rise of only five metres can be effective for screening purposes. The benefits to wildlife include lower noise levels, reduction of disturbance from vehicle lights and suppression of de-icing salt spray.
Although alignments on the skyline should generally be avoided, following the top of a major ridge can have environmental benefits by avoiding the valley bottom where it can have a major impact on sensitive wetland habitats.
Where infrastructure rises up or follows the side of a valley, the intrusion can be significant. In such situations earthworks need to be properly sited and designed. Major earthworks can be avoided by following the contours high up the valley side. Split carriageways and restored graded-out slopes are effective design solutions. Care must be taken not to grade out slopes where valuable habitats or species may be present.
There will be opportunities to place short sections of infrastructure on shallow viaducts in order to maintain an element of connectivity in the landscape. For further information see Section 7.3.1.
Following a valley bottom may be a satisfactory alignment only if the severance of watercourses and other linear features are avoided or minimised.
6.2.3 Crossing valleys
Infrastructure can be carried across valleys on embankments or viaducts. Viaducts have environmental advantages subject to the choice of the appropriate crossing point.
Viaducts are suited to narrow, steep-sided valleys as they:
- Minimise landtake and fragmentation within a valley by allowing watercourses and any existing nature conservation interest to continue under the structure;
- Maintain connectivity for species movement; and
- Retain views up and down the valley.
For further information see Section 7.3.1.
Embankments are suited more to wide, shallow valleys as they:
- Can maintain some degree of connectivity through the use of appropriately sited and dimensioned culverts and underpasses. In other situations lower alignment may allow the construction of an overpass.
- Can be integrated with the adjacent landform by good use of earthworks and planting; and
- Offer more scope for screen planting.
6.2.5 Junctions and roundabouts
Highway junctions and roundabouts can be wildlife traps or islands and are very intrusive unless well sited and designed with earthworks at a scale appropriate to minimise the impact of any signs, gantries, lighting and overbridges. They should be designed to avoid fragmentation with good connections above or below the carriageways as appropriate for the species native to the area.
- Major junctions should be sited on lowlying ground screened by landform, wherever possible.
- Roundabout earthworks generally need to be low, simple and rounded.
Land contained within junctions can be very extensive (up to 40 ha) and, in some cases, may offer opportunities for substantial planting and the creation of areas of nature conservation interest, e.g. species-rich grassland and wetland with balancing ponds. However, these areas should not be isolated from the adjoining land otherwise they become dangerous traps for fauna. They should not be considered for compensatory habitat creation.
- Connectivity between the segments of a major interchange may be important for the movement of fauna and can be achieved using culverts or tunnels.
- Fencing of the correct type may be required to prevent the larger mammals from crossing road carriageways and railway lines at these busy intersections, junctions and triangles.
- Exits from fenced areas should be provided for larger mammals.
- Whenever possible existing vegetation should be retained within the junction.
It is important to be aware of how these structures meet (crossing roads, railways and fences) to avoid confusing the fauna and unintentionally leading them onto the road.
