Why this route?
Work undertaken on Transmission Gully since 2006 provided the first real opportunity to conduct on-site, in-depth investigations into the impact of the alignment on ground and environmental conditions. These effects were considered in the development of the Transmission Gully Route.
The route provides several significant benefits, in addition to the identified cost savings. They include:
The route will largely avoid native forests and will have fewer and shorter stream crossings. This will minimise effects on aquatic life, freshwater and marine ecosystems. It will also fit well within the natural land form and will create minimal noise for nearby residents or those using recreational areas.
Improved route security
The route crosses the Ohariu Fault on an earth embankment which will be more resilient in a major earthquake and be easier and quicker to repair.
The route cuts less into the hills and follows flatter slopes to reduce the risk of landslides.
Improved highway safety and function
The route better suits the proposed 100km/h speed limit. It provides a median barrier along the entire route, crawler lanes on the steepest sections and auxiliary lanes for vehicles merging between interchanges.
Improved connections to local roads
An eastern Porirua interchange will connect to both James Cook Drive in Whitby and Warspite Avenue in Waitangirua, providing improved connections with the wider eastern Porirua area.
Opportunity to protect the environment
The Transmission Gully route allows a better opportunity to enhance the environment and preserve people's enjoyment of their properties and recreational areas.
Planting is well underway in a number of areas with native trees and shrubs. More than 150,000 have been planted with many now well established. Additional planting is planned and this will allow easier passage for birds within the area.
The route allows an opportunity to avoid adverse effects on aquatic life. It will have fewer stream crossings and will generally cut less into the hills, making it easier and simpler to treat run-off and stormwater along the route. Measures to control run-off will include the capture of sediment during construction and enhancements to the stormwater management system. Provision will be made for fish passage and natural debris flows in streams.
Work on environmental effects has identified the need for protection of several terrestrial, freshwater and marine environments and has included a detailed physical survey of the Te Puka and Horokiri Streams and Duck Creek.
Other specific social and environmental investigations included:
- Noise monitoring to determine measures to reduce project and traffic noise.
- Visual and social effects considered to enable landscaping which would be sensitive to existing landforms and which would minimise the visual impact of the highway.
- Assessment of stormwater management requirements.
- Archaeological investigations were undertaken in three areas most likely to have historic sites. No archaeological discoveries were made along the route.
Further detailed investigation work will be conducted and the team will be working with local environmental and community groups to ensure the best possible outcomes are delivered.
Reducing the impact of fault lines
Both the State Highway 1 coastal route and the Transmission Gully route cross fault lines, as is the case with many roads in New Zealand.
The Transmission Gully route offers improved security for the region's road network over the existing coastal route even though it crosses two fault lines.
The Transmission Gully route crosses the Ohariu Fault on an earth embankment which will be more resilient in a major earthquake and allow easier and quicker reinstatement than the previously planned viaduct.
Reducing the impact of storms
The route generally runs lower along the Gully to reduce the height and number of large cuts into the hillside, meaning the risk of a landslide is reduced.
Fewer bridges and culverts along the route mean that there will be less obstruction to the natural movement of debris in streams during storms. This means that the overall risk of storm damage to structures as a result of debris build-up or washouts is reduced.
Regional Parks near the route
- The route largely avoids native forest areas.
- Sensitive landscaping appropriate to land form will minimise the visual impact of the road.
- Bridges, viaducts and retaining walls will blend in with their locations.
- Appropriate route design and landscaping will reduce noise levels.
Battle Hill Farm Forest Park
Battle Hill Farm Forest Park is an important regional recreational park owned by Greater Wellington Regional Council. It is also the site of the last battle in the region between Maori and the Crown in 1846. It is widely used for walking, horse riding, equestrian events, mountain biking, camping and picnicking. It also includes a commercial farm operation.
The Transmission Gully route reduces earthworks and crosses the park on flatter land on the valley floor close in against the low rounded hills known as Gas Line Ridge. The ecological and environmental benefits include:
- Less risk of sediment entering Horokiri Stream.
- Less risk of sediment affecting Pauatahanui Inlet.
- Reduced intrusion of the highway on people's view and on the landscape.
- Reduced direct noise effects.
- Fewer structures such as viaducts.
The Transmission Gully route is located well away from the colonial homestead, cottage, woolshed, stockyards and gravestones, and the site of the battle itself on the ridge leading up to Battle Hill summit. The route will continue to allow full recreational access to the park. A bridge will be provided across the highway to the rear of the park so that park activities can continue largely as before.
The Transmission Gully Route
Heading south on State Highway 1, the Transmission Gully route starts just south of MacKays Crossing, curving left up the Te Puka Stream valley and over the Wainui Saddle. It follows Horokiri Stream down to Battle Hill Farm Forest Park and on past Pauatahanui Golf Course.
The route swings west to cross over State Highway 58 about 600 metres southeast of the existing Pauatahanui roundabout. It climbs to the headwaters of Duck Creek, runs above the creek, and crosses Cannons Creek near the Takapu Road electricity sub-station.
The route follows the hills above Ranui Heights before dropping down through plantation forest opposite Kenepuru Hospital to join State Highway 1 at Linden.
In more detail the route is as follows:
State Highway 1 MacKays Crossing/Paekakariki Interchange
The route to Transmission Gully from the north crosses above State Highway 1 south of MacKays Crossing. Traffic from Kapiti will still be able to access the existing State Highway 1. Traffic from Paekakariki township can access Transmission Gully via MacKays Crossing.
- View larger map (PDF, 6.47MB)
Crawler lanes for the steepest section of the route on the climb and descent between MacKays Crossing to Wainui Saddle are proposed to take into account the large difference in speeds between cars and trucks.
- View larger map (PDF, 6.17MB)
The road will generally run parallel to the Horokiri Stream over this section and has been positioned to lessen the impacts on the stream and its tributaries and the surrounding environment.
- View larger map (PDF, 6.89KB)
Battle Hill Farm Forest Park
The route through Battle Hill Farm Forest Park is on the valley floor and offers more opportunity to provide stormwater control. It also means that there is less risk of sediment entering the Horokiri Stream during construction. Its valley placement makes it less visible from the park and surrounding areas and reduces the level of road noise.
- View larger map (PDF, 6.41MB)
Pauatahanui Golf Course
This section of the route passes through rural land adjacent to the Pauatahanui Golf Course and Flightys Road. The road will cross a number of small tributaries of the Ration Stream but there will be no major stream crossings requiring significant bridge structures.
- View larger map (PDF, 6.45MB)
The Transmission Gully route will cross above the existing State Highway 58 route. There will be on and off ramps from Transmission Gully to a new two-lane roundabout on State Highway 58.
- View larger map (PDF, 6.20MB)
James Cook Interchange
The Transmission Gully route runs under the proposed interchange. The roundabout links both James Cook Drive and Warspite Avenue at Waitangirua. There are no connections proposed to local roads on the eastern side of the route in line with Porirua City Council land use development policies.
- View larger map (PDF, 6.48MB)
Throughout this section the route will run along the eastern side of Duck Creek valley which generally comprises undulating farm land that forms part of the Belmont Regional Park. There are four major bridge structures within this section including the 260m long Cannons Creek Bridge, which is the longest bridge over the Transmission Gully route.
- View larger map (PDF, 5.79MB)
The Transmission Gully route will cross above a roundabout with on and off ramps, and a connecting road to Porirua. Extra lanes will assist merging where appropriate. Porirua will be connected to Transmission Gully via a new 50km/h road over the existing State Highway 1 motorway and railway line to a new roundabout on Kenepuru Drive.
- View larger map (PDF, 5.95MB)