Tuesday, 19 June 2018

You SHALL pass...New Zealand backcountry bridges

A vital life saver, avenue to adventure...


Rivers pose one of the most dangerous obstacles to people enjoying the outdoors in New Zealand. Trampers have learnt over the years to give our rivers the respect they deserve. During the Colonial era so many people drowned crossing rivers that it became known as the 'New Zealand Death'.

Swing-bridge over the Boyle River at Windy Point, Lake Sumner Forest Park




Prior to the 1940's there were very few structures in the back country of New Zealand and those that existed were mainly built for private use. There were few bridges, so most rivers were crossed at fords with the resultant danger. 

From the late 1930's a herculean effort began to cull the destructive wild deer herds from the more remote areas of the country. This was also when the back-country started to be used for pure recreation. To facilitate both these undertakings huts and bridges were required.
Deer cullers crossing a three wire bridge, Waingawa River, 1957:from nzdeercullers.org.nz


The New Zealand Forest Service and Department of Lands & Survey hut-building era of the 1940s to 1970s coincided with a huge back-country bridge-construction effort too. Bridges changed the nature of back-country tramping almost as much as huts. The access afforded by bridges allowed tramping trips to occur in bad weather without the risk of trampers becoming  stranded by flooded rivers.

  Back-country bridges: a multitude of styles

There are a multitude of bridge designs used in the NZ outdoors, they are usually tailored to fit the particular conditions in the area to be bridged. Some are massive affairs while others hardly deserve to be called bridges at all. Obviously it is impossible to look at all the designs used but there are a few standardised types that stand out.

The symbol used on a topographic map to denote a back-country bridge

These bridge types are:

Fixed bridges

A fixed bridge will be a permanent rigid structure built to fit a particular location. These will be built of many materials: stone, wood, metal and concrete. These structures are often used to span smaller creeks and rivers but they can range in size from a couple of meters to tens of hundreds of meters.

Transportable rigid walk bridge, Travers Valley, Nelson Lakes NP

One material used for a rigid bridge is metal, normally steel this is often used on the well travelled tracks where the number of passing feet is greatest. There are now standardised bridge sections used by DOC around the country, similar to the two shown below in Nelson Lakes National Park. 

Steel and wood bridge crossing the Sabine River, Nelson Lakes National Park


A transportable walk bridge, Upper Travers Valley, Nelson Lakes NP

Wood is also used for smaller spans such as this example on the Travers Valley Track on the approach to Coldwater Hut. The creek is narrow so a stronger metal bridge is not required here. Wood is cheaper than metal and can be constructed on location more easily.

Chandler Stream bridge on the Travers Valley Track, Nelson Lakes NP
Wooden bridges are also used to span gullies that might otherwise cause soil erosion form all of the people passing that way. These spans are often used on the more popular walkways.

A new style wooden bridge on the Arthur's Pass Track, Arthur's Pass NP

These fixed bridges can be architectural features as well, designed to fit into the visual aesthetic of a particular location. They often feature interesting designs or ornament or can have an interesting shape. The bridge shown below is constructed of concrete and has both a curve and an arch so it compliments its location crossing a bend in the stream.

Architecturally designed bridge on the Arthur's Pass Track, Arthur's Pass NP

If the span of the bridge needed is long heavier steel girder bridges can be used. A good example is the girder bridge over the Devil's Punchbowl Falls outlet in Arthur's Pass. This bridge is long at +30 meters but also busy so it needs to be strong to handle all of the foot traffic it receives. 

Steel girder bridge over the outlet stream of the Devils Punchbowl Falls in Arthur's Pass

There are also combination bridges such as these examples...they uses strong steel girders for their base while the upper part of the bridge is constructed using wood. This type of tread-way bridge can be transported by helicopter sling load so you see a lot of them. Note the concrete caisson (foundation) used to anchor the bridge at the near end, the far end rests on a handy boulder.


Billy Goat Gruff bridge over the upper Maruia River, Lewis Pass Scenic Reserve
Speargrass Creek bridge, Nelson Lakes NP
The Sluice Box Bridge, Marble Hill camp-site, Lewis Pass Scenic Reserve

A fixed bridge will last for a long time so many of these will have been in their current location for 50-60 years now.

Suspension Bridges

There are many suspension bridges in the New Zealand back-country, they come in many sizes and styles but all have certain features. 

These are:

  • a tower at each end of the bridge to support the foot way
  • long arching cables from tower to tower...these suspend the foot way
  • a central foot way, often made of wood and cable with vertical stability cables
  • they often have ramps/steps for access on either end of the bridge

The photos below are of various suspensions bridges illustrating these points...


Good view of the support cables, Falls River bridge, Abel Tasman National Park


The support towers on the Falls River bridge, Abel Tasman NP


Steps at the start of the Waterfall Creek bridge, Abel Tasman NP. Note the Maori carving

Another example of this type of bridge is the Cedric Stream bridge in Nelson Lakes National Park. This bridge is an important link in keeping the Travers-Sabine Circuit track open in all weather. This bridge is over a stream with a history of extreme flooding so it rests high above the riverbed.  It has the standard towers at both end with the typical cable structure and a wooden walk way.

Support tower for the Cedric Stream suspension bridge, Nelson Lakes NP

You will notice chains have been used for the vertical stability supports, this is a common feature as they can be tensioned more easily than suspended cables. The chains also make the bridge more stable in high wind.


Cedric Stream suspension bridge, Nelson Lakes NP

Sometimes this type of bridge will have metal towers at each end, these tend to be use on smaller obstacles. The beauty of these are that they can be largely pre-fabricated and then lifted into place by helicopter or road. As you can see in the photo below these metal towers require bracing wires on each tower to support the weight of the heavy bridge-way.

A smaller metal framed suspension bridge over Hopeless Creek, Nelson Lakes NP
Detail of the cable supports, Hopeless Creek bridge, Nelson Lakes NP
As you can see above the legs of the tower are securely attached to concrete piles and backward tension is provided by the cables running over the top of the tower.

By the way all these suspension bridges could also be classed as swing-bridges, the terms are often used interchangeably in New Zealand. 

Swingbridges: A New Zealand tramping institution

Swing bridges are the most common type of bridge you will encounter while tramping in New Zealand. The vary in size from only a few meters to great river spanning monstrosities and are built using cables attached to a variety of anchors: trees, rock faces, boulders and towers.


Swing bridge over the Travers River, Nelson Lakes NP

The standard design features 10 cables, two support the bridge structure, these are attached to the foot-way with short vertical cables. There will also be horizontal cables above and alongside the foot way structure. The foot way itself are a series of cables with a metal or wooden platform built along their top. Most of these bridges will have netting along the sides to stop people falling off the structure. 

The bridge below (over the Upper Sabine River) is typical of the newer swing-bridges you will encounter, this bridge was only constructed in 2016 so is very new.


Swing bridge over the Upper Sabine river, Nelson Lakes NP
More typical are the following bridges, these are of all metal/cable construction and feature aluminium support stays between the arching support cables and the walk way. They also have metal towers at one or both ends.

Good side view of a swing bridge, this is near Mid Robinson Hut, Victoria FP
Detail of the Mid Robinson swing-bridge tower: note the metal tower, stay wires and access ramp all in metal

The long and bouncy swing bridge over the Henry River, St James Walkway


The tower and approach ramp on the Mid Boyle swing-bridge, St James Conservation Area

Lower Boyle River bridge, St James Walkway.

Swing-bridges don't always have that classic form of course, you will strike all manner of swing bridges built to fit the particular location they are constructed in. Here are a couple of unusual examples I have struck over the years...

Swing-bridge over a side creek on the St James Walkway near Cannibal Gorge Hut

Swing-bridge over the Nina River chasm, Lewis Pass Scenic Reserve


One of the new swing-bridges on the Paekakiriki Escarpment track

Walkwires: three wire bridges

Back in the deer culler years the type of bridge you would most often encounter was a classic two/three wire. A three wire is just that...three wires (cables), one for your feet and two for your hands. Words often used when talking about a two or three wire are 'bloody', 'bastard', 'flaming',  'effing' and '..that ho'er of a...'.

These are natural reactions as they are quite literally a complete bastard/ho'er to use.


The daunting view as you start walking a three wire...Hollyford Track, Fiordland

  They have a most unnerving tendency to sway from side to side as you are crossing them...it is not a pleasant sensation. The buttock clenching aspect of some of these bridges is that they are often a long way up in the air...to fall off may not be fatal but it would certainly spoil your day!

Symbol for a walk-wire bridge on a topographic map

It is very easy to fall off a three wire bridge, which is why DOC have strengthened most of them with triangular cable stays, board-walks and netting.


The three wire leading to Mystery Creek, Fiordland


Wire bridges are gradually being replaced by swing bridges and suspension bridges, you will still strike them but just not as frequently. Good spots to see/use them are Fiordland (Dusky Track, Hollyford Track), remote West Coast valleys, the Urewera Forest and in the Ruahines/Kaweka's.


A classic three wire on the Harpers Pass Track, Taramakau Valley, West Coast

Three wire with netting on the Dusky Track Fiordland

Some of the three wires in Fiordland are way above the waterline as they get torrential rain there for a lot of the year. Because of this you often need to climb onto them using aluminium ladders DOC have lashed to the start/end of the walk-wire. 


Detail of various three wire bridges...note the access ladder
If you look further down this post I have given you some ideas about how to cross both three and two wire bridges. 

Walkwires: two wire bridges

Another type of bridge you might encounter is the two wire, it is a cousin to the three wire and consists of a wire for your feet and one for your hands. These are the quickest and cheapest form of bridge to construct but can be devilishly hard to use. If there is any degree of slackness in either cable it is common for the user to fall off them.

An example of a two wire bridge

In Europe and some parts of North America these bridges are known as via ferante or "sky roads".

A via ferante two wire bridging system in the Canadian Rockies
The only place I have had to use a two wire was on the way to the old Nina Hut in the 1990's, there was one you had to use in heavy rain to cross some difficult side-streams. There are also a few up the more remote West Coast Valleys.
 
Upper Nina Bivouac, Nina River Valley, Lewis Pass Scenic Reserve

Two wires have largely been replaced across the DOC estate with other safer means of river crossing.
 

Cableways

Where a river was too wide or expensive to span using a conventional foot bridge, the builders often chose to construct a cable-way instead. These consist of a thick wire cable strung between winching stations on either side of a river.  

Letting gravity do the work..speeding to the low point of a cable-way




To cross the river the passenger sits in an open sided metal box, called a cage, basket, box or chair. The box is suspended from the main wire on two large pulley wheels, with a secondary cable and pulley to steady it. 

The passenger covers the first half of the crossing using gravity they are then hauled the rest of the way using the riverside winch attached to a smaller loop of wire. This is a similar system used for ship to ship transfer using a bosun's seat. 


Ship to ship transfer using a classic bosun's chair, image US Navy
 
Using a cable way is more challenging for a solo tramper, requiring you to use a lever with a couple of hooks on its end. To start, you climb into the basket with your gear and release the brake on the main cable. The basket once released will fly down the cable, eventually coming to rest at the lowest point mid river. 

You then hook the lever to the main cable and laboriously pull yourself 30-40 cms along the cable in the direction you want to go.

Detail of a cable-way: note the pulley system and the lever bar hanging from the main wire

Cable-ways are incredibly dangerous...there are a multitude of stories about people falling out of the chair, getting hair/fingers/clothing stuck in the winch system and people getting stuck halfway across rivers when they discovered they did not have the lever bar to haul themselves along.

Things not to do while using a cable-way:

  • get into the basket without your pack and the motion lever
  • stand up while in motion
  • swing the basket from side to side
  • go onto one with lose hair, clothing, straps etc. flapping around
  • and never, ever try to haul the basket by hand...this is a near guaranteed way to loose fingers/skin.


The now defunct cable way across the White River, on the Harman Pass Track, Arthurs Pass


Few cable ways remain in use today as they are gradually being replaced by safer bridges. Those that remain tend tom be clustered in the more remote valleys of the West Coast of the South Island in places like the Hokitika, Kokatahi and Whitcombe Valleys. 

How to cross a two/three wire bridge

A three wire is a bit easier to use than a two wire, you have two wires for your hands and one for the feet so it is easier to balance yourself as you cross. A lot of the existing three wires have had v shaped stays fitted to them to improve their stability. These stays anchor the cables in a fixed position and cut down on the 'wobble' effect these bridges naturally have.

Detail of the stability cable stays on a walk-wire on the Dusky Sound Track
Before jumping onto the walk-wire, store any trek poles etc. so you have two free hands and make sure there are no loose straps/equipment that could snag. Only one person on a walk-wire at a time.

The best way to cross a three wire is facing forward with your toes pointing out, your foot arch straddling the lower cable, heals on the outside of the cable. This way you can see where you are going while also maintaining the best balance possible on the bottom wire.

You slide/move your feet along the bottom cable while grasping the hand cables tightly. Watch those cable stays as I often find the edges are ragged and sharp

 
Good illustration of foot placement while crossing a three wire bridge

Two wire bridges are very, very difficult to use, that is why there are so few of them left. 

The most important thing when using a two wire bridge is tension, if those wires are slack then more than likely you are going to fall off. You shuffle your feet from side to side while holding the top wire and leaning slightly outwards to keep tension on the hand-wire. 

If you have a rope and harness it is a good idea to connect yourself to the top wire for safety. If you ever strike a two wire bridge only use it if you absolutely must, usually you will find the river it crosses can be forded in all but the worst conditions so just do that.



The side to side shuffle on a two wire bridge..the deep, ice blue abyss below!

 I sincerely hope you never have the opportunity to use one...

The bridge I have crossed the most is...

This is the bridge I have crossed the most number of times, a classic swing-bridge of course. It is the bridge over the lower Boyle River on the St James Walkway. I have crossed this bridge 15 times now on the way to/from some point in the surrounding area. She is a familiar friend now, it means either the start of a trip or the end.

Lower Boyle River bridge, St James Walkway, St James Conservation Area