Boardwalks of Pouakai

This article has been contributed by Logan Drummond, including preparation of the drawings that outline the construction methodology of these boardwalks. The impressions and images presented in this journal article were observed during a walk in to Ahukawakawa Swamp to see several rare species (notably Melicytus drucei and Olearia quinquevulnera) growing within distinctive habitats where the swamp meets the adjoining slopes.

Between all of us at O2 Landscapes, we watch a lot of documentaries. Which means that there are also a few too many Werner Herzog impressions (of varying quality) around here – including one occasion earlier this year as we ascended the flanks of a sleeping volcano.

This particular morning started with the kind of pervasive fog that is commonplace at Pouakai – a great sheet of suspended water blanketing the dark forest interior. Plants have been engaged in an evolving dialogue with these environmental conditions for hundreds of thousands of years, as Taranaki and its adjoining ranges forced their way skywards – becoming gradually enlarging magnets for cloud and eroding/erupting in alternating phases.

We were headed towards Ahukawakawa Swamp, a sphagnum moss swamp sitting a kilometre above sea level. A river once flowed down this volcano before lava was forced up, creating a wide dome that cut off the river and led to the formation of a large basin behind it. Mt. Taranaki is part of a volcanic chain that includes a range of interesting botanical habitats such as the Kaitake and Pouakai Ranges, Paritutu and the Sugar Loaf Islands.

Despite the extreme elevation change, the ascent of the Pouakai Range is rather low effort, thanks to a well-devised system of timber boardwalks installed by DoC over the last few years. The genius of these structures owes to their pre-fabricated nature, standardisation of materials and cantilevered installation. Given our interest in built landscape, these boardwalks became a separate focus during our walk, alongside the intended botanical one.

Pictured above are newly constructed sections of boardwalk (delivered by helicopter) awaiting installation. Prefabrication in the context of National Parks and inaccessible areas vastly increases the speed at which large of areas of trail can be formed and greatly decreases the physical requirements of workers who would otherwise need to transport heavy building materials across large distances and uneven ground.

The boardwalk has been standardised to utilise modular sizes of treated pine and matching spacings. The treads and stakes are formed from 50mm square batons; with the square profile and correct grain orientation providing a dimensionally stable piece of timber. Resistance to bowing, cupping and twisting is especially important where a high level of wetting, drying and freezing occurs. Few places are harsher environments for timber than the slopes of a volcano (particulartly one with annual rainfall this high).

Stringers are made with a 50mm x 100mm profile which is also used to form the first tread, helping to differentiate and highlight the change in level between sections, presumably as a safety measure. The total riser height is a relatively low 150mm, which makes ascending the stairs easy for a wide range of ages and the mountain quickly traversable.

The previously described fabrication lends itself to simple installation. Prefabricated sections can be cut into shorter lengths that create short runs of stairs. Each section sits atop the last (the one above holding the section below), and this is all reinforced by stakes. Because the timber batons are as wide as all spacings, stakes can be driven down to lock in multiple sections below.

The drainage function of the trail has also been integrated into the boardwalk construction. In some parts, sections of boardwalk are bridged by dual planks that turn the vertical sides of the boardwalk sections into water directing structures – an essential requirement in a landscape that can receive up to 8000mm of rainfall in a year, seven times greater than Auckland’s average yearly rainfall.