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Nelson Historical Society Journal, Volume 6, Issue 5, 2002

20th Century Alterations

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20th Century Alterations

In late 2001, because of its relationship to the problems being experienced at the eastern end of Tahunanui Beach, I found myself looking again at the seawall, this time much more closely than I had in 1977. Consultants to the NCC had referred, in their 2001 report on the state of the beach, to 'the concrete, close to vertical face, seawall retaining Rocks Road' and had continued by saying that 'Waves incident on the wall reflect off it with negligible energy loss'.

Standing on the beach looking eastwards to the wall, it was clear to me that it had been specifically designed to absorb wave action, and this prompted me to look into how it came to be built that way. In doing so I came to appreciate the way its newer western section had been deliberately arranged to absorb wave action and, by this means, greatly reduce the storm wave effects. These, I learnt, had on occasions made the original roadway near impassable when high tide and high onshore wind conditions coincided.

Given its obvious sophistication and knowing something of the complexity of the design task – it is by no means a plain straightforward seawall – I would not have been surprised to discover that experienced, international civil engineering consultants had been involved. My initial enquires led me to two previous City Engineers, Mac Crampton and Geoffrey Toynbee, both of whom confirmed that what could now be seen was, in fact, a combination of 19th and 20th century construction. The later work had been designed and supervised by Nelson City Council staff during the period when Charles Kidson was City Engineer.

In September 1958 the City Engineer advised his Council that, with Government financial assistance obtained largely through the advocacy of Nelson MP Stan Whitehead, improvements to the Sam Jickell-designed roadway and seawall were to be carried out in three stages. By then, work on the first stage, from Tahunanui Beach to Magazine Point, was already nearing completion.

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Looking to the west along the new wall under construction in 1961. Note the piles of hard material, evidently excavated from Magazine Point, on the beach. Collection of Geoff Toynbee.

Looking to the west along the new wall under construction in 1961. Note the piles of hard material, evidently excavated from Magazine Point, on the beach. Collection of Geoff Toynbee.

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The second stage was to be from Richardson Street to the Anchor Foundry and the third would be the formation of a 40ft dual carriageway from Port Nelson to Anzac Park. The planned overall project completion date was 1962. The last stage was eventually completed in late 1964, to the delight of the local Automobile Association, which described it as 'an excellent entrance to the city…as fine as any in New Zealand'.

This staged work programme explains the present three-part appearance of the face of the seawall. The primary purpose of the alterations was to raise and widen the roadway in order to provide greater protection from the sea for those using Rocks Road. The considerable length of wall left unaltered between the first and second stages lay within the zone sheltered from heavy seas by Haulashore Island and by the rocky shore in front of it, and thus did not need such alteration.

This means that the near-vertical face of the central third of the Rocks Road seawall reveals the original 19th century construction. The raking, ribbed and facetted western third was built against the face of the old wall in 1958/1959. The raking eastern third, much like the western section, but with more substantial ribs and no facets, was also built against the face of the old wall and was scheduled for completion in late 1962.

The need for a taller and more durable seawall arose not only because of the overtopping which occurred in adverse weather, but also because of the effects of sea action on the old one. This was washing away the jointing material in its concrete block construction and seriously eroding the fill under the roadway behind it. The still-visible part of the old wall face was given a sprayed-on plaster coating in order to protect it, but it is of concern to note that this plaster has been damaged in places.

The solution conceived by City Council engineering staff, led by Geoffrey Toynbee, in the late 1950s consisted of a new, relatively light-weight wall founded on the underlying rock base. It was designed to lie at an angle, braced against the face of the earlier one and supported in places by the sloping rock outcrops left exposed at the foot of the existing wall. This technique also enabled the engineers to accommodate a bulge in the old wall caused by an earlier slump in the cliff face behind it.

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The profile and structural details of the western and eastern sections of the new wall differed along their length. These differences depended on the state of the adjoining existing wall, its proximity to Tahuna Beach and the fetch, or degree to which parts of the wall were exposed to distant sources of wave action. At the eastern end of the wall the latter was considerably less, because of the protection provided by the Boulder Bank and Haulashore Island. Another influence on the design was the existence of natural rock outcrops in front of the wall in some places.

The arrangement arrived at in order to absorb wave action consisted of a reinforced concrete wall with an outer raking face at about 450 degree slope, a facetted vertical central section and an outward-projecting top profile at roadway level. The purpose of the outer slope was to reduce the initial wave forces, while that of the facetted section was to deflect downward the waves which rose above the sloping face. The purpose of the top section, built considerably higher than the old wall to reduce overtopping onto the new higher roadway, was to turn back those waves which reached it.

In addition to the effect of the new wall's profile on seas which arrive more or less at right angles to it, the prominent projecting concrete ribs, which stiffen its relatively thin concrete surfaces, also serve to modify wave action in its vicinity. They do this by deflecting outward those waves which meet it obliquely instead of straight-on. The deflection effect also assists in reducing the wave energy which eventually reaches the beach. This is because the most frequent wind direction produces waves which strike the wall from an angle directed towards the beach at its western end and thus travel westward along it.

This was the intention of its designers who, in addition to thoroughly understanding its function as a road-edge structure, were also well aware of the sea-state relationship between the seawall and the adjoining beach. A further indication of their recognition of this relationship shows in the purpose of the additional length of unribbed, unfacetted, plain seawall profile. This was built westward, beyond the water's edge and deep into the dunes, to provide for what they knew would be the sand's inevitable advance and recession as natural forces acted on it over the years.

On the landward side there were two significant design factors. The first was the distance from the new wall to the near edge of the reconstructed roadway proper. In places, the presence and width of the new footpath meant the road traffic loads on the wall were more distant, and thus less severe, than elsewhere. The second factor was the need to provide for page 13the increased wall loads at that part of the old wall where bulging had occurred due to slumping of the Tahunanui hillside.

To my surprise, given the numerous examples elsewhere of seawalls with upper sections continued a metre or so above the level of the adjoining roadway in order to protect passing vehicles and pedestrians from stormy seas, the structure of the new Rocks Road wall was not extended above footpath level. This decision, perhaps made as a cost-saving measure, was to have a profound effect on the appearance of the wall from seaward, from the beach, and when passing by on foot or in a vehicle.

Each of these thoughtfully arranged, sequential wall profiles employed by the engineers are, on their own, not unusual in seawall design, and one or another of them can be seen elsewhere in New Zealand. The Wellington Harbour region has examples of each one, for instance in Oriental Bay, Lyall Bay and Kio Bay. The Rocks Road combination of all four – sloping face, facetted vertical section, projecting top, and vertically ribbed surfaces – is rare indeed however, and I have not seen it anywhere else on our coasts.

Two further qualities make the Rocks Road seawall of special significance in aesthetic terms. One is the considerable visual variety achieved in these interacting, sea-deflecting shapes as they meet the differing circumstances occurring along the outer face of the sinuous curves of Sam Jickell's original wall. The other is the sturdy post and chain fence which, strung out along its top, contributes an impression of filigreed curvature to the complex forms already present in the sinuous concrete wall itself.

Many handsome seawalls go largely unnoticed by passers-by on the roadways and footpaths they support and protect, usually because the wall is continued up above road level, thus blocking views of its outer faces. Here, however, as a result of this local design decision, the whole of the ribbed, raking, winding, seaward surfaces of the Rocks Road wall is clearly visible between and over the posts and chains to all who travel to and from the city.

Later still, in about 1975 it seems, apparently because the height of the old central section was found to be inadequate in heavy sea conditions, a length of the outside edge of the adjoining roadway surface was raised, though the adjoining footpath was left at its existing level. At this time also, a plaster protective coating was applied to the face of the 19th century wall to prevent further erosion of the blockwork joints.

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In my 1977 article, impressed by its appearance but largely ignorant of its origins, I described this seawall as 'a priceless visual asset to Nelson' and went on to say 'I have not seen any quite like it. Its ribbed raking massive elegance is combined with considerable variety in its curvature, cast iron and chain balustrades, steps, and plain vertical profile at rock outcrops…all providing an object lesson in nautical engineering'.

To my delight, at the end of the 1970s refurbishment work was carried out on the posts and chains, some by then almost 80 years old and showing considerable deterioration. Later still, in the early 1980s, further repair and protection work was carried out on the old wall face where it was still exposed to the action of the sea.