Friday, October 3, 2008

Days Bay Seawall Restoration

Sandy beaches come and go. More so if they are on exposed coast lines. But Wellington harbour, with the harbour mouth facing south and the receptacle of stormy weather, also presents a challenge to those who would tame the sea.

Despite the harbour having a relatively shallow entrance that breaks the ferocity of the southerly storms, the eastern side of the harbour has continually taken a beating from the sea. The southerlies take material from the beaches and can also deposit new material from outside the harbour entrance on the foreshore.

This eastern side of the harbour has been subject to interesting changes over the last 150+ years. 160 years ago Days Bay beach was a swamp. Then a large earthquake in 1855 lifted the shoreline by 1200 – 1500 mm. That enabled a road to be formed where previously it was impossible. Prior to that the sea had relentlessly, for some hundreds of years, tickled away at the foreshore taking bits off here and there and depositing sand in other places. Most of Eastbourne village is built upon old sand dunes. In the 1950’s houses were literally being tipped into the sea at the southern end of Eastbourne. Millions of dollars were poured into the sea in the form of land protection work to maintain the values of the beach front properties, until the 1980s, when another piece of natures work surprised us. But more of that later.

Days Bay beach has long been popular – even before there was any good sort of road, or good road transport for day trippers and picnickers. But the increased use of cars in recent times and the need for more parking along side the beach in Days Bay for day ferry commuters, combined with the apparent need to protect the edge of the road from erosion, has lead to attempts to “hard edge” the contact line between sea and land. On sand this is extraordinarily hard to accomplish with success. Many years ago a small wooden retaining wall, on the northern side of the wharf, was erected. That has been largely successful to date, possibly because it is partially protected by the wharf structure and somewhat still in the shelter of the southern point of Days Bay.

Then a little bit more wooden retaining was put in. That has also been largely successful to date. (Small remedial action on both walls has had to be undertaken.) Remember though, we are talking a few years – not the time frame that sea erosion works to. Those sea forces might rip out yesterdays’ wall and then it might take 70 years or 100 years for the wind and wave direction etc to be such that it undoes mans’ best efforts. It is interesting to note in the photographs that while three palings are exposed at the southern end (in front of the Boat Shed), at the northern end four and half palings are exposed above the sand. The tops of the concrete foundation for the retaining posts are also becoming exposed. The beach is lower at the northern end, shown below.

The photo above shows the join of the third section of seawall. This is lower than that it joins and in this photo shows about one and a half palings above the beach level. Further north, showing in the following photo, two palings are showing. The beach is being lowered the further north along the beach one looks.

So then the Hutt City Council decided to be brave or foolish and further extend the wooden wall. It would ‘tidy’ the area, provide more parking and also allow the addition of a narrow walking track that was not there before. But that part of the beach is more exposed to the southerly wind-whipped sea. The beach is lower at that point (than further south) because of that greater exposure. Consequently the wall was a little higher than along by the wharf. This recent short winter the new wall was effectively wiped out. Undermined.

The photo below shows the recently installed walls being removed. Apart from very shallow footings other aspects of the construction were poorly designed. Gaps between the palings allowed erosion of the cloth backing and removal of the infill behind, by the sea. The wall capping was only held down onto the posts with 150mm stainless nails. In parts the thrashing by the sea (including floating logs) lifted that capping, simply pushing out the nails. The capping should have been held down by stainless steel strapping.

The first finger pointing must be at the engineering design. In Eastbourne there are now enough ‘lay engineers’ who could have told the council that if there was any hope of the wall surviving in sand it would need a very deep foundation. On a rocky shore line a footing would need to be excavated below the upper rock level to allow for some erosion. All along the Eastbourne coast line there are again examples of poor engineering design and poor implementation of wall construction. The photo below is a pile of piles! These have been dug out following the design failure.

I produced a series of photographs in the early 1980s along the marine drive to Eastbourne of the many failing bits of seawall at that time. That resulted in considerable seawall upgrading and considerable learning on the part of the people involved. That learning has unfortunately apparently been lost and design and installation workmanship deteriorated. More recent seawall ‘enhancement’ now has many wall ‘foundations’ hanging in mid-air with the loose material they were seated upon washed away. This can be seen in Mahina Bay and Sunshine Bay. The last couple of weeks has seen the most recent board wall in Days Bay totally removed and being replaced with a much deeper foundation. Appalling original design or construction oversight. I wonder if the new one will survive?

The problem now is likely to be that this greater height of seawall will lead to the flattening of the beach during storms to an even greater extent and make the wave attack on the beech and wall that much greater. Such a lowering of the beach would mean even heavier attack of the remaining un-walled section of the beach/road edge. A couple of weekends back I found Days Bay residents gallantly planting more Pingau in the hope they could arrest this possible sea action before it begins.

An interesting observation on which I can only speculate is the line of sand exposed currently by the excavation required to replace the wooden retaining wall. Does that represent an old beach height? Or is it simply the height of old wind blown sand on top of the up-lifted swamp land? The top of an old dune? Whatever the answer it does suggest that there is considerable ongoing beach erosion in this area. Erosion that is likely to be exacerbated by the new hard walls being created.

In Eastbourne, where the houses were falling into the sea in the 1950s, a massive concrete seawall was built on top of sheet steel piling. That essentially failed with the sea rapidly undermining the new concrete wall, leaving it hanging on the sheet piling. Groins, stretching out into the sea, were then placed and maintained for the next 30 years to assist the rebuilding of the sandy foreshore. The photo of that seawall and groins, below, was taken in May 1987.

Then the results of further earthquake action combined with river flooding and the old man in the sea began to deliver tonnes and tonnes of shingle from the Orongorongo river valley, south west around the coast from Eastbourne.

This is quite an extraordinary and on-going event. In 1981 the then Eastbourne Borough Council was very concerned at the possibility of loosing the road in front of the Bus Barns. The sea had taken out much of the foreshore and was within a couple of metres of the tar seal on the road - at the edge of the grass in the photo below. While the council deliberated on what to do and luckily made no hasty decisions to build yet another seawall, the first pebbles of the huge shingle bank that is now there, arrived – and saved the road. The shingle bank is now three time the size of what shows in the photo:

The apparently inexorable movement of the shingle bank then took it around Point Arthur to begin depositing shingle in front of the concrete seawall. In time, as it moved northwards, the groins were removed and the shingle bank grew. People managing the sports ground adjacent to the northern end of the seawall wanted to take advantage of the newly deposited land. There were proposals, plans, discussions, meetings and arguments. Luckily sense prevailed then (although who knows what will happen in ten or twenty years time when the history repositories have departed this bit of the land or this world) and the new beach has been left to expand or contract as natures forces would have it. The photo below was taken from approximately the same position as the earlier photo showing the groins. This one was taken Oct 08.

But now a new problem is presenting itself. This great moving mass of shingle is reaching into Days Bay where it can be seen as ‘destroying’ the good sandy beach. What is to be done? We so often see a problem and think, solution? I do it too. I have just seen the first house I built nearly 35 years ago nearly destroyed by a slip – a slip that I should have foreseen if I hadn’t so much wanted to build there. . .

We could try regular grading of the beach to gather up the shingle to be deposited some other place. We could build a great big break water from Windy point out into the harbour to catch the shingle slug. At least slow it down. That would leave it to build up a huge shingly beach in old Eastbourne for a few years. We could then move the sand dunes westward at Bishop park and widen the park. Or we could mine the shingle as it reaches the north end of the Eastbourne beach or build a mining platform out in the little bay between Eastbourne and Days Bay. I guess if we take the mining or beach grading idea we could be mining for a very very long time. We could mine it. Grind it to sand and replenish the northern end of Days bay beach with it. But where we put the mining plant and crushing machine is a tricky question to answer? NIMFY

Part of the trouble, of course, is that we don’t really know where that shingle might move to or how much more there is to come. Perhaps we do need to recycle it back to Eastbourne south, continually, to protect that end of the Borough.

No! Let us stop, try very hard not to get too emotionally tangled up in seeking a solution for the next 10 to 20 years. Maybe we have to accept nature’s way on this occasion. The hills uplift, erode, uplift, erode. The sea worries away at the shore giving some, taking some back.

Let us watch and marvel.

No comments: