TECHNICAL DEBATE

NOT RELEVANT TO THE CURRENT SEAWALL DEBATE

DATE: 12/05/2015

Rebuttal to Unnamed “Coastal Hazards Expert’s Policy Statement” No response received since 2010.

 (Note: see below for the original  statement from the ‘unnamed expert’)

As your coastal hazards expert has stated the seawalls constructed on the Kapiti coast as a popular means of protecting coastal property since the 1950’s were built with little understanding of coastal processes and with little regard to the environmental effects they have on the surrounding beach. That was then but this is now. I know about the seawall having been destroyed in 1976 at Paekakariki and later at Raumati as it was my company that undertook the work of rebuilding. And yes, continued scouring at the base of the wall has required additional work to the base of the wall.

I need to point out that the wall constructed with timber palings was a mere 1.8 meters in vertical length. And the beach had to drop a mere 600 mm (2 foot) for the wall to fail which was well outside the scope of cyclic tide sand scouring and replacement. For many years of the initial life of this wall it was mostly buried in places by the shifting sands particularly at northern Paekakariki.  In my mind this calls into question at least in this circumstance, the comment concerning the flow on effect of the impermeable barrier and the shortened swash zone which allegedly creates deeper water and prevents sand from being deposited on the foreshore as asserted by your coastal hazards expert. I am saying that these assertions whilst containing partial reality are often presented as fact which is not always applicable to every situation.

 

And of course it was mostly the council owned and constructed wall that failed, necessitating both replacement with longer palings, lengthening the palings with concrete footings, and installing the rock where appropriate. The lessons have been expensive but needed to be learned. The difficulty has been of course that the coastline was already developed with Paekakariki’s Parade needing to be maintained as access within the town for many properties that may be washed away after the road potentially has been taken out. And a similar situation had developed at the Esplanade in Raumati.

I can recall during the construction of the wall at northern Paekakariki, unearthing of the remains of a long forgotten Maori oven at the lowest excavation level. This provides a clue that the coast, was at some time significantly further seaward than at a time previously and causes one to wonder about the cyclic tendencies of the shore line in the light of rhetoric regarding rising sea levels.

 

I will put into perspective the difference between original wall height/depths of those that failed and those constructed nowadays.

Whereas the original walls were built 1.2 meters in height and 600mm below the beach surface level, nowadays the heights are less than 1.5 meters high and a fully 2.7 meters below the surface of the beach. That is deeper than the average house ceiling is high.

The walls are constructed with stainless steel fixings and long protected Reid bar ties back to large 200mm heavily tanalised poles embedded as the dead man. Scouring on a scale never before seen in living memory on the Kapiti Coast will need to take place for a wall so constructed to be undermined as occurred with the originally constructed walls. I write this special response to clarify the “cheaply constructed seawalls” comment of your coastal hazards person who in promoting a “well vegetated fore-dune” has not understood the situation faced by these Paekakariki property owners.

 

Your coastal hazards expert has given a lesson in backshore, foreshore, and near shore transfer of sediment deposits stating that wave energy is effectively absorbed through this process. “Sand deposited in the backshore acts as a buffer. During storm events some of the sand is given up to the foreshore and nearshore thereby enabling the beach to maintain its position”. I can testify that during some previous major storm events the sand buffer contained in the backshore and moving to the foreshore offered no protection whatsoever to certain properties which never recovered. There was no absorption of wave energy at all, other than the limited vulnerable absorption of those destroyed foliated banks exactly like the banks currently under threat above the proposed wall. Some property owners had to move altogether and others at great expense had to undertake major works stabilizing newly exposed sand banks.

 

To say that seawalls act as a barrier to the wind, causing a lot of sand grains that would otherwise nourish a dune system, to drop at the base of the seawall is not, or at least only partially correct in this instance as can be proved by experience gained of other walls directly adjoining and also nearby the proposed wall. The sand dunes have been static for many years but recently wind is further undermining the dry running sand from around the roots of existing pohutukawa trees, taupata trees and other ground cover.

The nearby walls are topped by waves rarely, perhaps twice each year in events depositing broken shell particles and sand which drains extremely well, whereas as stated above, the dunes are being steadily eroded due to vandals digging at the toe some years ago.  So the reason for the proposed wall is that the dune its self is under threat unlike neighbouring properties which have been stabilised by the presence of well constructed walls.

 

To say that dunes provide a natural store of sand that protects the hinterland area from erosion during a storm event is correct only in terms of a normal storm and not as with major storms of the past which have decimated whole properties in a single afternoon. And while it is nice to say that the dune is connected to the beach and forms part of a system of sediment transfer between the foreshore and the backshore it is also saying by inference that the property owner has to pay an horrendous price in not being allowed to defend against large scale property loss as a result of a major one off storm event.

 

To make the call that the adverse effects that seawalls exert on a beach are exacerbated by sea level rise because they hold the shoreline at a fixed position without allowing the beach to adjust naturally, and quote figures of global sea levels rising since the mid nineteenth century is to introduce a clinical and disputed opinion which carries no cost to the one offering the opinion but has radical impact to the embattled property owner.

 

The alternative protection structure of a rock revetment is appreciated and understood for its ability to absorb wave energy in a way that allows sand to deposit on the foreshore more readily. However this takes no account of the reduction of use and lifestyle to the property owner whose decision I believe it ought to be, as to how much expense they are willing to bear to modify and maintain a seawall which incidentally works very well with rock.

I understand that the advantage of the rock revetment does not inhibit sand deposition in the same way a solid wall does. The protective wall is situated at the backshore level seldom reached by the waves behind the existing tyres and rail irons. There is very little sand at this location other than the wind-blown sand of the dune which comprises finer particles and does not remain as a beach surface ingredient but sinks between the larger and more absorptive particles of the foreshore, or blows away, not in any appreciable way, to rejuvenate the sand bank from which it originally came. I say this from many years of observing other like situations on the Kapiti Coast. To put any hope in the accumulation of significant enhancement to the beach level to bring about a reduction in swash power from this source is unrealistic.

 

In short I find the overall tone of your letter unhelpful in that almost the entire comment from the coastal hazards expert focused on why a wall cannot and should not be built in spite of the fact that most of the coastline protection along the area is of solid construction. You (the Regional Council) seem to be attempting to implement policy that is designed to allow the natural inclinations of the sea to take it’s course across property seafronts which are not protected and which are certainly surrounded and adjoined by areas which are. This taken to it’s logical conclusion means that council policy wishes for all existing structures within (or outside) a council designated line need to go as the mood of the elements take it.

Question: Does this summation have any basis in reality with the Regional Council policy?

If so I would like to respectfully suggest that this expert comment be saved for areas such as Queen Elizabeth Park where there has never been significant development subject to a major storm threat, and these policies could be worked through with no unhelpful and unnecessary hurdles for those who need to do something to improve and secure their frontage. Also that council officials be encouraged to identify with the struggles of the property owner whose right it should be to protect his life’s work in many cases.

 

John Mills

 

 

Statement of Wellington Regional Council’s unnamed “Coastal Hazards Expert”

 

Received Re: application for resource consent 13/08/2010

 

The construction of seawalls has been a popular means of protecting coastal property for many years. However, they have often been built with little understanding of coastal processes or regard to the adverse environmental effects they have on the surrounding beach. Seawalls have been extensively used in the area since the 1950’s. In 1976, the seawall at Raumati beach was destroyed in a storm event and subsequently rebuilt. Since that time continued scouring at the base of the wall has required additional rock armouring to protect the wall from toe scour. Seawalls can sit on a beach for many years without experiencing any major storm activity, creating the perception that they are more effective than they really are. In many situations a well vegetated foredune will cope with most of the day to day beach activity and moderate storm activity. It is the big events that hard structures really need to be designed to withstand – and often cheaply constructed seawalls are not up to this task.

Beaches are extremely effective at absorbing wave energy, even though they are composed of loose unconsolidated material. An important mechanism by which this is achieved is through the transfer of sediment between the backshore (above mean high water level) the foreshore (area between high and low water level) and the nearshore (underwater area fronting the beach). Sand deposited in the backshore acts as a buffer. During storm events, some of the sand is given up to the foreshore and nearshore thereby enabling the beach to maintain its position. In the ensuing calmer period, this sand is returned to the backshore area by waves depositing sand from the nearshore and onto the foreshore where it is carried to the backshore by wind transport. Thus, a beach is a connected system between the nearshore, foreshore and backshore. When engineering works are placed across or along a beach they interrupt this natural system of sediment transfer between the respective parts of the beach.

A considerable amount of wave energy is dissipated in the nearshore surf zone, with the remaining energy being expended on the beach in the form of swash. A beach works to dissipate swash run-up by absorbing the water into the beach sediments. This process is effectively curtailed by the construction of a seawall, as it creates an impermeable barrier to the swash as it runs up the beach. The wave energy is then reflected back off the structure rather than being absorbed by the beach sediments. This enhances erosion of the beach and scouring at the toe of the wall in two ways. First it shortens the width of the swash zone, effectively shortening the width of the beach over which wave energy has to be dissipated. Second, because of this reduced beach width, the foreshore becomes saturated with water, reducing the ability of the sand to absorb run-up. Saturated sand is very easily eroded and this is enhanced by the reflected wave activity. As swash is reflected off the wall, it creates deeper water on the beach and consequently, higher current velocities that have greater capacity to remove sand from the foreshore. The flow on effect of this process is that it prevents sand from being deposited on the foreshore.

Seawalls also interrupt the process of sand deposition by wind activity and prevent the transfer of sediments between the backshore and the foreshore. Impermeable structures act as a barrier to the wind, causing a lot of sand grains that would otherwise nourish a dune system, to drop at the base of the seawall. Even in moderate conditions on a high tide, this sand is easily removed from the toe of the seawall by wave activity. On a natural sandy beach with no engineering works, wind blown sand can be transported above the mean high water mark, thereby providing material for dune development. Dunes provide a natural store of sand that protects the hinterland area from erosion during a storm event. Without a seawall, this dune is connected to the beach and forms part of system of sediment transfer between the foreshore and backshore.

Contrary to popular opinion seawalls do not prevent erosion, rather, they hold a shoreline in one position, whilst sand continues to be removed from the beach in front of the wall. Presently, there is no low tide beach in front of the seawall at Raumati Beach, Raumati South and Paekakriki because sand has continued to be removed from the beach. In this way walls become undermined and collapse. This is partly why the seawall at Raumati was destroyed in 1976. This is especially pronounced when they are constructed in isolation. As the beach retreats around the seawall, wave energy becomes focussed around the structure enhancing erosion on the surrounding beach. Furthermore, wave reflection occurs off the ends of the wall and exacerbates erosion either side of the structure – in a well documented process known as end effect erosion.

The adverse effects that seawalls exert on a beach are exacerbated by sea level rise because they hold the shoreline at a fixed position without allowing the beach to adjust naturally. Global sea level has been rising since the mid 19th Century at rates of 1.0-2.0 mm/yr, due to human induced climate change. Over the last 50 years the average rise has been in the upper part of this range at 1.7-1.8 mm/yr and in the past 10 years it has been running at 3.0 mm/yr. In fact, recent figures show that the rise in the South Pacific is presently the highest in the world. About half of the rise is due to thermal expansion from global warming, with the rest coming from melting continental ice (as opposed to sea ice). Data from satellite radar altimetry suggests the rate is accelerating slightly and is expected to continue accelerating over the coming century.

Sea level has been measured via tide gauges around New Zealand since the late 19th Century. The records indicate an average sea-level rise of 1.6 ±0.2 mm/yr, which is in line with the global mean. Around Wellington the rate over the past 100 years has been slightly higher at 1.78 mm/yr, which equates to a 17.8 cm rise over the past century.

Mid-range projections for future sea level rise by the Intergovernmental Panel for Climate Change are from 0.14-0.18 m by 2050 and 0.30-0.50 m by 2100. However, there is considerable uncertainty in these projections and if the accelerating trend continues, sea level rise will be higher than presently forecast.

An alternative protection structure that can be used on a beach that is in net long term erosion is to build a rock revetment. They still have an impact on the beach, but if designed properly they can be used to absorb wave energy in a way that allows sand to deposit on the foreshore more readily. The advantage of a rock revetment is that it does not inhibit sand deposition in the same way a solid wall does. The structure can be built to be more permeable by allowing water to flow through it, whilst trapping sand. Furthermore, a rock wall can be easily maintained and raised over time with the addition of extra rock if required. This is not the case with a seawall, which is much more difficult and expensive modify and maintain.

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