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Bibliography

Coastal Erosion on the Kapiti Coast

Kapiti Observer

Dominion Post

https://golearngeo.wordpress.com/2010/07/14/constructive-waves-destructive-waves/

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http://www.coastalsociety.org.nz/Newsletter/pdfs/nzcs32.pdf

http://www.stuff.co.nz/dominion-post/comment/8410415/Kapiti-erosion-Head-to-head

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http://www.stuff.co.nz/environment/80441421/Eating-the-shore-New-Zealands-shrinking-coastline

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http://www.stacey.peak-media.co.uk/NNorfolk/Horton2010/Horton1Apr2010.htm

Why?

Fiona Frazer

By combining multiple solutions to combat erosion it ensures that it will be the most effective. This combination of solutions is also better for the natural and cultural environments. Naturally, the coast will be maintained and by dune stabilization the area will become more abundant in vegetation and wildlife - promoting a sustainable, clean, green image along the Kapiti Coast. This will also benefit the cultural environment, more tourists may visit the area and more people may move to the area for its beauty and the coast. This would bring economic gains to the community which can go further in improving infrastructure and beaches. However, if you were to just use dune stabilization for the whole coast, in many areas south of the cuspate where erosion is significantly worse, the dunes wouldn’t develop in the time it would take erosion to severely damage the area - and therefore wouldn’t be cost effective.

Also, if the solution was to only use seawalls and gabions, that would cost a great deal and ruin the natural image of our beach. They could end eroding more areas of the beach because when the waves hit the wall, they don’t lose any energy and instead that is reflected back with the backwash. Seawalls and gabions would need to be maintained regularly to ensure that the beach was still a safe environment for people and animals. These large built structures would take away from the beauty of our coast and reduce the amount of visitors and beach users we have - which can ultimately affect the economic growth of Kapiti. By only using seawalls and gabions in high risk areas it provides enough safety and security for residents to relocate, without the ongoing costs of maintenance. They will also maintain the width of beach at present so that beach users still are able to enjoy these areas.

The downside to this solution would be the overall cost. Dune stabilization is relatively cheap, unlike sea walls. However, the seawalls don’t have to be reinforced walls of concrete that cost upwards of $500,000, but if the seawall only needed to be suitable for the short term, a wooden one would cost ⅕ of that. To only build a short term seawall on sections of the coastline that are at high risk until planned retreat takes place is far cheaper than the continued maintenance required. For example, in Wellington September last year, it cost $1.3million to repair the damaged seawall which had a 41 meter hole torn through it, in addition to that, it cost an extra $500,000 in compensation for water damage to nearby homes. And in a small West Coast town of Granity, a school which now juts out onto the beach has had to have the seawall which protects it repaired twice; costing a total of $200,000. To prevent those incidents from occurring on our coast, it would be best to retreat from areas that are at such high risk.

However, when relocating residents from their homes, compensation will have to be made.

In the long term, this would be the best solution although it would have a large initial cost, but it would be worth it to have a positive impact on people's lives by not having to worry about erosion on their doorstep at present. It will also ensure that future generations have a safe and sustainable environment to grow up in.

A Suitable Solution

After researching how coastal erosion interacts with the natural and cultural environments, I believe that in northern areas (like Paraparaumu Beach and Otaki) would benefit best from dune stabilization. Whereas, south of the cuspate in Raumati and Paekakariki, the most effective method would be using hard management like seawalls and gabions to secure the land that homes have been built on and can be considered high risk. I also think a level of planned retreat would be necessary because as climate change persists, storms are likely to increase in severity and it is best to ensure the community is safe, by relocating them from high risk areas.

Because the Kapiti Coast is so varied in its landscape and erosion is happening at different rates in different places, to get the most effective coastal erosion prevention would be to combine multiple methods. To use soft management in areas like Paraparaumu Beach and Otaki is a much more natural approach because erosion isn’t drastically changing the landscape at this time. By planting out the dunes with durable plants (spinifex and pingao) it stabilizes this sediment and creates a large enough buffer between homes and wave actions. So if there was a storm, the sand dunes would absorb the majority of the waves energy and decrease the amount of erosion. Also, by planting along the coastline, it also has better natural environment effects and increases habitats. Sand dunes are a durable and self sustaining solution which would add to the community’s natural beauty and possibly increase tourism to the area.

Hard management would be more effective in areas south of the cuspate in Raumati and Paekakariki. By building seawalls and installing gabions, it will benefit the residents by keeping their properties and homes stable and safe during storms. However, if you begin a planned retreat along this area for residents that are most at risk or affected by erosion, it would be a sensible and long term solution. While the seawalls are still effective and retaining the properties, residents would be relocated elsewhere in the district to ensure they are no longer at risk. By relocating residents and surrendering their land to erosion, the seawalls won’t have to be maintained and the land can be developed into dunes so that our coastline can find its equilibrium position and erosion would be minimal.

Effects on People

The effects on people vary along the coast. In Paraparaumu Beach right along Marine Parade for example, people aren’t worried as much about coastal erosion because their properties are not set directly on the sand and they have decent soft management in place to delay erosion. However, just 500m away, residents are fighting with the council to protect their homes. For example, Wendy and Graham Brown at 175 Manly Street have already retreated 15m since 1989 due to erosion, and now their house is “at risk of collapse.” They were unable to gain resource consent from the council to maintain a concrete block seawall and now fear that their $655,000 house will collapse within a year, leaving them land worth $20,000.

Residents in Raumati South and Paekakariki are equally as concerned. Their houses are situated right along the coastline and on top of vulnerable sediment. As more storms persist the erosion is becoming worse and in some extreme cases, families are being told it would be wise for them to relocate as their house is too at risk. The other option for them would be to raise their 2m high seawall to 4m which would cost $2,500 a metre. Stuart Whyle of Harveys Real Estate has said that the economic impacts affecting beachfront residents that would expect their homes to sell for $900,000; but due to erosion and seawall repairs needed, buyers are only willing to pay around $850,000. If coastal erosion processes persisted it could greatly affect the people in beachfront homes, they would lose profit value for their homes, or even have them damaged or destroyed by wave action. This could displace hundreds of people along the Kapiti Coast.

Nearly all beachfront residents are very concerned about the amount of erosion occurring as it would compromise the safety of their homes and therefore have economic perspectives on the issue. These homeowners have taken it upon themselves to protect their properties by building or extending seawalls, installing gabions or even compromising with concrete blocks. Such as Gavin Bradley in Raumati who had built a seawall to protect his property, but wanted to upgrade and extend it to ensure that his and his neighbours properties were secure enough to survive the winter storms. These residents concerns have got them involved with the council's efforts to delay erosion, and so when the council put out forum which took in suggestions from residents - they got 400 responses.

The Kapiti Coast District Council must manage coastal erosion because of the social perspectives such as, tourism attraction, property values, and recreational activities, while for local iwi it holds great significance, which all bring economic benefits to the community.

The KCDC acknowledge that managing the coastline is an important issue for residents. It was surveyed that the community wants a comprehensive approach to coastal management to treat the coast as an ecosystem instead of just protecting it. Therefore, the Coastal Strategy was developed in 2006 which acts as a guide to ensure that the coast is restored and enhanced.

Pat Dougherty, chief executive at KCDC has said that “Scientists say sea-level rise and more intense storms will worsen erosion and put properties, as well as roads and public infrastructure, at risk.” It is required by law that all councils plan for the impacts of climate change. This means that the council take a 10 yearly review of the District Plan, however, under the Resource Management Act, “the council must provide protection of outstanding natural features and landscapes from inappropriate subdivision”, and try to prevent new buildings/developments in areas that are at high risk from natural hazards. KCDC have already prepared for this and has coastal setback lines in place covering the majority of the coastline. The council are informing property owners how coastal erosion could impact their properties and are willing to listen to the public’s perspectives on how they think is best to handle the issue.

Soft Management

In 2012 after the council had put out the latest district plan which included a 50 and 100 year coastal erosion prediction that would affect 1800 homes, they commissioned three local iwi for their perspectives. Te Atiawa ki Whakarongotai, Ngati Toa and Ngati Raukawa, worked with the council to locate historic sites that could be of interest to them. 400 were identified, but only 40 were included in the district plan, including pa (Maori village land), urupa (burying grounds), and waahi tapu (places of ancestors death) sites. The local iwis have concluded that they would support the council in decisions to protect these sacred sites as long as there is minimal disturbance to the historic land.

Second, is soft management. This is when there are is minimal human interaction and no built structures are in place. Instead, it focuses on planning and utilizing the natural environment to reduce coastal erosion. This is done by beach replenishment, dune stabilization, and planned retreat.

Beach Replenishment: is when large amounts of sand are added to the beach area. This creates a wider beach and therefore replacing sand that has been eroded. By doing this, it needs to be constantly maintained and replenished. It may appear that after storms that the replenishing may have been a waste, but the majority of it is returned by the swash once the storm has passed. However, it does not prevent erosion from occurring on our coast, but instead delays it. The downside to beach replenishment is that it is very expensive to transport or redistribute vast quantities of sand to our beach areas. Also the amount of human interaction involved could greatly contribute to erosion (photo 10).

Photo 9: Beach replensihment using ships to pump sand from sea floor on to beach in Hayling Island, England.

Photo 10: Beach replensihment using diggers to redistibute sediment evenly across beach.

Hard Management

What is Coastal Erosion and how does it affect the Kapiti Coast?

Dune Stabilization: is the currently a very popular sustainable choice in managing coastal erosion along Paraparaumu Beach. This is when the sand dunes are planted out with very durable plants that thrive in beach conditions, such as, spinifex, pingao, beach grass, and carpobrotus (pigface) (photo 13). These plants have tough exteriors and sturdy roots which stabilises the sand within the dunes, thus making the sand less likely to erode by waves or wind. Not only do they stop sand from being eroded, they also trap sand in large swashes so that the beach continues to build up. Dune stabilization is also the sectioning off of dune areas to people. Councils choose to section off large areas of dunes because if people walk over them, it makes the sand looser and more susceptible to erosion, it also disturbs vegetation and native habitats which are developing (photo 12). The downside to this though is that some people in the community feel frustrated that they have limited accessibility onto the beach (photo 11).

Photo 13: Beach grass along Marine Parade, Pararparaumu Beach that works in preventing wind and wave erosion.

Photo 11: Dune stabilization along Marine Parade in Paraparaumu Beach. Large areas of beach a off limits to prevent loosening soil.

Photo 12: Dune stabilization allows for native vegetation to develop fully so that it is most effective in preventing coastal erosion.

There are two ways to manage the rate at which the Kapiti Coast is eroding.

First, there is hard management, which uses built infrastructure to prevent the erosion of the coast. Hard management includes structures like seawalls, gabions, groynes, and rip raps.

Planned Retreat: is probably the most drastic soft management strategy. This means relocating beach front property owners further inland, and letting the coast erode until it reaches its equilibrium. Often beaches have equilibrium points that maintain the size of the beach - equal amounts of accretion (build up of sediment) and erosion. However, since the cultural environment has built so close to the beachfront and preventive measures have been put in place to stop the coast from finding its equilibrium means there is a constant effort to keep the beach maintained. A planned retreat means that many homes will likely be destroyed, roads and infrastructure will also be compromised which community members may find unfair.

The Kapiti Coast is located 48km out of Wellington on the west coast of the lower North Island, it stretches up for 40km; from Paekakariki (in the south) and reaching up to Otaki (in the north) (map 1). Coastal erosion is an issue that wears away the natural beach environments by wave action, currents, and high winds. Erosion on the Kapiti Coast is threatening roads, beachfront homes, and changing our coastline. The cuspate at Paraparaumu Beach is not greatly affected by coastal erosion due to being sheltered by Kapiti Island 5km out from the coast which reduces wave energy. This cuspate has positive and negative effects on erosion along the coastline. Since it extends out 3km further than the coast, it is easier for it to collect sediment and balance out the effect of erosion, just like in Waikanae and Otaki where sediment is being brought down from more northern areas by longshore drift and balancing out the erosion. However, in Raumati and Paekakariki, the cuspate acts as a barrier and there is more erosion than sediment being deposited.

Coastal erosion has always been a process on the Kapiti Coast, however, in recent years, erosion has been increasing due to climate change. Global warming is causing the polar ice caps to melt which is rising sea levels, therefore causing higher tides and more intrusion into land and ultimately more erosion. The changing climate also means that there are more severe storms and flooding which will continue to damage our coastline and waterways that interact with the beach. Erosion on the Kapiti Coast is highly likely to continue since global warming is causing sea levels to rise constantly, therefore our coasts will continue to deteriorate, unless increased preventive measures are put in place.

Seawalls: are the most popular built structure along the Kapiti Coast to prevent erosion. They are most commonly made out of wood or concrete and built parallel to the coast so that it can withstand waves breaking on it. Seawalls are effective in a way that they absorb the energy of the wave and reflect it back and reduce the coastline eroding (photo 1). However, the problem with seawalls is that when the wave hits the wall, all the water still has its energy, but it is reflected to create a larger backwash and erodes the base of the seawall. Continual erosion like this will undercut the seawall causing it to collapse and need repair (as seen in photo 2).

Coastal erosion can drastically reshape the coastline of Kapiti and this could greatly affect the cultural and natural environments.

The effects on the natural environment are clearer to see as the damage after storms is very noticeable. This is evident from one storm in May 2015 when destructive waves and high winds carved out 4m of Ben Bowie’s backyard so that now the beachfront access stairs don’t reach the property. Another storm earlier this year in August had a combination of high winds and destructive waves which also washed out up to 10m of land from some properties. It also managed to wash out a seawall in Raumati Beach and destroyed a large portion of seawall that maintained Wharemauku streams exit onto the beach. Since these seawalls collapsed in the storm, a mass amount of sediment and vegetation were washed away, leaving some properties even more exposed to the waves. If coastal erosion persisted so heavily due to global warming, it would slowly carve into our natural and cultural environments and damage or destroy native plants and habitats which we didn’t think were at great risk. Such as in Kena Kena, they lie only slightly higher than the rest of the coast above sea level, which means they had been relatively unaffected by coastal erosion. However, now, due to global warming and sea level rise, some property owners have become concerned and started to take action by building their own seawalls and gabions.

Diagram 4 (above): A and B above show the way that seawalls reflect waves and cause erosion at the base of the wall.

Photo 1: Seawall at Raumati Beach in front of the Waterfront being rebuilt after a large storm washed it out.

Photo 2: This shows the difference a sea wall can make, the beach is at least 1m below the top of the sea wall while the rest of the sand being retained.

Photo 14: The remaining beach access that now does not lead up Ben Bowie's property

Photo 16: Mass erosion caused by the storm in August at Wharemauku stream

Photo 15: The broken seawall at Wharemauku stream exit after a strom in August

Photo 17: Residents build their own seawalls out of concrete blocks to protect their home along Rosetta Road in Raumati Beach.

Photo 3 (to the right): Gabions built on top of a seawall as a second line of densse by one property owner.

Gabions: are wire cages that are filled with crushed rocks and typically stacked in a seawall position, they can also be used in conjunction to seawalls (photo 3). Since they are not as solid as seawalls, the water will hit and go into the gaps, this will weaken the waves energy and therefore reduce erosion on the coast. Gabions are still very effective in protecting vegetation and sediment (photo 4). However, they do not last as long as seawalls, this is because the wires get rusty, break, and create a hazard to beach users (photo 5). This means that gabions need to be regularly maintained to ensure they are effective and the beach is a safe environment.

Photo 4: Gabions built in Raumati South, behing an Aotea Road property.

Photo 5: Broken gabions shown above produce a hazard for beach users. Sharp wire is spread out on the beach and sharp rocks are even more exposed.

Map 3: The white lines above show the wave direction which is pushed along by the wind direction (green arrow). The red arrow shows the fetch (distance travelled by the waves that hit our coastline).

Map 1: The Kapiti Coastline is shown above along the red line.

Map 2: The Kapiti Coast district is shown within the red outlined area.

Groynes: are structures that are built perpendicular and work by preventing sediment being continuously moved along the beach by longshore drift. Instead, it is trapped beside the groyne and builds up the beach (diagram 5). Groynes prevent the erosion of the beaches further up the coast and protect sea walls from wave erosion. The downside to Groynes is that sediment is still being eroded from the other side because nothing is able to replace it (diagram 6). The solution to this is to build more groynes, but unfortunately it would ruin the look and appeal of the Kapiti Coast because they are unattractive structures and reduce the accessibility along the beach (shown in photo 5).

Diagram 5: Groynes traps sediment moving with longshore drift

Diagram 6: Groynes don't prevent erosion on the opposite side and create uneven beach ground.

Photo 5: Groynes built on Great Yarmouth beach in England show how the beach can build up, but also be eroded. It also shows how unattractive lots of groynes can look on one beach.

Rip Raps: are barriers of large rocks at the base of cliff or beach (photo 7). They work similarly to gabions but on a larger scale, by reducing the waves energy by allowing water through gaps in the rocks but not creating a large backwash, and therefore preventing erosion of the coastline (photo 8). Rip Raps are easy to maintain and can host as fishing platforms, however they can intrude onto the beach which reduces walking space in high tide (photo 6). You can see some between Poplar Ave and Aotea Road in Raumati South.

Coastal erosion on the Kapiti Coast is an issue which originates from wave action and high winds across the Tasman Sea from Australia, 3,000km away. The energy of the waves is caused by the distance it has to travel; called the fetch (map 3). This combines with prevailing westerly winds to New Zealand and generates a lot of force which hits our vulnerable coastline.

When waves hit the Kapiti coastline, the amount of erosion is dependent on the size of the swash and backwash. Backwash is the pull of sediment with water from the beach back by the wave, whereas swash is the distribution of sediment from the wave onto the beach.

Constructive waves are created in calm weather with low crest and trough heights - this means that when the wave breaks on the beach, it has a strong swash but a weak backwash and the sediment is able to build up the beach. However, destructive waves have large amounts of energy; typically generated by long fetch distances and in storms. This means that when they break onto the beach they carve out sediment with the strong backwash and deposit a very little amount since it has a weak swash.

Long shore drift is also a wave process that affects erosion on the Kapiti Coast. It is when the wave hits the coast from the prevailing wind/wave direction (typically westerly on the Kapiti Coast) and deposits sediment in this direction, however when the wave retreats it does so perpendicular to the coast due to gravity. This repeats and moves sediment along the coastline.

Coastal erosion not only has an effect on the natural environment, but also the cultural environment. This is because many homes and community infrastructure has been built along the coastline and erosion is a constant threat to them. So, the ways people have been trying to manage this is through hard management and soft management. Hard management is built infrastructure like seawalls and gabions which some homeowners have built themselves to protect their homes and properties. Soft management is the use of plants and natural elements to minimise the effect of erosion.

Photo 8: Although rip raps aren't as effective as sea walls as preventing water getting through, they still prevent erosion.

Photo 7: Rip Raps are best used for long distances. This photo shows it stretching from Queen Elizabeth Park to Aotea Road

Photo 6: Rip Rap at a beach access of Hydes Road in Raumati South. It extends out into the beach which means that in high tide there is no beach space.

Diagram 3 (below): The processes involved in longshore drift

Diagram 2 (above): Destructive Wave Diagram and its process in coastal erosion

Diagram 1 (above): Constructive Wave Diagram and its process in coastal erosion

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