Coastal erosion is the removal of material at the coast causing the shoreline to retreat landward. The processes include not only the work of the sea, but also that of the wind, migrating river mouths and tidal inlets, coastal landslides and tectonics. Coastal erosion occurs as a short-term fluctuation in the shoreline and a long-term trend of shoreline retreat. Coastal erosion can also be caused, or exacerbated, by man-made structures placed in the coastal environment, which interfere with natural coastal processes.
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Coastal inundation is the flooding of low-lying coastal areas by seawater. This occurs when storm surges or heavy swells - often coinciding with high tides - overtop beach crests. Percolation through beach dunes can add to the volume of inundation flood water. Beach front properties can also suffer from direct wave attack causing damage and localised flooding.
Low-lying areas, which experience coastal erosion, can also be at greater risk of coastal inundation as natural barriers are weakened.
Healthy dune systems, gravel barriers and coastal wetlands and marshes can all help protect inland areas from erosion and inundation by buffering wave energy, slowing water velocities and preventing the transport of coastal sediments. Often where coastal development has occurred these natural systems are degraded or lost.
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| Erosion caused the loss of Norfolk pines at Haumoana in March 2005. Photo: N. Daykin - Hawke's Bay Regional Council |
Previous effects in Hawke's Bay Erosion has been causing damage to property in Hawke's Bay since at least the 1850's. Awareness and concern for coastal erosion and inundation has been increasing as the impacts on people's homes and public assets has become more significant and more wide spread. In particular, concern has grown at Clive, Waimarama, Haumoana, Te Awanga, and Westshore. The issue was particularly topical in the early 1970s, following several damaging storm events and a significant coastal inundation event and again more recently due to increased storminess in the early 2000's and changes to coastal hazard management. In August 1974, seawater flooded three hundred hectares of horticultural and urban land in East Clive. To prevent a re-occurrence a sea exclusion bank was constructed in 1976-77 along the coastal area. However, the shoreline continued to recede and erosion was accelerated by the Hastings sewer outfall constructed in 1979. By 1982 erosion had substantially decreased the ponding area between the beach berm and the sea exclusion bank and it was twice overtopped by the sea. The long-term vulnerability of the area was recognised and in 1985 a scheme was initiated to move the sea exclusion bank further inland. At Waimarama a community funded protection scheme, involving the construction of a rock revetment (wall) was undertaken in 1978. Ongoing work is necessary to maintain the wall. The sandy beach in front of the wall is lost from time to time as a result of this protection work, causing a loss of dry beach at high tides. However given the right conditions the sand does return to the beach. On the evening of 3rd April 2002 about 20 Haumoana residents had to leave their homes as the high seas threatened a dozen properties near the corner of East and Clifton Roads, with some properties receiving major structural damage. The rough seas destroyed fences, cracked doors and tossed up stones smashing windows. Further south, families camping at Kairakau Beach in Central Hawke's Bay had to abandon caravans and campsites at 7pm as huge waves began to hit the shore and camping ground. Water, driftwood and other debris had been tossed onto awnings, part of the retaining wall was destroyed and in some cases caravans were filled with sand and were pushed 2 or 3 metres back by the force of the waves. Many residents have taken their own action in an effort to prevent erosion and protect their properties - with only limited effect and at some cost to the environment. Erosion was first recorded on the north side of the Ahuriri Harbour entrance in the late 1800s. Reports quote coastal retreat of 200-240 m just north of the Ahuriri entrance and in excess of 90 m over six years at Westshore. This erosion continued at various rates until the 1931 earthquake, which caused uplift of the land in and around Napier and advancement of the shoreline. The beach and coastline have been re-adjusting to this uplift, with erosion and shoreline retreat again becoming a problem along Westshore Beach since the late 1950's. A beach nourishment scheme was adopted in 1987 as the most appropriate way to mitigate ongoing erosion at Westshore. Currently, beach nourishment is carried out each year at locations determined from beach profile monitoring. The rate of nourishment is approximately 10,000 m3 per year. ^Top^
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Future coastal erosion and inundation With sea level rise predicted to accelerate over the next 100 years coastal erosion and inundation will continue to occur in Hawke's Bay, but the extent remains difficult to accurately predict. Extensive research has been carried out over the last 10 years resulting in an improved understanding of coastal processes acting along the Hawke's Bay coast. As a result of this work coastal experts now believe that the shoreline at Bayview and Westshore is relatively stable with a zero net drift of beach material, but fluctuations in erosion and accretion of the shoreline will continue to occur. The shoreline from Clive to Clifton has a net northerly drift of beach material resulting in significant coastal retreat. The long term shoreline retreat at Clifton Beach is on average 0.75m per year; Haumoana and Te Awanga 0.30m-0.70m per year; and Waimarama 0.13m per year. Changes in long-term weather patterns are expected to bring increased storminess over the next few decades, resulting in increased frequency of heavy swell events from the North East to South East quarter and associated erosion, inundation and overtopping. The anticipated future sea level rise will also exacerbate the erosion and inundation hazard. Properties in coastal hazard zones will be destroyed, unless communities can find a cost-effective means of preventing beach erosion and inundation over the long-term. Preventing coastal hazards created by erosion and inundation is expensive. Hard engineering solutions such as groynes and seawalls are not preferred because of their adverse impact on the environment. It is increasingly important to protect remaining natural buffers, such as dunes, beach barriers and coastal wetlands, and to allow beach systems to absorb erosion events so that they can still provide protection to inland areas. For areas experiencing long-term shoreline recession and significant damage to property, retreat away from the coast may be the only viable long-term option. The experience of Hawke's Bay's coastal settlements shows that the force of the sea should not be underestimated and new development should be located a safe distance away from the coast to accommodate the natural processes of erosion and inundation and to protect existing natural buffers. The key to minimising the effects of these coastal hazards is to be able to predict the extent and likely frequency of erosion and inundation occurrence. Ongoing research will continue to improve our understanding of the coastal processes that cause erosion and inundation and our ability to quantify the coastal hazard risk created by the location of communities in areas subject to these processes. ^Top^ What can you do? One of the most important defences against coastal hazards are healthy natural buffers, including healthy dune and beach barrier systems, coastal wetlands, marshes and coastal vegetation. When these buffers are healthy they absorb wave energy and flood waters, helping to minimise damage to inland areas. When you visit the beach make sure you use designated walking and vehicle tracks to minimise damage to dune and barrier systems, don't throw garden waste into dune areas as this kills off dune vegetation and spreads weeds, and if you live near the coast don't place structures in active dune or gravel barrier areas as they need room to move back and forth in response to storm events. You could also join one of the many community restoration projects which protect and enhance dunes and coastal wetlands or you could start your own. For more information on local restoration projects or if you would like to start up a project in your area, contact the Hawke's Bay Regional Council or check out their website atwww.hbrc.govt.nz. ^Top^
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