Canterbury Bight
Encyclopedia
Canterbury Bight is a 135 km stretch of coastline between Dashing Rocks (north Timaru
) and the southern side of Banks Peninsula
(Birdlings Flat) on the eastern side of the South Island
, New Zealand
. The bight faces southeast, which exposes it to high-energy storm waves originating in the Pacific Ocean
(Kirk, 1967). The most frequent wave approach direction for the Canterbury Bight is from the southeast and the most dominant the south with wave heights of over 2m common (Kirk, 1967). The bight is a large, gently curving bend of shoreline of primarily mixed sand and gravel (MSG) beaches. The MSG beaches are steep, highly reflective (of wave energy) and composed of alluvial gravel deposits. The alluvial gravels are the outwash products of multiple glaciations that occurred in the Southern Alps
during the Pleistocene
(Kirk, 1967). Large braided rivers transported this material to the edge of the current continental shelf
, which, due to sea level rise is 50 km seaward of the coasts current position (Kirk, 1967). The MSG beaches of the Canterbury Bight therefore occur where the alluvial fans of the Canterbury Plains
rivers are exposed to high-energy ocean swells (Hart et al., 2008). The dominant rock ‘greywacke
’ in the Southern Alps is consequently the primary constituent of the MSG beaches (and Canterbury Plains), which is partially indurated sandstone of the Torlesse Supergroup (Hart et al., 2008).
mouth (Hemmingsen, 2004). Features of note along this section of the bight include the Washdyke Barrier and Washdyke Lagoon
.
mouth and Taumutu on the Southern end of Kaitorete Spit
. Unconsolidated alluvial cliffs interbedded with sands and silts, which back steep, narrow MSG beaches unify this zone (Hemmingsen, 2004). The cliffs are the result of erosion of the Rangitata River
, Ashburton River and Rakaia River
alluvial fans whose mouths are all encompassed by this region (Single, 2006). Continued cliff erosion contributes around 70% of the coarse material supplied to the MSG beaches of the Canterbury Bight (Hart et al., 2008).
and represents the ‘down-drift’ end of the Canterbury Bight. This zone is dominated by the Kaitorete Spit
(actually a barrier) and is backed extensively by dune systems (Hemmingsen, 2004). Kaitorete ‘Spit’ encloses Lake Ellesmere
(Waihora), the fourth largest lake in New Zealand
(Hemmingsen, 2004). This section of the bight is the only that is not in a long-term erosional state.
and backwash, with the larger storm induced waves creating stronger swash/backwash, which removes more eroded material. The amount of gravel provided to the coast from the cliffs is estimated at around 666,400m3/yr although this value is also speculative (Single, 2006).
of the continental shelf
is relatively flat with no major obstructions (Kirk, 1967). Because of this storm waves are thought to be capable of moving sediment onshore (by increasing water velocity near the bed) although due to the highly turbulent swash/backwash zone, only a small portion of sediment will remain onshore (Kirk, 1967).
and the basalt cliffs at Dashing Rocks prevent significant longshore transport out of the Canterbury Bight system by obstructing further transport. Secondly, longshore transport appears to have abated as little sediment has accumulated at the down-drift end of the bight since the 1950s suggesting sediment is lost from beaches before reaching Banks Peninsula (Hemmingsen, 2002). This has been attributed to sediment becoming finer after undergoing abrasion
, which allows it to be winnowed from the beach (Single, 2006). To further this conclusion, backwash is significantly weaker than swash as there is a lot of percolation
through the coarse beach material (Kirk, 1967). Weaker backwash means that sediment must be smaller in order for it to be removed from the beach. Estimations for the amount of sediment losses due to abrasion differ greatly with studies giving figures of 76%, 9-98% and 5-65% (Single, 2006).
and backwash, which act to move the material up and down the beach in a zigzag fashion. Almost all of the changes in beach morphology and sediment distribution observed are produced by swash and backwash (Kirk, 1967). The direction and rate of longshore sediment transport is a function of the angle of wave approach, wave strength and the time between successive waves (Hart et al., 2008). The end result of this is a net northward migration of coarse sediment, predominantly in the swash zone (Hart et al., 2008). This is because only swell/waves moving from south to north are generally strong enough to move large sediment (Kirk, 1967). Sediment removed from the backshore is predominantly moved offshore rather than along shore as only large southerly storm waves and their subsequent swash, which usually flow perpendicular to the beach are able to reach this area (Kirk, 1967). The direction of wave approach is often relatively perpendicular due to wave refraction
. This means that longshore transport mainly occurs in the nearshore swash zone.
on the Canterbury Bight, it is easy to assume that these waves will inherently cause an increase in sediment transport, particularly from south to north. But as discussed earlier, storm waves act more to remove sediment offshore than along it.
is occurring along 75% of the Canterbury Bight. In the long term, most MSG beaches are in an erosional state due to the lack of available coarse sediment needed to withstand the high-energy environments they reside in (Hart et al., 2008). In the Northern Zone however, from Taumutu to Birdlings Flat/Banks Peninsula conditions are relatively stable as longshore transport into the zone is small, but sufficient to maintain a relative equilibrium (Kirk, 1967). The Central Zone, from the Rangitata River mouth to Taumutu is experiencing the worst erosion along the Bight. Estimates vary on the rate of erosion but an average of 8m/yr is given, however this value may be influenced by high erosion levels at one site (Single, 2006). The height of the cliffs characterizing this zone and the size of the beach in front of them are a controlling factor for erosion rates (Single, 2006). The Southern Zone, from Dashing Rocks Timaru to the Rangitata river mouth is also undergoing erosion although at rates not as severe as seen along the Central Zone. The Washdyke Barrier is the major concern in this zone (Kirk, 1992).
Management of the Canterbury Bight is controlled and regulated by Environment Canterbury (Ecan). Ecan believes that in many instances, the increased risk of erosion and seawater inundation are caused by the inappropriate location of assets and activities and by a reliance on inadequate works to protect from the ocean (Ecan, 2005). To investigate coastal hazards, Ecan; establish and maintain co-operation with weather and tsunami forecasting agencies in the issuing of warnings about potentially damaging natural events, assess the effect of hazards on the coast and regularly collect data on sea/shoreline conditions to determine any changes in occurrence of hazards and the physical nature of the coast as well as determine areas requiring hazard mitigation (Ecan, 2005).
This process has created a significant hazard, as the Washdyke barrier is the only line of protection between the high-energy ocean and valuable infrastructure including State Highway 1, an important railway and a large industrial area. Additionally, the barrier protects the Washdyke Lagoon, which is a valued wildlife area (Kirk, 1992).
In 1980, to manage the erosion hazard of the Washdyke barrier beach crest heights were raised 2.0–2.5m to minimise washover, washover sediment was used to fill the body of the beach and river gravels were used to cap the beach crest (Kirk, 1992). This programme was monitored over five years and showed that erosion was decreased by 55%, with no retreat or washover. Untreated adjacent beaches experienced significant retreat over the five-year period showing the programme was very successful (Kirk, 1992).
. The Washdyke barrier re-nourishment has proved a successful venture for that area although it has only lessened the threat, rather than completely removed it. The success of the re-nourishment programme means it should be used again for this area. Along the Central Zone of the Canterbury Bight, different mitigation methods are needed in order to decrease erosion risks. A huge dilemma is created, as sediment is needed from this area to nourish the Northern Zone, which without it would itself begin to erode. Given this, only three options are left, either do nothing, retreat from the coast or constantly re-nourish the area with large sediment. Doing nothing is an option for some areas where there is no economic or cultural significance and erosion poses no risk to anything valuable. Objects that can be moved landward, without incurring significant losses should be moved in a managed retreat. Lastly, re-nourishment could be used sparingly in areas where objects cannot be moved or are have some form of value. Re-nourishment would be the ideal method used for the entire coastline but this is not plausible due to the size of the area and the cost of re-nourishment.
Timaru
TimaruUrban AreaPopulation:27,200Extent:Former Timaru City CouncilTerritorial AuthorityName:Timaru District CouncilPopulation:42,867 Land area:2,736.54 km² Mayor:Janie AnnearWebsite:...
) and the southern side of Banks Peninsula
Banks Peninsula
Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves...
(Birdlings Flat) on the eastern side of the South Island
South Island
The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. The bight faces southeast, which exposes it to high-energy storm waves originating in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
(Kirk, 1967). The most frequent wave approach direction for the Canterbury Bight is from the southeast and the most dominant the south with wave heights of over 2m common (Kirk, 1967). The bight is a large, gently curving bend of shoreline of primarily mixed sand and gravel (MSG) beaches. The MSG beaches are steep, highly reflective (of wave energy) and composed of alluvial gravel deposits. The alluvial gravels are the outwash products of multiple glaciations that occurred in the Southern Alps
Southern Alps
The Southern Alps is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the island's western side...
during the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
(Kirk, 1967). Large braided rivers transported this material to the edge of the current continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
, which, due to sea level rise is 50 km seaward of the coasts current position (Kirk, 1967). The MSG beaches of the Canterbury Bight therefore occur where the alluvial fans of the Canterbury Plains
Canterbury Plains
The Canterbury Plains are an area in New Zealand centred to the south of the city of Christchurch in the Canterbury Region. Their northern extremes are at the foot of the Hundalee Hills in the Hurunui District, and in the south they merge into the plains of North Otago beyond the Waitaki...
rivers are exposed to high-energy ocean swells (Hart et al., 2008). The dominant rock ‘greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
’ in the Southern Alps is consequently the primary constituent of the MSG beaches (and Canterbury Plains), which is partially indurated sandstone of the Torlesse Supergroup (Hart et al., 2008).
Zones of the Canterbury Bight
The Canterbury Bight can be split into three distinct regions; Southern Zone, Central Zone and the Northern Zone (Hemmingsen, 2004).The Southern Zone
The Southern Zone represents the southern most part of the Canterbury Bight, which runs from Dashing Rocks to the Rangitata RiverRangitata River
The Rangitata River is one of the braided rivers that helped form the Canterbury Plains in southern New Zealand. It flows southeast for 120 kilometres from the Southern Alps, entering the Pacific Ocean 30 kilometres northeast of Timaru...
mouth (Hemmingsen, 2004). Features of note along this section of the bight include the Washdyke Barrier and Washdyke Lagoon
Washdyke Lagoon
Washdyke Lagoon is a brackish shallow coastal lagoon approximately 1 kilometre north of Timaru, South Canterbury, New Zealand. The lagoon has drastically reduced in size since 1881 when it was approximately 253 hectares, now it is less than 48 hectares in area...
.
The Central Zone
The Central Zone is the largest of the three zones and runs between the Rangitata RiverRangitata River
The Rangitata River is one of the braided rivers that helped form the Canterbury Plains in southern New Zealand. It flows southeast for 120 kilometres from the Southern Alps, entering the Pacific Ocean 30 kilometres northeast of Timaru...
mouth and Taumutu on the Southern end of Kaitorete Spit
Kaitorete Spit
Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. it runs southwest from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere from the Pacific Ocean...
. Unconsolidated alluvial cliffs interbedded with sands and silts, which back steep, narrow MSG beaches unify this zone (Hemmingsen, 2004). The cliffs are the result of erosion of the Rangitata River
Rangitata River
The Rangitata River is one of the braided rivers that helped form the Canterbury Plains in southern New Zealand. It flows southeast for 120 kilometres from the Southern Alps, entering the Pacific Ocean 30 kilometres northeast of Timaru...
, Ashburton River and Rakaia River
Rakaia River
The Rakaia River is in the Canterbury Plains in New Zealand's South Island. The Rakaia River is one of the largest braided rivers in New Zealand...
alluvial fans whose mouths are all encompassed by this region (Single, 2006). Continued cliff erosion contributes around 70% of the coarse material supplied to the MSG beaches of the Canterbury Bight (Hart et al., 2008).
The Northern Zone
The Northern zone runs from Taumutu to Banks PeninsulaBanks Peninsula
Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves...
and represents the ‘down-drift’ end of the Canterbury Bight. This zone is dominated by the Kaitorete Spit
Kaitorete Spit
Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. it runs southwest from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere from the Pacific Ocean...
(actually a barrier) and is backed extensively by dune systems (Hemmingsen, 2004). Kaitorete ‘Spit’ encloses Lake Ellesmere
Lake Ellesmere
Lake Ellesmere / Te Waihora is located in the Canterbury Region of the South Island of New Zealand. It is actually a broad, shallow lagoon located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow sandy spit called Kaitorete Spit, or more correctly Kaitorete...
(Waihora), the fourth largest lake in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
(Hemmingsen, 2004). This section of the bight is the only that is not in a long-term erosional state.
Sediment Inputs
Generally speaking, there are six potential sediment sources for beach environments. These are longshore transport, onshore transport, wind transport, river transport (and alluvial cliffs for the Canterbury Bight), biogenous (mainly in shell form) deposition and hydrogenous deposition (Kirk, 1967). In the Canterbury Bight system, wind transport and biogenous and hydrogenous deposition can be excluded as agents of sediment inputs. Wind can be excluded as it acts to remove sediment from the beach although this is not a significant amount. Biogenous deposition can be excluded as the high-energy environment and coarse sediment deter shelled animals from occupying the area. Lastly, hydrogenous deposition is not considered important to the Canterbury Bight system (Kirk, 1967). This means that rivers, longshore transport and onshore transport are the main sediment sources for the Canterbury Bight.Rivers
Erosion of the alluvial cliffs (and subsequent longshore transport) through the Central Zone of the Canterbury Bight is believed to provide the majority of coarse material to the beach system (Kirk, 1967). This creates a conundrum, as rivers are generally accepted to be the main source of sediment to coasts and three large rivers (Rangitata, Ashburton and Rakaia) discharge into the Canterbury Bight. Furthermore, the total amount of sediment that the rivers transport to the coast is proportional to other rivers worldwide (Hemmingsen, 2004). The first reason the rivers do not provide a significant amount of sediment to the coast is that the coarse sediment (i.e. gravels) are transported offshore during floods where waves are unable to return it to the coast and/or it is deposited further inland within the river channel (Kirk, 1967). The second reason is that the material capable of nourishing the coastline (i.e. coarse material like gravels) provided by rivers is estimated to only be around 176,700m3/yr although this value is very speculative (Single, 2006). This estimate of coarse sediment supply only equates for less than 10% (by weight) of sediment supplied by the river systems. The remaining 90% (by weight) is fine material, which is unable to nourish the Canterbury Bight and is transported offshore (Kirk, 1967)Alluvial Cliffs
The erosion of alluvial cliffs found in the central zone is predominantly caused by sub-aerial processes followed by marine processes removing the eroded material (Kirk, 1967). This eroded material is then subjected to longshore transport, which in the case of the Canterbury Bight is predominantly from south to north. Estimates for the rate of erosion vary along the coast but are averaged at about 8m/yr (landward retreat), although high erosion levels at one site may influence this value (Single, 2006). The marine processes include swashSwash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...
and backwash, with the larger storm induced waves creating stronger swash/backwash, which removes more eroded material. The amount of gravel provided to the coast from the cliffs is estimated at around 666,400m3/yr although this value is also speculative (Single, 2006).
Onshore Transport
Onshore transport of sediment is considered a secondary sediment source for the Canterbury Bight. In the offshore zone, sediment movement is unimpeded as the local bathymetryBathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
of the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
is relatively flat with no major obstructions (Kirk, 1967). Because of this storm waves are thought to be capable of moving sediment onshore (by increasing water velocity near the bed) although due to the highly turbulent swash/backwash zone, only a small portion of sediment will remain onshore (Kirk, 1967).
Sediment Outputs
Evidence for longshore transport is readily apparent on the Canterbury Bight. These include the formation of the Kaitorete Barrier, a landform associated with longshore sediment transport (Hemmingsen, 2002). The Canterbury Bight is not eroding due to net longshore transport exceeding sediment inputs however as firstly, Banks PeninsulaBanks Peninsula
Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves...
and the basalt cliffs at Dashing Rocks prevent significant longshore transport out of the Canterbury Bight system by obstructing further transport. Secondly, longshore transport appears to have abated as little sediment has accumulated at the down-drift end of the bight since the 1950s suggesting sediment is lost from beaches before reaching Banks Peninsula (Hemmingsen, 2002). This has been attributed to sediment becoming finer after undergoing abrasion
Abrasion (geology)
Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, waves, gravity, running water or erosion. After friction, the moving particles dislodge loose and weak debris from the side of the rock...
, which allows it to be winnowed from the beach (Single, 2006). To further this conclusion, backwash is significantly weaker than swash as there is a lot of percolation
Percolation
In physics, chemistry and materials science, percolation concerns the movement and filtering of fluids through porous materials...
through the coarse beach material (Kirk, 1967). Weaker backwash means that sediment must be smaller in order for it to be removed from the beach. Estimations for the amount of sediment losses due to abrasion differ greatly with studies giving figures of 76%, 9-98% and 5-65% (Single, 2006).
Longshore Transport
Sediment removed from the cliffs, brought onshore from offshore and the sediment supplied by rivers that remains in the lower foreshore zone undergoes longshore transport. The main agents are swashSwash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...
and backwash, which act to move the material up and down the beach in a zigzag fashion. Almost all of the changes in beach morphology and sediment distribution observed are produced by swash and backwash (Kirk, 1967). The direction and rate of longshore sediment transport is a function of the angle of wave approach, wave strength and the time between successive waves (Hart et al., 2008). The end result of this is a net northward migration of coarse sediment, predominantly in the swash zone (Hart et al., 2008). This is because only swell/waves moving from south to north are generally strong enough to move large sediment (Kirk, 1967). Sediment removed from the backshore is predominantly moved offshore rather than along shore as only large southerly storm waves and their subsequent swash, which usually flow perpendicular to the beach are able to reach this area (Kirk, 1967). The direction of wave approach is often relatively perpendicular due to wave refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
. This means that longshore transport mainly occurs in the nearshore swash zone.
Wind
Wind also plays a part in the transportation of sediment. The most frequent velocities reached by winds on the Canterbury Bight are capable of moving medium to coarse sized sand particles (Kirk, 1967). These winds are attributed with the movement of sand from the beach towards the dunes including the extensive dune ridges along the Kaitorete Barrier (Kirk, 1967). Wind also has a secondary effect of sediment transport, especially strong winds following southerly swell. These winds force the wave crests of the southerly storm waves to spill (spilling breakers). Spilling breakers produce a longer and stronger swash (Kirk, 1967). Given that swash is a component of longshore driftLongshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...
on the Canterbury Bight, it is easy to assume that these waves will inherently cause an increase in sediment transport, particularly from south to north. But as discussed earlier, storm waves act more to remove sediment offshore than along it.
Current Erosion Conditions
ErosionErosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
is occurring along 75% of the Canterbury Bight. In the long term, most MSG beaches are in an erosional state due to the lack of available coarse sediment needed to withstand the high-energy environments they reside in (Hart et al., 2008). In the Northern Zone however, from Taumutu to Birdlings Flat/Banks Peninsula conditions are relatively stable as longshore transport into the zone is small, but sufficient to maintain a relative equilibrium (Kirk, 1967). The Central Zone, from the Rangitata River mouth to Taumutu is experiencing the worst erosion along the Bight. Estimates vary on the rate of erosion but an average of 8m/yr is given, however this value may be influenced by high erosion levels at one site (Single, 2006). The height of the cliffs characterizing this zone and the size of the beach in front of them are a controlling factor for erosion rates (Single, 2006). The Southern Zone, from Dashing Rocks Timaru to the Rangitata river mouth is also undergoing erosion although at rates not as severe as seen along the Central Zone. The Washdyke Barrier is the major concern in this zone (Kirk, 1992).
Management of the Canterbury Bight is controlled and regulated by Environment Canterbury (Ecan). Ecan believes that in many instances, the increased risk of erosion and seawater inundation are caused by the inappropriate location of assets and activities and by a reliance on inadequate works to protect from the ocean (Ecan, 2005). To investigate coastal hazards, Ecan; establish and maintain co-operation with weather and tsunami forecasting agencies in the issuing of warnings about potentially damaging natural events, assess the effect of hazards on the coast and regularly collect data on sea/shoreline conditions to determine any changes in occurrence of hazards and the physical nature of the coast as well as determine areas requiring hazard mitigation (Ecan, 2005).
Current Coastal Management Initiatives
Erosion, and subsequent sea water inundation poses a serious threat along the length of the Canterbury Bight. To date, erosion has led to the loss of agricultural land, threatened valuable infrastructure and some holiday settlements, and reduced coastal lagoons and wetlands (Ecan, 2005). One of the main areas of concern is the Washdyke barrier. The shoreline at Washdyke was naturally eroding before construction of the Timaru harbour commenced in 1879. The harbour has prevented sediment being transported from the south meaning that no coarse sediment is able to nourish the Washdyke beach/barrier. The material that is currently on the beach is undergoing abrasion (discussed above), which has reduced grain sizes and lowered the berm heights increasing the amount of washover, which further increases erosion (Kirk, 1992).This process has created a significant hazard, as the Washdyke barrier is the only line of protection between the high-energy ocean and valuable infrastructure including State Highway 1, an important railway and a large industrial area. Additionally, the barrier protects the Washdyke Lagoon, which is a valued wildlife area (Kirk, 1992).
In 1980, to manage the erosion hazard of the Washdyke barrier beach crest heights were raised 2.0–2.5m to minimise washover, washover sediment was used to fill the body of the beach and river gravels were used to cap the beach crest (Kirk, 1992). This programme was monitored over five years and showed that erosion was decreased by 55%, with no retreat or washover. Untreated adjacent beaches experienced significant retreat over the five-year period showing the programme was very successful (Kirk, 1992).
Recommendations for Coastal Management
There is a clear need for further mitigation to coastal erosion hazards along the bight through coastal managementCoastal management
In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...
. The Washdyke barrier re-nourishment has proved a successful venture for that area although it has only lessened the threat, rather than completely removed it. The success of the re-nourishment programme means it should be used again for this area. Along the Central Zone of the Canterbury Bight, different mitigation methods are needed in order to decrease erosion risks. A huge dilemma is created, as sediment is needed from this area to nourish the Northern Zone, which without it would itself begin to erode. Given this, only three options are left, either do nothing, retreat from the coast or constantly re-nourish the area with large sediment. Doing nothing is an option for some areas where there is no economic or cultural significance and erosion poses no risk to anything valuable. Objects that can be moved landward, without incurring significant losses should be moved in a managed retreat. Lastly, re-nourishment could be used sparingly in areas where objects cannot be moved or are have some form of value. Re-nourishment would be the ideal method used for the entire coastline but this is not plausible due to the size of the area and the cost of re-nourishment.
See also
- Coastal ManagementCoastal managementIn some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...
- Coastal ErosionCoastal erosionCoastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...
- Longshore transport
- Lake Ellesmere / Te Waihora
- Washdyke LagoonWashdyke LagoonWashdyke Lagoon is a brackish shallow coastal lagoon approximately 1 kilometre north of Timaru, South Canterbury, New Zealand. The lagoon has drastically reduced in size since 1881 when it was approximately 253 hectares, now it is less than 48 hectares in area...