Washdyke Lagoon
Encyclopedia
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 (0.48 square kilometres). It is enclosed by a barrier beach that is 3 kilometres long and 3 metres above high tide at its largest point (see Figure 1). The reduced lagoon size is due to the construction of the Timaru Port breakwater which is preventing coarse sediments from reaching and replenishing Washdyke Barrier. This is important as the lagoon and the surrounding 250 hectares are classified as a wildlife refuge
and it demonstrates the role human structures have on coastline evolution.
builds up sediment
across the mouth of a water source cutting it off or diverting it from directly reaching the ocean. In order for a lagoon to form the barrier needs both a coarse and fine sediment source, the fines usually from longshore transport offshore and the coarse usually from transport of sediment from river mouths. The barriers created rely on the accretion of sediment to at least equal the rate of erosion in order for the lagoon to remain separate from the ocean. This balance is also effected by sea level changes, rising causing the barrier to be overtopped and falling allowing a smaller barrier keep the lagoon separate.
There are many examples of these relatively closed lagoon systems around the world. Most have significant ecological value, so preserving the balance between fresh and salt water is important. Examples include the Coorong
(Australia, 260 km2, Laguna de Araruama (Brazil, 10,360 km2), Lake St. Lucia (South Africa, 312 km2) and Lake Songkhla (Thailand, 1,040 km2). If the sediment sources for these barriers was to be removed their barriers would be eroded away and bays would form in the lagoons place with inundation.
The lagoon is situated at the southern end of the Canterbury Bight
. This coastline is characterised by its mixed sand and gravel beaches and southerly longshore currents. These currents transport coarse greywacke sediments that have reached the coast down braided rivers that originated at the Southern Alps and fine sediments from further offshore. Southerly currents push them north and they are then deposited on Washdyke Barrier causing it to stay above the high tide mark and form the lagoon.
The southerly current travelling up from South of the South Island of New Zealand forms many lagoons along the east and south coast. These form a chain of lagoons including Lake Ellesmere
(Canterbury) and Waituna
(Southland). Like Washdyke, these closed lagoons are also facing the possibility of becoming much more open systems in the near future.
Currently Kaitorete Barrier
(enclosing Lake Ellesmere) is undergoing erosion, especially at its southern end, due to coastal rotation of currents. It is also possible that it will permanently breach forming an open bay in the coming decades.
Waituna Lagoon is also under threat from breaching but in this case it is machinery not erosion that is opening the coastal lake to the ocean. This is so that it can be used for hunting and allow grazing at its banks. However, it is now been opened more frequently increasing the natural salinity of the lagoon. The effects of this on native fish and bird species are still not fully understood.
The drastic reduction in area of the lagoon can be at least partially attributed to the construction of the Timaru Port, which started in 1878. In building the port's 700 metre long breakwater (Figure 3) the sediment supply from the south is blocked from reaching the Washdyke Lagoon area. Since the construction the Washdyke-Opihi coast has lost 2.62 million cubic metres of sediment per year, whilst the coast to the south of the port is accumulating sediment forming 80 hectare of new land. This is a large loss to the north of the port that can not be replaced by current longshore transport mechanisms.
The port's breakwater starves the barrier of coarse sediments from the rivers to the south. As shown in Figure 4, prior to breakwater construction fine and coarse sediments were transported from the south and deposited on Washdyke Barrier. However, after the breakwater was built the coarse sediment is unable to reach the barrier as it accumulates at the port's southern side. Some sediment is supplied further offshore and through bypassing around the breakwater but this is a much reduced load.
Without sediment supply erosion
of the barrier is rapid as normal processes such as washover continue. This happens when waves hit the barrier and push sediment from the ocean side to the lagoon side, there is no incoming sediment on the ocean side to rebuild the barrier. This means that the barrier migrate landward as the barrier ridges are reworked. The ratio of fine to coarse sediments making up the barrier is also increased. The coarse sediments armour the barrier structure against ocean swell
and wind waves, without this sediment the fines are much more vulnerable to being removed leaving the barrier structure more vulnerable. Maximum wave height for this section of coast during one October was found to be 6.3 metres, and other studies have recorded frequent breakers around 5 metres high. This means that the barrier would frequently have to deal with overtopping events.
Another factor that is preventing the supply of coarse sediment to the lagoon barrier is the damming of the Waitaki River
. The dam was constructed by 1934 as part of a hydroelectric scheme and is the only braided river in New Zealand that is dammed. It is estimated that the dam stops 50% of the bed load transport.
The amount of sediment that is supplied to the barrier is the most important aspect in determining barrier alignment and breakdown. This was shown studying the coastal barriers on the Atlantic coast of Nova Scotia. This is particularly important as these barriers are one of the few other MSG beaches internationally.
Another, more local site very relevant to the issues at Washdyke Lagoon is Waimataitai Lagoon. This lagoon was situated just north of the Port of Timaru (South of Washdyke Lagoon and Dashing Rocks) prior to 1933. The barrier enclosing the lagoon began to erode rapidly after the construction of the port resulting in the complete destruction of the lagoon, leaving behind an open bay in its place.
Internationally, sediment starvation is causing the erosion of many important coastal features. Breakwaters and other man-made structures in Thailand are responsible for accelerated beach erosion which may start to effect tourism in the coming years.
In Hallsands, Devon, England, a combination of an existing seawall and new gravel dredging to form a harbour from 1986 to 1902, caused the erosion of the whole village by 1917. This was due to sediment starvation causing the erosion of the mixed sand and gravel barrier beaches which protected the village. They are not alone, the effects of man-made structures on restricting or diverting natural sediment transport are now well known and to some extent there is now a move away from these.
There is also an issue surrounding the pollution of the lagoon. Nearby freezing works have previously pumped waste water into the lagoon. Workers in charge of checking this discharge pipe also noted thinning of the barrier as early as 1897. More recently there has been concern about the pollution runoff from the Washdyke industrial area immediately to the west of the lagoon.
Another source of pollutants was the sewage outfall pipe which ended on the ocean side of the middle of the barrier and was in use until 1998. This pipe was constantly at risk to exposure and damage as the barrier eroded and would have contributed pollutants and caused eutrification in the lagoon.
From 1979 to 1985 the Timaru City Council commissioned a length of the Washdyke Barrier to be renourished in order to protect a sewage outfall pipe and ascertain whether or not the system would viably protect the barrier structure. A 300 metre section of the barrier was raised 2-2.5 metres using sediments which have rolled over the barrier and are now on the lagoon side, then armouring them with coarse sediments brought in from Opihi River (12 kilometres away). Despite the overall findings of the report being that the project was 'technically and economically feasible' no further renourishment work of the barrier has been undertaken. However, the effects of the renourishment on this section of the barrier is still evident (as of 2006).
Further work is needed in order to show the best way to maintain the bar economically and environmentally. This work needs to be site specific due to the unique morphodynamics and challenges that the lagoon is facing. Mixed sand and gravel barriers are an area that is being continuously studied and some long term behaviours are still not understood. A challenge to future work is that the Timaru Port is a major economic driver in the region and sections of the community may feel that Washdyke Lagoon is a necessary sacrifice.
s and sandpipers, heron
s and other species of water birds (Figure 5). Gull
s and dotterels nest on the shingle barrier of the eastern side. However in recent years black-fronted dotterel numbers have been declining. Saltmarsh ribbonwood, rushes and exotic plants can be found on the margins of the lagoon. A reef borders part of the coast just off the barrier, attracting oystercatcher
s and turnstone
s. The area is also popular for catching whitebait
off the barrier.
The ecology of lagoon systems is lost if the sediment sources for these barriers is removed and their barriers erode away, bays then form in the lagoons place with inundation. This is what will happen at Washdyke Lagoon and other lagoons around the world if human activities continue to starve natural sediment supplies. This will have ecological implications and will also allow for flooding of land previously protected by the lagoon, such as the Washdyke industrial area.
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
approximately 1 kilometre north of Timaru
Timaru
TimaruUrban AreaPopulation:27,200Extent:Former Timaru City CouncilTerritorial AuthorityName:Timaru District CouncilPopulation:42,867 Land area:2,736.54 km² Mayor:Janie AnnearWebsite:...
, South Canterbury
Canterbury
Canterbury is a historic English cathedral city, which lies at the heart of the City of Canterbury, a district of Kent in South East England. It lies on the River Stour....
, 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 lagoon has drastically reduced in size since 1881 when it was approximately 253 hectares, now it is less than 48 hectares in area (0.48 square kilometres). It is enclosed by a barrier beach that is 3 kilometres long and 3 metres above high tide at its largest point (see Figure 1). The reduced lagoon size is due to the construction of the Timaru Port breakwater which is preventing coarse sediments from reaching and replenishing Washdyke Barrier. This is important as the lagoon and the surrounding 250 hectares are classified as a wildlife refuge
Wildlife refuge
A wildlife refuge, also called a wildlife sanctuary, may be a naturally occurring sanctuary, such as an island, that provides protection for species from hunting, predation or competition, or it may refer to a protected area, a geographic territory within which wildlife is protected...
and it demonstrates the role human structures have on coastline evolution.
Classification
Washdyke Lagoon is a coastal lake, ‘choked’ type lagoon as it is separated from the sea by a built-up bar, and has only minimal tidal input (<5% of total tidal impact to coast within the lagoon). Lagoons of this type usually form along high energy, microtidal coastlines, have high ratio of fresh to salt water and rarely open to the ocean. This type of lagoon differs from more ‘open’ lagoons which are open to the ocean through one or more channels, have a lower percentage of fresh water and form directly at the end of large river mouths. Lagoons form when longshore transportLongshore 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...
builds up sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
across the mouth of a water source cutting it off or diverting it from directly reaching the ocean. In order for a lagoon to form the barrier needs both a coarse and fine sediment source, the fines usually from longshore transport offshore and the coarse usually from transport of sediment from river mouths. The barriers created rely on the accretion of sediment to at least equal the rate of erosion in order for the lagoon to remain separate from the ocean. This balance is also effected by sea level changes, rising causing the barrier to be overtopped and falling allowing a smaller barrier keep the lagoon separate.
There are many examples of these relatively closed lagoon systems around the world. Most have significant ecological value, so preserving the balance between fresh and salt water is important. Examples include the Coorong
Coorong National Park
The Coorong is a national park and lagoon ecosystem in South Australia , 156 km southeast of Adelaide. Its name is thought to be a corruption of the local Aboriginal people's word kurangh, meaning "long neck"; a reference to the shape of the lagoon system...
(Australia, 260 km2, Laguna de Araruama (Brazil, 10,360 km2), Lake St. Lucia (South Africa, 312 km2) and Lake Songkhla (Thailand, 1,040 km2). If the sediment sources for these barriers was to be removed their barriers would be eroded away and bays would form in the lagoons place with inundation.
Formation and Setting
Washdyke Lagoon was created by the formation of a mixed sand and gravel (MSG) beach barrier (Figure 2) allowing fluvial water from Washdyke Creek to accumulate behind it, with some tidal infiltration through the barrier. The MSG barrier type is relatively rare and has its own unique set of processes and morphology. MSG barriers form mostly on paraglacial coasts where there is a large amount of coarse grained sediments available and storm wave conditions during which more sediment load is transported ashore. They also form steep slopes in high wave energy environments and have very high levels of seepage of freshwater into the ocean, due to the large grain size range increasing permeability. Another characteristic is the zonation of sediments on the barrier. Coarse, flat sediments form the crest, whilst large, spherical and rod shape sediments form the outer part and the barrier infill is made up of finer sediment sizes. Washdyke Lagoon is a useful example of a barrier that displays all of these characteristics.The lagoon is situated at the southern end of the Canterbury Bight
Canterbury Bight
Canterbury Bight is a 135 km stretch of coastline between Dashing Rocks and the southern side of Banks Peninsula 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...
. This coastline is characterised by its mixed sand and gravel beaches and southerly longshore currents. These currents transport coarse greywacke sediments that have reached the coast down braided rivers that originated at the Southern Alps and fine sediments from further offshore. Southerly currents push them north and they are then deposited on Washdyke Barrier causing it to stay above the high tide mark and form the lagoon.
The southerly current travelling up from South of the South Island of New Zealand forms many lagoons along the east and south coast. These form a chain of lagoons including 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...
(Canterbury) and Waituna
Waituna Wetlands Scientific Reserve
The Waituna Wetlands Scientific Reserve is a peatland area of in the Southland Region of New Zealand. It was designated as having international significance under the Ramsar Convention in 1976...
(Southland). Like Washdyke, these closed lagoons are also facing the possibility of becoming much more open systems in the near future.
Currently Kaitorete Barrier
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...
(enclosing Lake Ellesmere) is undergoing erosion, especially at its southern end, due to coastal rotation of currents. It is also possible that it will permanently breach forming an open bay in the coming decades.
Waituna Lagoon is also under threat from breaching but in this case it is machinery not erosion that is opening the coastal lake to the ocean. This is so that it can be used for hunting and allow grazing at its banks. However, it is now been opened more frequently increasing the natural salinity of the lagoon. The effects of this on native fish and bird species are still not fully understood.
Management Issues
Currently the lagoon is shrinking at an alarming rate. The barrier has retreated 400 metres between 1865 and 1987 (3.2 metres per year). In 1992 it was predicted that by the year 2000 the lagoon would have totally disappeared. Whilst this has not yet happened it is still a possibility in the near future. This would cause flooding of the Washdyke industrial area and the loss of habitat for the wildlife within the refuge.The drastic reduction in area of the lagoon can be at least partially attributed to the construction of the Timaru Port, which started in 1878. In building the port's 700 metre long breakwater (Figure 3) the sediment supply from the south is blocked from reaching the Washdyke Lagoon area. Since the construction the Washdyke-Opihi coast has lost 2.62 million cubic metres of sediment per year, whilst the coast to the south of the port is accumulating sediment forming 80 hectare of new land. This is a large loss to the north of the port that can not be replaced by current longshore transport mechanisms.
The port's breakwater starves the barrier of coarse sediments from the rivers to the south. As shown in Figure 4, prior to breakwater construction fine and coarse sediments were transported from the south and deposited on Washdyke Barrier. However, after the breakwater was built the coarse sediment is unable to reach the barrier as it accumulates at the port's southern side. Some sediment is supplied further offshore and through bypassing around the breakwater but this is a much reduced load.
Without sediment supply erosion
Coastal erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...
of the barrier is rapid as normal processes such as washover continue. This happens when waves hit the barrier and push sediment from the ocean side to the lagoon side, there is no incoming sediment on the ocean side to rebuild the barrier. This means that the barrier migrate landward as the barrier ridges are reworked. The ratio of fine to coarse sediments making up the barrier is also increased. The coarse sediments armour the barrier structure against ocean swell
Swell (ocean)
A swell, in the context of an ocean, sea or lake, is a series surface gravity waves that is not generated by the local wind. Swell waves often have a long wavelength but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with...
and wind waves, without this sediment the fines are much more vulnerable to being removed leaving the barrier structure more vulnerable. Maximum wave height for this section of coast during one October was found to be 6.3 metres, and other studies have recorded frequent breakers around 5 metres high. This means that the barrier would frequently have to deal with overtopping events.
Another factor that is preventing the supply of coarse sediment to the lagoon barrier is the damming of the Waitaki River
Waitaki River
The Waitaki River is a large river in the South Island of New Zealand, some 110 km long. It is the major river of the Mackenzie Basin.It is a braided river which flows through Lake Benmore, Lake Aviemore and Lake Waitaki. These are ultimately fed by three large glacial lakes, Pukaki, Tekapo,...
. The dam was constructed by 1934 as part of a hydroelectric scheme and is the only braided river in New Zealand that is dammed. It is estimated that the dam stops 50% of the bed load transport.
The amount of sediment that is supplied to the barrier is the most important aspect in determining barrier alignment and breakdown. This was shown studying the coastal barriers on the Atlantic coast of Nova Scotia. This is particularly important as these barriers are one of the few other MSG beaches internationally.
Another, more local site very relevant to the issues at Washdyke Lagoon is Waimataitai Lagoon. This lagoon was situated just north of the Port of Timaru (South of Washdyke Lagoon and Dashing Rocks) prior to 1933. The barrier enclosing the lagoon began to erode rapidly after the construction of the port resulting in the complete destruction of the lagoon, leaving behind an open bay in its place.
Internationally, sediment starvation is causing the erosion of many important coastal features. Breakwaters and other man-made structures in Thailand are responsible for accelerated beach erosion which may start to effect tourism in the coming years.
In Hallsands, Devon, England, a combination of an existing seawall and new gravel dredging to form a harbour from 1986 to 1902, caused the erosion of the whole village by 1917. This was due to sediment starvation causing the erosion of the mixed sand and gravel barrier beaches which protected the village. They are not alone, the effects of man-made structures on restricting or diverting natural sediment transport are now well known and to some extent there is now a move away from these.
There is also an issue surrounding the pollution of the lagoon. Nearby freezing works have previously pumped waste water into the lagoon. Workers in charge of checking this discharge pipe also noted thinning of the barrier as early as 1897. More recently there has been concern about the pollution runoff from the Washdyke industrial area immediately to the west of the lagoon.
Another source of pollutants was the sewage outfall pipe which ended on the ocean side of the middle of the barrier and was in use until 1998. This pipe was constantly at risk to exposure and damage as the barrier eroded and would have contributed pollutants and caused eutrification in the lagoon.
Remediation and Future Management
Every ten months the entrance to the Timaru Port is dredged in order to keep the channel deep enough for use as a commercial port. Each time around 100,000m3 of fine grained sediments are removed. It was proven that 20% of these tailings are coarse enough to benefit Washdyke Barrier, so tailings have been dumped off the Washdyke coast since the early 1990s. However 80% of this material is unsuitable due to the fine, weathered nature of the sediments, making them less resistant to wave activity than the sediment source prior to port construction.From 1979 to 1985 the Timaru City Council commissioned a length of the Washdyke Barrier to be renourished in order to protect a sewage outfall pipe and ascertain whether or not the system would viably protect the barrier structure. A 300 metre section of the barrier was raised 2-2.5 metres using sediments which have rolled over the barrier and are now on the lagoon side, then armouring them with coarse sediments brought in from Opihi River (12 kilometres away). Despite the overall findings of the report being that the project was 'technically and economically feasible' no further renourishment work of the barrier has been undertaken. However, the effects of the renourishment on this section of the barrier is still evident (as of 2006).
Further work is needed in order to show the best way to maintain the bar economically and environmentally. This work needs to be site specific due to the unique morphodynamics and challenges that the lagoon is facing. Mixed sand and gravel barriers are an area that is being continuously studied and some long term behaviours are still not understood. A challenge to future work is that the Timaru Port is a major economic driver in the region and sections of the community may feel that Washdyke Lagoon is a necessary sacrifice.
Ecological Value
Washdyke Lagoon is a feeding ground for a variety of birds including waterfowl, handfuls of migratory godwitGodwit
The godwits are a group of large, long-billed, long-legged and strongly migratory wading birds of the genus Limosa. They form large flocks on coasts and estuaries in winter....
s and sandpipers, heron
Heron
The herons are long-legged freshwater and coastal birds in the family Ardeidae. There are 64 recognised species in this family. Some are called "egrets" or "bitterns" instead of "heron"....
s and other species of water birds (Figure 5). Gull
Gull
Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...
s and dotterels nest on the shingle barrier of the eastern side. However in recent years black-fronted dotterel numbers have been declining. Saltmarsh ribbonwood, rushes and exotic plants can be found on the margins of the lagoon. A reef borders part of the coast just off the barrier, attracting oystercatcher
Oystercatcher
The oystercatchers are a group of waders; they form the family Haematopodidae, which has a single genus, Haematopus. They are found on coasts worldwide apart from the polar regions and some tropical regions of Africa and South East Asia...
s and turnstone
Turnstone
Turnstones are the bird species in the genus Arenaria in the family Scolopacidae. They are closely related to calidrid sandpipers and might be considered members of the tribe Calidriini....
s. The area is also popular for catching whitebait
Whitebait
Whitebait is a collective term for the immature fry of fish, typically between 25 and 50 millimetres long. Such young fish often travel together in schools along the coast, and move into estuaries and sometimes up rivers where they can be easily caught with fine meshed fishing nets...
off the barrier.
The ecology of lagoon systems is lost if the sediment sources for these barriers is removed and their barriers erode away, bays then form in the lagoons place with inundation. This is what will happen at Washdyke Lagoon and other lagoons around the world if human activities continue to starve natural sediment supplies. This will have ecological implications and will also allow for flooding of land previously protected by the lagoon, such as the Washdyke industrial area.
See also
- LagoonLagoonA lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
- Longshore DriftLongshore driftLongshore 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...
- Sediment transportSediment transportSediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
- 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...
- Kaitorete SpitKaitorete SpitKaitorete 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...
- Lake Ellesmere / Te Waihora
- Canterbury BightCanterbury BightCanterbury Bight is a 135 km stretch of coastline between Dashing Rocks and the southern side of Banks Peninsula 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...
- Waituna Wetlands Scientific ReserveWaituna Wetlands Scientific ReserveThe Waituna Wetlands Scientific Reserve is a peatland area of in the Southland Region of New Zealand. It was designated as having international significance under the Ramsar Convention in 1976...