Washdyke Lagoon is a brackish shallow coastal
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...
approximately north of
Timaru
Timaru (; mi, Te Tihi-o-Maru) is a port city in the southern Canterbury Region of New Zealand, located southwest of Christchurch and about northeast of Dunedin on the eastern Pacific coast of the South Island. The Timaru urban area is home to ...
, South
Canterbury
Canterbury (, ) is a City status in the United Kingdom, cathedral city and UNESCO World Heritage Site, situated in the heart of the City of Canterbury local government district of Kent, England. It lies on the River Stour, Kent, River Stour.
...
,
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
. The lagoon has drastically reduced in size since 1881 when it was approximately , now it is less than in area. It is enclosed by a barrier beach that is long and 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 are classified as a
wildlife refuge
A nature reserve (also known as a wildlife refuge, wildlife sanctuary, biosphere reserve or bioreserve, natural or nature preserve, or nature conservation area) is a protected area of importance for flora, fauna, or features of geological or ...
and it demonstrates the role human structures have on coastline evolution.
[Kirk, R. M. & Lauder, G. A. (2000). Significant coastal lagoon systems in the South Island, New Zealand. ''Science for conservation 146.'']
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.
[Kjerfve, B. (Ed.) (1994). Coastal lagoon processes. ''Elsevier Oceanography Series 60.'' Elsevier, Amsterdam.] Lagoons form when
longshore transport
Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction ...
builds up
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
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.
[Masselink, G. & Hughes, M. G. (2003). Introduction to coastal processes and geomorphology. Hodder Education, London.]
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.
[Barnes, R. S. K. (1980). Coastal lagoons: the natural history of a neglected habitat. Cambridge University Press, London.] Examples include the
Coorong, Australia, , Laguna de Araruama (Brazil, ),
Lake St. Lucia (South Africa, ) and
Lake Songkhla (Thailand, ). 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.
[Kjerfve, B. (1986). Comparative oceanography of coastal lagoons. ''Estuarine Variability.'']
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.
[Carter, R. W. G., Orford, J. D., Forbes, D. L. & Taylor, R. B. (1987). Gravel barriers, headlands and lagoons: An evolutionary model. ''Coastal Sediments 87:'' 1776 - 1792.] They also form steep slopes in high wave energy environments
[Kirk, R. M. (1980). Mixed sand and gravel beaches: Morphology, processes and sedimentation. ''Progress in Physical Geography 4:'' 198 - 210.] and have very high levels of seepage of freshwater into the ocean, due to the large grain size range increasing permeability.
[Carter, R. W. G. & Orford, J. D. (1984). Coarse clastic beaches: A discussion of the distinctive dynamic and morphosedimentry characteristics. ''Marine Geology 60:'' 377 - 389.] 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.
[McKay, P. J. & Terich, T. A. (1992). Gravel barrier morphology: Olympic National Park, Washington State, U. S. A. ''Journal of Coastal Research 8(4):'' 813 - 829.] 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
The Canterbury Bight is a large bight on the eastern side of New Zealand's South Island. The bight runs for approximately from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm w ...
. 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.
[Eikaas, H. S. & Hemmingsen, M. A. (2006). A GIS approach to model sediment reduction susceptibility of mixed sand and gravel beaches. ''Environmental Management 37:'' 816 - 825.] 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 / Te Waihora is a broad, shallow coastal lake or waituna, in the Canterbury region of the South Island of New Zealand. It is directly to the west of Banks Peninsula, separated from the Pacific Ocean by the long, narrow, sandy K ...
(Canterbury) and
Waituna Lagoon
__NOTOC__
The Waituna Lagoon is on the southern coastline of the South Island of New Zealand. It forms part of the Awarua Wetland, a Ramsar site that was established in 1976. It gives it name to waituna, a type of ephemeral coastal lake.
The la ...
(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.
[Soons, J. M., Schulmeister, J. & Holt, S. (1997). The Holocene evolution of a well nourished gravelly barrier and lagoon complex, Kaitorete "spit", Canterbury, New Zealand. ''Marine Geology 138:'' 69 - 90.]
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.
[Thompson, R. M. & Ryder, G. R. (2003). Waituna Lagoon: summary of existing knowledge and identification of knowledge gaps. ''Science for conservation 215.'']
Management Issues
Currently the lagoon is shrinking at an alarming rate. The barrier has retreated between 1865 and 1987 ( per year).
[Single, M. (2001). ‘Human Impacts on the Physical Environment.’ In Sturman, A. & Spronken – Smith, R. (Ed.). The physical environment: A New Zealand perspective. Oxford University Press, Victoria.] 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 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 of sediment per year, whilst the coast to the south of the port is accumulating sediment forming 80 hectare of new land.
[Tierney, B. W. (1977). Coastal changes around the Port of Timaru, New Zealand. ''New Zealand Geographer 33:'' 80 - 83.] 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 by bypassing the breakwater but this is a much reduced load.
Without sediment supply
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
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.
[Woodroffe, C. D. (2002). ''Coasts: Form, process and evolution.'' Cambridge University Press, Cambridge.] 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
In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...
, without this sediment the fines are much more vulnerable to being removed leaving the barrier structure more vulnerable.
[CERC (1984). The shore protection manual (4th Ed.). Coastal Engineering Research Centre, US Army Corps of Engineers, Washington.] Maximum wave height for this section of coast during one October was found to be ,
[Hastie, W. J. (1983). Wave height and period at Timaru, New Zealand. ''Journal of marine and freshwater research 19:'' 507 - 515.] and other studies have recorded frequent breakers around 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 Waitaki River is a large braided river that drains the Mackenzie Basin and runs some south-east to enter the Pacific Ocean between Timaru and Oamaru on the east coast of the South Island of New Zealand. It starts at the confluence of the Å ...
. The dam was constructed by 1934 as part of a hydroelectric scheme and is the only braided river in New Zealand that is dammed.
[Ligon, F. K., Dietrich, W. E. & Trush, W. J. (1995). Downstream ecological effects of dams. ''BioScience 45:'' 183 - 192.] It is estimated that the dam stops 50% of the bed load transport.
[Murray Hicks, D., Shankar, U., Duncan, M. J., Rebuffe, M. & Aberle, J. (2006). Use of remote-sensing with two-dimensional hydrodynamic models to assess impacts of hydro-operations on a large, braided, gravel-bed river: Waitaki River, New Zealand. In Sambrook Smith, G. H., Best, J. L., Bristow, C. S. & Petts, G. E. (2006). ''Braided rivers: Process, deposits, ecology and management.'' Association of Sedimentologists special no. 36.]
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.
[Orford, J. D., Carter, R. W. G. & Jennings, S. C. (1996). Control domains and morphological phases in gravel-dominated coastal barriers of Nova Scotia. ''Journal of Coastal Research 12(3):'' 589 - 604.]
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 affect tourism in the coming years.
[Siripong, A. (2008). The beaches are disappearing. ''Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes 2008.'']
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.
[Timaru Herald, 7 December 1897. http://paperspast.natlib.govt.nz/cgi-bin/paperspast?a=d&d=THD18971207.2.7&l=mi&e=-------10--1----0-- Accessed 27/4/2011.] 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 nitrification 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 of fine grained sediments are removed.
[Inglis, G., Gust, N., Fitridge, I., Floerl, O., Woods, C., Hayden, B. & Fenwick, G. (2003). Port of Timaru: Baseline survey for non-indigenous marine species. ''Niwa biosecurity New Zealand technical paper no. 2005/06.''] 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.
[Hastie, W.J. (1983). Sediment transport in the nearshore environment, Timaru, New Zealand. ''University of Canterbury Ph.D. thesis''] 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 section of the barrier was raised 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 ( 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
godwit
The godwits are a group of large, long-billed, long-legged and strongly migratory waders of the bird genus ''Limosa''. Their long bills allow them to probe deeply in the sand for aquatic worms and molluscs. In their winter range, they floc ...
s and
sandpiper
Sandpipers are a large family, Scolopacidae, of waders. They include many species called sandpipers, as well as those called by names such as curlew and snipe. The majority of these species eat small invertebrates picked out of the mud or soil. ...
s,
heron
The herons are long-legged, long-necked, freshwater and coastal birds in the family Ardeidae, with 72 recognised species, some of which are referred to as egrets or bitterns rather than herons. Members of the genera ''Botaurus'' and ''Ixobrychus ...
s and other species of water birds (Figure 5).
Gull
Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century, m ...
s and
dotterel
The Eurasian dotterel (''Charadrius morinellus''), also known in Europe as just dotterel, is a small wader in the plover family of birds.
The dotterel is a brown and black streaked bird with a broad white eye-stripe and an orange-red chest ban ...
s nest on the shingle barrier of the eastern side. However, in recent years black-fronted dotterel numbers have been declining.
[Southey, I. (2009). Numbers of waders in New Zealand 1994-2003. ''Department of Conservation research and development series 308''] 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
The oystercatchers are a group of waders forming 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. The e ...
s and
turnstone
Turnstones are two bird species that comprise the genus ''Arenaria'' in the family Scolopacidae. They are closely related to calidrid sandpipers and might be considered members of the tribe Calidriini.
The genus ''Arenaria'' was introduced by t ...
s. The area is also popular for catching
whitebait
Whitebait is a collective term for the immature fry of fish, typically between long. Such young fish often travel together in schools along coasts, and move into estuaries and sometimes up rivers where they can be easily caught using fine-m ...
off the barrier.
[Washdyke Wildlife Refuge, Dept. of Conservation status report. http://www.doc.govt.nz/upload/documents/about-doc/role/policies-and-plans/j39005_washdukelagoonwildref.pdf. Accessed 1/4/2011.]
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
*
Lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...
*
Longshore Drift
Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle incoming wave direction ...
*
Sediment transport
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural system ...
*
Coastal erosion
Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward ...
*
Kaitorete Spit
*
Lake Ellesmere / Te Waihora
Lake Ellesmere / Te Waihora is a broad, shallow coastal lake or waituna, in the Canterbury region of the South Island of New Zealand. It is directly to the west of Banks Peninsula, separated from the Pacific Ocean by the long, narrow, sandy ...
*
Canterbury Bight
The Canterbury Bight is a large bight on the eastern side of New Zealand's South Island. The bight runs for approximately from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm w ...
*
Waituna Wetlands Scientific Reserve
References
{{Reflist, 2
External links
Washdyke Lagoon Wildlife RefugeMap of Washdyke LagoonWashdyke Lagoon restoration project
Lagoons of New Zealand
Landforms of Canterbury, New Zealand
Wetlands of Canterbury, New Zealand