Washdyke Lagoon (known in Māori as Waitarakao) is a brackish shallow coastal lagoon approximately north of Timaru, South Canterbury, New Zealand. 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 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 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.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 environmentsKirk, 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. 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 has formed a chain of lagoons along the east and south coast of the island, including Waituna, Wainono and Washdyke Lagoons, Coopers Lagoon / Muriwai, and Lake Ellesmere / Te Waihora. Like Washdyke, these closed lagoons are also facing the possibility of becoming much more open systems in the near future.
Currently Kaitorete Spit (enclosing Lake Ellesmere / Te Waihora) 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 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, 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 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.