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 (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.
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 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.
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.
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