On a good day (which occurs more often than the locals will let on) Wellington Harbour is more like a lake than a harbour, being up to 28 metres deep and occupying nearly 50 square kilometres. It is a major feature of Wellington city, and some pretty awesome forces were involved in its creation.
New Zealand straddles a segment of the collision boundary between two of Earth’s 15 major crustal plates – the Australian Plate (which is moving northwards) and the Pacific Plate (going west). The entire North Island sits on the crust at the very eastern edge of the Australian Plate. The collision between the two plates is responsible for almost all topographic features of the New Zealand landscape.
In the Wellington region, the plate collision is compressing Earth’s crust in an east-west direction, almost like squeezing a corrugated sheet – the ridges go up and the valleys go down. But at the same time, the crust is being sheared sideways so that when the faults move they do so both vertically and sideways at the same time.
There are several major faults in the Wellington region, all of which are considered active – in other words they have all moved in the last 100,000 years. The habour is formed between a master fault, the Wellington Fault, and a set of smaller faults at an angle to it. These faults are all sideways faults with significant vertical movement and they all cut right through the crust, which is about 25 km thick beneath Wellington.
While the harbour originally was natural, a significant amount of reclamation has taken place since the 1850's, which has added around 155 hectares to the city.
A number of water sources end in the harbour, which can vary in colour from aqua blue on a brilliant summer day, turquoise on a cloudy day, and brown when the Hutt River has been in flood.
Physically, the colour of water is affected by factors like the light source, absorption and scattering of light, as well as suspended materials in the water. The major light source for Wellington Harbour is the Sun. Sunlight comprises different colours of light, from red light with the longest wavelength to violet light with the shortest. The colour of an object mainly depends on the colour of light emitted from it. Absorption by water is stronger for red light, but weaker for blue light. But this effect is only apparent when the water is reasonably deep (exceeding 1 metre). This is why, a cup of pure water appears colourless under white light whereas a big body of water, like a swimming pool, looks bluish through the thickness of water. Under different weather conditions, and different light, the colour of the sea changes. The sea appears blue under a clear sky because the blue light from the sky is reflected by the water surface. As the amount of light reduces, the amount of blue light from the sky reflected also reduces, affecting the colour of the sea.
Two main sources of Wellington's water end in the harbour, the Waiwhetu Aquifer, and the Hutt River.
The Waiwhetu artesian aquifer is a zone of water-holding sand, gravel and boulders beneath the Hutt Valley. Water from the Hutt River starts to flow underground around Taita Gorge. From Melling southwards, the water becomes naturally pressurised beneath a layer of hard clay. This pressurised zone, the Waiwhetu artesian aquifer, stretches as far south as the harbour. It is estimated to be up to 70 metres thick at its western edge against the Wellington fault line, and 20 metres thick at the eastern edge of the harbour. The pressure in the aquifer has resulted in several fresh water springs in the harbour floor.
Water takes more than 12 months to pass through the aquifer to our wells and is naturally filtered while underground, making it free from disease causing micro-organisms. The Waiwhetu aquifer provides around 40 percent of Wellington's annual water supply.
The Hutt Water Collection Area covers almost 9,000 hectares of bush-clad mountains and valleys at the southern edge of the Rimutaka Ranges. The collection area is about equal in size to a square with the length of each side the same as the distance between Wellington's railway station and Petone. Rainwater collects in tributary streams that flow into the Hutt River. There is a weir (low dam) at Kaitoke, just north of Upper Hutt, where water is taken from the river, strained to remove sticks, leaves and silt, and piped through tunnels to the Te Marua Water Treatment Plant. Up to 150 million litres of water is taken per day from the Hutt River, provided an adequate flow is maintained downstream of the weir. The Hutt Water Collection Area provides about 40 percent of Wellington's annual water supply.
At ground zero, you will find some Wellington icons that (tenuously) links to the name and theme of this earth cache. Not actually alien line forms, or a real threat to Wellingtonians (unless they topple over in an earthquake...) the historic cranes on Queen's Wharf are surviving examples of cranes that used to line the Wellington waterfront.
You also have a long-distance view of both the Hutt River and Waiwhetu Aquifer, to the north.
To log the cache, complete the following:
1. visit ground zero and describe the colour of the water in the harbour today - what do you think has influenced the colour of the water today?
2. estimate Wellington's total daily water usage from the information above, assuming an adequate water flow is maintained downstream from the Kaitoke weir
3. who is responsible for maintaining water quality in the Wellington area?
Email your answers to the cache owner, prior to logging your find