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Sedimentation

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The Auckland Unitary Plan has identified areas of Te Atatū Peninsula for high intensification. These zones include the 'mixed housing urban zone' (which allows for up to three stories) and a 'terrace housing and apartment building zone' (where developers can build up to seven stories). As quiet, suburban streets turn into huge construction zones, the risk of sedimentation entering Te Wai o Pareira increases. So what is sedimentation? What does it do to our waterways? What can WE do about it?

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What is sedimentation?


Like the seas, our rivers are constantly changing in appearance. From glassy and still, swift and swirling, to roiling and turbid. Likewise, the water colour can range from blue to green to brown, especially in our wetter months. Why? Because of sedimentation – when soil, sediment and rock fragments erode and enter the water during rain events.

Natural or not?


A certain amount of erosion and sedimentation is natural. For example, estuaries are shaped by the mixing of water and sediments from both a waterway and the ocean. Natural sedimentation events can alter the channel size, shape and bed material naturally over time. But too much sedimentation can accelerate these processes and dramatically alter these environments. Contributors include inappropriate land-use and management practices, building dams, bridges or crossings, uncontrolled livestock access and removing riparian vegetation (which stabilise riverbanks).


Smothering our species


Te Wai o Pareira suffers a double-whammy of sedimentation because it is a tidal river. After a rain event, sedimentation enters the awa, and can also be brought in from the sea as the tide comes in.  As the river’s flow begins to slow, coarse and sandy sediment sinks to the river floor and smothers bottom-dwelling invertebrates such as damselfly and mayfly larvae, reducing a food source of fish. It can also smother fish spawning grounds.


Meanwhile, the finer sediment stays suspended in the water column for longer, reducing the penetration of light. This reduces the ability of algae and aquatic plants like eelgrass to photosynthesize. It also clogs the gills of fish, while the reduced visibility makes and it difficult for them to detect predators and prey. Fine sediment and the nutrients they carry are also associated with blue-green algal blooms which disrupt natural food webs.


Sediment also affects the river as a whole, changing its flow and depth over time. And of course, it has an effect on the estuaries and lakes it flows out to, causing infilling.

Building sites – what are the rules?


Building sites have a big part to play in keeping sediment from entering our waterways via stormwater drains. Contractors are bound by resource and building consents to install sediment and erosion controls to contain sediment on site. All the information can be found in Auckland Council’s document ‘Building on small sites’.

The do’s:


- A silt fence: a temporary barrier used on the downhill side of the site to intercept dirty water and retain sediment from entering stormwater drains.
- Earth bunds: on the outer edges of the site to divert clean rainwater away from the exposed worksite.

- Stabilised entranceways: using geotextile cloth and large washed aggregate to keep vehicles off exposed soil and clay.
- Minimise exposed areas: ensuring that wherever possible, grass berms and vegetation be retained, and stockpiles are covered with mulch, straw, plastic sheets or tarpaulins to minimise exposed soil.

- Dewatering: dirty water from footings, trenches and pile holes must be pumped through a filtering device to a stabilised surface (grassed or metalled surface – not concrete) above a silt fence. If this is not possible, a vacuum truck can be used to remove water from the construction site.
- Drain/catchpit protection: sand socks in the kerb channel to slow the flow of the water, allowing more sediment to drop out of the water. This measure only works in conjunction with the above measures.


The don’ts:


- Pumping unfiltered dirty water to the kerb: pumping unfiltered dirty water from trenches or pits directly to the kerb and channel or stormwater system is prohibited – see above for dewatering.
- Concrete kills: Never put concrete or wash concrete down stormwater drains. Concrete and cement are extremely toxic because they have a high lime content with a pH of 12 out of 14. This can easily kill fish, eels and other wildlife.
- If there is a spill, do not wash it down the drain! Instead, follow the steps on page 30 of Building on small sites.


Videos


- Best practice guidance for site management
- Building a silt fence and bunds
- Stabilising entranceways
- Working with concrete
- Catchpit protection

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With such a dramatic increase in new developments on Te Atatū Peninsula, we need your eyes and ears on these construction sites to ensure they are following the rules. Please either notify us, or call the Pollution HOTLINE 09 377 3107 or Auckland Council on 09 301 0101.