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A word about rainwater tanks

Waikawa Beach doesn’t have town water, so everyone uses either or both of rainwater, collected from the roof, or bore water. In our case, we have a bore, but it’s rich in iron and not suitable for household use. It’s great for watering the garden though.

When you rely on water collected from the roof you’re very aware of rainfall — both amount and frequency. After a period of drought, as we’ve had in late 2017 and early 2018, every drop of rain is welcomed.

We have what must be a roughly 20,000 litre tank that collects water from our nearly 70 square metre roof.

Rainwater collection tank.

Rainwater collection tank.

For every 1 millimetre of rain we can collect around 65 litres of water.

As a guide to collection capacity, consider that each 1mm of rain = 1 Litre (L) of water per square metre (m2) of roof area, then allow a 15% wastage factor. — Roof surface & area | Rain Harvesting.

So, with a roughly 20,000 litre tank and let’s say 65 square metres of roof space, we need around 310 millimetres of rain to completely fill the tank.

What about water quality?

Radio New Zealand’s Our Changing World said:

Further research into various physical methods for collecting clean roof water showed that a first flush diverter was the single most effective way of maintaining good water quality. The first flush diverter diverts the first 50 to 100 litres of water collected during a rain event, ensuring that contaminants don’t make it into the tank.

It works like this: the first rain in any spell washes bird poop, sand, leaves and whatnot off the roof, down the main pipe and into the flushing tank where it accumulates. The debris falls to the bottom and a plastic ball floats on the surface. When the water reaches the top of the flushing tank the ball blocks the pipe and any additional rainfall goes back out the secondary pipe at the side and into the main storage tank.

The first flush diverter connected to the downpipe captures dirt and debris, keeping it out of the rainwater tank.

The first flush diverter connected to the downpipe captures dirt and debris, keeping it out of the rainwater tank.

Meanwhile, the water collected in the flushing tank slowly drips out through a tiny hole in a small disc at the top of a small pipe at the bottom, draining the tank so it’s ready to collect more debris in the next fall of rain.

From time to time you need to clean the filter inside the first flush diverter, to keep the system operating smoothly. These before and after shots show why.

Cleaning the first flush diverter filter - before.

Cleaning the first flush diverter filter — before.

Cleaning the first flush diverter filter - after.

Cleaning the first flush diverter filter — after.

How much water do we use?

This is a tricky question and I don’t know a good answer for measuring it precisely. Perhaps there’s a meter of some kind that can be installed between tank and house? I’d love something that could send to my smartphone via an app and alert me to high usage. (Something to investigate …)

Meanwhile, Kāpiti District Council include guidance on their water rates bills for residents there. A friend sent this screenshot:

Water use guidance.

Water use guidance.

In our case, we have a 2 person household. We try to conserve water and we don’t need to use rainwater for the garden. It’s reasonable to think we may use around 400 litres per day.

That would mean our 20,000 litre tank would last 50 days without refills. It’d probably be safer to reckon on 40 days as water is drawn from above the bottom of the tank to avoid sucking sediment into the system. That gives us around 6 weeks of water from a full tank.

Luckily, here at Waikawa Beach, we have AquaGold to top up our tank if the weather doesn’t cooperate.

Aqua Gold.

Aqua Gold.

Water tank fill.

Water tank fill.

Horowhenua District Council have recently mentioned in passing perhaps connecting Waikawa Beach up to the reticulated water supply (at a huge cost in rates). As far as I’m concerned, that’s a No Thanks.

Horowhenua District Council summer water restrictions.

Horowhenua District Council summer water restrictions.