storm water detention tanks

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2798 posts

Uber Geek

#3041991 25-Feb-2023 21:59

Your detention requirements for holding back anything greater than 10% AEP (classical 1 in 10) is going to be huge. A single catchpit with 300mm outlet and 900mm sump will carry approimately 90 linear meters of road with a width of 3.5m which is 315 litres per mm of rain per hour, anything above this and it will overflow.

mentalinc

3240 posts

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#3041992 25-Feb-2023 22:06

I was going to make my own post, but will tag into this one.

I have two tanks (Retention & Detention), they are connected with say a 100mm pipe.

The problem as I see it is the roof rainwater goes into the Retention tank, which is always full to the bottom of the connecting pipe (per mspaint picture below), and as such has very little capacity before filling up the detention tank, which has a soak maybe and then an overflow to the stormwater to the street.

This doesn't seem like the correct setup and they would be better off both being empty to handle the full rain load each time?

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dantheperson

174 posts

Master Geek

#3041996 25-Feb-2023 22:34

That setup looks correct. You want to fill up your retention first, and then overflow any excess to the detention which slowly releases into the council combined or stormwater.

The detention is the capacity of the 2nd tank, as well as the capacity of the 1st above the connecting pipe. When the 2nd tank is full to the level of the connecting pipe, then the 1st tank will start filling above the connecting pipe. After the rain event the 2nd tank will drain, and the top area of the 1st tank will drain into the 2nd tank. So that area above the connecting pipe is also detention.

Retention is so you can water your garden, flush the toilets etc with the rainwater. If the first tank was kept it empty it would certainly detain more water in a heavy rain event, but then you wouldn't have a retention tank, you'd have two detention tanks.

fe31nz

1232 posts

Uber Geek

#3042004 25-Feb-2023 23:08

Benoire:

My goal for my next house is to plumb in the rainwater as the primary source for my house and then use the mains when it runs low; it will be cleaner and less metals compared to the current watercare supply and generally a standard rain event in winter in Titirangi will supply ~10 days worth of water for me (nearly 5000L) with a historic summer level in feb providing about 12 days worth of fresh water.

If you do that, make sure to take fluoride pills or your dental bills will skyrocket.

Handle9

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#3042015 26-Feb-2023 06:39

fe31nz:
Benoire:

My goal for my next house is to plumb in the rainwater as the primary source for my house and then use the mains when it runs low; it will be cleaner and less metals compared to the current watercare supply and generally a standard rain event in winter in Titirangi will supply ~10 days worth of water for me (nearly 5000L) with a historic summer level in feb providing about 12 days worth of fresh water.

If you do that, make sure to take fluoride pills or your dental bills will skyrocket.

Nah, just use fluoride toothpaste.

mentalinc

3240 posts

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#3042023 26-Feb-2023 07:57

thanks @dantheperson agree with the logic, however the retention tank isn't plumbed to do anything so all the water just sits there in the tank, and more often than not the tank overflows out onto the ground around it vs if it was nearly empty, would capture alot mroe water.

Might consider hooking it up to water the garden however.

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MadEngineer

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#3042094 26-Feb-2023 10:47

Amongst the houses in Whanganui we were looking at buying was this one and if you look through the photos you’ll spot a large green tank taking up space in the small back yard:
https://www.oneroof.co.nz/24-buckingham-place-springvale-whanganui-manawatu-whanganui-1858441

Its location I’d describe as a culdesac off a culdesac. The houses are all on a shared driveway that opens up into a large space that’d be great for kids to hoon around on or street bbqs. One oddity I noticed was that all the houses there had rainwater tanks obviously because there’s too many houses in one spot feeding onto the end of the street.

Street view shows one manhole in the middle of the road and instead of gutters down each side there’s a small ridge in the middle of the road.

It was a nope for me.

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Bung

6489 posts

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#3042095 26-Feb-2023 10:56

There could be another reason. Years ago a friend in Wanganui was getting ready to put her washing out. I said what's the point it's about to rain. That was the point, Wanganui water was so crap you used rain as a final rinse.

mattwnz

20164 posts

Uber Geek

#3042221 26-Feb-2023 14:13

MadEngineer: Amongst the houses in Whanganui we were looking at buying was this one and if you look through the photos you’ll spot a large green tank taking up space in the small back yard:
https://www.oneroof.co.nz/24-buckingham-place-springvale-whanganui-manawatu-whanganui-1858441

Its location I’d describe as a culdesac off a culdesac. The houses are all on a shared driveway that opens up into a large space that’d be great for kids to hoon around on or street bbqs. One oddity I noticed was that all the houses there had rainwater tanks obviously because there’s too many houses in one spot feeding onto the end of the street.

Street view shows one manhole in the middle of the road and instead of gutters down each side there’s a small ridge in the middle of the road.

It was a nope for me.

Watertanks maybe a requirement on the covenant, and the council may have required that of the developers. But why not require underground tanks and larger tanks, so they are no visible. That was a requirement of the property in my parents subdivision. Their local council had the opportunity to require new build watertanks in a district plan update, but didn't bother. Now they are having water shortages every year... The stupidity and lack of planning in NZ is so frustrating.

mattwnz

20164 posts

Uber Geek

#3042224 26-Feb-2023 14:27

Does your stormwater go into the councils system? In many areas this is not permitted, and people instead have to install soak pits. In my parents areas, they have a water tank underground, which then goes into a soak pit if it overflows.In the recent storms, the manhole on the watertank popped off when it filled up.
But soak pits with urban intensification is more and more difficult as it needs land. Then are the council increasing their stormwater network capacity and pipes to cope with the increase of new houses connecting to it.

Jase2985

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#3042230 26-Feb-2023 14:46

mattwnz:

But why not require underground tanks and larger tanks, so they are no visible. That was a requirement of the property in my parents subdivision. Their local council had the opportunity to require new build watertanks in a district plan update, but didn't bother. Now they are having water shortages every year... The stupidity and lack of planning in NZ is so frustrating.

because its expensive and noone wants to pay for it

mattwnz

20164 posts

Uber Geek

#3042231 26-Feb-2023 14:53

Jase2985:

mattwnz:

But why not require underground tanks and larger tanks, so they are no visible. That was a requirement of the property in my parents subdivision. Their local council had the opportunity to require new build watertanks in a district plan update, but didn't bother. Now they are having water shortages every year... The stupidity and lack of planning in NZ is so frustrating.

because its expensive and noone wants to pay for it

I think it cost my parents 10k for 10,000Lconcrete tank installed, which was a small fraction of the land cost, and that will pay for itself over time with less council infrastructure needed, and water redundancy. Maybe it means less money for developers if they have to install them themselves, but having this requirement made no difference to people buying the land vs buying land without this requirement, despite the person building having to pay for it. But one reason we have infrastructure problems in NZ is because they haven't done this sort of thing, and haven' increased infrastructure to cope with the growth. So someone will pay for it in the future. So it is false economy to just connect more and more people up to existing infrastructure until it is overloaded and another crisis occurs. That said 50% of water usage in my area is from leaks in council pipes.

COcunils can get small 200L PP watertanks for half the price in the Wellington region, so by buying in bulk, other types could also be sourced cheaper if they got serious about people having their own rainwater storage.

Jase2985

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#3042243 26-Feb-2023 15:03

with a 10kL tank i would expect you will still need council water, stormwater (unless you go for a soak pit) and sewage.

will take a long time to pay for its self. and will cost a lot more on uneven ground

mattwnz

20164 posts

Uber Geek

#3042244 26-Feb-2023 15:13

Jase2985:

with a 10k tank i would expect you will still need council water, stormwater (unless you go for a soak pit) and sewage.

will take a long time to pay for itsself. and will cost a lot more on uneven ground

It is in the centre of town so is on town supply and other services. The council required it to take load off the network for watering the gardens. So all exterior taps all are connected to it, but not the drinking water. It does mean they can water the garden when there is a hose ban on because their water comes from the tank..

SirHumphreyAppleby

2847 posts

Uber Geek

#3042245 26-Feb-2023 15:13

mattwnz:

But one reason we have infrastructure problems in NZ is because they haven't done this sort of thing, and haven' increased infrastructure to cope with the growth. So someone will pay for it in the future. So it is false economy to just connect more and more people up to existing infrastructure until it is overloaded and another crisis occurs. That said 50% of water usage in my area is from leaks in council pipes.

COcunils can get small 200L PP watertanks for half the price in the Wellington region, so by buying in bulk, other types could also be sourced cheaper if they got serious about people having their own rainwater storage.

PP tanks at 50% off are still a complete rip off. Injection moudling and the raw materials are dirt cheap, especially at volume. Our 5000L metal tank was only a little more expensive than a plastic one, but at least that had an element of labour involved, with someone using a rolling machine and putting it together by hand.

As for infrastructure, more people isn't the solution.

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