Just doing rainwater harvesting calculations and thought I'd apply the math to an urban setting, namely a typical Victorian terrace house in London that my family owned many years ago.
Surprisingly, result based on average UK rainfall is encouraging.
Because these houses are terraced, its viable to install large collection vessels in the back gardens. These can potentially capture around 50% of all the water that falls on the roof.
3,000 litres of pre-stored water, topped up with harvested rainwater, has the potential to keep a family of 5 or 6 supplied indefinitely with sufficient water for survival.
This is a bit of a surprise to me. It needs looking at more closely based on actual house dimensions and historic rainfall patterns in specific areas.
But food for thought???
Over the course of a year, yes you should easily be able to collect enough water to survive on, but the biggest issue is that most of the water falls together in a few big storm events, rather than being spaced out through the year.
For example since the beginning of May this year, we have not had enough rainfall (in my area) to refill 2 No. 210L water butts that are connected to the roof of a double garage - Yes they keep getting topped up by rain showers (but not fully) and I keep using the contents for the garden, which is obviously draining it further. But between December and February I could have probably easy filled them 100 times over.
So the biggest issue for me is that of sufficient storage to see you through the drier months.
So its more a case of how many Water Butts/IBC's do you realistically need in a survival situation to save you having to go out and collect water.....
I'd be interested to see how you got to your 3000L calculation?
Exactly right Dev.
The calculation I made was based on 50% of the UK average monthly rainfall, so I captured some seasonality, although maybe not the reality of the exact microclimate of that part of London.
Using a catchment area of 50 sqm, and harvesting 50% of it, there would be just about enough to meet demand, without dipping into reserves, which would act as a buffer.
I also calculated on the basis of a frugal 5L per person per day. This equates to 930 L per month 6 people. If a buffer of 3,000 L is put in place, this will effectively give 3 months insurance against un expected drought.
Back to your point, that is precisely why local average rainfall needs to be determined, and also the degree of variation that can be expected too.
On paper it looks viable, volume-wise.
In addition to reservoirs, and ensuring that the roofing surfaces are free from toxic metals, urban preppers will also have to make provision for sterilising and filtering their harvested water before considering it drinkable.
(8 August 2014, 17:10)Devonian Wrote: [ -> ]I'd be interested to see how you got to your 3000L calculation?
You are kidding, correct?
M x mm=L
roof measurement (sq/M), times millimeters of rain per year, equals liters of water collected.
(8 August 2014, 18:41)Mortblanc Wrote: [ -> ] (8 August 2014, 17:10)Devonian Wrote: [ -> ]I'd be interested to see how you got to your 3000L calculation?
You are kidding, correct?
M x mm=L
roof measurement (sq/M), times millimeters of rain per year, equals liters of water collected.
You mis-understand MB!
I was more interested in understanding LS's calculation of the following, which he has now explained:
"...3,000 litres of pre-stored water, topped up with harvested rainwater, has the potential to keep a family of 5 or 6 supplied indefinitely with sufficient water for survival...."
Not area of roof, area of building footprint that the roof covers in sq Metres. I worked with 50 Sqm.
Say rainfall is 3 cm per month. 3cm expressed as metres is 3/100 =.03m
So depth of rain in Meters x building footprint in Sqm =volume in Cubic meters.
That's 50 x 0.03 = 1.5 Cu meters
1 Cu Meter is 1000 Litres.
So multiply 1.5 x 1000 = 1,500L
Half of that roof would yield 750L
6 inhabitants at 5L per day each would consume 6 x5 x 31 =930L
750L minus 930L = 180L of water that would have to be drawn from the buffer, beyond what was harvested.
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3000L is an arbitrary volume that I used as a what if reserve.
Average UK monthy rainfall figures that I found showed lowest value around 60mm per month and highest values a little over 90mm per month.
If there was zero rainfall this buffer would last our preppers 3 months, possibly double that if they are very careful.
Playing around with it some more, on an assumption that rain does indeed fall every month, but at a bad time for our preppers, I'm seeing that the lowest reserve they'd see would be around 1200L
That would work out to one or two months life line for them.
If someone would like to research urban centre rainfall averages by month, and report them back here, I'll plug them in to evaluate a little more realistically.
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Incidentally this rainwater calculating is driven by our living under a Tyvek tent at the moment. In the last three days we've had nearly 60mm of rain which generated a HUGE amount of water in the garden and yard. Around 12,000L actually. And most of it went down the drain..... That got me thinking, and scheming...