## What Size Rainwater Tanks Do You Need – 1

With sustainability being a majot topic you may be thinking about a rainwater tank. . . but what size of tank do you need?

The bigger tank you get the cheaper the cost of storage per kilolitre (1,000L or 1kL) but you don’t want to buy one too big!

Not only will it cost more, but it will take up more space on your block.

### How Much Rainfall is There in Your Area?

The first thing you need to do is find out what is the average rainfall for your area.

Remember that average rainfalls can change over quite small distances. (For example the rainfall in some Melbourne’s Western suburbs can be 30% lower than parts of the Eastern suburbs).

If you go to this web page and enter any Australian town it will give you the nearest weather stations.

http://www.bom.gov.au/climate/data/

For other counties try your national weather centre.

Once you have selected a weather station click on the get data button and you will get heaps of data. What you want is the mean monthly rainfall in mm.

This should look something like the table below, which is the rainfall for Werribee Racecourse, my local Weather station.

 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual 40.1 38.8 34.9 45.3 48.9 34.3 40.1 46.6 52.2 59.7 49.6 45.0 542.4

Remember the above figures are the mean so for example one year there may be a summer thunderstorms and January may get 80mm of rain, the next year in a drought it may get no rain at all.

### How Much Rain Will You Get ?

Now you need to measure the roof plan area that will be drained to the tank/s in m2.

Multiply the area in m2 by the rainfall in mm for each month and divide by 1,000 will give you the volume of rain in kilolitres.

For example; An 180 m2  roof in Werribee Household will get a mean of 97.6kL per year. (The bottom row of the table below shows the monthly means as well as the annual mean)

 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual mm 40.1 38.8 34.9 45.3 48.9 34.3 40.1 46.6 52.2 59.7 49.6 45.0 542.4 kL 7.2 7 6.3 8.1 8.8 6.2 7.2 8.4 9.4 10.7 8.9 8.1 97.6

The above totals are theoretical!

In practice you will usually collect 80-90% of these amounts.

This is due to losses through evaporation, and any first flush filtering systems.

The next question is how much water do you need?….To find out see Post 2 on this subject

## Rainfall Intensity

When most people think about rainfall they think about the amount of rainfall . . . . . . . . . but it’s just as important to understand how hard it can rain, the  ‘Rainfall Intensity’.

The ‘Rainfall Intensity’ is important when thinking about things like; Roof Gutters, Down Pipes, Stormwater Pipes , Tank Overflows, and even how high your house is above ground level.

## Recurrence Interval

When talking about heavy rainfall you will frequently hear talk on the news about things like a ‘1 in a 100 year storm’. What that means is that a statistical calculation indicates that a storm of that strength is only expected to occur once in any 100 year period, the ‘Recurrence Interval’.

There are two rainfall intensities that the building codes normally consider. They are based on the peak 5 minutes during a storm for vaious ‘Recurrence Intervals’.

Intervals can be from: 1 in 20 years to: 1 in 100 years (See this link for more information: Roof Choices)

## Intensity Calculation

Statistical intensities have been calculated for all locations in Australia and are available at the Bureau of Meteorology (BOM) Website.

Search for ‘Intensity Frequency Duration (IFD)’

### To Use the IFD

1. Enter “your town name ”  “coordinates”  into Google and write down the coordinates
2. Click on the Create An IFD button
3. Enter the coordinates into the Decimal Coordinate box
4. Enter the town name in the Location Name box
5. Tick that you accept the Conditions of Use
6. Press the Submit button
7. Click on the Table tab
8. The following is the top line of the table for Werribee in Victoria.
 Duration 1EY 1 year 50% 2 years 20% 5 years 10% 10 years 5% 20 years 2% 50 years 1% 100 years 5 mins 44.9 60.1 83.4 99.7 121 153 179

The units are mm of rain in 5 mins (Multiply by 12 to get an hourly flow rate)

## Low Rainfall Intensity

Parts of the Plumbing and Building Codes refer to areas of “Low Rainfall Intensity”.

A location  of “Low Rainfall Intensity” means the 5 minute rainfall intensity for an average recurrence interval of 20 years is not more than 125 mm/hour.

## Rainwater Collection – Quality Issues

This is a fairly typical rainwater collection installation.

Gutters discharging to a pipe which discharges onto a screen fitted to the tank access point, cheap and cheerful!

I must have seen it hundreds of times . . . . . . but it has some negative implications on the water quality you will get from the tank!

## Problems

1. Although the screen keeps leaves, mozzies, and other debris out of the tank it lets sunlight in. This will encourage algae growth inside the tank.
2. At the top of the tank the screen is out of view and consequently hardly ever gets cleaned. As a consequence the leaves rot and organic materials gets washed into the tank providing nutrients for that algae.
3. Discharging water into the top of the tank isn’t the best location (see this link:  Inlet Improvements).

## Solutions

• Install a cover over the access point (Something like a round dustbin lid with a hole for the inlet pipe) This will keep the light out, but the issue of the rotting leaves will remain.
• Screen the leaves out at the gutter discharge using a rainwater diverter with a self cleaning screen (for an example see this link: Supadiverta), and pipe the inlet  flow to the bottom of the tank.

## Rainwater Collection Fail

How Not to collect rainwater!

I suppose its cheap…….but it’s not the most convenient for filling your watering can from.

You can also see that algae is growing in all those open containers.

It’s not something most of us would want on our front gardens!

On the plus side it probably looks very spectacular in heavy rain as the water overflows into the various containers.