1. Getting value for money
You can reduce the costs of installing rainwater tanks and rain-gardens in a number of ways: for instance, by doing as much of the labour as possible yourself, or by buying a second-hand pump or by using recycled materials for your rain-garden.
But for any fixed cost for a tank or rain-garden, there are many different ways that you can arrange and use the system, and each set-up will give you a different environmental benefit (EB) score.
Stormwater Tender will pay a set price per EB score, so the better EB score you can get for your property, the better your chances of being a successful bidder, and the more money you will receive from Stormwater Tender.
The EB calculator will help you find the best set-up (tank size, water uses, garden design) for your property and circumstances, and the tips on this page aim to give you some ideas for getting the best EB possible.
2. Already have a rainwater tank?
Then you are already ahead of the game. If you have only been using the tank for garden watering, or if it is only connected to a small part of your roof, then you might be able to earn a substantial EB score for relatively little outlay.
For instance, let's say your 4-person family has 250 square metres of roof, and 300 square metres of garden that you want to water. If you have a 2500-litre tank draining just a quarter of your roof and you are only using it for garden watering, you would currently be saving 21,000 litres each year, and have a current EB score of 0.32 (Try this calculation yourself on the EB calculator!).
You could buy another 2500-litre tank and put it on the opposite corner of your house, and re-arrange your downpipe connections so that both tanks now drain your entire roof.
You could connect the two tanks by a charge pipe so that they fill together (you would then treat them as a single 5000 litre tank in the calculator), and you could get a plumber to connect your tank system with a pump to your toilet, laundry and hot-water system. The new system would earn an EB score of 1.94, and would save 121,000 litres of water each year. For stormwater tender, you would need to subtract your existing EB score of 0.32 to make a score of 1.62.
Your reduced costs in having a tank system already partly set up would make a bid seeking to recover most or all of your costs to Stormwater Tender competitive.
3. Non-standard uses for tank water?
a) Estimating EB scores for non-standard uses
The following hints are designed to help you estimate the benefit of including non-standard uses in your bid using the EB calculator. If you would like help, please contact Darren Bos; dbos@unimelb.edu.au
Uses that demand water all year round will get you a better score than uses that are seasonal, like garden watering.
If you would like to use the EB calculator to estimate water savings or an EB score that could be earned with a water use that the calculator doesn't list, you could trick it by entering values for the listed uses that match your planned use. The calculator assumes that toilets use 6,903 litres per person per year, and you can enter a fractional number of people living on the property. So if you had a 30,000 litre use of water that will be steadily required all year round, you could estimate its effect by ticking toilet flushing and entering the number of people as 30,000/6903 = 4.35. (But please do not use this method for submitting a bid to stormwater tender: explain your use in words in the space provided, and only include true toilet usage in the appropriate box on the Tender application form)
Water usage for topping up a swimming pool (without a pool cover) can be estimated by counting your pool-filling as garden-watering. To estimate this water use, multiply your pool area by 6, and add that area to your garden watering area (Again, please do not use this method for submitting a bid to stormwater tender: explain your use in words in the space provided, and only include true garden usage in the appropriate box on the Tender application form)
b) More than one toilet?
The critical factor for working out water use for toilets is the number of people using them. If you have two toilets in a four-person house, and are only connecting one of the toilets to a tank, then you need to estimate the number of people who would use that toilet on average over a year.
If the toilet is used mainly by 2 people, and the toilet will be the only indoor use for the tank then to estimate your EB score using the calculator, enter the number of people on your property as 2.
If you are using your tank for other indoor purposes, compare your EB scores for toilet flushing as the only indoor use with 2 people and with 4 people, and work out the difference. You can then calculate your EB score including all your planned uses for 4 people. To adjust for not everyone using your connected toilet, subtract the difference you worked out earlier (note this result will only be an approximation).
3. Ways to disconnect as much of your roof as possible.
a) Connecting a tank to all of your roof
There are a number of possible approaches to connecting all of your roof area to a tank.
One way is to use a storage system at roof level. See www.rainsaver.com.au as an example. Such systems have the advantage of reduced pumping costs.
A more common way is to use a charge system, which uses your downpipes as water storage that allows the tank to fill to the same level by gravity. To install such a system, you would need to replace your existing stormwater pipes with stronger, sewerage grade pipes.
A similar approach that would require less replacement of existing pipes would be to have two or more tanks draining different parts of your roof and to have the tanks joined by a charge pipe system.
b) First flush diverters
In some ways, the most damaging stormwater flows to the creek are those small, frequent flows that run off our roofs and roads after just a few millimetres of rain. Diverting just those small events can earn a substantial environmental benefit, at a very small cost. For instance, if you have a 250-square-metre roof with 5 downpipes, and you put a first-flush diverter on each downpipe, each to divert the first 60 litres of runoff, then you could score an EB score of 0.39 (try this yourself in the EB calculator, by entering a tank volume of zero and a first flush volume of 300 litres).
Note, however, that water diverted by first-flush diverters must not go to the underground stormwater drainage system. To be eligible for funding from stormwater tender they must go to the garden or perhaps to your rain-garden.
4. Getting the most out of a rain-garden
For roof runoff, a rainwater tank (with high usage) will almost always score better than a rain-garden. However, rain-gardens are generally the easiest, and often the only, solution to treat runoff from paved areas (or roof areas that can't be connected to tanks).
If you have paved areas that are connected to the underground drainage system, then you should consider a rain-garden as an option to help get the best EB score for your property.
There are many factors that help a rain-garden perform well, however a few basic tips will help you on your way.*
*Note that these rain-garden tips apply to Mt Evelyn, which has heavy soils with low infiltration capacity. In other areas with more porous soils, different rules of thumb will apply.
Vegetated systems will perform better than unvegetated systems (unvegetated loamy sand systems are especially discouraged). Make sure that at least 50% of the plants are from the recommended species list.
Unlined systems will perform better than lined systems because they allow infiltration into the soils, and systems with unlined walls will perform better than systems with only an unlined bottom. However, fully unlined systems are only appropriate if the garden is more than 5 metres from your house. If the garden is less than 5 metres from the house on a downhill slope, you might be able to line just the side nearest the house: seek advice on this. If you do want to consider a rain-garden that is lined on only one or some sides, you can estimate its EB score using the EB calculator by selecting irregular shape (and selecting unlined bottom and sides), entering the filter area in square metres, and entering the length of unlined walls for the filter perimeter.
If the rain-garden has unlined walls, a long, skinny rain-garden will perform better than a more square or round rain-garden. This is because a longer perimeter allows more area for infiltration into the surrounding soil.
Sand is the best medium for nitrogen removal (which contributes to a better EB score), while in some cases, gravel can score better because there is more storage space between the gravel than between the sand particles. The EB calculator doesn't permit more than one medium type, even though rain-gardens can have two or more layers of media. If you wish to have more than one medium in your rain-garden, try calculating your EB score twice, one for each medium and take the average score of the two.
A system without an outlet pipe will usually perform better than one with an outlet pipe. Systems with outlet pipes near their bottom will always perform more poorly than a system without a pipe. However, you will likely find that a system with an outlet pipe at or near the top of the filter will be almost as good, or perhaps slightly better than a system without a pipe at all.
Following the above tips, rain-gardens that are at least 5-10% of their catchment area (i.e. the paving or roof area that they drain) will perform best.
If you are considering a rainwater tank and a rain-garden together, you need to consider if the overflow from the tank should go to the rain-garden. Generally, if the rain-garden performs better than the tank, then you will improve your EB score by directing the tank overflow to it, but otherwise the reverse will apply. You are best to try the two alternatives on the EB calculator to see which approach is best.
5. Do you have any ideas not covered in this website?
Please contact Darren Bos; dbos@unimelb.edu.au
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