Rainwater Harvesting

Our initial idea around our house renovation was to build a smaller dwelling at the top of the block for my in-laws to live in. It was going to be all on one level, easy access and set up for aged-care. This house would have incorporated large, holding tanks under the dwelling to hold the rainwater harvested off the roof area. In turn this would be plumbed into the lower house (the current renovation) for use there also. 

This is not how things have gone to plan, so with the renovation of the lower house taking precedence we are having to place our holding tanks in a temporary position on the top of the block for incorporation into a future dwelling at some point.

The under house tanks waiting for connection

To achieve this we looked at installing rainwater tanks under the house. There was enough room and we wanted to make the most of this available area. So water from the existing house roof enters a series of four fiberglass tanks under the house as its first stop. We chose fiberglass tanks as they were a suitable material to hold water. They came in a size that could be configured under the house between the existing stumps and they were light weight and manageable enough to get under the house to begin with.

These four tanks will be plumbed together and act as a small holding tank ensemble that will be fitted with a pump to deliver water up the block slope to the two 23000 litre water tanks sitting in their temporary position. These header tanks will then deliver water back down the slope and into the main water supply for the house.

From here we will use this rainwater for our daily activities – washing, drinking and cooking. Blackwater from the toilets will go straight into the sewer system. I was keen to look at alternative toilet systems but this was one area that is certainly far easier with more space available under the immediate toilet area and much more time to jump through the necessary hoops required. Alas, it was not to be for this house, but I hold hope for future developments.

Greywater from virtually everywhere else in the house excepting the kitchen area will be diverted to the greywater system as detailed in a previous post. This water is then planned to enter a filter network that will employ a sand filter and reed bed along with a network of ponds. I am planning to harvest some of this water to fill an extra rainwater tank that will supply water to our poultry and animals as well as supplement the vege garden/green house with extra water if necessary.

Once the ponds and tank are topped up, any overflow from this network will be fed back into the swale design that I am working on. This in turn will supply water to the orchard area and is a major part of the rainwater harvesting system of the garden.

After observations of the rain event we had back in early autumn I was reminded of the potential damage that water can do if not properly managed. This has always been at the front of my mind when designing the garden area. Added to this is the potential for further water harvesting from a second dwelling in the future and designing a system that will also be able to accept this extra water if it eventuates.

I have also seen water running off the neighbour’s shed roof where a downpipe has disconnected. The gutters are filled with sand and grass and even if the downpipe is connected it runs onto the lawn. This to me is also a potential water supply to divert into the swale system and solve an existing problem very easily. I will address this situation when the swales are in place if it hasn’t been resolved beforehand.

I am viewing the swale system as being necessary in several ways. It will prevent a general downpour of water from eroding the soil on the slope and reaching the lower property and it will instead channel water to where it can be managed by the trees and vegetation on the property.

Without these swales capturing and slowing the water, rainwater is free to move down the steep slope encumbered only by the vegetation and rough surface of the soil. This does nothing to halt its journey as was observed in the autumn storm event. Rainfall gathered into a channel that eroded part of the soil and then directed itself straight down the slope, around the partially finished retaining wall and pooled at the bottom of the newly extended part of the building where it gradually rose up until it was level with the first course of bricks. In and of itself, not a problem in this case, but a strong warning of potential problems in the future if there is no water management in place.

Typically, what would happen on a site like this is that very little would be done in the way of water management at the top of the slope. Water would fall and wash onto what is to be the brick paved area of the house and from there be diverted into storm-water drains which in turn would flow into the sewer system.

By using swales to harvest this water from the whole top of the block it is intended to allow the water to be slowed and captured and then to slowly dissipate into the soil. I am particularly interested to see how the phenomena of a water plume plays out on the site over time. The concept being that as water is regularly captured and sunk into the ground that an eventual reserve of water if formed under the soil which can be utilised by trees and plants.

Having the greywater diverted from the house will add to this regular deposit of water into the system so that even during times of no rainfall, some water will still be entering the system as long as there are people using the house. This is the next stage of the system that needs to be looked at. That is, finding a sustainable strategy for allowing a timed and automatic release of water into the greywater system periodically, should there be no occupants in the house to contribute to greywater usage. A timer set up to ‘dump’ water into the outside trough periodically should be enough to keep the system ticking away.


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