Climate change has had a huge impact on our weather and rainfall globally. The warming and cooling phases of sea surfaces (El Nino and La Nina) are increasing in variability which has led to a larger proportion of rainfall now occurring in single events of greater intensity.
The impact of this variability differs depending on location. We anticipate seeing reduced average rainfall in most regions of Australia, little change in New Zealand and wetter conditions in Europe, including the UK and Ireland. In all regions, however, extreme rainfall events are likely to become more frequent and severe.
This unpredictability of when and how much rain is on its way expedites the need for buildings to capitalise on significant downpours through rainwater harvesting to guarantee supply during times of drought. It’s also important we consider rainwater attenuation to manage increased stormwater flows and mitigate damage to infrastructure during intense weather events.
Prioritising water
Water is cheap and considered abundant. But, in reality, less than one per cent can be used by people. The rest is inaccessible or undrinkable – think polar ice caps and salt water from oceans.
Earth may always have the same volume of water, due to our water cycle, but it doesn’t always return to the same place or in the same quantity.
We’re faced with the challenge of an increasing population, rise in temperatures and severe changes in rainfall. This all adds up to global water scarcity.[1]
Net zero water
Reducing energy consumption features strongly in the construction industry. Businesses are highly focused on operational energy with this being a key part of a strategy towards net zero carbon or carbon neutral, terms we commonly hear these days. So how does water play into this? A similar reduction hierarchy can be taken to reduce our water usage, recycle water and then, as a last resort, offset the remaining consumption.
Net zero water (or water neutrality) needs to be a priority for the built environment as it journeys towards net zero carbon.
Water intensity and water scarcity
As we strive to think holistically about rainfall and how we might use it to be more resourceful, we need to consider two different aspects.
High rainfall intensity
Flooding needs to be avoided so buildings stay dry during periods of high rainfall intensity. Roofs and roof drainage systems are designed to agreed intensities set by building codes and standards.
These codes and standards are typically based on an Average Recurrence Internal (ARI) of 100 years, colloquially known as the 100-year storm. Now known better as the Average Exceedance Probability (AEP), this means there’s a 1 in 100 change that the rainfall intensity that a building’s roof drainage system is designed to accommodate will be exceeded in that year.
While the ARI/AEP depends on location, and is based on models using historical data, it predicts that a building, generally, has a one per cent chance of exceeding its drainage system capacity, potentially leading to water ingress and damage.
But, if the climate is changing, how do we use historical data like this? It’s now common to factor in a 30 to 40 per cent increase to help futureproof a new building that might last 60 years.
Buildings also need to consider rainwater attenuation to reduce flowrates and reduce stormwater impact on infrastructure during these more intense weather events.
Drought
It’s essential to conserve water to provide resilience during droughts. This means reducing consumption and replacing potable (or drinking) water with recycled water or rainwater where possible. This reduces pressure on our dams and groundwater sources, where most of our potable water comes from, and the threat of them becoming empty during droughts.
Buildings should consider rainwater harvesting for reuse to reduce water consumption and cater for a reduced average rainfall.
Unpacking rainwater harvesting and rainwater attenuation
First up, we need to determine the optimum volume and size of a rainwater tank. We do this by modelling the rainwater that could be captured and re-used for applications such as cooling tower make-up, irrigation and toilet flushing.
The optimal rainwater tank size balances the maximum amount of rainfall able to be captured with the physical constraints of its location and the demands that it serves.
We want rainwater to supply a high proportion (or contribution) of the demand to reduce non-rainwater usage. For example, in a rural area where the tank is the sole water supply, we would make sure there is rainwater always available when needed – a 100 per cent contribution.
As the tank size increases, these performance metrics improve. We ensure sizes provide good outputs for these metrics, balanced with practicalities such as available space.
Due to the variable nature of rainfall around the world, approaches to rainwater collection differ markedly, depending on location.
Rainwater harvesting
It’s easy to think of rainwater harvesting in a domestic context, with a rotomolded plastic or corrugated steel tank which collects water from a localised section of a roof. However, for commercial projects, tanks are usually much larger and there are often multiple tanks.
These tanks can be made of plastic, glass reinforced plastic, concrete or bolted steel.
Their location, or locations, needs to consider the best way to capture as much rainwater from a building’s roof as possible, relying on graded pipework between outlets or gutter and the tank. This may be on the roof, in-ground or elsewhere.
Siphonic roof drainage systems enable tanks to capture more roof area and to be located more centrally where treatment is possible. Outlets are tested to drainage greater flowrates and the pipework can run flat so can run greater distances.
Rainwater attenuation
When considering how best to manage rainwater, we also need to consider reducing flowrates to reduce pressure on infrastructure and treatment.
Sustainable drainage systems deal with surface water runoff through capture, use, absorption, storage and transportation in ways that reflect nature.
In London, we’re working on both new and major refurbishment projects where rainwater attenuation is being designed in as close to the source as possible, for example roof level. This reduces the impact of rainfall on infrastructure. Peak flows can be reduced significantly by storing water on the roof, also known as a blue roof, with outlets slowly releasing rainwater into the drainage system.
These measures help to reduce flooding, sewer overflow and reduce the need to upgrade infrastructure for new developments. The surfaces above them are often used as green roofs and terraces.
Shifting rainwater storage from basement tanks to dispersing the volume across roof level frees up basements for end of trip facilities including cycling and electric vehicle parking. It also eliminates the need for excavation, particularly in existing buildings.
Additional measures mitigate the risk of blockage, for example overflows and secondary outlets. Waterproofing for zero gradients is a top architectural priority and, as we see an increasing use of timber in construction, we need to ensure a small leak doesn’t lead to catastrophic damage of a timber structure.
Creative rainwater solutions
Conversely, we’ve also been looking at where we can remove roof drainage. In the centre of historic London, we’re designing a high-rise office building with perimeter balconies for occupant amenity.
Conventionally, all these balconies would have rainwater outlets and pipework down the building. The project team is considering removing the downpipes to allow more space on the floorplates and let any wind-driven rainwater run down the façade and re-disperse to the atmosphere.
Rainwater attenuation in refurbishments
While heritage, listed and existing buildings need careful consideration, we can still make improvements and incorporate rainwater attenuation tanks at basement level. Reducing peak runoff, even if limited, is still valuable.
Benefits of rainwater harvesting and rainwater attenuation
Putting forward the case for capturing rainwater has never been easier. With the low cost of water, there is rarely an overall financial payback for all but the simplest of systems. However, bills can be reduced and businesses recognise the importance of reducing their water use.
Reduce the cost of water bills
Using recycled rainwater for these functions can significantly reduce water bills by decreasing demand on mains water supply.
Cooling
One way to reduce energy usage in cooling applications, whether industrial or cooling of a commercial building, is by using water, either in a traditional cooling tower or using adiabatic pre-cooling. But, this can use large amounts of water. On a peak summer day in Australia, a large commercial office building can use 100,000 litres and a hospital can use 500,000 litres, just for cooling.
Irrigating green spaces
Landscaped green areas in buildings are on the rise. Offices, public buildings and apartments increasingly feature vertical gardens and green terraces to enhance occupant wellbeing and support biodiversity. This leads to greater water demand. Re-using rainwater for irrigation is one of the easiest solutions to meet the increased water demand as it requires minimal treatment.
Flushing toilets
The first dual-flush toilets were developed in Australia in the 80s, and now feature around the world with progressively reduced flushing volumes down to between 3 and 4.5 litres a flush. The next step in water conservation is substituting drinking water with a recycled water supply, or with rainwater captured on site.
While vacuum toilets have crossed-over from the transport sector (ships and planes) using even less water (0.5 to 1.5 litres per flush), the energy intensity for the saving in water (kWh/L) to operate the vacuum pumps needs to be considered. Limited data is currently available.
Reusing rainwater for toilet flushing has become the norm for many projects that we work on.
Go green and be seen
Reputation
Harvesting rainwater also shows a commitment to sustainability, suggesting that a business is charting towards net zero. And, of course, that’s good for building reputation, attracting tenants and appealing to investors.
Traditionally, rainwater tanks have been installed in an area where they’re concealed from view, for example plant areas, car parks or underground. However, integrating rainwater tanks into high traffic areas helps to publicly demonstrate sustainability commitments and engage with the local community.
We’ve been talking with architects on a project in Brisbane about how we may make the rainwater tank visible in a premium location. The client wants to go lean, clean, green and be seen.
Achieve higher sustainability accreditation
Implementing rainwater reuse will provide benefit to projects targeting sustainability certifications such as Green Star and NABERS Water. Green Star Buildings sets a minimum expectation for projects to reduce potable water use by 15 per cent – and while efficient fixtures and fittings may contribute, some projects will need to look at onsite water capture and reuse to meet this requirement.
Points are available for increased reduction therefore appropriate tank sizing and connection to all possible sources (toilet flushing, irrigation, washdown and more) is beneficial.
Address local pollution
By reducing stormwater discharge volumes, we improve run off which helps to mitigate flooding and the subsequent pollution. Stormwater detention on-site also reduces the level of treatment required when discharged to an authority’s water supply.
Design water infrastructure with climate change in mind
While current codes and standards vary globally, there’s a strong message to reduce water use wherever possible.
Our hydraulics team at NDY is always trying to think one step ahead. How might codes and standards evolve as our climate becomes more extreme? What might we need to do to meet the needs of future codes and standards? Could rainwater harvesting and attenuation become an imperative for every new build?
Buildings use 20 per cent of the world’s available drinking water.[2] It’s imperative we measure and reduce our water usage. As the property and construction industry accelerates on its journey towards net zero, rainwater harvesting and attenuation can help to reduce water usage and improve sustainability performance.
[1] https://www.epa.gov/watersense/how-we-use-water
[2] https://www.nabers.gov.au/ratings/our-ratings/nabers-water
