By collecting water from impermeable surfaces, rainwater harvesting can reduce the volume of runoff and peak flows and so have a positive impact on surface water flooding. It can vary in scale from single water butts installed on private properties to underground storage tanks on commercial areas. Costs and effectiveness of the intervention depend on its scale and design, but benefits from reduced runoff are only significant for larger systems.
To read more, have a look at some of the References for Rainwater Harvesting as an Intervention.
Rainwater Harvesting Systems provide no possibility of access or the benefits of accessing green space.
Rainwater Harvesting Systems have no impact on air quality.
High peak flow and volume reductions can be achieved depending on the size and design of the system/butt and the saturation of the system. An estimated 24,000l/a can be saved from the average roof (11). However, there is little evidence on the scale of this impact on flooding. (1,3,8)
Rainwater Harvesting systems are unlikely to contribute to reducing fluvial flooding apart from reducing runoff into water courses.
Rainwater harvesting can have positive impacts by saving water and thus energy, but if pumps are used the emissions might outweigh the benefits. (3,6)
Rainwater Harvesting Systems have no capacity to provide habitats for wildlife.
Infiltration allows groundwater recharge or releases water slowly into the water bodies. This can mean a positive impact on low flows.
Rainwater Harvesting provides no opportunity for reducing pollution and may even deteriorate the quality of water. However, it does intercept water initially and can so reduce the first flush effect.
Water butts can be used as planters and so provide aesthetic benefits. Tanks can be stored underground so as to not impact on the landscape or be designed to provide amenity value. (8, 10)
Rainwater Harvesting Systems provide no opportunity for cultural activities or further cultural benefits.
Rainwater Harvesting Systems may be able to add value to a property, especially if they are extensive.
Due to their impact on surface water flooding, Rainwater Harvesting Systems may influence the extent of flooding downstream.
Considering the Bigger Picture
Rainwater Harvesting acts as source control and storage. It prevents runoff by taking it up at its source. In one year, it is estimated that 24,000l can on average be saved from a roof in the UK, preventing this additional runoff. Once RWH systems have reached their capacity, they cannot contribute any more to reducing runoff. Ways of dealing with overflows have to be incorporated – this could be infiltration systems like Rain Gardens, for example, taking up water spilling out of water butt outlets.
The impact of RWH is mostly realised on a local scale, but cumulative effects where RWH is implemented on as many properties as possible are to be expected.
On the left, you can find an example of how different interventions can be incorporated into the urban landscape.
To provide a comprehensive treatment and management of surface water, rainwater harvesting should be seen within the wider landscape. While it is able to intercept rainfall before it becomes runoff, it is important to understand that its ability to take up existing runoff and infiltrate it is limited and that once the tank is filled, they cannot contribute to runoff management at all.
Especially where large quantities of surface runoff are expected, for example where large impermeable areas are located upstream, rainwater harvesting should be complemented with additional interventions and exceedance flow management systems. They can be combined with raingardens that are planted at overflow pipes, for example. Water can further be lead into retention ponds or wetlands to undergo additional treatment or to be stored over a longer period.
Below, you see the benefits of rainwater harvesting systems illustrated on a local scale.
£10+ for water butts, £2000-4000 for a complete domestic system. Retrofit is possible but likely more expensive for entire systems. Depends on scale and type of the system (e.g. gravity fed or pump system) and existing connections.
Context: Residential, Industrial. Retrofit and use in high density urban areas possible.
Connection to rainwater pipes is necessary. More water is collected from sloping roofs. (7)
Costs: £0.1-0.4 per m2 (4,7)
Typical maintenance activities: cleaning and inspection. Depends on context and type of system.
Trade-offs and Potential Dis-services
Even if there is no increase in property value, rainwater harvesting systems and water butts can save significant amounts on water bills (depending on type of water use and intensity of use).
During periods of hosepipe bans, as they can happen more frequently, harvested rainwater can be used to water vegetation and keep it beautiful. For bigger systems, the ability to meet water demand independent of mains water can provide sustainability and resilience benefits.
Where complete RWH systems are installed with pumps, the intensity of energy use can be increased compared to mains water. This can have net negative impacts on emissions from the system. This is not the case for water butts and other storage systems without pumps.
While it is generally not an issue, water harvested from roofs can hold high concentrations of pollutants, especially after long dry periods. However, there is little evidence of this occurring in significant frequency. It is important to connect drain pipes correctly, so only rainwater is discharged into the water storage.
- Ashley, R. M., Nowell, R., Gersonius, B., & Walker, L. (2011). Surface Water Management and Urban Green Infrastructure, 44(0), 1–76.
- Berwick, N., & Wade, D. R. (2013). A Critical Review of Urban Diffuse Pollution Control : Methodologies to Identify Sources , Pathways and Mitigation Measures with Multiple Benefits.
- (2014). Demonstrating the multiple benefits of SuDS – a business case.
- Environment Agency. (2015). Cost estimation for SUDS – summary of evidence. Bristol.
- Helmreich, B., & Horn, H. (2009). Opportunities in rainwater harvesting. Desalination, 248(1-3), 118–124.
- Parkes, C., Kershaw, H,, Hart, J. Sibille, R., Grant, Z (2010):Energy and carbon implications of rainwater harvesting and greywater recycling. Bristol: Environment Agency.
- Woods Ballard, B., Wilson, S., Udale-Clarke, H., Illman, S., Ahsley, R., Kellagher, R. (2015): The Suds Manual. London: CIRIA.
- Susdrain (2016): http://www.susdrain.org/delivering-suds/using-suds/suds-components/source-control/rainwater-harvesting.html
- info (2016): http://www.savetherain.info/media-centre/rainwater-harvesting-faa qs.aspx
- Rainwaterharvesting Ltd (2016): http://www.rainwaterharvesting.co.uk/rainwaterharvesting-simple-guide.php
- BBC (2016) http://www.bbc.co.uk/gardening/basics/techniques/watering_savingwater1.shtml
- co.uk (2016), Rain harvesting http://www.yougen.co.uk/energy-saving/Rain+Harvesting/