Rainwater Harvesting and Sustainable Drainage Systems

A number of organisations are involved in designing the waterworks of a new development. Planners, the Local Sustainable Drainage System (SuDS) Adoption Board, designers, and civil specialists all have to come to a consensus on what’s best for the project.

It’s important that self-builders know the technical issues involved, and that they share the vision of how their project will benefit from the water-related work, and how these will be maintained during the building’s lifespan. Ultimately, the result must comply with the requirements of the 2010 Flood & Water Management Act and the new SuDS Standard. Cost-effectively adopting the SuDS is now a vital consideration.Waterfall

Future-proofing

The country’s national regulations, standards, and procedures are dictated by its predicted future needs. The 2010 Environment Agency Report Water for People and the Environment shows that current stresses on water supplies will be worsened by our predicted population growth.

Climate change is also predicted to change weather patterns, leading to longer dry spells and more intense downpours. This kind of pattern will lead to more of the kind droughts we saw in 2012 and the kind of floods we saw in 2014.

An obvious way of dealing with this problem is to manage the excessive amounts of rainfall in a way that can also help us out in times of drought.

Reusing water and SuDS systems

The simplest way to do this is to manage rainfall as it comes, making it usable for non-drinking purposes such as gardening, car-washing, toilet-flushing, etc. Involving these measures from the beginning of a project also makes it a lot more cost-effective than implementing stand-alone SuDS and water reuse solutions, particularly once the related adoption charges are considered.

Case Study

A 2010 government-funded Developer Guidelines Study provides a good example of how this can be done. In the illustration below, surface water (rainwater which runs off roofs and paving, including water that drains to the public sewer from activities such as car washing or shrub-watering) is controlled at each individual home, using features such as soakaways (where possible), permeable paving, and rain gardens. RWH Diagram Crop

Rainwater harvesting (RWH) systems can play an important role in limiting the amount of water that runs off the land by keeping everything that lands on large roof areas stored for later use. Even so, the main holding tank might have to overflow during very heavy downpours. This overflow can be sent to a communal system serving lots of smaller homes whose roof sizes don’t justify having individual systems. The communal tank will then have to overflow into a SuDS.

What is a soakaway?

Soakaways disperse surface and storm water where connection to the storm water system is impractical or unnecessary. The basic principle is that of a ‘reverse well’ i.e. a ‘hole-in-the-gound’ that lets water out rather than collecting it.  Soakaways enable storm water to be dealt with at source rather than being diverted into the sewer systems. 

What is a rain garden?

A simple rain garden is a shallow depression, with absorbent, yet free draining soil and plants that can cope with occasional temporary flooding. Rain gardens help our gardens to deal with heavy rain, but they also filter and clean run-off from fields etc.

Cost Effectiveness

Water management trains along these lines would ordinarily be designed to cut mains water consumption by around 40% more than current water efficiency measures, so helping to reduce the risk of future droughts. This would also help reduce the amount of water needing SuDS management. Because the RWH systems form part of the household water supplies, they will require their own maintenance regimes, and they will help properties to meet the water quality aspects of the 2010 Flood and Water Management Act.

Simply, SuDS management only needs to begin from the overflow outlets of the communal tank(s), which can also hold excess water during a storm. These systems can considerably reduce the complexity and cost of implementing a SuDS, particularly when balancing ponds and/or swales are included in the system.

What is a balancing pond?

In urban areas, the extent of hard surfaces such as roofs and roads means that rainfall flows directly to the drainage system as opposed to being soaked up by soft ground. This can cause widespread flooding downstream. The purpose of a balancing lake or pond is to contain this surge of water and slowly release it. 

What is a swale?

Swales are shallow, broad, and planted channels which store and/or convey run-off and remove any pollutants. They may be used to pass the run-off to the next stage of the treatment system and can be designed to encourage infiltration of water into the soil where possible.

Careful design

A carefully designed and integrated rainwater harvesting/SuDS system will help deal both with floods and droughts in line with the Environment Agency’s climate change predictions, at no greater cost than a stand-alone SuDS once its adoption costs are included.

Considering these issues from the outset is essential, but with a lack of any current governmental policy on the matter many of us are unfortunately left unaware of how effective such systems can be.

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