Sustainable Drainage Systems CPD – Friday 21st February
Acheson and Glover (AG) Sustainable Drainage Systems
The success of Xflo permeable paving depends on ‘no fines’ construction, with larger aggregates which filter and clean water and allow for the controlled release of attenuated water into the ground or drainage system.
AG are committed to minimising their impact on the environment:
- A short distance between the quarry (source) and manufacturing point.
- As a UK company (based in Northern Ireland), there is a short journey from the manufacturing site to UK construction sites. The movement and transport of products is carefully controlled to ensure maximum efficiency and minimum environmental impact (full lorry loads, ‘piggy back’ transportation).
- Local aggregates are used; these are non-primary (recycled) aggregates.
- Energy is provided by renewable sources (wind farms, bio-digesters).
- Biodiversity management schemes exist for each quarry.
- High quality concrete flagstones and pavers, in a variety of colours.The concrete has recycled content. Any cementitious material has as much cement replacement product as possible (up to 70% can be replaced). This has environmental benefits as high carbon emissions are associated with cement production.
- Concrete facing bricks come in a range of colours; the use of high quality pigments ensures that the colour will not fade.
- Dry build retaining walls – these can be stacked up without the need for foundations or mortar.
SuDS (Sustainable Drainage Systems)
What are SuDS and why are they necessary?
- SuDS mimic the natural drainage of a site as closely as possible.
- They attenuate (retain) water, releasing it at a controlled rate into the ground or drainage system.
- SuDS clean water and remove pollutants through filtration.
- The SuDS ‘triangle’ encourages us to take account of water quality, water quantity and amenity/ biodiversity.
- SuDS are becoming increasingly important to deal with issues of flooding. Two thirds of extensive flooding in 2007 was caused by surface water run off, with the remaining one third being attributable to rivers. Flooding has both an emotional and financial impact and therefore it is in everyone’s interest to minimise it as much as possible. Many surface water drains are close to full capacity.
- Flooding (caused by increased rainfall) is a consequence of climate change. Since 1961, the number of occurrences of 3 consecutive days of rainfall has increased by 50%. We design for a 1 in 100 year storm – do we need to add an extra 40% for climate change?
- Hard surfaces increase urban rainwater runoff. In cities and towns, 5% of rainwater is absorbed by the land, while 95% of this rainfall runs to rivers. This is in stark contrast to the countryside, where 95% of rainfall is absorbed by the land and 5% runs into rivers.
- The written statement made by The Secretary of State for Communities and Local Government (Eric Pickles) in 2014 outlines that SuDS are only applicable to developments of more than ten houses in England. In Scotland and Wales, SuDs must be used where more than one house is proposed, or for a single development of greater than 100sqm.
- The NPPF states that developments should not increase flood risk.
- The typical build up for the Xflo system incorporates 80mm pavers (laid with 7mm joints, filled with 2-6.3mm jointing material) on a 50mm laying course (with 2-6.3mm clean washed angular grit), on a transfer layer (composed of two layers of 150mm clean washed angular stone (4-40mm). The aggregates should become larger towards the bottom of the SuDS construction.
- There should be no fines within the build up (this is key to successful performance).
- Water filters through the joints between the pavers, not through the pavers themselves.
- The depth of the sub-base (transfer layer) ultimately depends on the required holding capacity and should be determined by hydraulic design (carried out by an engineer).
- On sloped sites, it may be necessary to introduce a series of dams/ baffles within the sub base.
- Xflo offers various systems depending on the specific ground conditions:
- System A – full infiltration – typically used where there is a low water table on site and therefore water can discharge into the ground below the SuDS construction, with no discharge from site. A permeable geotextile fabric below the sub base prevents the stone from mixing with the ground beneath the construction.
- System B – partial infiltration – allows partial discharge from site.
- System C – no infiltration – used in clay soil or where there is a high water table. No water is absorbed by the ground; water is fully attenuated and released into a drainage system. An impermeable membrane below the sub base prevents any water from passing into the ground below the construction.
Improvement in water quality
- There are many different sources of pollution – cars, agriculture, industry etc.
- These contribute many different types of pollution – hydrocarbons, oils etc – which are contained within water.
- SuDS have mechanisms to remove these harmful pollutant particles from the water. As water filtrates through the layers, adsorption takes place (pollution molecules are held on the outside surface of stones within the transfer layer). Aerobic bacteria within the soil contribution to biodegradation of the pollutant particles.
- A SuDS system will remove 60-95% of total suspended solids in water, 70-90% of hydrocarbons, 50-80% of phosphorus, 65-80% of nitrogen and 60-95% heavy metals.
Expense, installation and maintenance
- A SuDs system will not require standard drainage infrastructure (manholes, gullies, pipes), which can reduce/ offset the cost of their installation.
- Pavers can be machine laid (300-600m2 per day, per machine).
- Dry construction, therefore Xflo can be laid in any weather conditions.
- It is better to use pavers than slabs since there are fewer joints when slabs are used which reduces the potential for water to infiltrate through the system.
- There is no additional space requirement for Xflo – it can be incorporated within hard surfaces which would be there anyway.
- Xflo can be laid in a variety of patterns.
- There is very little maintenance required with Xflo paving, with an expected lifespan to match that of a typical building – 60 years.