Guide to Passive Irrigation

GUIDE TO PASSIVE IRRIGATION

Rethinking water

As our urban population continues to grow, so the need for local green space in our towns and cities becomes more important to experience a thriving and diverse natural landscape is essential to maintaining good natural and physical wellbeing – a fact now recognised by the governments and the World Health Organisation.

Yet in the face of climate change and increasing pressure on natural resources, creating these green spaces in urban environments remains a challenge.

The solution lies in how we manage water, the lifeblood of the natural world, only by understanding how we can unlock its full potential will we be able to create future landscapes capable of enhancing resilience, natural ecology, and the way we live.

Introducing Green Urbanisation

Green Urbanisation makes a step change in sustainable water management, allowing the creation of green spaces that diminish environmental while supporting greater biodiversity.

Harnessing and inspired by natural processes, this next generation of sustainable drainage systems capture and reuse stormwater through passive irrigation to sustain green assets. As well as reducing potable water demand, enhancing biodiversity and vitally, increasing resilience, such systems comprise a net zero passive process that’s scalable in any application from a single plot to an entire river catchment.

In this guide, we explore the role of passive irrigation-led technologies in creating resilient landscapes for the future, the evidence base behind such solutions and their best practice in a wide range of applications.

PASSIVE IRRIGATION

What is passive irrigation?

Simply put, passive irrigation is self-watering. Passive irrigation is achieved without energy-consuming systems, for example pumps, using gravity alone to direct roof water and stormwater runoff to get to where it needs to be to irrigate vegetation by the process of capillary action.

Supporting plant growth

To understand the significance of passive irrigation-led solutions, we must start with the fundamental needs of a plant – water, oxygen, light and nutrients – and the role of natural forces in their survival and growth.

Water is essential for a plant’s survival, helping to carry nutrients from the soil or compost through the roots to the stems and leaves.

Plants use capillary action to draw water through the roots to the plant. The sticky, cohesive nature of water molecules adhere to the internal cellulose structure of the plant, enabling water to move from root to tip. Evaporation at the leaves supports this process further, drawing more water upwards.

Optimising water volumes for plant health

While subsurface systems can reduce water demand, it’s also important that there isn’t an excess of water or that the soil is constantly wet, as this can reduce air pockets and limit the supply of oxygen to the plant that is essential for growth.

Passive irrigation techniques enable water to be controlled more effectively, achieving the careful equilibrium of water that is essential to enable the plant to thrive.

The role of capillary rise in practice

Also known as capillary action or capillary motion, capillary rise occurs when water is transported through surface tension and the interaction of adhesive and cohesive forces – the same capillary action used by plants.

Wicking cylinders and/or membranes are designed and integrated into Green Urbanisation solutions to utilise capillary rise, encouraging water to rise upwards, despite gravitational force, into the growing medium.

The benefits of subsurface passive irrigation

The graph above [figure 1] compares top down and subsurface irrigation methods on a turf grass sports pitch in Brisbane, Australia. As well as achieving more consistent soil moisture content, the results were realised with a saving of between 20% to 60% in water volume used.

The benefits of passive irrigation

• Sustaining plant growth: by providing access to soil moisture storage, plants are maintained during dry weather.

• Increased urban cooling: increased soil moisture and healthy vegetation can help reduce urban heat through evapotranspiration and shade, therefore reducing the need for energy consuming air conditioning.

• Reducing the volume entering the sewer network: passive irrigation can protect downstream environments, by reusing rainwater it helps remove stormwater that would otherwise go into the main sewer network.

• Low cost and net zero: no energy is required in the passive irrigation process, which keeps cost lower than conventional irrigation methods.

• Biodiversity Net Gain benefits: passive irrigation delivers measurable improvements for biodiversity by creating highly durable habitats capable of surviving extended dry periods allowing ecosystems to thrive in the optimised growing environment.

• Carbon Sequestration: by enabling the implementation of tree planting programs in the urban environment and sustaining their growth, naturally reducing the amount of carbon in the air through photosynthesis and achieving long-term carbon storage.

• Improved air quality: by creating green spaces in dense urban environments, trees and vegetation filter airborne impurities and pollutants in the air, improving air quality and the health and wellbeing of communities.

• Reducing potable water and energy consumption: passive irrigation reduces the levels of potable water used to maintain healthy landscapes and urban cooling lowers energy consumption required for air conditioning.

Soil & Surface temperatures

Permavoid Geocelluar System

Permavoid is a highly versatile sub-base replacement system designed for water attenuation and infiltration. The sub-surface geocelluar crates manage the water at source, collecting rainwater for re-use via passive irrigation to sustain green assets and create a resilient, fully integrated sustainable water management network.

Certified to British Standard BS 7533-13 (Pavements constructed with clay, natural stone or concrete pavers), the system can be used to attenuate and reuse water at roof, podium or ground level and is scalable from a small housing plot to industrial warehousing where we combine loaded, high trafficked areas alongside green assets to create versatile urban and industrial landscapes that deliver maximum natural capital return.

Permavoid is also an ideal solution for projects where there is a high water table, contaminated land or where there is a shallow outfall – a situation where conventional deep buried tanks would require additional pumping.

Figure 2 - Permavoid passive irrigation system

Sustaining green assets

Due to the close proximity of the surface, Permavoid enables the connection of harvested rainwater to green assets through subsurface passive irrigation.

Permavoid features a series of wicking cones, finely hollow and vertically structural columns, which mimic the natural process of capillary rise allowing the vegetation to draw water as required for optimum health. The additional geomembrane aids the distribution of water beneath the growing medium across a larger surface area, ensuring the right amount of moisture to promote growth and prevent wilting of the local greenery, making the landscape more resilient in extended dry spells.

Over the next pages, we will explore the research and testing behind Permavoid and specifically the performance of its wicking geotextile in a range of growing mediums.

Supporting resilience

By utilising natural capillary action, Permavoid uniquely combines high-strength sub-surface attenuation with net zero passive irrigation to create next generation landscapes capable of helping mitigate climate change.

Additionally – and critically – re-using water in this way also helps remove stormwater that would otherwise go into the sewer network that is under increasing pressure from greater population, urban density and more extreme weather events.

Future-proofing

Permavoid can also be integrated with Polysync, a gamechanging intelligent surface optimisation system that responds to forecast weather events.

Polysync uses real-time high resolution weather forecasting data to optimise water volumes stored in Permavoid attenuation tanks. If heavy rainfall is forecast, the system will instruct the tank to reduce its stored water volume ahead of time to accommodate storm flow without surcharging the local sewer or river network.

If a prolonged dry spell is predicted, the system will maintain maximum volume for re-use in either non-potable applications or for the irrigation of green assets. Polysync can be installed in new, as well as retrofit projects, and data collected through the solution is protected by ISO 27001 compliant information security management systems.

TESTING

The geotextile and patented wicking cylinder has been tested in a range of applications to assess their effectiveness in subsurface irrigation.

Tests have been completed both in-house and by STRI (formerly the Sports Turf Research Institute), a consultancy responsible for the installation of some of the world’s most iconic sports surfaces, including the All England Lawn Tennis Club and Twickenham Stadium.

Capillary cone tests

Geotextile wicking tests

Permavoid capillary cones. Trial demonstrated wicking potential with blue dye.

Growing substrate tests

Permavoid filter/wicking geotextile. Trial demonstrated wicking potential with blue dye.

Design Considerations

Name: Design Considerations

Rootzone

Generally, implies manufactured origin. Normally sand based materials with some organic content added to the mixture.

Common in turf applications where good drainage and structural stability is needed.

Substrate

Capillary rise on two contrasting substrates.

On the left, less capillary rise is observed in a coarser gravelsoil based substrate. On the right, greater levels of capillary rise are observed in a fine sand soil substrate.

Growing medium

Implies manufactured origin. Should be applied to all green roof applications. High levels of chunky mineral content and low organic content.

Catch-all phrase which can be used in any application where plants are supported by the material.

Topsoil

Test conditions

Not always suitable for rooftop and podium application. Always check infiltration rate, wicking potential and clay/silt content.

The wicking efficiency is measured using a bespoke test in STRI’s soil laboratory with samples of air-dried growing media compacted to mirror realistic field conditions in clear Perspex columns, and a water reservoir provided from below. The rate of water rise is then measured over a period of a week.

In real installations, this level may rise further, but this test gives a definitive value for the wicking efficiency of a particular growing medium, for use in design evaluation.

APPLICATION

Application best practice

Applying learnings from this laboratory testing, Polypipe Civils & Green Urbanisation has developed best practice advice for the application and design of Permavoid into a wide range of growing media.

System Design Notes

• System works more effectively if cones and soil are pre-wetted. This often means a temporary irrigation system should be used during installation

• The use of an approved wicking/filter geotextile is key

• Direct contact between soil and wicking geotextile is vital

• Use of an approved rootzone is vital for system to work effectively in the long term

• Unapproved rootzones can cause poor drainage or poor wicking – compromising the integrity of the whole system

Recognising that the health and survival of a green roof can be dependent upon unpredictable weather patterns. With new techniques and technologies we are able to create a system which automatically monitors, and manages water supply to provide optimum soil moisture, temperature and salinity conditions , allowing the green roof to flourish – maximising its benefits. Controlled and monitored remotely online via a smartphone, tablet, or computer, the Cloud Water Control System combines monitoring sensors, remote valve operation technologies and cloud computing software to provide adaptive irrigation, creating the ideal conditions for plant life, while maximising water usage efficiency. The result is a Blue-Green roof which can be more closely monitored and controlled than ever before, bringing together the best of nature, technology and engineered water management products, in one streamlined system. Using our capillary cones – which draw water upwards and are unique to Polypipe Geocellular systems – stored rainwater irrigates the green roof, while sensors within the cells monitor available water levels. If a pre-determined ‘low soil moisture level’ is reached, the system will remotely open a solenoid valve to add water from the Rainwater recovery tank until the correct moisture level is reached. The system also monitors the level of water available in the rainwater recovery tank and controls a back-up water supply into the recovery tank. This is to ensure there is always sufficient water available in the recovery tank for irrigation. During high-rainfall events, excess water in the wet cell spills over into the raft. When the tank reaches capacity this water overflows to the drain.

London Wall Place was a design undertaken by EPG for HurleyPalmer-Flatt and we worked with Brookfield Multiplex contractor to deliver the project. A 1931 m2 podium was used to store 182 m3 of water. In certain areas the Permavoid layer utilised the passive irrigation system to irrigate the green areas. Although the decision was taken to not hold 10 mm at all times to maximise the effects of passive irrigation, the units still had capillary cones in the soft landscape areas to provide a reduced, but clearly effective means of reusing water migrating through the system. As seen in the picture taken during the heat wave of 2018 the passive irrigation system is still sustaining the green areas Given the statement above with regards to not holding back water intentionally in all areas it shows how effective the system can be.

Permavoid was used initially to alleviate flooding from an adjacent retaining wall. 120 tonnes of earth had to be removed from the garden to create the new profile and formation levels. Access was restricted to a relatively narrow strip of ground at the side of the house. The site was very wet, with poorly drained clay soil and large volumes of water coming through the retaining wall from adjoining properties. A number of decorative, pleached trees were installed within raised planters to create a decorative feature and natural screen.

The three elements of the Permavoid drainage/ irrigation system installed were:

1. Retaining wall drainage

2. Planter drainage & irrigation

3. Lawn drainage & irrigation

Interconnected pipes and overflows, were designed to work as a cascade system towards the outfall point. Water was collected from the back roof of the property to provide a “free, sustainable” source of water to use for washdown of the patio area.

Details of what was installed on this site

Permavoid 150 Unit installed in the planters at the front face of the retaining wall to provide drainage to intercept and convey the water coming through drainage holes in the wall.

Permavoid2 85 Irrigation Panels Installed at the base of Planters to provide drainage and Irrigation.

Permavoid2 85 Irrigation Panels Installed at the base of lawn below 250mm of carefully selected top soil to provide drainage and Irrigation for natural turf Lawn.

Permavoid 85 Unit installed as part of the sub-base below the patio paving to create a Rainwater Harvesting System collecting water from the roof of the property.

Figure 2: Install of Permavoid under turfed area

Figure 1: Permavoid install with wicking cones at density of four per tile.

London during intense / regular storm events would flood areas as the local sewer was already at capacity. The London Boroughs were brave in their approach and intercepted the storm water at source with shallow attenuation tanks to alleviate the sewer. They included functionality benefits with the addition of localised rain gardens, which incorporated again passive irrigation at the base.

BRUNTWOOD WORKS BLOC ROOF -MANCHESTER

Sited on Bloc – a city centre workspace that has been transformed by Bruntwood Works as part of its £50m Pioneer redevelopment programme – the blue-green roof installation from Polypipe Civils & Green Urbanisation, Machester’s first wild flower roof, will help to develop a greater understanding of how new construction and data technologies can help cities and urban developments to mitigate the impact of climate change and population growth, while enhancing biodiversity.

The initiative is a joint venture between environment management consultants EPG, United Utilities, Bruntwood Works and Polypipe Civils and Green Urbanisation to assess how storing and re-using rainwater at roof level can reduce the volume of surface run-off entering its sewer network. As a result, it will help to lower the flood risk associated with the prolonged high-intensity storm events that are becoming increasingly frequent as the climate changes.

Unlike conventional green roofs, which use a drainage layer to simply remove rainwater, the 525 sq m blue-green structure retrofitted to Bloc’s flat roof stores rainwater beneath the planted surface where it lands. Capillary cones and wicking geomembrane form part of the Permavoid attenuation layer, draw water up through the structure to the underside of the green roof substrate to support surface planting via passive irrigation.

The breakthrough technology protects green areas during periods of drought, reduces potable water demand during hot weather and enhances biodiversity by maintaining flora in optimum growing conditions. In the case of Bloc, planting has been specially selected by partners of the project, STRI, from local varieties to help attract pollinators and a particularly rare butterfly, the Manchester Argus.