Bioretention Devices Explained

Richmond West Development Rain Garden Nelson
Richmond West Development Rain Garden Nelson

Bioretention Devices Explained

Bioretention devices, such as raingardens, swales, filter strips and stormwater planters are landscaping features which are developed specifically for stormwater management. They use plants and substrate to detain and filter stormwater to reduce the negative impacts of excessive runoff from roads, roofs and paved surfaces on lakes, streams and the ocean.
 

Bioretention devices mitigate the impact of building developments on waterways, moderating flooding, peak flows and spills.  This prevents erosion and scouring of streams and rivers. Bioretention devices reduce water pollution by removing suspended solids, nutrients and dissolved metals contaminants from stormwater runoff.  Collaterally, they provide aesthetic amenity by enhancing natural landscape and biodiversity elements in the urban environment.
 

A popular bioretention device is a raingarden. This is a shallow depression, strategically located to collect, infiltrate and filter rain that falls on hard surfaces like roofs, driveways or streets.
 

In New Zealand, a raingarden typically includes specifically-selected natives planted into a specialised landscaping media, which attenuates water flow and volume by slowing percolation. This media also filters out around 90% of suspended solids, (or sediment), and heavy metals. Nitrogen and phosphorus are absorbed by the plants and the microbial life that live around plant roots. Experiments have shown that even harmful bacteria, such as faecal coliform, can be reduced by up to 92% through the use of such bioretention devices.
 

However, to realise this high treatment performance in practice, stringent quality controls and assurance processes need to be in place during a bioretention device’s design, manufacture, installation, commissioning and maintenance stages.
 

Studies have shown that for compost to not leach nutrients and copper in a bioretention device, it needs to be well-decomposed compost made from an organic source matter. Having a stringent quality control process for media blending is the most critical criterion to achieve a consistently performing rapid bioretention device in the field. The media specification directly controls the saturated hydraulic conductivity rate, and indirectly influences the pollutant removal capacity by controlling the contact time between the stormwater influent and media. Plant-support capacity, leaching potential and susceptibility to clogging are all dependent on media design, blend constituents used and the homogeneity of the final product.
 

Wholesale Landscapes’ BioRetain meets all the criteria required for a highly-engineered bioretention media. Manufactured in their purpose-built potting mix complex, which includes an onsite laboratory, Wholesale Landscapes’ BioRetain has the best qualities of compost-soil blends with the extra addition of wetting agents and water crystals. The application of BioRetain means a growing tree or shrub is not solely dependent on rainfall. The water crystals are able to hold 400 times their own weight in water, absorbed from excess irrigation, so that in the dryer months plants can thrive in a moist and nutrient-rich environment.
 

BioRetain contains 100% screened soil sourced from safe and non-contaminated sites. It has a well-conditioned mushroom-compost base to promote microbial activity and aged sheep and chicken manures as a rich nutrient source.
 

Bioretention devices demonstrate how the landscape can be used to protect ecosystem integrity as well as providing aesthetic benefits and space efficiency. They use land area both as a treatment system and to provide appealing amenities. Through good design, and the use of specialist media, such as Wholesale Landscapes’ BioRetain, bioretention devices can blend into the site environment, provide for increased biodiversity and add tangible and intangible value to the local community.

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Full truck load volumes
  • Bark: 7m³
  • Compost: 4m³
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  • Compost: 10m³
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  • Wholesale Landscapes – Stoke Yard (that's us!)
  • Stoke Garden & Landscapes
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  • Garden Box – Middleton
  • Urban Paving & Landscape Supplies – Harewood
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  • Styx Mill Landscape & Nursery  – Styx
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  • Odering Garden Centre Christchurch - Philpotts
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  • Fulton Hogan Timaru
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We welcome pick-up of product by the public for volumes 1m³ and over. Please check your estimated trailer weight rating below. If your trailer is unable to hold the weight of our minimum pick-up volume, please book a delivery. *We require a minimum order quantity of 3m³ for any bark, compost, soil or shell products and a minimum order quantity of 1.5m3 for any stones or aggregates.

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Recommended depths

Typically you need a finishing layer of screened soil of 100mm when working with an uneven surface. 100mm of soil will allow to even out any hollows and give enough depth to create a flat surface

A general rule for compost is to allow for the depth of the current root structure of your plants with an additional 30-50% to mix in with the base. If you’re planting into holes or filling up a vege garden  you will need to accommodate for this prior to filling up

We recommend 100mm (10cm) deep for new bark gardens and planting into bare areas. For existing gardens with some cover, we recommend 75mm-100mm (7.5cm-10cm)

Note: Barked areas for playground impact protection need to meet NZS5828:2015 standards and is calculated on the fall areas and the height of the playground. Please contact us to discuss your barked playground areas

For paths and driveways which are often exposed to car and foot traffic, we recommend 100mm-125mm (10cm-12.5cm) on newly prepared areas due to settling of product.

This excludes preparation, compaction or drainage gravel underneath. For existing areas 75mm thickness is recommended for areas needing a small top-up.