Urban Intensification, Biophilic Design & SW Management in SEQ. Understanding Blue Green Roofs.

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  • The Blue Green roof combines blue and green roof technologies. Conventional green roofs use a drainage layer to provide lateral drainage and irrigation. Blue roof technology, however, aims to increase both the volume of water stored and control the amount of water released.
  • Combining the technologies can increase the overall benefits of greening roof-scapes.
  • Combining blue green roof technology and encouraging the wide spread adoption would mitigate flood events and better manage storm water runoff in SEQ.
  • Blue Green roof tops will capture and slowly release rainwater in order to slow the rate of runoff and reduce the potential for related flooding.
  • During and after rainfall, weirs at roof drain inlets restrict the flow rate of the captured water, creating temporary ponding before gradually releasing stormwater to the main SW system. Most weirs allow for adjustment of the flow rate.
  • The ratio of captured runoff is typically designed to closely mimic the pre-construction hydrology of the site. A blue roof acts as a temporary sponge, replacing the prior capacity of undeveloped, pervious ground to absorb rainwater.
  • The stored rainfall can be reused for irrigation purposes, as makeup water for cooling, or in recreational contexts for communal spaces.
  • Blue roofs are often incorporated with extensive green roofs to provide hydration to vegetation.
  • Blue Green Roofs reduce or eliminate the need for underground storage, which in turn reduces excavated material and underground construction cost and time.
  • Roof top Gardens will increase real estate value, biodiversity, rainwater management and re-use, building energy performance, urban resilience, human health or reduction of the urban heat island effect.

Glossary of Green Roof Types

Extensive

Extensive green roofs are lightweight with a shallow layer of growing substrate of less than 200 mm deep -  Generally groundcovers, sedums and grasses.

Intensive

Intensive green roofs are intended to replicate what can typically be found at ground level in the natural landscape, and in green spaces such as parks or cultivated gardens. (A deeper or intensive growth medium substrate is greater than 200mm, to accommodate plants with deeper root zone requirements).

Semi Extensive

A semi-extensive green roof is a hybrid of the two systems (extensive and intensive). Allowing a combination of both shallow and deeper planting requirements.

Recommendations

  • Government to create specific Green Infrastructure funding Grants for Roof tops.
  • Incentives to developers to adopt Green & Blue Roof Initiatives via tax concessions.
  • Incentives to developers through Increased building height approvals which would raise floor space yield ROI and counter balance any upfront costs associated with provision of green (blue) infrastructure.
  • Approval relaxations on Site Cover ratio, Car Park Spaces required, sub-grade SW retention requirements when sustainable Blue Green Roofs are design and Installed.
  • Mandated Green (Blue) Roofs in High Density Inner City precincts under certain Neighbourhood plan policy / City Plan 2014.

 

James Worth, Director of Worth Design, June 2018

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