Reducing impervious surfaces Print

Design Outcome

​​​​​​​​​Reducing impervious surfaces will increase ​opportunities for rainfall to be attenuated within vegetation and soils. ​

This will moderate stormwater volumes and reduce the capacity requirements for infrastructure and treatment practices in the catchment. Reduced imperviousness is also likely to reduce the contaminant load, since there is less surface area for deposition of contaminants and more vegetated areas to capture and transform contaminants. Reducing impervious surfaces may include the following approaches: 
  • Shared driveways
  • Shared road surfaces for low traffic environments (as discussed in Section D5.3)
  • Replacing impervious surfaces with pervious paving, living roofs, etc.
  • Aggregating buildings and ancillary structures to reduce total footprint and access requirements
  • Setting building configuration and heights to provide for optimal and flexible uses, including underground parking and mixed use development as appropriate
  • Locating proposed developments close to existing access.

The redevelopment of a brownfield site allows an opportunity to re-evaluate impervious extents for current and future uses. Retrofitting catchments provides opportunities to extend the design capacity of existing stormwater management infrastructure or to reduce existing impacts on receiving environments.

​ Challenges and solutions
The table below describes some of the common issues relating to reduced impervious surfaces. In all circumstances there is a potential design solution, which must be bal​anced against other objectives for the project.
that WSD approaches and their requisite functions are protected. ​​


Challenge

Solution

Some land uses require a high level of imperviousness

In highly impervious areas such as civic and industrial sites, the key is to direct stormwater from reticulated systems to vegetated landscape areas or to WSD stormwater practices that utilise natural systems and processes (raingardens, living roofs, etc).

Pervious paving should also be considered (refer Section E4.5).

Emergency service access requires specific road widths and turnarounds

Turnarounds can be avoided through a connected roading grid, accommodated by no-parking restrictions in specific areas, or replaced by ‘T’ turning points, or reinforced grass areas at road ends.

Parking is sometimes required for visitors, events, or future proofing for increased densities

Parking locations should be evaluated for their appropriateness to the site, taking into account the neighbourhood as a whole, potential for garaging under buildings, and associating future high density or commercial areas with public transport hubs.

Where parking is deemed appropriate, potential strategies to reduce imperviousness include shared area for opening car doors, pervious area forward of wheel stops, and one way angled parking.

Parking substrates, especially ‘overflow’ parking can utilise pervious pavement or reinforced grass materials. Refer to the Auckland Transport Code of Practice (ATCOP) for more information.

Roading responds to Council standards

ATCOP describes a broad function for roads to contribute to quality urban design. Appropriate carriage widths are an expected outcome of this design approach.

Although councils have roading standards they do have flexibility to reduce road widths. The widths are assessed on a case-by-case basis, dependent on development form and potential effects to the environment (narrowing roads to protect significant features or limiting earthwork impacts to slopes). Alternative carriage widths are particularly appropriate for structure planning, where broader scale roading networks, street hierarchy and streetscape environments can be simultaneously addressed.

Urban design networks promote a connective urban grid

There is a balance between roading effects on the environment and ensuring connectivity and free flowing traffic within a community. A connected road grid can reduce arterial roads and their associated wide carriage widths and provide opportunities for more low traffic roads with enhanced landscapes.

Footpaths are required for pedestrian-oriented communities

Footpaths can be eliminated in many instances through shared surfaces in community streets and laneways.

Where dedicated footpaths are appropriate, consider using pervious materials or directing impervious surfaces to landscape areas or vegetated practices such as raingardens and swales.

Kerbs are perceived as neat edges for residential streets

Flexibility should be maintained around kerb edges, to utilise a combination of flush kerbs, broken kerbs, or directing kerbs to WSD practices. The Auckland Transport Code of Practice (ATCOP) requires kerbing for urban streets, however allowances are given to drain the road to stormwater control devices.

Driveway lengths are affected by conventional approaches to building setbacks

Back lanes can reduce the need for individual driveways (refer to ‘WSD streets’ in Section D5.3), otherwise the extent of driveways can be reduced by a single shared access to multiple garages or multiple lots.

Setbacks can be reduced to bring houses close to roads. Other alternatives include providing wheel tracks rather than a single service, using pervious materials such as porous concrete or pavers, or directing stormwater runoff to raingardens or filterstrips on individual lots.

Gradual increases in imperviousness can result from subsequent land owners accumulating paths, sheds, etc.

Pre-empt the needs of future occupiers through site designs, or set expressed limits on ancillary structures, protection of green space and extent of earthworks.

Building and structure footprints

Consider living roofs and walls for structural surfaces (refer to Section E4.4 on living roofs that follows), or direct runoff from roofs to landscape areas or dedicated stormwater practices.

Consider clustering buildings and ancillary structures to share access and foundations, or build upwards for infill situations.

Soils to be re-opened from under impervious surfaces may have limited infiltration capacity, especially where they have been earthworked

The restoration of topsoil and subsoils over relatively impermeable subsoil layers will still attenuate significant quantities of stormwater. Modified subsoils can also sometimes be rehabilitated.

Structural strength of the subbase may be affected by removing impervious surfaces and introducing groundwater flows​

When removing impervious surface consider set back distances to structures, use additional drainage adjacent to structures, or investigate the use of pervious structural soils. ​

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