Typology 9 – Outstand with surrounding permeable pavement

A structural soil trench extends beneath the parking bays and outstands. Permeable asphalt is used for the parking bay surface. Stormwater runoff from one or more driving lanes flows towards the permeable parking bays and infiltrates into the underlying structural soil where it can be accessed by tree roots. During peak events road runoff will flow across the surface of the permeable pavement and be conveyed along the kerb and channel.

Capital and operational costs are influenced by site specific factors including proximity to underground services and ease of access for construction and maintenance. The indicative cost estimate below is for a straightforward site (i.e. assuming service relocation not required and access is unconstrained) and the infrastructure dimensions shown in the standard drawings. The indicative cost estimates will be updated as we gain more experience with this typology.

Capital cost – $36,000
Operational cost – $250 per year

Outstands with surrounding permeable paving are similar to those currently installed along the west side of Eades Place in West Melbourne.

Vegetation – Tree only. These outstands are designed to provide moisture to the soil adjacent to the trees so that tree roots can access this water during dry periods. The water that infiltrates into the permeable asphalt is unlikely to benefit any understorey vegetation planted around the trees due to the depth of the available water/moisture.

Outstand area- The outstand dimensions can be adjusted depending on the space available. It is recommended that the minimum dimension is 1.5 m by 1.5 m.

Permeable asphalt area – Ideally the permeable asphalt area should be at least 20% of the contributing impervious catchment area.

Lining – There is no liner between the tree plot soil and structural soil so tree roots can extend into the structural soil.

Species selection – Refer to Urban Forest Planting Program.

Other

• The tree plots are elevated relative to the road to maximise soil volume. The tree is surrounded by compacted granitic gravel. The surface of the gravel is set lower than the kerb around the perimeter of the outstand to minimise egress of the gravel onto the surface of the permeable asphalt and therefore minimise the risk of clogging the permeable paving.
• Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

The performance of Outstand with surrounding permeable paving can be assessed using MUSIC. The recommended modelling parameters are shown in Table 1. The typical performance of Outstand with surrounding permeable paving is shown in Table 2.

Table 1 Recommended MUSIC parameters for Outstand with surrounding permeable paving

Table 2 Typical performance of outstand with surrounding permeable paving (assuming five parking bays of permeable paving which forms 50% of catchment area)

The long-term performance of assets can be strongly influenced by practices and procedures adopted during the construction and establishment phase. It is recommended that hold-points are used to allow City of Melbourne, or the system designer, to check that the design intent is met during critical stages of the construction and establishment period.

Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

It is the contractor’s responsibility to provide a minimum of 24 hours weekday’s notice (unless agreed to prior) to the superintendent to arrange for an inspection for the hold points.

Recommended construction and establishment phase hold points:

It is important to ensure that assets are functioning properly before responsibility for them is transferred to the asset owner (generally at the end of the establishment phase). The following aspects should be checked prior to handover.

Documentation – Construction documentation AND as constructed survey received.

Permeable paving hydraulic conductivity – At least 500 mm/hr

Vegetation growth – Average height of trees > 2000 mm

Tree condition – Tree healthy and free from disease

Recommended routine inspection and maintenance activities for permeable footpath.

Recommended monitoring activities for permeable footpath.