Typology 3 – Covered back of kerb raingarden tree pit

Covered back of kerb raingarden tree pits are suitable for heavily used footpaths where there is limited space behind the back of kerb that is not essential for pedestrian movement or other street functions.

The cover above the filter media ensures minimal impact on the width of the footpath available for pedestrians. Stormwater flows from the street gutter through a kerb inlet into a sunken tree pit and percolates through the soil to perforated pipes in the base of the tree pit.

The purpose of this information is to assist you in developing your Blue Green Infrastructure project. Some of the drawings associated with this typology are not yet included in the Engineering Standard Drawings. We are currently identifying sites for potential trials. You will need work with our Infrastructure and Assets team and Urban Forest team to finalise your design and obtain relevant approvals.

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Design and construction notes

How does the system work?

Stormwater flow from the street gutter is directed into a grated inlet and onto the filter media surface. The filter media surface is set lower than the adjacent gutter channel to facilitate temporary ponding of water above the filter surface. When the temporary ponding capacity is exceeded, runoff will bypass the raingarden by continuing to flow down the channel.

The drainage layer can be configured as a retention, detention or free draining system to support optimal vegetation growth.

Covered back of kerb raingarden tree pits contain a tree and no understorey vegetation (due to the cover).

Indicative cost

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 – $16,000
Operational cost – $450 per year

Examples of similar systems

Covered back of kerb raingarden tree pits are similar to those currently installed in various locations in the CBD including the south side of Little Collins Street immediately west of Spring Street.

PLANNING AND DESIGN

Key design considerations

Vegetation – Tree only

Filter media depth – At least 600 mm (ideally 700 mm)

Filter media surface area – The raingarden dimensions can be adjusted depending on the space available and the size of the contributing catchment. It is recommended that the minimum dimension is 1 m by 1 m.

The filter media surface area should be 11-30% of the contributing impervious catchment area to ensure adequate soil moisture.

Lining – If a detention or retention outlet is used, the lower 300 mm of the drainage/transition layer has an impervious liner to detain/retain water between rainfall events.

Sides of filter media unlined to allow tree roots to access soil adjacent to raingarden

Filter media soil specification – City of Melbourne Tree Planter Soil Specification (contact Urban Forest team)

Drainage layer – Outlet configured to:

  • detain water (Detail 5, Option 1 with 10mm hole in cap) OR
  • retain water, if agreed to by Urban Forest (Detail 5 Option 1 with sealed cap) OR
  • freely drain (Detail 5 Option 2)

Species section – Refer to Urban Forest Planting Program

Other design elements

  • The filter media surface is set 300 mm lower than the adjacent gutter channel to facilitate temporary ponding of water above the filter surface.
  • No mulch is used on the filter media surface.
  • The cover is configured to allow the diameter of the tree trunk opening to be expanded as the tree matures.
  • Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

MUSIC modelling parameters

The performance of Covered back of kerb raingarden tree pits can be assessed using MUSIC. The recommended modelling parameters are shown in Table 1. The typical performance of a covered back of kerb raingarden tree pit is shown in 2.

Table 1 Recommended MUSIC parameters for a Covered back of kerb raingarden tree pit

Table 2 Typical performance of a Covered back of kerb raingarden tree pit

CONSTRUCTION PHASE

Construction and establishment advice

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:

Handover checklist

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.

Extended detention depth (i.e. height between top of filter media and street gutter) – At least 75% of depth shown on drawings measure using a tape measure.

Filter media condition – No significant mounds or depressions. Visual judgement.

Drainage layer water level (measured via inspection opening) – At least 600mm below filter media

Filter media hydraulic conductivity – Between 50 and 100mm/hr for tree

Tree growth – Average height of trees > 2000mm

Tree condition – Tree healthy and free from disease

ONGOING MANAGEMENT

Maintenance checklist

Recommended routine inspection and maintenance activities for Covered back of kerb raingarden tree pits.

Monitoring specification

Recommended monitoring activities for Covered back of kerb raingarden tree pits.

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