Typology 2 – Rectangular back of kerb raingarden

Rectangular back of kerb raingardens are suitable to use when there is at least 1.2 metres of space behind the back of kerb that is not essential for pedestrian movement or other street functions.

Stormwater flows directly from the gutter into the raingarden, percolates through the soil and is collected in perforated pipes in the base. The soil is set down to allow for temporary ponding.

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?

The interface between Rectangular back of kerb raingardens and the adjacent footpath should be designed with consideration of anticipated pedestrian numbers, footpath width and proximity of raingarden to rubbish bins or other street furniture. Detailed design and placement should ensure there is no pedestrian trip hazard.

Stormwater flow from the kerb is directed down grouted rocks onto the filter media surface. When the temporary ponding capacity is exceeded, runoff will bypass the raingarden by continuing to flow down the channel. The drainage layer, at the bottom of the system, can be configured as a retention, detention or free draining system to support healthy vegetation.

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 – $4,000
Operational cost – $100 per year

PLANNING AND DESIGN PHASE

Key design considerations for raingarden with a tree and understorey

Vegetation – Tree and understorey

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 a sealed cap) OR
  • freely drain (Detail 5 Option 2)

Species section 

  • For tree: Urban Forest Planting Program
  • For understorey: Biodiversity Planting Palette and consultation with Parks Maintenance team

Other design elements

  • The overflow weir is set 150 mm higher than the filter media surface to ensure temporary ponding of water above the filter surface.
  • No mulch is used on the filter media surface (because it could float away and or impede the removal of accumulated sediment).
  • The upper edge of the grouted rock strip is set 50 mm below the adjacent gutter channel so that if sediment accumulates on the rock strip it will not impede the flow of water from the channel onto the raingarden surface.
  • Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

Key design considerations for raingarden with understorey only

Vegetation – Understorey only

Filter media depth – At least 300 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 1-2% 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 can be lined

Filter media soil specification – Appendix C of Adoption Guidelines for Stormwater Biofiltration Systems (Payne et al, 2015)

Drainage layer –  Outlet configured to retain water (Detail 5, Option 1 with sealed cap or Option 3)

Species section – Biodiversity Planting Palette and consultation with Parks Maintenance team

Other design elements

  • The overflow weir is set 150 mm higher than the filter media surface to ensure temporary ponding of water above the filter surface.
  • No mulch is used on the filter media surface (because it could float away and or impede the removal of accumulated sediment).
  • The upper edge of the grouted rock strip is set 50 mm below the adjacent gutter channel so that if sediment accumulates on the rock strip it will not impede the flow of water from the channel onto the raingarden surface.
  • 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 Rectangular back of kerb raingardens can be assessed using MUSIC. The recommended modelling parameters are shown in Table 1. The typical performance of a rectangular back of kerb raingarden is shown in Table 2.

Table 1 Recommended MUSIC parameters for Rectangular back of kerb raingarden

Table 2 Typical performance of Rectangular back of kerb raingarden 

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 600 mm below filter media for systems containing trees and at least 300 mm below filter media for systems containing no trees.

Filter media hydraulic conductivity 

  • Between 100 and 300 mm/hr for understorey vegetation
  • Between 50 and 100 mm/hr for tree

Vegetation growth

  • Average height of understorey vegetation > 300 mm
  • Average height of trees > 2000 mm

Vegetation condition – Plants healthy and free from disease

ONGOING MANAGEMENT

Maintenance checklist

Recommended routine inspection and maintenance activities for Rectangular back of kerb raingardens

Monitoring specification

Recommended monitoring activities for Rectangular back of kerb raingardens

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