Typology 4 – Large outstand raingarden

Stormwater flow from the driving lane is collected in the spoon drain and directed down grouted rocks onto the filter media surface (refer Detail 3). During large runoff events, the capacity of the spoon drain will be exceeded and driving lane runoff will flow across the parking bays into the gutter channel.

When the raingarden’s temporary ponding depth is exceeded, flows will overtop an overflow weir between the raingarden and the gutter channel (refer Detail 17). The distance between the overflow weir and face of kerb is set to allow sufficient peak flow conveyance.

In the past, many civil contractors have had difficulty constructing kerb outstand raingardens that allow for water from the kerb and channel to flow onto the surface of raingarden. The advantage of this design is that the levels of most roads will enable water from the kerb and channel to flows onto the raingarden surface.

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 capital cost estimate excludes the spoon drain. The indicative cost estimates will be updated as we gain more experience with this typology

Capital cost – $14,000
Operational cost – $300 per year

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 2 m by 2 m.

When a tree is present, 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 

• 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 100 mm higher than the filter media surface to ensure temporary ponding of water above the filter surface.
• Vegetated soil batters are used around the perimeter of the raingarden.
• 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 is set 50 mm below the inlet spoon drain invert (Detail 15) so that if sediment accumulates on the grouted rock it will not impede the flow of water into the raingarden.
• Bollards are used to define raingarden edges and prevent vehicles from driving over the raingarden corners as they enter/exit car parking bays.
• Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

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 2 m by 2 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 100 mm higher than the filter media surface to ensure temporary ponding of water above the filter surface.
• Vegetated soil batters are used around the perimeter of the raingarden.
• 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 is set 50 mm below the inlet spoon drain invert (Detail 15) so that if sediment accumulates on the grouted rock it will not impede the flow of water into the raingarden.
• Bollards are used to define raingarden edges and prevent vehicles from driving over the raingarden corners as they enter/exit car parking bays.
• 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 large outstand raingardens can be assessed using MUSIC. The recommended modelling parameters are shown in Table 1. The typical performance of a large outstand raingarden is shown in 2.

Table 1 Recommended MUSIC parameters for Large outstand raingardens

Table 2 Typical performance of large outstand raingarden 

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.

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

Recommended routine inspection and maintenance activities for Large outstand raingardens

Recommended monitoring activities for large outstand raingardens