Northshore City Council Bioretention Guidelines

Transcription

Bioretention Guidelines
First Edition
July 2008
North Shore City Bioretention Guidelines
First Edition
July 2008
PREPARED BY:
Michelle Malcolm of SINCLAIR KNIGHT MERZ
Level 12, Mayfair House, 54 The Terrace, PO Box 10-283, Wellington, New Zealand
T +64 4 473 4265
F +64 4 473 3369
www.skmconsulting.com
and
Mark Lewis of BOFFA MISKELL
Level 3 IBM Centre, 82 Wyndham Street, PO Box 91250, Auckland 1030
T +64 9 358 2526
F +64 9 359 5300
www.boffamiskell.co.nz
EDITED BY:
Chris Stumbles
REVIEWED BY:
Robyn Simcock, Tom Schueler, Earl Shaver
GRAPHICS BY:
BOFFA MISKELL
Level 3 IBM Centre, 82 Wyndham Street, PO Box 91250, Auckland 1030
T +64 9 358 2526
F +64 9 359 5300
www.boffamiskell.co.nz
APPROVED FOR
RELEASE:
__________________________________
Jan Heijs (Infrastructure Planning Manager)
July 2008
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Contents
Glossary
iv
1.
Background
1
2.
What is bioretention and how does it work?
2
2.1
2.2
2.3
2
2
6
3.
Limitations
3.1
3.2
3.3
3.4
4.
5.
6.
7.
ii
History
Bioretention process
Performance and design
8
Geology
Maximum grades
Connection to the stormwater network or receiving environment.
Location
8
8
10
10
Bioretention gardens
12
4.1
4.2
4.3
4.4
12
14
16
18
Rain gardens
Stormwater planters
Tree pits
Bioretention swales
Engineering design
21
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
21
22
22
23
27
29
31
33
Location
Impervious liner
Geotextile liner
Inlet design
Surface storage and high flow overflow/bypass
Soils
Under drainage
Connections
Landscape design
34
6.2
6.3
35
37
Landscape
Plant Selection
Construction
40
7.1
7.2
7.3
7.4
7.5
7.6
40
40
40
40
41
41
Excavation
Timing
Geotextile and liners
Backfilling gravel
Backfill soil
Erosion Checks
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7.7
7.8
7.9
8.
9.
Planting
Tolerances
Construction Checklist
41
42
43
Maintenance
44
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12
44
44
44
45
45
45
45
46
46
46
46
47
Access
Under drain
Fertilizing
Harvesting
Watering
Weeding
Pest damage
Mulching
Standing Water Problems
Rubbish and Debris
Pre-treatment
Maintenance Schedule
References
48
Appendix A Plant Specifications
Appendix B Hydraulic Design
Appendix C Bioretention Growing Media Specifications
Appendix D Typical Details
Appendix E Practice Notes
Appendix F Owners Manual
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Glossary
Adsorption: The gathering of a gas, liquid, or dissolved substance on the surface or
interface zone of another substance.
Bioretention: A vegetated depression located on the site that is designed to collect, store
and infiltrate runoff. Typically includes a mix of amended soils and vegetation.
Evapotranspiration: Loss of water from the soil both by evaporation and by transpiration
from plants.
Filtration: The process of removing particulate matter from water by passing it through a
porous medium such as sand
Flow regime: The pattern and volume of river or stream flow throughout the course of a year.
Hydrostatic: A term associated with fluids at rest or to the pressures they exert or transmit.
Hydraulic conductivity: The rate at which water can move through a permeable medium.
Infiltration: Water movement into the soil.
Microbes: Microscopic living organisms, including bacteria, protozoa, viruses, and fungi.
Microbial: Of, relating, or caused by microorganisms.
Permeability: Water movement through the soil.
Percolation: Water movement into the groundwater.
Sedimentation: The settling of solids in a body of water using gravity.
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1.
Background
Protection of the natural environment of North Shore City has been identified by the
community as its number one priority. Of particular concern is the health of streams within the
city and protection of these receiving environments from the effects of stormwater
discharges.
Urban stormwater runoff has adverse effects on the ecological, recreational and amenity
values of stream corridors. Urban development adds hard surfaces to catchments, creating
increased levels of runoff in storm events, whilst also reducing base flows during dry weather,
due to reduced ground soakage of rain water. This additional runoff is conveyed rapidly to
streams in piped stormwater systems. This change in flow regime results in: increased
stream flows, scouring of stream banks, a reduction in stream biodiversity and opportunities
for habitats, and degradation of amenity and recreation values.
Urban development also creates increased contaminant loads which are transported in storm
water runoff to urban streams. This results in the reduction of the life supporting capacity of
urban streams and the rendering of urban streams as unsuitable for contact recreation.
Bioretention gardens are engineered gardens designed to harness the natural ability of
vegetation and soils, they can be used to reduce stormwater volumes, peak flows and
contaminant loads, which result from the urbanisation of streams.
This guidance offers design, construction and maintenance advice to enable the construction
of bioretention gardens that are effective, attractive and enduring. In many locations where
conventional gardens would be used, bioretention gardens can be used instead, and could
include small herbaceous gardens within private sections, modern landscape planting within
commercial sites or tree pits within the streetscape. The widespread adoption of bioretention
gardens for the management of urban runoff will contribute to North Shore City Council’s
vision for attractive, landscaped catchments draining to healthy urban streams.
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2.
What is bioretention and how does it work?
2.1
History
Pioneered in Maryland USA in the early 1990s, bioretention gardens are now used widely
throughout the USA, Europe, Australia and New Zealand. TP10 has included rain gardens for
the treatment and attenuation of urban stormwater in the Auckland region since 2003.
Bioretention devices are no longer experimental technology - they have been used
successfully throughout the world for over fifteen years. Over this period, lessons have been
learnt on how best to manage urban stormwater through the use of bioretention.
This guidance document brings together design advice from guidance produced in New
Zealand, Australia and the United States, as well as research on the performance of
bioretention devices undertaken both in New Zealand and overseas, and translates this
information so it is relevant for designing, constructing and maintaining bioretention devices
within North Shore City.
This guidance is aimed specifically at the design of bioretention gardens that serve new
impervious areas less than 1000m2. Bioretention gardens serving new impervious areas
greater than 1000m2 must be designed to meet TP10 design standards 1 .
2.2
Bioretention process
Bioretention systems are planted areas that filter stormwater runoff through a vegetated soil
media layer. Water is then collected through perforated pipes at the base of these systems to
be directed to an approved outlet.
Bioretention systems slow stormwater flows, and allow for some reduction in the total volume
of runoff by transpiration and infiltration. Bioretention gardens are designed to capture all of
the stormwater from small storms, and the initial stormwater flow from larger storms. The
remaining flow from large storms that overtops bioretention systems leads to a piped
stormwater system or overland flow paths.
Bioretention systems remove suspended solids by filtration through vegetation, these solids
then settle within the ponded surface. Microbial processes occur at the interface of plant roots
and soil media to intercept, metabolise and sometimes transform a range of pollutants.
1
Auckland Regional Council , 2003 Technical publication 10, Design Guideline Manual: Stormwater Treatment
Device
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This guidance document is focused on the following types of bioretention gardens:

Rain gardens

Stormwater planters

Tree pits, and

Bioretention swales.
These bioretention gardens vary in scale and application. The specific design aspects and
applications of each of these gardens are discussed in detail in section 4. The principals that
govern the performance of bioretention gardens are common, and are discussed below in
sections 2.2.1 to 2.2.4
2.2.1
Evaporation and transpiration
Bioretention gardens reduce the volume of storms through transpiration and evaporation. The
plants in bioretention gardens use some of the rainwater that is directed into the rain garden,
and it is transpired back into the atmosphere. The ponding of stormwater on the surface of
bioretention gardens is shallow, generally 200mm – 300mm, which facilitates the evaporation
of some of this ponded water into the atmosphere.
2.2.2
Groundwater recharge
If bioretention gardens are situated on relatively flat, stable slopes, and are not within the
zone of influence of a structure, they do not require an impervious liner. This enables some of
the stormwater directed to the bioretention garden to percolate into the groundwater.
Disposing of a portion of stormwater runoff to soakage, reduces stormwater peak discharges
and runoff volumes to downstream catchments, and increases groundwater flows to augment
seasonal water tables in streams.
2.2.3
Reducing peak discharge
Increased imperviousness results in increased peak flows due to less water being lost to
evaporation and infiltration and a reduced time of concentration. Bioretention gardens are not
designed to provide peak flow mitigation in large events. The benefit of bioretention gardens
for peak flow mitigation is in small events, where increases in peak flows can result in stream
erosion and changes to stream habitat. The temporary detention storage provides attenuation
of flows therefore reducing post-development storm peaks in small events. The depth of
water detained on the surface of a garden should be limited to 200-300mm (plus 100mm
freeboard) across the garden’s surface area. All bioretention gardens should be designed to
enable flows from large storm events to either bypass or overflow.
2.2.4
Water quality treatment
Bioretention gardens remove pollutants using physical, chemical, and biological mechanisms.
Specifically, they use adsorption, microbial action, plant uptake, sedimentation and filtration.
In addition, bioretention gardens sited in appropriate soils can be designed to infiltrate
stormwater runoff, thus replenishing groundwater. In the Auckland region the focus of
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stormwater quality management is on the removal of total suspended solids and metals.
Bioretention gardens are effective at removing total suspended solids, metals and nutrients.
Adsorption
Adsorption is a chemical process that removes some forms of metals and phosphorus. The
process takes place on mulch and soil particles in the upper layers of the bioretention garden.
Soil particles have charges similar to magnets, as do dissolved metals and soluble
phosphorus. When these charges are complementary, dissolved metals and phosphorus are
attracted to the open soil particles. This process is called adsorption. The limit to adsorption
is the finite number of charged soil particles within a bioretention garden. Researchers
monitoring bioretention gardens in Maryland, USA, suggest that the capacity of soil to retain
pollutants could last for more than 10 years 2 . Re-spreading decomposed mulch at the
surface of bioretention gardens could help to replenish the soil’s adsorptive capacity.
Microbial action
Microbes found in bioretention gardens break down organic substances and digest harmful
pathogens. Microbes are found throughout a bioretention garden but occur most commonly at
the interface between soil and plant roots. Plant roots provide a medium and a source of
oxygen for these microbial processes to occur.
The inherent design of bioretention gardens requires them to dry out quickly, and this also
helps to remove pathogens, which typically prefer wet conditions.
Plant uptake
The vegetation in bioretention gardens uses the nutrients found in stormwater as it grows.
Plants also take up metals, organics and other pollutants to be used by the plant, stored as a
by-product in specialised cells, or transformed through enzymatic action by plant cells. Plant
litter can contribute to nutrient loads because decaying vegetation does release nutrients and
stored contaminants back into the bioretention garden. Regular removal of plant litter from a
bioretention garden, including the coppicing, pruning or heading of plants, should keep this
problem to a minimum.
Sedimentation and filtration
Sedimentation and filtration are physical processes that remove soil particles, litter, and other
debris from water. This is achieved by slowing water down inside the bioretention gardens
allowing the settlement of suspended particles. Because the inflow to the bioretention garden
passes through vegetation and mulch layers, pollutants can be filtered within the spaces
between soil particles. Plants also offer some filtration as water passes through them.
2
North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local
governments cooperating: Designing Rain Gardens (Bio-Retention Areas)
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Sedimentation and filtration are primary mechanisms for removing total suspended solids
(TSS), litter, debris and nutrients and metals attached to sediment particles.
Table 1 Pollutant removal mechanisms 3

Pollutant Removal Mechanism
Pollutants
Adsorption to soil particles
Dissolved metals and soluble phosphorus
Plant uptake
Small amounts of nutrients including phosphorus and nitrogen
Microbial processes
Organics, pathogens
Sedimentation and filtration
Total suspended solids, floating debris, trash, soil-bound
phosphorus, some soil bound pathogens, soil bound metals
Figure 1: Bioretention processes
3
Brix, H. 1993. Wastewater treatment in constructed wetlands system design, removal processes, and treatment
performance. Pp. 9-22 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed). Boca Raton,
Fla.; CRC Press, 632 pp.
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2.3
Performance and design
Bioretention gardens should be designed to capture the first flush of rainfall. TP10 defines the
rainfall depth used for calculating the first flush in Auckland as 1/3 of the 2 year, 24 hour
rainfall depth. For the North Shore this equates to 26.6mm.
The sizing for the bioretention devices in North Shore City is provided within the Proposed
District Plan Change 22. To meet the permitted standard, devices must be sized as follows:

Bioretention devices that do not discharge to a pond designed to meet TP10 standards
must have a surface area of 8% of the increased impervious area (excluding any
additional roof area that is treated by a rainwater harvesting system).
Bioretention devices that discharge to a pond designed to meet TP10 standards must
have an area of 5% of the increased impervious area (excluding any additional roof area
that is treated by a rainwater harvesting system).
For commercial areas 4m3 of on-site detention must be provided per 100m2 of impervious
area less the rainwater harvesting volume (which should be a minimum of 2m3). This can
be provided as detention storage over the bioretention device, or provided by a separate
device.
The minimum size of bioretention to be provided in accordance with any permitted,
controlled or limited discretionary activity shall be 2m², with a minimum depth of at least
600mm.
The surface area of a bioretention garden is more important than the bioretention garden’s
volume for achieving stormwater volume reduction, peak flow attenuation and water quality
treatment. The method of sizing bioretention devices provided in Plan Change 22 is based on
an equation provided by the North Carolina Natural Resources Conservation Service 4 , which
calculates for an entirely impervious catchment, a bioretention device with a surface areas
sized at 8% of the contributing catchment area in order to capture a first flush of 26.6mm. To
ensure bioretention gardens achieve optimum performance, where ever possible bioretention
gardens should be located to minimise the pervious catchment draining to them.
The side slopes of a bioretention garden do not need to be vertical, and for construction
purposes battered slopes may be desirable. However to ensure sufficient contact between
the soils and stormwater runoff battered sloped should not exceed 1:1. In addition the surface
area of the garden for the purposes of meeting the District Plan criteria, applies to the surface
above a depth of 300mm.
4
North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local
governments cooperating: Designing Rain Gardens (Bio-Retention Areas)
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The North Shore City Council’s Plan Change 22 applies to developments less than 1000m2.
Bioretention gardens serving new impervious areas greater than 1000m2 must be designed to
meet TP10 design standards, which are slightly different. 5
The depth of bioretention gardens is controlled by the practicalities of providing bedding for
the under drainage, sufficient soil depth to support vegetation, and sufficient ponding depth
for detention. If desired, an additional layer of storage can be provided beneath the underdrain to increase the amount of infiltration achieved, which further increases the overall depth
of the garden.
Table 2 illustrates that most bioretention gardens designed to meet the permitted standard as
proposed by Plan Change 22, will have a minimum soil depth of close to 0.6m. Provided
these gardens are designed with adequate surface area, they are likely to provide the same
level of water quality treatment as a deeper garden.
Table 2 Typical depth for bioretention layers
Bioretention layer
Depth
Comments
Detention
Layer
Detention layer
0mm -400mm (including
100mm freeboard for over-flow
designs)
200mm – 300mm of detention should be
applied. The overflow should be
designed to discharge high flows with
100mm freeboard.
Bioretention
layer
Mulch layer
50-75mm
Organic decomposed mulch, if a rock
surface finish is desired this is additional.
Bioretention filter
media
500 - 1000mm
300 mm minimum soil depth required for
grasses and small shrub, 1m depth is
minimum required for trees.
Transition Layer
100mm
Sand/ coarse sand.
Drainage layer
200 - 300mm
A minimum 50mm gravel surrounding
the pipe on all sides.
Storage depth
beneath the underdrain
0mm – 300mm
Optional layer to provide greater
infiltration.
Drainage layer
5
Device
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3.
Limitations
Bioretention gardens are an important tool in stormwater management practices, as they can
be used throughout urban catchments to mitigate the effects of urban stormwater discharges.
However, bioretention gardens may not be suitable on all sites, and require particular
attention to their design to ensure they will not result in adverse effects.
3.1
Geology
North Shore City is predominately underlain by two geologies, residual soils of the Waitemata
Group of Miocene age and alluvial deposits of Pliocene to Holocene age. However, the
Takapuna and Milford areas are an exception as they are underlain with volcanic deposits.
The Waitemata Group soils are typically clays and silts with low permeabilities. The deposits
have formed from in-situ weathering of the parent rock. Many of the slopes and cliffs in North
Shore City are formed by Waitemata Group soil and rock.
The lower lying areas are generally underlain by alluvial deposits. The alluvial deposits are of
varying soil type including clays, silts and sands and consequently varying permeability.
The volcanic deposits are ash, tuff and basalt and form part of the Auckland Volcanic Field.
When soils are present they generally comprise silts and sands which are more permeable
than the Waitemata Group residual soils. The basalt deposits will typically require rock
breaking or controlled explosion to excavate. The basalt is generally fractured and vesicular,
providing high, apparent permeability. Disposal of stormwater into basalt should be
considered.
While there is none mapped within North Shore City, there is potentially in the northern most
parts some Northern Allochthon (previously known as Onerahi Chaos Breccia). If the site is
found to be underlain by this geology a specific assessment of the use of bioretention by a
geotechnical engineer should be made as these soils are known to creep even at gentle
gradients especially in poorly drained sites.
Localised areas of fill, where fill is either imported of re-worked material, are present across
North Shore City. The nature and quality of fill will vary greatly. If significant quantities of fill
are present on-site then the suitability of locating a bioretention garden within the fill should
be specifically determined by a suitably experienced engineer.
3.2
Maximum grades
Infiltration gardens should not be installed on steep slopes as this may lead to saturation and
slope failure. Installation of unlined bioretention gardens that allow for infiltration on slopes
steeper than 1V:5H is not recommended, unless a detailed geotechnical engineering analysis
is undertaken at the design stage.
For slopes between 1V:5H and 1V:4H a lined bioretention garden may be used. The liner
should be an impermeable sheet that prevents water in the bioretention garden from
saturating the surrounding soils.
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A bioretention garden may be used on slopes steeper than 1V:4H if the effects have been
assessed by a Chartered Geotechnical Engineer, who recommends the use of such a device.
Under North Shore City Council’s ‘Infrastructure Design Standards’, section 2.4.2 (d)
‘Analysis must be carried out where the slope is steeper than 1V: 4H’. Practically, this has
meant that a geotechnical report has been required for any building consent application for a
new structure or addition to a structure that includes additions outside of the existing
structure. Consequently, the use of bioretention gardens should be included as part of a full
geotechnical report for site development.
Lined bioretention gardens are required for sites that are part of an overall sloping terrain. For
larger sites, lined or unlined bioretention gardens can used provided they are at least 5m
upslope from the rest of slope.

Table 3 Maximum slope
Slope Inclination
Liner
Less than 1V:5H
No
1V:5H - 1V:4H
Yes
Steeper than 1V:4H
Yes *
•

Use of bioretention gardens with slopes steeper than 1V:4H is subject to specific
geotechnical analysis and design.
Figure 2: Maximum slope
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3.3
Connection to the stormwater network or receiving environment.
All bioretention gardens except those designed for soakage will have to be located so the
invert of the garden can drain via gravity to the public stormwater system or the receiving
environment, via an approved outfall or overland flow path.
3.4
Location
Bioretention gardens should be located away from travelled areas such as public pathways to
avoid compaction. Where ever possible the bioretention gardens should be located to
minimise the pervious areas draining to them, and therefore they should not be located in
overland flow paths. The sizing of the bioretention garden must take into consideration the
potential contributing catchment for the calculation of the design storm capacity of the
garden.
3.4.1
Set back
Bioretention gardens should ideally maintain a 1m minimum distance from property lines.
Bioretention gardens must not be installed within the zone of influence of foundations or
within 3m of the edge of any structure, with the exception of stormwater planters, which are
designed to abut buildings. If a bioretention garden is installed upslope and within 6m of a
structure it should be lined (may only need to be lined one side) to prevent potential
saturation of the foundation soils. These distances may be reduced on the advice of an
engineer.
It is recommended that bioretention gardens installed adjacent to roads have an impermeable
lining on the side adjacent to the road, to prevent stormwater migrating from the garden into
road sub-grade. In addition, while a concrete wall structure is unlikely to be required around
the whole garden, it is advisable to use a concrete edge beam or wall to provide support on
the side adjacent to the road.
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Figure 3: Setback limitations
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4.
Bioretention gardens
4.1
Rain gardens
Rain gardens are planted garden beds with a specifically formed porous soil media. In most
situations rain gardens are directly connected to impervious surfaces, although sometimes
there is an intermediary filter strip or rock apron to reduce scouring or to capture entrained
sediment. In some situations where it is not possible to directly connect the rain garden to the
impervious areas, stormwater may be piped into the garden.
As stormwater enters the rain garden it is filtered through plants specifically selected to
tolerate the hydrologic conditions and to provide water quality treatment. The stormwater then
receives additional treatment as it permeates through an organic mulch layer, the root zone
of the plants, and through a sequence of soil layers. These soil layers are organic in the top
layers, such as a sandy loam enriched with compost, followed by porous sandy soil, to a
gravel drain with a transition layer. Treated water in the gravel layer is then collected via
perforated pipes. These pipes flow to an approved outlet to enter the receiving environment
or reticulated systems.
As well as filtering and infiltrating stormwater, rain gardens also provide temporary ponding
on the surface of the rain garden. Storm events that are greater than the design storm,
overflow from the rain garden into a grated overflow and connect to the reticulated system at
the base of the rain garden. Alternatively, excess stormwater may overflow from a rain
garden to an overflow path or a sequence of stormwater management devices in a treatment
train.
Rain gardens can be in used in new developments or retrofitted to post-development
conditions. They are suitable for site specific applications, serving single dwellings or
commercial premises. They can also be designed to serve larger catchments, and be located
within roading reserves or car parks.

Table 4 Rain Garden Design Summary
2
Minimum size
2m
Minimum depth
600mm + under drain +detention
Slope limitations
Runoff Type
Slopes 1:12 – 1:4 incorporate benched berms
Slopes 1:5 use an impervious liner

Slopes 1:4 and greater are not suitable without
geotechnical design
Roof and surface runoff
Applications
Single residential lot, commercial lots, roadways and carparks

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Figure 4: Rain garden
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4.2
Stormwater planters
These gardens are essentially planter boxes (e.g. an above-ground pre-cast concrete unit)
with a specifically formed soil media in which plants are grown. Stormwater planters operate
as follows:
1) Roof water is discharged into the planter from a downpipe, this can either be via surface
discharge or a bubble-up inlet.
2) The ‘first-flush’ of stormwater infiltrates soil layers and is then collected in a drainage
layer to be directed to a discharge point.
3) Ponding occurs as soils become saturated to the top-of-wall level in the planter box. This
storage serves to further attenuate flows. An outlet rise comes into operation when the
ponding capacity is full. Excess runoff, after the ‘first flush’ has been retained is
discharged through the outlet riser and standpipe to reticulated systems.
4) If planters are adjacent to buildings they should be above ground. Stormwater planters
can be partially sunk, but if they are within 3m of a buildings foundation, this should only
be undertaken based on the advice of an engineer.
5) The device should have a horizontal surface

Table 5 Bioretention Planter Design Summary
2
Minimum size
2m
Minimum depth
600mm + under drain +detention
Slope limitations
Runoff type
Slopes 1:4 and greater are not suitable without geotechnical
design
Roof runoff
Applications
Residential and commercial
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Figure 5: Bioretention Planter
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4.3
Tree pits
Bioretention gardens can be constructed to accommodate street trees. Tree pits are similar to
rain gardens, except they require a greater surface area and/or soil depth to accommodate
tree growth. Trees should be planted a minimum of 1 meter away from any perforated pipe
under-drain and a root barrier may also be required.
Stormwater runoff is collected in the tree pit where temporary ponding occurs. Water
infiltrates through the bioretention filter media before being collected by an underlying
perforated pipe for subsequent discharge to a stormwater system.
In most situations it should be possible to design the tree pits so larger flows bypass the tree
pit and are conveyed downstream by the curb and channel to the nearest road sump. In
situations where this is not possible the tree pit should have an overflow within the garden to
convey larger flows into the piped stormwater system.
Additional benefits can be achieved for the establishment of trees if the tree pits can be
extended as linear trenches. Paving can be placed over the top of the linear soil trench.
Tree pits do not require concrete lined walls, although the use of a concrete edge for support
on the road side is recommended. The tree pit does not need to be completely lined with an
impermeable lining, but on the side adjacent to the road it is advisable to provide an
impermeable liner to prevent stormwater from migrating from the bioretention filter media into
road subgrade.

Table 6 Tree Pit Design Summary
2
Minimum size
2m , although many trees will require a larger area
Minimum depth
1m + under drain + detention
Slope limitations

Runoff type
Slopes 1:12 – 1:4 incorporate bench berms,
Slopes 1:5 use an impervious liner,

Slopes 1:4 and greater are not suitable without
geotechnical design
Surface runoff
Applications
Roadways and carparks

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Figure 6: Tree Pit
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4.4
Bioretention swales
Bioretention swales provide both stormwater treatment and conveyance functions by
incorporating specific plants and soil media into a conventional swale design. A swale
component provides pre-treatment of stormwater to remove coarse to medium sediments,
while the bioretention function removes finer particulates and associated contaminants.
Bioretention swales attenuate the flows of frequent storm events and are particularly efficient
at removing nutrients. The bioretention component of the swale can be located along the
length of the swale or closer to an outlet.
To design the system, separate calculations are required for the swale and the bioretention
component to ensure appropriate criteria are met in each section.
Flow needs to be uniformly distributed over the full surface area of the filter media to achieve
maximum pollutant removal performance. Swale design should incorporate a flow-spreading
device at the inlet such as a shallow weir across the channel bottom or a stilling basin.
When the bioretention trench is located along the full length of the swale base, the desirable
maximum longitudinal grade is 4%. To ensure stormwater has sufficient time to filter into the
bioretention layers, check dams should be used along the swale length.
A common way to design bioretention swales is to use a system of discrete cells, with each
cell having an overflow pit that discharges to the piped stormwater system. Bioretention
systems can then be designed upstream of the overflows, thus allowing for a depth of
ponding over the bioretention medium.
When the bioretention trench is located at the most downstream part, the swale part should
have a grade of between 1% and 4%, if the grade of the swale is greater than 4% check
dams must be used to prevent scour of the swale. The desirable grade of the bioretention
zone is horizontal, to encourage uniform distribution of stormwater flows over the full surface
area of the bioretention filter media and to allow for temporary storage of flows for treatment
before bypass occurs.
When check dams are included in swale design to facilitate the creation of discrete cells,
consideration must be given to potential conflicts with pedestrians or mowers
The type of vegetation varies according to the landscape requirements. Generally, the denser
and higher the vegetation within the swale, the greater the filtration provided. It may not be
possible to mow bioretention swales and therefore native grasses, tussocks and sedges are
likely to more appropriate than lawn grass species. Occasional tree planting may occur as
long as it complies with acceptable sight lines and safety requirements, and is located at the
top of the bioretention swale to avoid the roots damaging the bioretention component.
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Table 7 Bioretention Swale Summary
Maximum bottom width
2m
Maximum side slope
1:3
Minimum depth
600mm + under drain + detention
Slope limitations
Longitudinal slopes between 1 – 4%
Runoff type
Surface runoff
Applications
Roadways and carparks
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Figure 7: Bioretention Swale
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5.
Engineering design
5.1
Location
Bioretention gardens should be located away from travelled areas such as public pathways to
avoid compaction. Where ever possible the bioretention gardens should be located to
minimise the pervious areas draining to them, and therefore they should not be located in
overland flow paths. The sizing of the bioretention garden must take into consideration the
potential contributing catchment for the calculation of the design storm capacity of the
garden.
Access needs to be provided to ensure that the bioretention garden can be maintained in
future.
5.1.1
Set back
Bioretention gardens should ideally maintain a 1m minimum distance from property lines.
Bioretention gardens must not be installed within the zone of influence of foundations or
within 3m of the edge of any structure. If a bioretention garden is installed upslope and within
6m of a structure it should be lined (may only need to be lined one side) to prevent potential
saturation of the foundation soils. These distances may be reduced on the advice of an
engineer.
It is recommended that bioretention devices installed adjacent to roads have impermeable
lining, to prevent stormwater migrating from the bioretention filter media into road subgrade.
In addition, while a concrete wall structure is unlikely to be required around the whole device,
it is advisable to use a concrete edge to provide support on the side adjacent to the road.
If trees are to be planted within gardens consideration should be given to over-head setbacks to ensure that mature trees do not interfere with power lines or other utilities.
5.1.2
Road reserve
Bioretention gardens are typically constructed within the parking lane and verge of road
reserves. This can potentially result in conflicts with existing or future services (both above
and below ground). Services do not preclude the use of bioretention gardens, however,
disturbance from periodic or ongoing maintenance of services may severely reduce the
functionality of the system. It is recommended that all services shall be clearly shown on
design plans and detailed on “as-built” drawings to ensure that subsequent maintenance
does not cause problems.
Road safety requirements must be taken into account when locating bioretention gardens
within roadways, including clearways and acceptable verge gradients. It may be desirable to
locate bioretention gardens within roundabouts. However, consideration will need to be given
to ensuring the bioretention gardens are planted with sufficiently tall vegetation to ensure the
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roundabout is visible, and that the cross-fall required to drain the road to the roundabout, is
acceptable in the context of the road’s design traffic.
5.1.3
Existing retaining walls
Bioretention gardens should not be installed so that they are above a 1V: 1H plane taken
from the toe of any retaining wall to the ground surface retained behind it. If a bioretention
garden is installed within this zone a specific design should be undertaken for the retaining
wall, as it may be subjected to surcharge loading.
Care should be taken to ensure that bioretention gardens are not short-circuited by nearby
retaining wall drainage blankets. Drainage blankets for retaining walls are typically not
designed or capable of handling significant quantities of stormwater, and locating the garden
in such a manner could lead to hydrostatic loading of the retaining wall.
5.2
Impervious liner
Bioretention gardens are intended to assist infiltration and recharge of groundwater where
possible, and therefore, in many cases bioretention gardens do not need to be lined. On
stable sites infiltration into the soil will reduce stormwater flows and recharge groundwater
without causing adverse effects.
In some situation impervious liners must be used. See chapter 3 for a full discussion of slope
limitations and the requirement for liners for devices located on sloping ground and near
buildings.
Where the bioretention garden lies within close proximity to infrastructure such as building
foundations or a road, an impermeable liner is likely to be required.
A liner must be installed in any bioretention garden situated on slopes steeper than 1V:4H.
The liner should be impermeable and prevent water retained in the garden from saturating
the natural soils. If an impervious liner is required then geotechnical advice should be
obtained, and as a minimum a 0.25mm thick polypropylene liner should be used.
In most cases, it is not necessary to use concrete lining for bioretention gardens. Exceptions
to this may be stormwater planters which are raised above the surrounding ground level and
concrete edging as support for devices installed adjacent to roadways.
5.3
Geotextile liner
The use of a permeable geotextile to line the base and walls of the bioretention garden may
be used to reduce the migration of in-situ soil particles into the bioretention filter media
thereby extending the life of the garden. The liner should be a light weight, non-woven,
needle punched geotextile.
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Geotextile liners shall not be used between layers, and the perforated pipe shall not be
socked. A transition layer of finer gravel between the soil and gravel surround will prevent soil
from entering the perforated pipe.
5.4
Inlet design
Bioretention gardens require design features so that either:
1) The catchment falls towards the garden where stormwater is captured as distributed flow
(particularly applicable for swales), or
2) The flow will enter the garden at concentrated discharge points, through kerb and
channel, swale, or piped systems.
Advice on the hydraulic aspects of inlet design is provided in Appendix B.
5.4.1
Pre-treatment
Once a bioretention area exceeds about 50 square meters in area, it will require a structural
form of pre-treatment to trap sediments, litter and debris. In these situations the pretreatments should involve a two cell design, with the first cell designed as a forebay, with a
500mm ponding depth before spilling over to second cell, which is designed in the standard
manner for a bioretention garden. In most cases, bioretention gardens are likely to be smaller
than 50 square meters.
In addition, for catchments such as roadways, carparks and commercial sites, where runoff is
likely to have a high contaminant load, the use of pre-treatment upstream of the bioretention
device should be considered to reduce the maintenance requirements and extend the life of
the bioretention garden. Pre-treatment can include a grass filter strip or a small forebay. For
some sites, it may be appropriate to consider using a gross pollutant trap or other engineered
device upstream of the bioretention garden
5.4.2
Distributed inflow
An advantage of flows entering a bioretention swale system in a distributed manner (i.e.
entering perpendicular to the direction of the swale) is that stormwater enters as shallow
sheet flow, which maximises contact with vegetation, particularly on the batter receiving the
distributed inflows. This batter slope is often referred to as a filter strip. The filter strip requires
dense vegetation to function most efficiently and requires shallow flow depths below the
vegetation height. The filter strip provides good pre-treatment (i.e. significant coarse sediment
removal) prior to flows being conveyed along the swale.
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Figure 8: Distributed inflow examples
5.4.3
Concentrated inflow
Concentrated inflows to a bioretention garden can be in the form of a concentrated overland
flow or a discharge from a piped drainage system. For all concentrated inflows, energy
dissipation at the inflow location is an important consideration to minimise any erosion
potential. For small gardens this can be achieved with rock benching and/or dense
vegetation, for larger gardens a flow distribution weir or small forebay may be required.
For bioretention gardens serving roads or carparks, inlets are typically formed from a cut-out
of the kerb. The width of the opening is governed by the design flow rate entering the system.
Kerb inlets aligned perpendicular to the flow path should be designed using the broad-crested
weir approach. However, where the inlet is orientated parallel to the flow path, the length of
opening must be increased (or multiple inlets used) to minimise the potential for bypass of
design flows. The shape of the inlet can also greatly affect the behaviour of both low and high
flows. Desirable attributes of a kerb inlet are provided below:

Rounded or tapered kerb edges (with sufficiently large radius for the design flow rate).
Concrete apron with a grade of approximately 10% to prevent localised ponding and
sediment build-up on the road.
Energy dissipation at the toe of the apron using grouted and/or wire mesh encased rock
(spacing of rock should not create channelled flow).
Flow diversion using raised structures within the kerb and channel should not be used as
this poses a potential hazard to bicycles and motor vehicles.
Where flush kerbs are to be used, a set-down from the pavement surface to the vegetation is
to be adopted. This allows a location for sediments to accumulate that is off the pavement
surface. Generally, a set down from kerb of 60 mm to the top of vegetation (if turf) is
adequate. Therefore, total set-down to the base soil is approximately 100mm (with turf on top
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of base soil). This set-down can be part of the set-down required for the provision of
detention storage above the surface of the bioretention garden.
Another important form of concentrated inflow in a bioretention swale is the connection with
the bioretention component, particularly where it is located at the downstream end of the
overlying swale and receives flows concentrated within the swale. Depending on the grade
and its top width and batter slopes, the resultant flow velocities at the transition from the
swale to the bioretention filter media may require the use of energy dissipation to prevent
scour of the bioretention filter media. The best method of achieving this is the use of a level
weir structure that reduces slope and distributes flow to the bioretention filter.

Figure 9: Concentrated inflow examples
5.4.4
Inlet scour protection
It is good practice to provide erosion protection for flows as they enter a bioretention garden.
Typically, flows will enter the bioretention garden from either a surface flow system (i.e.
roadside kerb, open channel) or a piped drainage system. In most cases, these flows will
enter a bioretention garden as ‘concentrated’ and as such, it is important to effectively slow
and spread the inflows. Rock beaching is a simple method for achieving this.
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Figure 10: Inlet scour protection
5.4.5
Inlet planting
The surface of the bioretention garden immediately downstream of the inlet should be
densely planted with vegetation to create a localised sediment and litter deposition area for
ease of maintenance. Pre-treatment areas also act to dissipate energy and spread flows prior
to contact with the bioretention filter surface, reducing scour potential.
5.4.6
Surcharge riser inlets
The most common constraint on pipe systems discharging to bioretention gardens is bringing
the pipe flows to the surface of a garden. In situations where ‘free’ discharge of the pipe to
the surface of the bioretention garden is not possible, a ‘surcharge’ riser can be used.
Surcharge riser inlets are a good solution because they prevent discharge water entering the
garden in manner that is likely to cause erosion.
However surcharge riser inlets can result in standing water, this is especially a risk in the clay
soils of the North Shore. Riser inlets can be designed so that they are as shallow as possible
and have pervious bases to avoid long term ponding in the pits. Where possible the riser
inlets should be designed so that water which seeps from the bottom or sides of the
surcharge pit is routed through at least part of the bioretention garden, and then is drained by
the bioretention under drain, to prevent short-circuiting of the bioretention garden, which
could undermine the treatment provided by the garden.
The riser inlets need to be accessible so that any build up of coarse sediment, and debris can
be monitored and removed as necessary.
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Figure 11: Surcharged riser inlet
5.5
Surface storage and high flow overflow/bypass
Advice on the hydraulic aspects of surface storage and overflow and bypass design is
provided in Appendix B.
5.5.1
Ponding storage
Ponding of stormwater above the surface of the bioretention filter media promotes settling of
coarse to medium sediments. The detention depth is controlled by the kerb inlet level (for
offline systems) and the bypass inlet level (for online systems).The detention depth should be
approximately 200 - 300mm, with an additional 100mm freeboard.
Batter slopes around the bioretention are preferable to steep or vertical sides. Bioretention
gardens installed immediately behind a roadside kerb where batter slopes cannot be
incorporated must have a 300mm wide concrete kerb support.
Mosquitoes need at least 4 days of standing water to develop as larva. The soil specification
has a minimum infiltration rate of 1.2m per day, if the maximum detention depth of 400mm is
applied, water should only stand in the bioretention garden for a few hours.
5.5.2
High flow bypass
In most cases a dedicated high flow bypass inlet will need to be incorporated into any
bioretention garden’s design. This high flow bypass inlet may be configured in following two
ways:

A grated riser within the detention zone, to convey flows in excess of the first flush into
the public stormwater system.
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An inlet designed to enable flows in excess of the first flush to bypass the bioretention
garden, and be conveyed into the public stormwater system. Where possible this is the
preferred option because it reduces potential damage to gardens in large events.
All types of high flow bypass inlets must be non-blocking to minimise the risk of flooding. In all
cases, a protected overland flow path is required to safely carry away excess flows to another
stormwater treatment device, or to an approved overland flow path for flows in excess of the
10% AEP in residential development and the 5% AEP for commercial areas.

Figure 12: Overflow options

Figure 13: High flow bypass illustration
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5.6
Soils
The soil used for bioretention gardens has an important role in water quality treatment, water
attenuation, and supporting associated vegetation. The soil of bioretention gardens must be
permeable enough to allow runoff to filter through the garden, whilst being able to promote
and sustain a vegetative cover. Soils must balance chemical and physical properties in order
to support biotic communities above and below ground.
5.6.1
Hydraulic conductivity
The saturated hydraulic conductivity of the bioretention garden should be between 50mm –
300mm/hr.
This range provides sufficient water retention to support vegetation and sufficient drainage to
ensure that the first flush of runoff from the catchment can be passed through the bioretention
filter media, rather than bypassing via the overflow.
TP10 has a minimum hydraulic conductivity rate of 12.5mm per hour 6 , however this rate has
become a default target. The bioretention filter media North Shore City are recommending is
more free-draining, and is consistent with the Australian Facility for Advancing Water
Biofiltration’s soil media specification, which requires a hydraulic conductivity rate of between
50 – 300mm/hr. 7
Australian research on the performance of bioretention gardens 8 , has found that of 12 sites
tested, 55% had infiltration rates below the desired minimum of 88mm/hr and 45% below
40mm/hr. This research indicates that poorer hydraulic conductivity than planned is often the
achievement. This is likely to be the result of one or more of the following:

The bioretention filter media does not fulfil the specification,

Compaction at the time of construction, and/or

The ingress of fines over time.
6
Device
7
Facility for Advancing Water Biofiltration, 2006, Bioretention and Tree pit media specification
8
Land and Water Constructions, 2006 Kingston City Council and Better Bays and Waterways - Institutionalising
Water Sensitive Urban Design and Best Practice Management Project Review of street scale WSUD in Melbourne
– Study Findings
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5.6.2
Bioretention filter media
Bioretention performance is maximised by using soils with high carbon content, low fertility
and high phosphate retention. Such soils have high water storage, aeration and ability to
remove metals, and low risk of nitrogen and phosphorus leaching. The specifications for the
best planting soil are listed below:

A mix of compost (approx. 30%), topsoil (approx.30%)and sand (approx.40%).
A uniform mix, free of stones, stumps, roots or other similar objects. 90-100% of the
medium should pass through a 10mm sieve and 97-100% should pass through a 25mm
sieve.
Free of brush or seeds from noxious plants.
The soil should contain the following properties:
Organic matter
10 – 30%
Seed germination score (out of 7)
≥6
Total water holding capacity
≥ 50%
Moisture
30% to 50%
-3
Bulk density -wet wt basis (kg per m )
750 – 1300
pH range
5.2 – 7.5
Magnesium
40kg/ha min
Phosphorus(P205)
80kg/ha min
Potassium (K20)
95kg/ha min
5.6.3
Transition layer
The particle size difference between the bioretention filter media and the underlying drainage
layer should be not more than one order of magnitude to avoid the bioretention filter media
being washed through the voids of the drainage layer.
A transition layer made up of 100mm of sand/coarse sand should be provided above the free
draining gravels provided around the pipe. An indicative particle size distribution is provided
below 9 :
Sieve Size
% passing
1.4 mm
100%
1.0mm
80%
0.7mm
44%
0.5mm
8.4%
.
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5.7
Under drainage
Much of the North Shore is situated on soils with low permeability rates. In a natural situation,
some water does permeate slowly into clay soils, but as these soils do not naturally have a
high rate of permeability they are not considered appropriate ground conditions for the use of
bioretention gardens which are reliant on soakage for stormwater disposal. For this reason all
bioretention gardens in North Shore City must be constructed with an under-drain which
connects to an approved stormwater outlet, unless the designer of the soakage bioretention
device can demonstrate with site specific geotechnical investigations and soakage tests, that
the site is suitable for the disposal of stormwater to soakage.
Important aspects of bioretention under-drainage are listed below:

The drainage layer and under-drain must be graded at a minimum of 0.5% towards the
outlet.
Under drains must lie on the base of the gravel drainage layer unless infiltration is an
output of the design.
Under-drains extending outside of the drainage layer (through in-situ soils) must be nonperforated.
Under-drains should be connected no less than 200mm above the invert of a stormwater
gully pit or manhole.
Under-drains should not be located within the groundwater zone of saturation.
Presence of water pooling at the base of the excavated facility may require a field
modification and possibly a plan revision.
Advice on the hydraulic aspects of under-drain design is provided in Appendix B.
5.7.1
Under drainage gravel layer
A layer of clean, washed gravel (5mm -14mm diameter or pea gravel) should be provided
beneath the transition layer to surround the perforated pipe. A minimum 50mm bedding layer
beneath the pipe should be provided.
The size of the drainage gravel should be determined in conjunction with the size of the
perforations of the under drain pipe. The under drain media should be sized so that d85 > 1 x
size of the perforation.
5.7.2
Storage layer
To facilitate groundwater recharge, a storage layer can be provided under the perforated pipe
in areas which are not at risk from geotechnical instability. The depth of this layer can vary
but a 300mm layer will provide some storage without significantly increasing the depth of the
excavation. If a storage layer is provided then an impermeable liner should not be installed.
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Figure 14: With and without storage layer
5.7.3
Perforated sub-surface drainage pipes
Perforated pipes can be either a PVC pipe with slots cut into the length of it, or a flexible pipe
with smaller holes distributed across its surface, both are suitable. A geofabric wrapping
should not be used around perforated pipes as this is a potential location for blockage. Where
perforated pipes are supplied with geotextile wrapping, it is to be removed before installation.
The diameter and number of perforated pipes required to drain a bioretention garden should
be sized so that the conveyance of water in the perforated pipe is not a control on the
system, to ensure this is the case, it is recommended that perforated pipes are sized to
convey peak flows an order of magnitude greater than the peak infiltration rate the
bioretention filter media is capable of delivering to the pipe. Appendix B provides the
equations for Darcy’s law to calculate the peak infiltration rate, the orifice equation to calcuate
the rate at which water can enter the pipe through the perforations and Mannings n equation
to calcuate the peak flow the pipe can convey.
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A single 110mm perforated pipe at 5% grade will be sufficient for a bioretention garden with
an area of 6m2, assuming there is a peak saturated hydraulic conductivity of 100mm/hr. For
larger gardens or more free flowing bioretention filter media a larger pipe is likely to be
required.
Bypass
5.8
Connections
Pipe joints and storm drain structure connections must be adequately sealed to avoid piping
conditions (water seeping through pipe or structure joints). Pipe sections must be coupled
using suitable connection rings and flanges. Field connections to storm drain structures and
pipes must be sealed with polymer grout material that is capable of adhering to surfaces.
Under drain pipe must be capped (at structure) until completion of the garden.
All bioretention gardens must be designed with an overflow. The overflow must either be
connected to an approved stormwater outlet or to an approved overland flow path. For
residential applications the overflow should divert runoff in up to the 10% Annual Exceedence
Probability AEP event into the public stormwater system (5% AEP for commercial
applications). In some instances, the overflow can be directed as sheet flow to other
stormwater gardens in a ‘treatment train’ (e.g swale to pond or wetland.)
For events between the 10% AEP (5% for commercial areas) and the 1% AEP runoff can be
diverted onto an approved overland flow path.
5.8.1
Observation/cleanout standpipe
An observation/cleanout standpipe should be installed in every bioretention garden that
services multiple properties or is larger than 10m2. The standpipe’s primary functions
provides:

An indication of how quickly the bioretention garden de-waters following a storm, and

A connection to the under drain system to facilitate cleanout.
The observation standpipe must consist of a rigid non-perforated PVC pipe, 100mm in
diameter. The top of the well should be capped with a screw, or flange type cover to
discourage vandalism and tampering.
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6.
Landscape design
6.1.1
Mulch/compost
Having a mulch, or compost layer on the surface of the ground can play an important role.
This layer assists in maintaining soil moisture and avoids surface sealing, which reduces
permeability. Mulch helps prevent erosion and provides a micro-environment suitable for soil
biota at the mulch/soil interface. Mulch should be:

Coarse grade shredded wood chips.

Well aged, free of other materials such as weed seeds, soil, roots etc.

Applied in a uniform thickness of between 50 and 75 mm deep.

Dense enough to avoid floating.
Chemical mulches can be applied hydraulically to adhere to the soil, and potentially assist
with flocculation. These are most relevant for larger areas. Compost is another consideration,
important for plant growth and water quality treatment, but fertilisers should be avoided as
they may compromise the water quality function of the bioretention facility.
In situations where the overflow is located within the bioretention garden, special attention
should be paid to the mulch to ensure that it is not prone to floating, and is not likely to cause
the overflow to block.
It is also possible to use rock or pumice as surface covers. These should be applied above
the mulch/compost layer.
6.1.2
Soil depth
The depth of the soils/mulch layer determines what plant species can be successfully grown
in the bioretention garden. Below are a range of soil depths and appropriate plant types:

300mm is a sufficient depth of to support grasses and small shrubs, up to 1m in height
(Note the roots of plants may reach the gravel layer).

300mm to 600mm is a sufficient depth of soil for larger shrubs.

Minimum of 1000mm of soil depth is required to support a tree.
North Shore City Council requires a minimum soil depth of 600mm for bioretention devices.
This includes the depth of the transition layer. However the edges of the garden may be
sloped, and in these areas the soil depth may be less than the minimum of 600mm. In these
areas of shallower soil it is important that appropriate plant selections are made.
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6.2
Landscape
Landscape design is an important consideration in the construction of bioretention gardens.
An attractive bioretention garden will increase the likelihood of it becoming a permanent
feature, with landowners taking pride and stewardship over the maintenance of their facilities.
6.2.1
Integrate
One design approach for bioretention is to integrate facilities into the finished landscape. This
is achieved through working within the existing landform, placing bioretention gardens on
existing terraces and tapering their berms into the slope. However, it is not advisable to
locate bioretention gardens behind existing retaining walls, see discussion in section 5.1.3.
Bioretention gardens also have the potential to reinforce existing landforms and/or reference
visible landforms nearby, in this way enhancing the experience of the site. Plant choices can
also reflect the proposed future landscape of the site, integrated with an overall planting
scheme. Where the constraints prevent particular species from being used in bioretention
facilities, it may still be possible to emulate the qualities in form, colour and/or texture of
plants in other areas of the landscape (see below).
6.2.2
Edge
To prevent vehicles driving on bioretention gardens, it is necessary to consider appropriate
traffic control solutions as part of the design, providing physical barriers such as kerb and
channel (with breaks to allow distributed water entry to the swale) or bollards and/or street
tree planting.
On sites with a slope greater than 1:12 water flowing into the bioretention garden will
naturally try to run off the downhill edge. Berms provided on the down slope side of the
bioretention garden must be benched into the slope. For slopes greater than 1:4, a retaining
wall structure may be required. Advice should be sought from a suitably qualified
geotechnical engineer for the design of any bioretention garden on a slope greater than 1:4.
While in some instances a formalised edge around the bioretention garden may be desirable
to delineate the space and ensure the bioretention garden is maintained appropriately, in
other locations, such as private yards, it may be more suitable to integrate the bioretention
garden into the character of the existing garden.
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6.2.3
Planting
Using the height, form, colour, and texture of plants a landowner can emphasize or give
depth to background plantings or structures. Patterns and rhythms can be formed, or
reinforce those within the existing landscape.
By planting in groups it is possible to emphasize the qualities of individual plants. Keep to odd
numbers and offset spacings to allow plants to blend as they reach their mature forms.
Provide for seasonal interest.
A list of appropriate native plants is provided in Appendix A. Native plants are good choice for
bioretention gardens because native plants are adapted to local conditions and have
ecological benefits. However, exotic plants can also be suitable for bioretention gardens, and
provided the plants are not pest species, and can tolerate the wetting and drying that occurs
in bioretention gardens, there is no reason that exotic plants cannot be used if they are the
land owners preference.
6.2.4
Form spaces and circulation
Bioretention gardens have the potential to create or connect spaces using the media of
landform and plants. Low grasses can form a virtual space, low shrubs a physical barrier,
larger specimens and trees a visual break and/or a ceiling for an outdoor space.
Trimmed hedges, rambling shrubs, grasses or hard materials such as stone, can create
edges, depending on the quality of the space desired. The spatial sequence within a
landscape can be directed by the size, shape and placement of the bioretention garden.
Bioretention gardens are not limited to amorphic curves, but can take on geometric forms that
relate to existing architecture or deliberate axes within the landscape.
6.2.5
Celebrate
An alternative to integrating bioretention gardens into the landscape is to form them into a
deliberate feature. This can reveal water flow, or water quality treatment. Forms can be an
expression of eco-technology with bands of regimented plants in tight rows. Another
possibility is to utilise the bioretention garden, downpipe or filter strip as water-play in the
landscape, using fountains and cascades to form landscape follies.
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Figure 15: Bioretention Planting
6.3
Plant Selection
6.3.1
Ecology
If a bioretention garden is large enough it can form multiple tiers of planting, effectively
creating a microclimate on the site. The minimum size for this to be achieved is approximate
6-10m2. Bioretention gardens may also provide a transitional edge to existing vegetation,
providing a buffer against damaging winds and overland flows, and allowing adjacent habitats
to remain intact.
Native species have distinct genetic advantages in bioretention gardens, having evolved
naturally with the resultant suitability for climate and soil types. This allows greater
survivorship, less maintenance, and greater competition with weed species. Native plants
also provide better habitat for native fauna within urban areas.
6.3.2
Function
Plants in a bioretention garden should generally have extensive root systems to encourage
biological activity (and thereby maximise water quality treatment). They should be adaptable
to varying levels of inundation and exposure (generally floodplain and upper coastal marsh
species are appropriate, but the species must tolerate soils that dry out). Plants need to
compete against weeds while not dominating other planting, and tolerate well-drained soils of
a moderate pH.
When planting a bioretention garden it is necessary to consider the function of the
bioretention garden in a technical sense and the tolerance of the individual plants to resulting
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conditions. For example, leggy shrubs and flax are effective for water quality treatment, but
they can act as an impediment to water flows if they are placed near inflow or outflow
structures.
Hardy plants tolerant to dry soil conditions should be planted densely along the perimeter of
bioretention gardens to form an edge to discourage foot traffic and mowing. Plants in the
centre of a bioretention garden require the greatest tolerance to inundation and should be
selected for dense root mats that coalesce the soil and inhibit weeds. These plants should
also lay flat under large sheet flows. Plants in streetscapes and roundabouts must be of a
size and form that takes into account sight lines of traffic.
Larger trees with extensive root systems should not be planted above existing infrastructure
pipes, or where future access is an issue
6.3.3
Materials
Ideal plant specimens will have well developed root systems and a well-shaped trunk, stem
and head (or apical shoot). Plants should be free of disfiguring knots, bark abrasions, wind,
freezing injury or any other disfigurements, and free from pests and disease. Plants should
have been previously ‘hardened off’ to cope with the climatic conditions of the site, which
usually requires one to three months placed in similar conditions to the intended site.
Roots should be just touching the edge of their containers and should be rejected if they are
wound round. For the rapid establishment of the bioretention garden, plant sizes should be
greater than PB2 as per nursery standards, with larger specimen trees greater than PB5.
Usually one or two-year-old plants will have root systems that are beginning to circle or get
matted. (Note: use only nursery-propagated plants; do not collect plants from the wild).
6.3.4
Planting
Plant in the shade when the ground is workable, but avoid compaction of moist soils. In some
instances it may be necessary to re-rake the soil following planting to prevent soil clogging.
Position plants before loosening them from their pots in accordance with a predetermined
planting plan.
When planting, protect the roots from the sun or drying winds and keep plants in the shade
and well watered until they are ready to set in. Set plants at 1200mm spacings for shrubs and
large grasses and between 300mm and 750mm for small grasses and herbs. Spacings and
densities are dependent on plant species and varieties for which advice should be sought.
Dig each hole twice as wide as the plant plug and deep enough to keep the crown of the
young plant level with the existing grade. Place 25mm of proprietary compost in the base of
each hole for plants. Make sure the crown is level and then fill the hole and firmly tamp
around the roots to avoid air pockets.
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Water immediately after planting and continue to water twice a week (unless rain does the
job) until plugs are established. You should not have to water your bioretention garden once
the plants are established. Plugs can be planted anytime during the growing season as long
as they get adequate water.
The suggested planting lists provided in Appendix A, are adapted from multiple sources,
including the ARC TP10 ‘Stormwater Management Gardens: Design Guidelines Manual’
2003. The plants mentioned all occur naturally within the Tamaki ecological district and within
the North Shore area specifically. Plants were chosen for their abilities to tolerate a range of
moisture conditions, and where possible a range of sun to shade conditions. Although the
selected plant species are suitable for bioretention, not all plants will be suitable for all sites
and the advice of a specialist should be sought wherever possible.
As well as planting within bioretention gardens there are opportunities to provide dead logs
as upright or horizontal features, which can provide initial structure to a garden, act as a
check dam, or provide bird roosts or habitat for herpertofauna and invertebrates.
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7.
Construction
7.1
Excavation
A standard bioretention garden is likely to have total depth of approximately 1.3m or less. The
minimum depth acceptable is 600mm. In some situations, for example for tree pits where a
minimum of 1m depth of soil is required, or on sloping sites, it may be necessary to construct
a pit with a depth greater than 1.3m.
For any excavation that is greater than 1.5m, shoring may be required unless the side slope
can be battered to a safe slope. A safe slope for slopes above the water table is 1:1.
It may be necessary to rip the bottom soils to promote greater infiltration.
Bioretention gardens should not be placed in the same excavated area used for construction
sediment controls unless these are excavated to remove fine sediments before the
bioretention filter media is added. Another possibility is to place a sacrificial sand layer in the
base of sediment control features to be removed at the completion of construction works
7.2
Timing
Designated bioretention areas must be fully protected by silt fence or construction fencing to
prevent compaction by heavy equipment during construction.
Defer building bioretention gardens until the contributing catchment has been stabilised, site
construction work is completed, and construction equipment and stockpiles are removed.
This is very important to ensure that bioretention facilities are not impacted by construction
activities prior to the operational phase. If bioretention facilities are constructed before the site
has been stabilised then they should be covered with a geotextile and left unplanted until
such time as construction activity has ceased and the site has been fully stabilised.
7.3
Geotextile and liners
Where geotextile and liners are to be used these should be installed carefully to prevent
damage and to ensure at least 15mm of overlap.
7.4
Backfilling gravel
Placement of the gravel over the under-drain must be done with care. Avoid dropping the
gravel from height. Spill directly over under-drain and spread manually.
Ensure drainage media is washed to remove fines prior to placement in bioretention system.
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7.5
Backfill soil
Australian research has indicated that many existing bioretention gardens have less than the
design permeability rate 10 . This is also thought to be an issue in Auckland with existing
bioretention gardens. This is often due to fill material being used that does not meet the
specification. This highlights the importance of quality control at the construction stage.
It is thought that another reason for soils not meeting permeability rates, is due to the
compaction of the bioretention filter media at the time of backfilling. When the bioretention
filter media is placed in the device it is essential it is not compacted.
It is recommended that soils is placed in 300mm lifts, and tapped gently with a back hoe.
Watering down can also be used to settle soils. It is suggested that following to initial filling, to
wait a few days to check for natural settlement and to add additional material as required.
7.6
Erosion Checks
It is good practice to check the operation of inlet erosion protection measures following the
first few rainfall events. It is important to check for these early in the systems life, to avoid
continuing problems. If problems occur, erosion protection must be enhanced. As well, be
sure to compact retention berms and apply erosion control fabric and planting to keep them in
place where applicable.
7.7
Planting
Prepare planting holes for any trees and shrubs, install vegetation, and water accordingly.
Install any temporary irrigation.
Lay down surface cover which will vary depending on the design but may be compost, mulch,
or stone or a combination.
In the Auckland region, the planting season is from May to September. Planting at this time of
the year will improve establishment and survival of plants and reduce the amount of
establishment maintenance required
10
Land and Water Constructions, 2006 Kingston City Council and Better Bays and Waterways Institutionalising Water Sensitive Urban Design and Best Practice Management Project Review of street scale
WSUD in Melbourne – Study Findings
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7.8
Tolerances
Conduct a final construction inspection, checking inlet, pre-treatment cell, bioretention cell
and outlet elevations.
Ensure the base of the garden and surface of the bioretention filter media is free from
localised depressions and low points resulting from earthworks finishing is particularly
important to achieve even distribution of stormwater flows over these treatment surfaces. For
swales, continuous longitudinal slopes (along the invert of the swale component) will reduce
the likelihood of local ponding within the swale. Generally, an earthworks tolerance of plus or
minus 50 mm is considered acceptable.
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7.9
Construction Checklist
DURING CONSTRUCTION
A.
Preliminary Works
Erosion/sediment control plan adopted
Temporary traffic/safety control measures
Location same as plans
Critical root zones (0.5m beyond drip line) of nominated trees are protected
B.
Earthworks
Bed of garden correct shape and slope
Batter slopes as plans
Dimensions of bioretention area as plans
Confirm surrounding soil type with design
Provision of liner as designed
Perforated pipe installed as designed
Drainage layer media as designed
Transition layer media as designed
Bioretention filter media specifications checked
Detention depth as designed
Compaction process as designed
C.
Structural Components
Location and levels of excavation as designed
Public safety protection provided
Pipe joints connections as designed
Concrete and reinforcement as designed
Inlets appropriately installed
Inlet erosion protection installed
Set down to correct level for flush kerbs
D.
Vegetation
Stabilisation immediately following earthworks
Planting as designed (species/densities)
Weed removal before stabilisation
E.
As Built Drawings
Engineer certifies the construction according to the consented design
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8.
Maintenance
Bioretention gardens require some regular maintenance to ensure they continue to perform
as stormwater management devices and as attractive landscape features.
Many design features can minimize the maintenance burden and maintain pollutant removal
efficiency. Key examples include: limiting drainage area, providing easy site access,
providing pretreatment, and utilizing native plantings.
The construction phase is another critical step where many maintenance problems can be
minimized or avoided. The most important maintenance guideline to follow during
construction is to make sure that the contributing drainage area has been fully stabilised prior
to bringing the practice “on line”.
8.1
Access
Sufficient access must be provided at the design stage, and protected throughout the
bioretention garden’s design life, to ensure the ongoing inspection, maintenance and
landscape upkeep of the bioretention garden is possible.
8.2
Under drain
For bioretention gardens larger than 10m2, or that serve more than one property, a manhole
should be provided at the connection between the overflow riser, the perforated under drain
and the non-perforated stormwater that conveys water away from the garden to the public
stormwater network.
For devices less than 10m2 that serve an individual lot, a manhole is not required, it is
sufficient to provide a riser connection for the overflow, which connects with a junction the
perforated under drain and the non-perforated private stormwater pipe that conveys water
away from the garden to the public stormwater network. This riser should be designed to
enable rodding of the under drain should it become blocked.
If the depth to the invert of the bioretention garden is greater than 1m then the manhole
should be sized to allow access, e.g. 1050mm diameter manhole. If the garden is 1m deep or
less, a mini-manhole will be sufficient. The manhole should be located close enough to the
bioretention garden to enable maintenance of the under drain.
8.3
Fertilizing
The bioretention filter media is designed to incorporate approximately 30% organic matter,
and will support plant growth. Excess fertilization, (besides compromising the facility’s
pollutant reduction effectiveness) leads to weak plant growth, promotes disease and pest
outbreaks, and inhibits soil life.
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8.4
Harvesting
Like any garden area that includes grasses or woody plant materials, harvesting and pruning
of excess or diseased growth will need to be done occasionally. Trimmed materials may be
recycled back in with replenished mulch material, composted elsewhere on the site, or taken
to landfill in the case of hot spot locations.
Trees and shrubs may also be pinched, pruned, thinned or dead-headed for shape or to
maximize fruit or flower production. Pruning of trees should occur before bud-break in the late
winter. Pruning of flowering shrubs should be performed immediately after the plants have
finished blooming. For specific pruning instructions for particular species consult your
nurseryman.
8.5
Watering
Typically, watering of the bioretention garden will not be necessary once plants have become
established, except during drought conditions. However, watering will be needed during the
plant establishment stage.
8.6
Weeding
Weeding of the facilities is not absolutely necessary for the proper functioning of the
bioretention facility. However, unwanted plants can be invasive, consuming the intended
planting and destroying the aesthetic appeal and biodiversity benefits of the bioretention
garden. Therefore, weeding is encouraged to control growth of unwanted plants, especially
where bioretention gardens are placed in prominent settings. Non-chemical methods (hand
pulling and hoeing) are preferable.
8.7
Pest damage
Trees and shrubs should be monitored for the appearance of pests and/or damage caused
by pests or disease. Monitoring should occur once a week during the first growing season.
For identification of specific pests and diseases, and for treatment recommendations, consult
the ARC biodiversity team
(http://www.arc.govt.nz/index.cfm?26D815A8-E018-8BD1-32D6-C3EF43975B56).
It is important to keep in mind that insects and soil micro-organisms perform a vital role in
maintaining soil structure. Therefore, the use of pesticides should be avoided so as not to
harm beneficial organisms. An alternative approach is to use a combination of biological,
physical, and chemical controls.
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8.8
Mulching
The mulch materials placed in the facility will decompose and blend with the soil medium over
time.
Mulch layers should not exceed 75mm in depth around trees and shrubs and should be
placed away from the base of trunks. Mulch can be spread to 50mm depths around
perennials. Grass clippings or animal waste should not be used as mulch in bioretention
gardens.
Avoid blocking inflow entrance points with mounded mulch. Mulch material should be reapplied once every 6 months during the first three growing seasons. Once a full groundcover
is established, re-mulching can be programmed annually, with the mulch scraped off and
removed every 5 years.
8.9
Standing Water Problems
Bioretention facilities are designed to have water standing for up to 24 hours. If this period is
routinely exceeded, the facility may not be functioning properly. Should standing or pooling
water become a maintenance burden, minor corrective action can usually correct it. Pooling
water is usually caused by clogging or blockage of the surface layer. The surface blockage
problem may be corrected by removing the mulch layer and using a flat-bottomed shovel and
skim off the top 50 mm of media, and then replace the mulch. If this is done several times,
then additional media may be needed in the future
It should be noted that careful adherence to the bioretention filter media specification can
mitigate the risk of standing water and save maintenance cost and effort.
8.10
Rubbish and Debris
Runoff flowing into bioretention gardens may carry litter and debris with it, particularly in
commercial settings. Rubbish and debris should be removed regularly both to ensure that
inlets do not become blocked and to keep the area from becoming unsightly. Inspect
bioretention areas after rainstorms to ensure drainage paths are free from blockages. Curb
cuts in parking areas will need to periodically be cleared of accumulated sediment and debris.
8.11
Pre-treatment
For bioretention gardens larger than 50m2 and for bioretention gardens serving busy roads or
commercial areas we suggest the use of pre-treatment. This pre-treatment may be in the
form of a forebay or engineered silt trap or a filter strip. These pre-treatment devices should
be checked 6 monthly and silt removed as required.
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8.12
Maintenance Schedule
Effective long-term operation of bioretention gardens requires dedicated and routine
maintenance tasks performed to a consistent timetable.
Monthly
Remove weeds and replace dead plants. Eradicate
noxious/pest weeds and undesirable growth
6 Monthly
12 Monthly
5-yearly
9
Litter removal
9
Inflow, overflow/outlets-check overflow for clogging.
Remove accumulated sediments. Check overflow
spillway
Summer- monitor and water vegetation in extended
dry periods
9
9
Pruning or thinning
9
Compost/Mulch
seasons)
replenishment
(first
3
growing
9
Remove accumulated sediments; reinstate plants,
soil and mulch. Check for ponding /clogging and
blockage of filter media
Inspect trees and shrubs and replace any dead or
severely diseased vegetation
Scour/erosion evident: check for erosion signs.
Check dams/capping system areas and correct as
required
Sump- accumulated sediments not more than 50%
full
Outlet manholes-check
manhole sumps
Pre-treatment,
required
inspection
and
remove
silt
9
9
9
9
from
9
and
silt
removal
as
9
Compost/Mulch replenishment (after first 3 growing
seasons)
9
Check for restrictions/clogging/failures in pipes
9
Scrap off top 100mm of soil and mulch, dispose to
landfill, replace
9
Replace transition layer or filter fabric if warranted
9
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9.
References
Auckland Regional Council, 2003 Technical publication 10, Design Guideline Manual:
Stormwater Treatment Device
Brix, H. 1993. Wastewater treatment in constructed wetlands system design, removal
processes, and treatment performance. Pp. 9-22 in Constructed Wetlands for Water Quality
Improvement, G. A. Moshiri (ed). Boca Raton, Fla.; CRC Press, 632 pp.
Brisbane City Council, 2005 Draft Water Sensitive urban Design Engineering Guidelines
Brisbane City Council, 2006, Stormwater Gardens Bioretention Basins for Urban Streets
City of Melbourne, 2004 Water Sensitive Urban Design Guidelines
North Carolina DENR. 1997. Stormwater Best Management Practices Manual. Raleigh, N.C.:
North Carolina Department of Environment and Natural Resources. Division of Water Quality.
85 pp.
North Carolina State University, North Carolina A&T State University, U.S. Department of
Agriculture, and local governments cooperating: Designing Rain Gardens (Bio-Retention
Areas)
North Shore City Council, 2005 Long Bay Practice notes 204 – Rain gardens
Facility for Advancing Water Biofiltration, 2006, Bioretention and Tree pit media specification
Land and Water Constructions, 2006 Kingston City Council and Better Bays and Waterways Institutionalising Water Sensitive Urban Design and Best Practice Management Project
Review of street scale WSUD in Melbourne – Study Findings
Prince George’s County, Maryland, The Bioretention Manual
City of Sydney, 2004. Water sensitive design in Sydney region, Technical Guide
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Appendix A Plant Specifications
Ground cover Plants
Botanical Name
Apodasmia similis
Common Name
oioi
Height
(mm)
1500
Spread
(mm)
1000
Appearance & Tolerance
A reed with fine grey leaves and brown markings at
intervals along the length of leaves.
Grows well in damp ground, tolerates open water,
direct sun and salt spray.
Carex lessoniana
Spreading
sedge
swamp
1000
2000
Wide green bi-folded leaves and long hanging green
spikes.
Prefers damp or periodically damp areas, tolerant of
direct sun and semi-shade.
Carex secta
purei
1500
2000
Bright green sedge that forms in clumps, developing
a trunk.
Tolerates damp and short periods of dryness, direct
sun and semi-shade. Prefers wet feet.
Carex virgata
purei
800
800
Upright, fine-leaved bright green sedge with long
seed heads.
Works well at both the centre and edges of
raingardens in wet and dry exposed conditions.
Cortaderia fulvida
toetoe
1500
2000
The smaller of the toetoe. Branching from the base
to 1500mm high (flowers to 2000mm). Long strapshaped leaves with red-orange coloured veins.
Prefers good drainage and semi-shade but can
tolerate wet conditions and full sun.
Cyperus ustulatus
toetoeupokotangata,
umbrella sedge
1500
1500
Large, pale olive-green curving sedge with large
dark brown or purple spikes.
Prefers wet edge conditions but can tolerate periods
of dry and exposure to full sun.
Dianella nigra
turutu
500
1000
Lily with reddish leaves, small white flowers, and
striking violet-blue fruit.
Does well in shade and open areas but prefers welldrained situations.
Gahnia xanthocarpa
tarangarara
3000
3000
Very stout sedge that can grow to over 3000mm
high. Sharp leaves may be used to dissuade public
access.
Grows well in boggy conditions and can tolerate
semi-shade and direct sun.
Gleichenia dicarpa
tangle fern
500
1500
Forms springy interlacing and compact thickets.
Prefers wet soils, but is comfortable in shady or
exposed conditions. Can be difficult to established.
Haloragis erecta
toatoa
1000
1000
A low growing shrub, with fine green serrated
leaves.
Grows in well drained damp soils but can tolerate
periods of dryness and direct sun as well as semishade.
Isolepis inundata
swamp club rush
300
500
Juncus pallidus
wiwi
1700
2000
Soft bright yellow-green erect tufts.
Does well in wetter soils in the sun or semi-shade.
An attractive light green rush up to 1500mm tall that
forms large clumps.
Prefers wet and wet edge conditions, and can
tolerate sun or semi-shade.
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Libertia grandiflora &
L. ixioides
mikoikoi, native iris
400
400
Clump forming native irises with narrow, upright
leaves. Small white flowers form on tall spikes in
spring.
Prefers well drained wet soils. Tolerates sun or
shade and periods of dryness.
Machaerina sinclairii
tuhara
1000
1500
Large and leafy pale green sedge with graceful
seed head.
Grows well in seepage areas and prefers shade, but
can tolerate edge conditions.
Muehlenbeckia
complexa
pohuehue
1000
2000
Dense sprawling divaricating shrub, good habitat
value and weed suppressant but can take over if
inter-planted.
Tolerates very dry to well-drained wet conditions in
full sun or semi-shade.
Phormium cookianum
mountain flax
1500
1500
wharariki,
Clump-forming flax with drooping yellow –green
leaves.
Tolerates full exposure and sun. Prefers edges and
well-drained soils.
Phormium tenax
harakeke, flax
3000
3000
Clump-forming flax with large stiff leaves and red
flowers that attract birds.
Prefers full exposure to sun and salt spray and
survives in boggy and dry environments.
Schoenus tendo
wiwi
1000
1000
Rush like sedge up to 1000mm tall light green culms
that turn to orange in the sun.
Prefers edge conditions and tolerates direct sun and
semi-shade.
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Small Trees and Shrubs (6m2 rain garden recommended for trees)
Botanical Name
Carpodetus serratus
Common Name
putaputaweta
Height
(mm)
600
Spread
(mm)
300
Appearance and Tolerance
Horizontally branching shrub with marbled leaves
and a white flowers..
Light shade will encourage dense undergrowth and
prefers damp free draining soils. Tolerates direct
sun.
Coprosma propinqua
mingimingi, black scrub
3000
2000
Bushy, dark green divaricating shrub with small
leaves.
Sun & shade, in well drained & boggy soils.
Coprosma robusta
karamu
5000
3000
A shrub or small tree up to 6000mm, dense and
lustrous green foliage and orange berries that
attracts wildlife.
Prefers sun but tolerates wet to dry conditions.
Quick to establish.
Cordyline australis
ti kouka, cabbage tree
8000
3000
1200mm height. Palm-like in appearance with large
heads of linear leaves and panicles of scented
flowers.
Dense taproots, so they must be kept away from
under drains. Sun to semi-shade. Prefers damp to
moist soil.
Cordyline banksii
ti
ngatere,
cabbage tree
Fuchsia excorticata
Hoheria populnea
forest
5000
2000
kotukutuku
5000
2000
houhere, lacebark
8000
3000
Branching from the base and forming a clump. Long
strap leaves with red-orange veins.
Prefers good drainage and semi-shade
A deciduous small tree up to 12000mm high that
has a very distinctive stripped orange bark and redpurple flowers.
Prefers cool shade and moist, well drained soils.
A fast growing tree with abundant flowers.
Prefers well draining soils. Tolerates sun but prefers
protection from wind exposure.
Leptospermum
scoparium
manuka and varieties
4000
2000
Shrub or small tree growing to 4000mm in height.
Natural forms have white to pinkish flowers.
Hardy and tolerant of difficult conditions including
salt and wet edges.
Melicytus ramiflorus
mahoe
5000
3000
A white barked tree with a spreading habit and large
serrated leaves.
A very hardy tree, tolerating sun and semi-shade,
dry and wet edge conditions.
Myrsine australis
mapou
5000
2000
A hardy tree with light green crenulated foliage and
red stems. Can be clipped to form a hedge.
Grows well in direct sun or shade but prefers well
draining or drier soils.
Olearia solandri
coastal tree daisy
4000
3000
A small leaved shrub with an upright habit and
autumn flowers with a pleasing fragrance. Can be
clipped to form an attractive hedge.
Tolerant of salt spray and
conditions, sun and semi-shade.
Plagianthus
divaricatus
makaka,
ribbonwood
saltmarsh
2000
1500
temporarily
wet
A tough divaricating and bushy shrub with attractive
red-copper branches.
Performs well in diverse soil conditions, tolerating
boggy soils and salt spray.
Rhopalostylis sapida
nikau palm
8000
3000
A New Zealand palm that may take some time to
form a trunk.
Provide light shade to encourage dense growth, but
can tolerate open conditions when established.
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Large Trees (10m2 rain garden recommended for one tree)
Botanical Name
Alectryon excelsus
Common Name
titoki
Height
(metres)
8
Spread
(metres)
6
Appearance and Tolerance
A tree that is common for streetscapes,
spreading from a single trunk with light green
glossy leaves.
Slightly frost tender, but tolerant of shade
and direct sun. Prefers well drained moist
soils.
Cyathea medullaris
mamaku
12
4
Hardy, fast growing tree fern with 3m fronds
Comfortable in moist soils and exposed
places, shade and frost tolerant.
rimu
Dacrydium
cupressinum
60
10
A beautiful tree with hanging needles and
weeping branches in juvenile form.
Prefers well drained soils, sun and semishade.
kahikatea
Dacrycarpus
dacrydioides
20
10
Kahikatea are tall native pines with upright
needles. The tree will take a few seasons to
develop robust foliage.
Able to tolerate waterlogged soils with
buttressed roots in sun and semi-shade.
Dysoxylum spectabile
kohekohe
12
10
An attractive tree of lush broadleaf foliage
and panicles of white flowers.
Requires some shelter or companion
planting when young, with low tolerance of
direct sun. Prefers intermittently wet soils
only.
Laurelia
zelandiae
novae-
pukatea
10
3
A distinct buttressed tree with glossy green,
erect foliage.
Can be slow growing but performs well in
damp areas. Tolerates sun and shade.
Podocarpus totara
totara
10
9
A hardy tree of dense spindle like foliage
that can achieve large sizes or can be
clipped to form a hedge.
Easy to establish and tolerant of open or
shade, wet or dry conditions.
Schefflera digitata
pate
5
4
Seven-finger foliage with massing of fruit for
birds.
Prefers damp and shady conditions but can
tolerate direct sun in a sheltered
environment.
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Appendix B Hydraulic Design
The sizing methodology provided in the District Plan Change 22, provides a relationship
between the area of the bioretention garden and the area of imperviousness to be managed
by the bioretention garden.
To ensure the garden achieves the performance standards hydraulic analysis is required to
enable the designer to ensure:

The contributing catchment area is correcting delineated and grades have been
assessed.
The inlet is sized to convey the peak first flush flow into the bioretention garden without
causing ponding upstream or bypassing the filter medium.
Sufficient storage is provided above the bioretention garden for attenuation of the first
flush volume to be stored, without bypassing the filter medium.
The under-drain is sized so that the bioretention filter medium is the control on the flow of
stormwater through the bioretention garden.
The garden either has an overflow system, designed to ensure that flows in excess of the
treatment flow bypass the system and are conveyed to an approved outlet or overland
flow path.
Bioretention swales are designed to convey design flows in a manner without causing
erosion.
Inlet design
To determine the width of the opening in the kerb to allow flows to enter the bioretention
garden, Manning’s equation can be used with the kerb, gutter and road profile to estimate the
flow depth in the kerb and channel at the entry point. Once the flow depth for the minor storm
(e.g. 1/3 2 year Average Recurrence Interval (ARI)) is known, this can then be used to
calculate the required width of the opening in the kerb by applying a broad crested weir
equation. The opening width is estimated by applying the flow depth in the gutter (as h) and
solving for L (opening width).
Where

Q = flow (m3 /s)

Cw = weir coefficient (= 1.7)

L = length of opening (m)

h = depth of flow (m)
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This method ensures the kerb opening does not result in an increase in the upstream gutter
flow depth, which in turn ensures the bioretention basin does not impact on the trafficability of
the adjoining road pavement.
Ponding depth and under drainage
The ponding depth of the bioretention garden provides an initial storage volume to capture
stormwater runoff to the bioretention garden, allowing the water to spread and infiltrate over
the entire facility area. The maximum depth of the ponding should be 300mm with a 100mm
freeboard. The duration of ponding after a storm event has passed should be less than 24
hours to ensure survival of the plants and to satisfy aesthetic criteria, although the exact
duration required will depend on the plants selected.
The effective drawdown rate is approximately equivalent to the sum of the hydraulic
conductivity of the limiting soil layer (Kn) and the maximum flow through the under-drain (Ku),
if applicable. The drawdown time can be estimated as the ratio of the ponding depth to the
effective drawdown rate.
A key hydraulic design consideration for a bioretention garden is the delivery of stormwater
runoff from the garden onto the surface of a bioretention filter media. This can be achieved by
creating shallow temporary ponding (i.e. detention) over the surface of the bioretention filter
media via the use of a check dam and raised field inlet pits. To ensure the perforated pipes
fitted within the under-drainage are of adequate size, checks are required, these are:

Ensure perforations are adequate to pass the maximum infiltration rate.

Ensure the pipe itself has capacity to convey the design flow/ infiltration rate.

Ensure that the material in the drainage layer will not be washed into the perforated
pipes. (Use a transition layer as discussed in section 0.)
The maximum infiltration rate represents the maximum rate of flow through the bioretention
filter media and is calculated by applying Darcy’s equation as follows:
where

Qmax = maximum infiltration rate (m3/s)

Ksat = hydraulic conductivity of the soil filter (m/s)

Wbase = base width of the ponded cross section above the soil filter (m)

L = length of the bioretention zone (m)

hmax = depth of pondage above the soil filter (m)

d = depth of bioretention filter media (m)
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The capacity of the perforated under-drains needs to be greater than the maximum
permeability rate ensuring the bioretention filter media drains freely and the pipe(s) do not
become the hydraulic ‘control’ in the bioretention system (i.e. to ensure the bioretention filter
media sets the travel time for flows percolating through the bioretention system rather than
the perforated under-drainage system).
To ensure the perforated under-drainage system has sufficient capacity to collect and convey
the maximum permeability rate, it is necessary to determine the capacity for flows that enter
the under-drainage system via perforations in the pipes. To do this, orifice flow can be
assumed and the sharp edged orifice equation can be used. Firstly, the number and size of
perforations needs to be determined (typically from manufacturer’s specifications) and used
to estimate the flow rate into the pipes using the maximum driving head (being the depth of
the bioretention filter media if no detention is provided or if detention is provided in the design
then to the top of detention).
It is conservative but reasonable to use a blockage factor to account for partial blockage of
the perforations by the drainage layer media. A 50 % blockage of the perforation is
recommended.
Where

Qperf = flow through perforations (m3/s)

B = blockage factor (0.5)

Cd = orifice discharge coefficient (assume 0.61 for sharp edge orifice)

A = total area of the orifice (m2)

g = gravity (9.79 m/s2)

h = head above the perforated pipe (m)
If the capacity of the drainage system is unable to collect the maximum permeability rate then
additional under drains will be required. After confirming the capacity of the under-drainage
system to collect the maximum permeability rate it is then necessary to confirm the
conveyance capacity of the under-drainage system, ensuring it is sufficient to convey the
collected runoff. To do this, use Manning’s equation. Manning’s roughness is dependent on
the type of pipe used. The under drains should be extended vertically to the surface of the
bioretention garden to allow inspection and maintenance when required. The vertical section
of the under-drain should be unperforated and capped to avoid short circuiting of flows
directly to the drain.
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Manning’s n roughness
Grassed Swales 11
For 150 mm grass and d < 60 mm n = 0.153 d-0.33 /(0.75 + 25s)
d > 60 mm n = 0.013 d-1.2 / (0.75 + 25s)
For 50 mm grass and d < 75 mm n = (0.54-228 d2.5) / (0.75 + 25s)
d > 75 mm n = 0.009d-1.2 /(0.75 + 25s)
Where:
d = depth of flow (m) for water quality storm
s = longitudinal slope as a ratio of vertical rise/horizontal run (m/m)
Under-drains
The Manning’s n for perforated pipe is as follows

Corrugated perforated pipe
0.021 to 0.025

Smooth PVC pipe with perforations
0.015
Size Overflow
In a bioretention system, a high flow overflow can be provided with a riser crest raised above
the level of the bioretention filter media to establish the design’s detention depth.
Grated risers are typically used and the allowable head for discharges into the pits is the
difference in level between the riser’s crest and the maximum permissible water level to
satisfy minimum freeboard requirement of 100mm.
To size an overflow grated riser, two checks should be made to test for either drowned or free
flowing conditions. The tests are:

A broad crested weir equation can be used to determine the length of weir required
(assuming free overflowing conditions), and
An orifice equation used to estimate the area between openings required in the grate
cover (assuming drowned outlet conditions). In addition, a blockage factor is to be used,
that assumes the grate is 50 % blocked.
11
Auckland Regional Council , 2003 Technical Publication 10, Design Guideline Manual: Stormwater Treatment
Device
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For free overfall conditions (weir equation):
Where

Qweir = Flow into the riser(weir) under free overfall conditions (m3/s)

B = Blockage factor (= 0.5)

Cw = Weir coefficient (= 1.66)

L = Length of weir (perimeter of pit) (m)

h = Flow depth above the weir (pit) (m)
Once the length of weir is calculated, a standard sized riser can be selected with a perimeter
of at least the same length of the required weir length.
For drowned outlet conditions (orifice equation):
Where

B, g and h have the same meaning as in Equation 3.4

Qorifice = flow rate into riser under drowned conditions (m3/s)

Cd = discharge coefficient (drowned conditions = 0.6)

A = area of orifice (perforations in inlet grate) (m2)
Where possible, ‘above design’ flows should enter overflow risers located near the inflow
zone or bypass channels, rather than pass over the bioretention’s surface. In situations
where both the minor (2 year ARI) and major flood (100 year ARI) flows need to be conveyed
over the bioretention’s surface, it is important to ensure that sufficiently low velocities are
maintained (preferably below values of 0.5 m/s and not more than 1.5 m/s for major flood) to
avoid scouring of the bioretention filter media and vegetation.
Conveyance
To calculate the flow capacity of a swale, use Manning’s equation. This allows the flow rate
and flood levels to be determined for variations in swale dimensions, vegetation type and
longitudinal grade. Manning’s n is a critical variable in Manning’s equation relating to
roughness of the channel. It varies with flow depth, channel dimensions and vegetation type.
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Flow must not scour the bioretention surface and needs to be uniformly distributed over the
full surface area of the bioretention filter media. In steeper areas, check dams may be
required along the swale to reduce flow velocities discharged onto the bioretention filter
media. It is important to ensure that velocities in the bioretention swale from both minor (1/3 2
year ARI) and major (100 year ARI) runoff events are kept sufficiently low (preferably below
0.5 m/s and not more than 2.0 m/s for major flood) to avoid scouring.
As bioretention swales are generally accessible by the public, it is important to check that
depth x velocity within the bioretention swale at any crossings and adjacent pedestrian and
bicycle pathways satisfies the following public safety criteria:

Depth x velocity < 0.6 m2/s for low risk locations and 0.4 m2/s for high risk locations

Maximum depth of flow over crossing = 0.3m
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Appendix C Bioretention Growing Media
Specifications
Guidelines for Growing Media in Raingardens
DRAFT
June 2008
The following guidelines for have been prepared for growing media in Raingardens with
consultation from ARC, NSCC, LEL, and Landcare Research. Work undertaken by the
Facility for Advancing Water Biofiltration (FAWB) from Monash University has also been
helpful in designing these guidelines. The objective is to define the attributes of a media
suitable for use in Raingardens that will :
o
o
o
maintain plant growth and health
retain contaminants
reduce flow to stormwater system
Testing Requirements
The media must meet a standard for quality in terms of physical and chemical parameters.
These are;
o
hydraulic conductivity
o
particle size distribution
o
organic matter content
o
pH, electrical conductivity, nutrient levels on a total basis
o
heavy metal content e.g. copper, zinc
Approved Testing Laboratories
For the analysis of organic matter, pH, electrical conductivity, nutrients and contaminants: Hill
Laboratories in Hamilton.
For the testing of physical properties such as hydraulic conductivity and particle size
distribution: Landcare Research in Auckland.
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PRODUCT SPECIFICATION
RAINGARDEN MIX
(Residential and Commercial Only1)
Saturated Hydraulic Conductivity
100 – 300mm/hour (Infiltration rate)
Particle Size Distribution
100%
<25mm
90-100%
<10mm
Total Water Holding Capacity
≥ 50%
Moisture
30% to 50%
Organic matter
10 – 30%
pH Range
5.5 – 7.5
Electrical Conductivity
<2.5 dS/m
Magnesium
40kg/ha min
Phosphorus
(P205)
80kg/ha min
Potassium (K20)
95kg/ha min
Seed Germination score (out of 7)
≥6
Total Copper mg/kg
80
Total Zinc mg/kg
200
1 – for all sites other than high risk industries as defined in the Proposed Auckland Regional Plan: Air Land Water
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Living Earth Limited
Raingarden Quality Procedure
Compost
Compost is a component in Raingarden mix and is subject to a schedule of quality checks to
ensure that it meets specifications for use in Raingarden mix.
Compost shall comply with the New Zealand Standard NZ4554 or attached specification
Compost is made from clean greenwaste. Materials not accepted for composting include;
o
treated timber and sawdust
o
orchard prunings from commercial orchards
o
material from contaminated sites
Weekly quality control checks on the screened compost include
o
Seed germination.
o
Moisture content
o
Visual
Monthly quality control checks on the compost include
o
pH,
o
electrical conductivity,
o
water extractable nutrients
o
seed germination
o
plant growth trials.
Heavy metal, organochlorine and pesticide testing is done annually on a composite sample
which represents the years production.
Topsoil
The topsoil used in Raingarden mix is tested for
o
heavy metals
o
soil texture, ie clay loam, sandy clay loam
Sand
Before a sand can be used in Raingarden mix it is tested for
o
electrical conductivity
o
seed germination
This is a one-off test, performed on a representative sample of sand from a pile.
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Raingarden Mix
The Raingarden mix is subject to regular quality control checks. Results are to be in line with
attached specifications.
Each batch
o
pH
o
electrical conductivity
o
seed germination
o
moisture
o
visual
Six Monthly
o
Infiltration
o
Particle size distribution
o
Total water holding capacity
Annually
o
Total nutrients including organic matter
o
Heavy metals
Annual testing is done on a composite sample and is representative of that year’s production.
Sampling Procedure
A standardised sampling procedure is necessary to ensure that test results accurately reflect
the pile being sampled. The key is taking small grab samples around the pile at varying
depths and heights to produce a sample that best represents the original pile.
1.
2.
3.
4.
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Label sample bag with name and date
Sampling equipment (scoops, forks) must be clean to ensure no cross-contamination
Sample must be a composite consisting of at least 6-8 grab samples or handfulls
taken randomly from around the pile at varying heights and depths.
Grab samples are placed in sample bag and mixed thoroughly to blend.
First Edition
July 2008
COMPOST SPECIFICATION
pH
6.0 - 8.5
-1
Conductivity (mS cm )
≤ 5.0
Ammonium*
≤ 200ppm
Germination score (out of 7)
≥6
Weeds
absent
Moisture
≥ 39% to 51%
Particle size % pass 15mm x 15mm orifice
100%
Plant Growth Index
≥3
3
Bulk Density –wet basis (kg/m )
550 - 650
* analysed in a 1:1.5 water extraction
Nutrient Analysis (average)
% by weight (D.M)
July 2008
N
P
K
Ca
Mg
S
OM
1.5
0.30
1.09
2.2
0.8
0.2
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Appendix D Typical Details
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Typical raingarden detail
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Typical raingarden detail including storage layer
Note that an impervious liner will not be required if a storage layer is provided beneath the level of the underdrain pipe. Storage should only be provided
where groundwater recharge is intended and ground conditions are suitable.
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Typical stormwater planter detail
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Typical tree pit detail
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Typical tree pit detail including storage layer
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Typical bioretention swale detail
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Typical bioretention swale detail including storage layer
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Appendix E Practice Notes
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Stormwater Management Practice Note NSC 10:
Bio-Retention
This practice note has been developed to provide guidance in meeting the minimum
requirements in the District Plan Rule 8.4.8 for the use of bio-retention for on-site stormwater
management. It is applicable to all areas of the city except the Long Bay Structure Plan Area.
10.1
Introduction
Bio-retention systems are shallow, planted areas which filter stormwater runoff through a
vegetated soil media layer. It is a concept that was developed by Prince Georges County,
Maryland in the United States in the early 1990’s and is now widely accepted as one of the best
stormwater management practices. It makes uses of the chemical, biological and physical
properties of plants, microbes and soils and is an effort to mimic nature.
Figure 10.1 : Award winning rain garden (Ellerslie Flower Show 2007)
10.2
Description
Bio-retention in the North Shore City context encompasses a number of different devices: rain
gardens, bio-retention swales, stormwater planters and tree pits. Green roofs are similar to bioretention but are dealt with separately in Practice Note NSC 12. North Shore City Council has a bioretention guidelines manual available on the council web site.
A rain garden is essentially a sunken
garden with good well drained soil and
an underdrain to which stormwater is
directed.
Figure 10.2 : Typical Rain garden
Stormwater Management Practice Note NSC 10 – Bio-Retention
September 2009
A bio-retention swale is a long narrow sloping
swale with a bio-retention system along the base
of the swale. It can be used to convey as well as
treat stormwater.
Figure 10.3 : Bio-retention swale
A stormwater planter is an above ground garden in a
large container with bio-retention soil media and an
underdrain to which stormwater is directed.
Figure 10.4 : Stormwater planter
Tree pits which are used for planting of street
trees can also be used for bio-retention as long
as they are designed with an appropriate
underdrain system.
Figure 10.5 : Tree-pit
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September 2009
10.3
When are Bio-Retention Devices Required?
Bio-retention devices may be used to meet the permitted activity standards for on-site
stormwater mitigation required by district plan Rule 8.4.8. Under this rule the following bioretention sizes are required for managing stormwater runoff from residential and non-residential
impervious areas:

Where the site does not drain directly to an approved stormwater detention facility, 8m2
of bio-retention area per 100m2 of impervious area to capture, hold and release in a
controlled manner.

Where the site does drain directly to an approved stormwater detention facility, 5m2 of
bio-retention area per 100m2 of impervious area.
(Note that an approved detention facility refers to a stormwater device which has been designed
to accommodate the proposed development on your site and provides adequate stormwater
mitigation for the 2 and 10 year rainfall events as well as providing extended detention for stream
protection)
Refer to the following stormwater management practice notes to determine if or when a bioretention is required:

NSC 03 Permitted Activity Route

NSC 04 Controlled Activity Route
10.4
Advantages of a Bio-Retention Device
Bio-retention offers a number of benefits which is why it is one of the preferred methods for onsite stormwater mitigation. The benefits include:

Improved water quality by filtering out contaminants

Improved hydrological response of stormwater peak flow and volume, especially for
smaller rainfall events

Providing amenity and increased vegetative cover

Utilises the same space as is normally used for gardens

Providing better growing conditions (free draining soils) for plants and thus improves the
garden

Mimics nature and uses natural processes
10.5
Minimum Design Requirements
A bio-retention device must meet the following minimum design requirements to qualify for the
permitted or controlled activity routes through the resource consent process as required under
section 8.4.8 of the district plan.
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The bio-retention area comprises the following typical layers:
Figure 10.6 : Typical bio-retention layers
1. Size and Shape: The bio-retention device can be any shape. Shapes that blend in with
the site are more pleasing on the eye and look more natural.
The bio-retention device size refers to the area that is filled with planting soil mix and
should be sized so that it has an area equivalent to 8% of the impermeable area draining
to it, and may be reduced to 5% if it drains to an NSCC approved stormwater pond. The
minimum size for a bio-retention device is 2m2. Note that if the calculated size for bioretention is less than 1m2 then no device is required, if it is between 1m2 and 2m2 then a
2m2 device is required. Refer to Stormwater Management Practice Note NSC 03 for sizing
requirements on your site.
2. Location : Devices should be located so that stormwater can flow to the device under
gravity and setback a minimum of 1m from property boundaries. Care should also be
taken to avoid underground services. Devices should be at least 3m from building
foundations or else a specific design and impermeable liner will be required. A
stormwater planter is suited for these situations. Devices should be set back from existing
retaining walls a distance equal to the height of the wall. Remember that plants need light
to grow and you need access for maintenance.
In areas with a slope of greater than 1:5 or geotechnically unstable areas an impermeable
liner is required.
3. Inlet Design : Good inlet design is essential for a bio-retention device to work. The
most important thing to consider is that the water must be able to flow into the device.
(Water flows down-hill!) Flow should be directed into the device in a sheet flow over a
vegetated filter strip if possible, and should always flow into the device in a manner which
avoids concentrated flows and scour. If possible high flows should bypass the device.
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4. Planting Soil Mix : The main component of the bio-retention device is the planting soil
mix. This must be from an NSCC approved commercial source. The minimum soil
depth is 600mm (500mm of planting soil mix and 100mm transition layer) which should
be increased to at least 1m when trees are planted.
For rain gardens, stormwater planters and tree pits the surface of the planting soil should
be flat and level to avoid localised ponding and blinding, while for bio-retention swales
the surface should be gently sloping.
Soil compaction should be avoided, allowing natural compaction only. Install materials in
300mm layers and soak with water to aid natural compaction. Depending on the soil
material, up to 20% natural compaction may occur over time.
5. Plants : Native plants are preferable but not essential. Any plants which suit your garden
may be used. The plants should be able to tolerate short periods of inundation and
longer dry periods, be perennial rather than annual, have deep fibrous root systems and
have spreading rather than clumped growth forms. Note that the use of wetland plants is
not recommended as these plants are not well suited to free draining soils.
Why native plants? These are best adapted to local climate, and attract and provide
habitat and food for local wildlife including birds and insects.
6. Mulch : 50-100mm surface mulch layer. Note that the depth of mulch should be taken
into account when setting the overflow level.
7. Under Drain : All bio-retention devices require under drains in North Shore City due to
the clay soils that occur. The under drain should be a perforated PVC pipe with a
minimum diameter of 100mm and should have a minimum slope of 0.5% (5cm over
10m). For bio-retention areas up to 10m2 a single 100mm diameter pipe will suffice, for
areas between 10m2 and 20m2 a single 150mm or two 100mm diameter pipes will suffice.
For areas larger than 20m2 a site specific design is required. Under drain spacing - there
should be one under drain per 3m width of bio-retention device.
Under drains should be evenly spaced along the length of the device. They should be
placed 75 to 300mm above the bottom of the drainage layer where no liner is present
(Top figure below) to allow for infiltration into the in-situ soils, or on top of the liner if
one is used (Bottom figure). There should be at least 25mm of drainage layer above the
top of the under drain.
Figure 10.7 : Location of underdrain
(top without liner)
(bottom with a liner)
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8. Liner : An impervious liner is required when bio-retention is used in geotechnically
unstable or steep sites greater than 1V:5H. The use of stormwater planters should be
considered in these situations.
9. Geofabric : The use of geofabric in the construction of bio-retention devices is generally
not recommended.
10. Root Barrier : The use of a root barrier should be considered when there are susceptible
services such (as sewers), or foundations which are likely to be at risk from root
penetration. The root barrier should only be placed adjacent to the services which require
protection and not around the whole device.
11. Filter Layer : A filter layer is required between the planting soil and the drainage layer.
A geofabric is not recommended for this purpose. The filter zone should consist of
50mm of pea-gravel (US #8 (1.18mm - 9.5mm) with 50mm of washed sand (0.5mm –
1mm) on top of it.
12. Ponding Depth : Ponding should normally be designed for a depth of 200mm above the
bed of the device.
13. Overflow : Ideally high flows should be by-passed to an overflow located outside the rain
garden. If an overflow is located within the overflow can be by means of a pipe/manhole
which connects into the underdrain system, or it could be a gravel curtain which connects
into the drainage layer.
14. Access: Suitable access needs to be provided for routine maintenance. For small
residential gardens may require access for a wheel barrow, while larger commercial
gardens will require more substantial access, suitable for a small excavator.
10.6
Construction
Ideally bio-retention devices must not be built until the rest of the site has been constructed and
the site stabilised. They must be protected from stormwater flows carrying high sediment loads
during construction activities from your site or neighbouring sites. If they are not protected then
the planting soil mix will need to be replaced. If work on the bio-retention device needs to
commence before the rest of the site is stabilised then the device should be constructed, but not
planted, and covered with a geofabric and topsoil. This will later be removed and the device
planted once the rest of the site has stabilised.
10.7
Maintenance
One of the important considerations with bio-retention devices is long-term maintenance.
Remember that a bio-retention device is a garden and not just a drainage system – they are
generally low maintenance, not NO maintenance.
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September 2009
They need water when it doesn’t rain until the plants are established. During dry periods the
under drain in the bio-retention devices may cause the planting soil to dry out. Watering the
vegetation on an as needed basis helps ensure a healthy condition and appearance.

Mulch annually with hardwood mulch as this suppresses weeds and retains moisture.

Every few years excess mulch may need to be removed.

Weed regularly as you would with any garden.

Don’t park or drive on the device as this causes compaction and leaves ruts.

Don’t let (fine) sediment build up – if a crust forms remove it & rework the top layer of
soil.

Keep an eye on the plants – if they are unhappy they may need moving. Plants may need
pruning, thinning or replacing from time to time.

Strong water flows may cause erosion, this will need to be repaired and measures put in
place to prevent recurrence.

Check the overflow for clogging and remove any build up of rubbish or debris regularly.
10.8

Additional Information
North Shore City Council Bio-retention Guidelines, First Edition, July 2008
10 - 7
Appendix F Owners Manual
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