New Zealand; Bioretention Guidelines (Rain Garden) - North Shore City Council

8/3/2019 New Zealand; Bioretention Guidelines (Rain Garden) - North Shore City Council

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Bioretention Guidelines


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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
http://www.skmconsulting.com/http://www.skmconsulting.com/


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ContentsGlossary iv

1. Background 1

2. What is bioretention and how does it work? 2

2.1 History 2

2.2 Bioretention process 2

2.3 Performance and design 6

3. Limitations 8

3.1 Geology 8

3.2 Maximum grades 8

3.3 Connection to the stormwater network or receiving environment. 10

3.4 Location 10

4. Bioretention gardens 12

4.1 Rain gardens 12

4.2 Stormwater planters 14

4.3 Tree pits 16

4.4 Bioretention swales 18

5. Engineering design 21

5.1 Location 21

5.2 Impervious liner 22

5.3 Geotextile liner 22

5.4 Inlet design 23

5.5 Surface storage and high flow overflow/bypass 27

5.6 Soils 29

5.7 Under drainage 31

5.8 Connections 33

6. Landscape design 34


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7.7 Planting 41

7.8 Tolerances 427.9 Construction Checklist 43

8. Maintenance 44

8.1 Access 44

8.2 Under drain 44

8.3 Fertilizing 44

8.4 Harvesting 458.5 Watering 45

8.6 Weeding 45

8.7 Pest damage 45

8.8 Mulching 46

8.9 Standing Water Problems 46

8.10 Rubbish and Debris 46

8.11 Pre-treatment 46

8.12 Maintenance Schedule 47

9. 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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GlossaryAdsorption: 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 transpirationfrom 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. BackgroundProtection 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 Councils

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 usedsuccessfully 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 deviceswithin 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 standards1.

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


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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 thatgovern 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 ofbioretention 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


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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 mechanisms3

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 filtrationTotal suspended solids, floating debris, trash, soil-boundphosphorus, some soil bound pathogens, soil bound metals

Figure 1: Bioretention processes


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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 standardsmust 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 imperviousarea 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 2m, with a minimum depth of at least

600mm.

The surface area of a bioretention garden is more important than the bioretention gardens

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 , 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.

4

The side slopes of a bioretention garden do not need to be vertical, and for construction


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The North Shore City Councils Plan Change 22 applies to developments less than 1000m2.

Bioretention gardens serving new impervious areas greater than 1000m2 must be designed tomeet 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 under-

drain 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

DetentionLayer

Detention layer 0mm -400mm (including100mm freeboard for over-flowdesigns)

200mm 300mm of detention should beapplied. The overflow should bedesigned to discharge high flows with100mm freeboard.

Mulch layer 50-75mm Organic decomposed mulch, if a rocksurface finish is desired this is additional.

Bioretention filtermedia

500 - 1000mm 300 mm minimum soil depth required forgrasses and small shrub, 1m depth isminimum required for trees.

Bioretentionlayer

Transition Layer 100mm Sand/ coarse sand.

Drainage layer 200 - 300mm A minimum 50mm gravel surroundingthe pipe on all sides.

Drainage layer

Storage depthbeneath the under-

drain

0mm 300mm Optional layer to provide greaterinfiltration.


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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


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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 Councils 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 slopeSlope 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 tominimise 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 anengineer.

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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11 First Edition July 2008


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 viaperforated 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 raingarden 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

Minimum size 2m2

Minimum depth 600mm + under drain +detention


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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 drainagelayer 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

Minimum size 2m2

Minimum depth 600mm + under drain +detention

Slope limitations Slopes 1:4 and greater are not suitable without geotechnicaldesign

Runoff type 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

Minimum size 2m2, although many trees will require a larger area

Minimum depth 1m + under drain + detention

Slope limitations Slopes 1:12 1:4 incorporate bench berms,

Slopes 1:5 use an impervious liner,

Slopes 1:4 and greater are not suitable withoutgeotechnical design

Runoff type 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. Bioretentionsystems 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,


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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 thegarden.

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 set-

backs 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


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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 roads 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 chapter3 for a full discussion of slope

limitations and the requirement for liners for devices located on sloping ground and nearbuildings.

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


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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 soilfrom 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 pre-

treatments 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 islikely 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


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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

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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 levelweir structure that reduces slope and distributes flow to the bioretention filter.

Figure 9: Concentrated inflow examples


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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


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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


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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 thepreferred 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, waterattenuation, 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 hour6, 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

Biofiltrations 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.


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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 non-

perforated.

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 layerbeneath 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 d 85 > 1 x


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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 pipewith 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.

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A single 110mm perforated pipe at 5% grade will be sufficient for a bioretention garden with

an area of 6m

2,

assuming there is a peak saturated hydraulic conductivity of 100mm/hr. Forlarger 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 coupledusing 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 ExceedenceProbability 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 standpipes 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



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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 streettree 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



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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 createedges, 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


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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 asize 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, stemand 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 begreater 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.


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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 thebase 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, siteconstruction 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.



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7.5 Backfill soil

Australian research has indicated that many existing bioretention gardens have less than the

design permeability rate10. 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 thefirst 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



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7.8 Tolerances

Conduct a final construction inspection, checking inlet, pre-treatment cell, bioretention celland 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 V t ti



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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 thebioretention gardens 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 publicstormwater 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



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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 (handpulling 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

http://www.arc.govt.nz/index.cfm?26D815A8-E018-8BD1-32D6-C3EF43975B56http://www.arc.govt.nz/index.cfm?26D815A8-E018-8BD1-32D6-C3EF43975B56


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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 re-

applied 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 poolingwater 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 canmitigate 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



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8.12 Maintenance Schedule

Effective long-term operation of bioretention gardens requires dedicated and routinemaintenance tasks performed to a consistent timetable.

Monthly 6 Monthly 12 Monthly 5-yearly

Remove weeds and replace dead plants. Eradicatenoxious/pest weeds and undesirable growth Litter removal

Inflow, overflow/outlets-check overflow for clogging.Remove accumulated sediments. Check overflowspillway Summer- monitor and water vegetation in extendeddry periods Pruning or thinning

Compost/Mulch replenishment (first 3 growing

seasons) Remove accumulated sediments; reinstate plants,soil and mulch. Check for ponding /clogging andblockage of filter media Inspect trees and shrubs and replace any dead orseverely diseased vegetation Scour/erosion evident: check for erosion signs.Check dams/capping system areas and correct asrequired Sump- accumulated sediments not more than 50%full Outlet manholes-check and remove silt frommanhole sumps Pre-treatment, inspection and silt removal asrequired Compost/Mulch replenishment (after first 3 growing

seasons) Check for restrictions/clogging/failures in pipes

Scrap off top 100mm of soil and mulch, dispose tolandfill, replace



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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 Environmentand 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 Georges 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 Common Name Height(mm)

Spread(mm)

Appearance & Tolerance

Apodasmia similis oioi 1500 1000 A reed with fine grey leaves and brown markings atintervals along the length of leaves.

Grows well in damp ground, tolerates open water,direct sun and salt spray.

Carex lessoniana Spreading swampsedge

1000 2000 Wide green bi-folded leaves and long hanging greenspikes.

Prefers damp or periodically damp areas, tolerant ofdirect sun and semi-shade.

Carex secta purei 1500 2000 Bright green sedge that forms in clumps, developinga trunk.

Tolerates damp and short periods of dryness, directsun 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 ofraingardens in wet and dry exposed conditions.

Cortaderia fulvida toetoe 1500 2000 The smaller of the toetoe. Branching from the baseto 1500mm high (flowers to 2000mm). Long strap-shaped leaves with red-orange coloured veins.

Prefers good drainage and semi-shade but cantolerate wet conditions and full sun.

Cyperus ustulatus toetoeupokotangata,umbrella sedge

1500 1500 Large, pale olive-green curving sedge with largedark brown or purple spikes.

Prefers wet edge conditions but can tolerate periodsof dry and exposure to full sun.

Dianella nigra turutu 500 1000 Lily with reddish leaves, small white flowers, andstriking violet-blue fruit.

Does well in shade and open areas but prefers well-drained situations.

Gahnia xanthocarpa tarangarara 3000 3000 Very stout sedge that can grow to over 3000mmhigh. Sharp leaves may be used to dissuade publicaccess.

Grows well in boggy conditions and can toleratesemi-shade and direct sun.

Gleichenia dicarpa tangle fern 500 1500 Forms springy interlacing and compact thickets.

Prefers wet soils, but is comfortable in shady orexposed conditions. Can be difficult to established.

f



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Libertia grandiflora &L. ixioides

mikoikoi, native iris 400 400 Clump forming native irises with narrow, uprightleaves. Small white flowers form on tall spikes inspring.

Prefers well drained wet soils. Tolerates sun orshade and periods of dryness.

Machaerina sinclairii tuhara 1000 1500 Large and leafy pale green sedge with gracefulseed head.

Grows well in seepage areas and prefers shade, butcan tolerate edge conditions.

Muehlenbeckiacomplexa pohuehue 1000 2000 Dense sprawling divaricating shrub, good habitatvalue and weed suppressant but can take over ifinter-planted.

Tolerates very dry to well-drained wet conditions infull sun or semi-shade.

Phormium cookianum mountain flax

wharariki,

1500 1500 Clump-forming flax with drooping yellow greenleaves.

Tolerates full exposure and sun. Prefers edges andwell-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 andsurvives in boggy and dry environments.

Schoenus tendo wiwi 1000 1000 Rush like sedge up to 1000mm tall light green culmsthat turn to orange in the sun.

Prefers edge conditions and tolerates direct sun andsemi-shade.



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Small Trees and Shrubs (6m2 rain garden recommended for trees)

Botanical Name Common Name Height(mm)

Spread(mm)

Appearance and Tolerance

Carpodetus serratus putaputaweta 600 300 Horizontally branching shrub with marbled leavesand a white flowers..

Light shade will encourage dense undergrowth andprefers damp free draining soils. Tolerates directsun.

Coprosma propinqua mingimingi, black scrub 3000 2000 Bushy, dark green divaricating shrub with smallleaves.

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 thatattracts 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 largeheads of linear leaves and panicles of scentedflowers.

Dense taproots, so they must be kept away from

under drains. Sun to semi-shade. Prefers damp tomoist soil.

Cordyline banksii ti ngatere, forestcabbage tree

5000 2000 Branching from the base and forming a clump. Longstrap leaves with red-orange veins.

Prefers good drainage and semi-shade

Fuchsia excorticata kotukutuku 5000 2000 A deciduous small tree up to 12000mm high thathas a very distinctive stripped orange bark and red-purple flowers.

Prefers cool shade and moist, well drained soils.

Hoheria populnea houhere, lacebark 8000 3000 A fast growing tree with abundant flowers.

Prefers well draining soils. Tolerates sun but prefersprotection from wind exposure.

Leptospermumscoparium

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 includingsalt and wet edges.

Melicytus ramiflorus mahoe 5000 3000 A white barked tree with a spreading habit and largeserrated 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 andred stems. Can be clipped to form a hedge.

Grows well in direct sun or shade but prefers welldraining or drier soils



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Large Trees (10m2 rain garden recommended for one tree)

Botanical Name Common Name Height(metres)

Spread(metres)

Appearance and Tolerance

Alectryon excelsus titoki 8 6 A tree that is common for streetscapes,spreading from a single trunk with light greenglossy leaves.

Slightly frost tender, but tolerant of shadeand direct sun. Prefers well drained moistsoils.

Cyathea medullaris mamaku 12 4 Hardy, fast growing tree fern with 3m fronds

Comfortable in moist soils and exposedplaces, shade and frost tolerant.

Dacrydiumcupressinum

rimu 60 10 A beautiful tree with hanging needles andweeping branches in juvenile form.

Prefers well drained soils, sun and semi-shade.

Dacrycarpusdacrydioides

kahikatea 20 10 Kahikatea are tall native pines with uprightneedles. The tree will take a few seasons todevelop 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 foliageand panicles of white flowers.

Requires some shelter or companionplanting when young, with low tolerance ofdirect sun. Prefers intermittently wet soilsonly.

Laurelia novae-zelandiae

pukatea 10 3 A distinct buttressed tree with glossy green,erect foliage.

Can be slow growing but performs well indamp areas. Tolerates sun and shade.

Podocarpus totara totara 10 9 A hardy tree of dense spindle like foliagethat can achieve large sizes or can beclipped to form a hedge.

Easy to establish and tolerant of open orshade, wet or dry conditions.

Schefflera digitata pate 5 4 Seven-finger foliage with massing of fruit forbirds.

Prefers damp and shady conditions but can

tolerate direct sun in a shelteredenvironment.



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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 ofstormwater 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, Mannings 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 tocalculate 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 forL (opening width).



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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 24hours 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 bio

New Zealand; Bioretention Guidelines (Rain Garden) - North Shore City Council

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Transcript of New Zealand; Bioretention Guidelines (Rain Garden) - North Shore City Council