Bioretention Guidelines

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1 First Edition July 2008

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3 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 , Wellington, New Zealand T F and Mark Lewis of BOFFA MISKELL Level 3 IBM Centre, 82 Wyndham Street, PO Box 91250, Auckland 1030 T F EDITED BY: REVIEWED BY: GRAPHICS BY: APPROVED FOR RELEASE: Chris Stumbles Robyn Simcock, Tom Schueler, Earl Shaver BOFFA MISKELL Level 3 IBM Centre, 82 Wyndham Street, PO Box 91250, Auckland 1030 T F Jan Heijs (Infrastructure Planning Manager) July 2008 First Edition i

4 Contents Glossary iv 1. Background 1 2. What is bioretention and how does it work? History Bioretention process Performance and design 6 3. Limitations Geology Maximum grades Connection to the stormwater network or receiving environment Location Bioretention gardens Rain gardens Stormwater planters Tree pits Bioretention swales Engineering design Location Impervious liner Geotextile liner Inlet design Surface storage and high flow overflow/bypass Soils Under drainage Connections Landscape design Landscape Plant Selection Construction Excavation Timing Geotextile and liners Backfilling gravel Backfill soil Erosion Checks 41 ii First Edition July 2008

5 7.7 Planting Tolerances Construction Checklist Maintenance 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 July 2008 First Edition iii

6 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. iv First Edition July 2008

7 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. July 2008 First Edition 1

8 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 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 1000m 2. Bioretention gardens serving new impervious areas greater than 1000m 2 must be designed to meet TP10 design standards 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 2 First Edition July 2008

9 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 to 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 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 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 mm (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 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 July 2008 First Edition 3

10 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) 4 First Edition July 2008

11 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 Microbial processes Sedimentation and filtration Small amounts of nutrients including phosphorus and nitrogen Organics, pathogens Total suspended solids, floating debris, trash, soil-bound phosphorus, some soil bound pathogens, soil bound metals Figure 1: Bioretention processes 3 Brix, H Wastewater treatment in constructed wetlands system design, removal processes, and treatment performance. Pp in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed). Boca Raton, Fla.; CRC Press, 632 pp. July 2008 First Edition 5

12 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 4m 3 of on-site detention must be provided per 100m 2 of impervious area less the rainwater harvesting volume (which should be a minimum of 2m 3 ). 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 4 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. 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) 6 First Edition July 2008

13 The North Shore City Council s Plan Change 22 applies to developments less than 1000m 2. Bioretention gardens serving new impervious areas greater than 1000m 2 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 Bioretention layer Drainage 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. Mulch layer 50-75mm Organic decomposed mulch, if a rock surface finish is desired this is additional. Bioretention filter media mm 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 mm A minimum 50mm gravel surrounding the pipe on all sides. Storage depth 0mm 300mm Optional layer to provide greater beneath the underdrain infiltration. 5 Auckland Regional Council, 2003 Technical publication 10, Design Guideline Manual: Stormwater Treatment Device July 2008 First Edition 7

14 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. 8 First Edition July 2008

15 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 (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 Less than 1V:5H 1V:5H - 1V:4H Liner No 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 July 2008 First Edition 9

16 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 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. 10 First Edition July 2008

17 Figure 3: Setback limitations 11 First Edition July 2008

18 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 Minimum size 2m 2 Minimum depth 600mm + under drain +detention Slope limitations 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 Runoff Type Roof and surface runoff Applications Single residential lot, commercial lots, roadways and carparks July 2008 First Edition 12

19 Figure 4: Rain garden 13 First Edition July 2008

20 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 Minimum size 2m 2 Minimum depth Slope limitations Runoff type Applications 600mm + under drain +detention Slopes 1:4 and greater are not suitable without geotechnical design Roof runoff Residential and commercial July 2008 First Edition 14

21 Figure 5: Bioretention Planter 15 First Edition July 2008

22 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 Minimum depth 2m 2, although many trees will require a larger area 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 without geotechnical design Runoff type Surface runoff Applications Roadways and carparks July 2008 First Edition 16

23 Figure 6: Tree Pit 17 First Edition July 2008

24 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. July 2008 First Edition 18

25 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 Applications Surface runoff Roadways and carparks July 2008 First Edition 19

26 Figure 7: Bioretention Swale 20 First Edition July 2008

27 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 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 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 21 First Edition July 2008

28 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 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. 22 First Edition July 2008

29 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 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 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. July 2008 First Edition 23

30 Figure 8: Distributed inflow examples 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 24 First Edition July 2008

31 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 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. July 2008 First Edition 25

32 Figure 10: Inlet scour protection 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 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. 26 First Edition July 2008

33 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 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 mm, 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 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. July 2008 First Edition 27

34 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 28 First Edition July 2008

35 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 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 mm/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 Auckland Regional Council, 2003 Technical publication 10, Design Guideline Manual: Stormwater Treatment 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 July 2008 First Edition 29

36 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 % of the medium should pass through a 10mm sieve and % 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% Bulk density -wet wt basis (kg per m -3 ) ph range Magnesium 40kg/ha min Phosphorus(P ) 80kg/ha min Potassium (K 2 0) 95kg/ha min 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%. 30 First Edition July 2008

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