Curbing the Flow by Reuben McPeak

CURBING THE FLOW How can high gradient land be better utilised to treat stormwater runoff in Auckland?

Reuben McPeak 2015 Bachelor of Landscape Architecture Unitec Institute of Technology

CONTENTS

iii iv iv

List of Figures List of Tables Image Credits

Abstract 1.0

THE AUCKLAND FLOW:

1.1 1.2 1.3 1.4 1.5 1.6

2.0

HIGHER GRADE THINKING:

2.1 2.2 2.3 2.4 2.5 2.6 2.7

3.0

APPLICATION THROUGH SITE TRIAL: Three Sites, Three Contexts

3.1

Stormwater Presence & Causes Contaminants & Their Effects Auckland Going Forward Te Aranga Principles Case Studies Aucklandâ&#x20AC;&#x2122;s Inclined Land

Precedent Research Case Studies Expression Trials Device Development Hydrology Calculation Device Layout The Living Elements

Vaughans Road 3.1.1 Stormwater Source & Flow 3.1.2 Site Location & Opportunity

Stormwater in a Local Context

2 4 5 7 9 11 Approaching WSD on Sloping Land

15 17 20 27 29 33 37 41

45 51 53

3.2 3.3

3.1.3 Wider Treatment Train Potential Penguin Drive 3.2.1 Stormwater Source & Flow 3.2.2 Site Location & Opportunity 3.2.3 Wider Treatment Train Potential Phyllis Reserve 3.3.1 Stormwater Source & Flow 3.3.2 Site Location & Opportunity 3.3.3 Wider Treatment Train Potential

4.0

THE AUCKLAND POTENTIAL:

Mapping Possibilities

4.1 4.2

5.0

REFLECTION: Retrospective Planning

Hibiscus & Eastern Bays Local Board Albert-Eden Local Board

56 62 64 67 69 76 79 81 84 89 92 96 99

LIST OF FIGURES

iii

FIGURE 1-2

Developable Land Categorisation - based on Bohard (1995)

FIGURE 1.1

Urban Water Management Transitions Framework - reproduced from Brown et al. (2008)

FIGURE 2.1

Swale Components

FIGURE 2.2

Conceptual Slope Terracing and Stormwater Flow

FIGURE 2.3

Conceptual Terrace Weirs

FIGURE 2.4

Slope Retention

FIGURE 2.5

Overland Flowpath in High Rainfall Events

FIGURE 3.1.1

North-West Link

FIGURE 3.1.2

Okura/Long Bay Regional Analysis

FIGURE 3.1.3

Okura Area Stormwater Flow and Drainage

FIGURE 3.1.4

Cross-Section A1: Vaughans Road Site Analysis & Potential

FIGURE 3.1.5

Vaughans Rd Proposed Wider Treatment Train

FIGURE 3.1.6

Cross-Section A2: Vaughans Road Proposed Treatment Slope: Sectional Analysis

FIGURE 3.1.7

Vaughans Road Treatment Slope - Perspective View

FIGURE 3.2.1

Zoning for Taiorahi Catchment in the Proposed Unitary Plan

FIGURE 3.2.2

Murray’s Bay Wider Hydrology Analysis

FIGURE 3.2.3

Cross-Section B1: Penguin Drive Site Analysis & Potential

FIGURE 3.2.4

Penguin Drive Proposed Wider Treatment Train

FIGURE 3.2.5

Cross-Section B2: Penguin Drive Proposed Treatment Slope - Sectional Analysis

FIGURE 3.2.6

Penguin Drive Treatment Slope - Perspective View

FIGURE 3.3.1

Phyllis Reserve Contextual Analysis

FIGURE 3.3.2

Phyllis Reserve Stormwater Flow & Drainage

FIGURE 3.3.3

Cross-Section C1: Phyllis Reserve/Oakley Creek Site Analysis & Potential

FIGURE 3.3.4

Phyllis Reserve Proposed Wider Treatment Train

FIGURE 3.3.5

Cross-Section C2: Phyllis Reserve/Oakley Creek Proposed Treatment Slope – Sectional Analysis

FIGURE 3.3.6

Phyllis Reserve Treatment Slope - Perspective View

FIGURE 4.1

Hibiscus & Bays: Significant Ecological Areas

FIGURE 4.2

Hibiscus & Bays: Conservation & Informal Recreation Zoning

FIGURE 4.3

Hibiscus & Bays: Catchment Analysis

FIGURE 4.4

Hibiscus & Bays: High-Gradient Land

FIGURE 4.5

Hibiscus & Bays: Steep Land X Conservation / Informal Recreation Zoning

FIGURE 4.6

Albert-Eden: Significant Ecological Areas

FIGURE 4.7

Albert-Eden: Conservation & Informal Recreation Zoning

FIGURE 4.8

Albert-Eden: Catchment Analysis

FIGURE 4.9

Albert-Eden: High Gradient Land

FIGURE 4.10

Albert-Eden: Steep Land X Conservation / Informal Recreation Zoning

LIST OF TABLES TABLE 1.1

Vehicle Related Contaminants

TABLE 1.2

Environmental Effects of Stormwater Contaminants

TABLE 2.1

WSD Treatment Device and Stormwater Control

TABLE 2.2

Slope Limitations of WSD Devices

TABLE 2.3

Swale Dimensioning through Hydrology Calculation

15 32

IMAGE CREDITS All images were photographed or created by the author with the exception of those listed below: p9,

Image 2

Google satellite imagery

p10,

Image 1

Retrieved October 18, 2015 from http://wikimapia.org/12666951/Barry-Curtis-Park

Image 2 Retrieved October 18, 2015 from http://www.landcareresearch.co.nz/science/living/cities,-settlements-and-com munities/urban-stormwater-management/catchment-management-and-hydrology

p17

Images 1 & 2

Izembart & Le Boudec, 2008

p18

Image 3

Opus, 2015

p42

Image 1

Google satellite imagery

p43

Image 1

Google satellite imagery

ABSTRACT Aucklandâ&#x20AC;&#x2122;s existing stormwater reticulation systems are practical in the Drained City mindset, but are ecologically harmful. Current water sensitive design (WSD) practice centres on integrated catchment management within which catchment runoff is calculated and remediation techniques implemented. This provides clear and objective deliverables for the defined area, but relies on generally flat land to do so within. The potential for alternate uses of this flat land raises the question:

How can high gradient land be better utilised to treat stormwater runoff in the Auckland region? The potential for stormwater treatment on high gradient (2540%) sloping land was analysed, comparing and contrasting with existing WSD practices. Three sites were selected as representative of some of the cityâ&#x20AC;&#x2122;s range of catchment and urban development characters. Analysis and design of slopebased stormwater remediation were considered for each site and across a wider treatment train. Design expression in each context achieved volume and contaminant remediation capabilities while adding value to the social and ecological construct.

1.0

THE AUCKLAND FLOW

STORMWATER IN A LOCAL CONTEXT

The population of Auckland is growing at a rate far greater than anywhere else in the country. Within the next 30 years, Auckland Council (2014) anticipates population levels to increase by between 700,000 and one million people. Existing residential area is expanding to cater to present and future growth through subdivision, in-filling, and high-rising. Debate exists as to whether the best option for Auckland is to give priority to expanding laterally to cater to this growth, or to focus on intensifying the existing urban space. The benefits of lateral growth centre around maintaining land package sizes in urban areas to a quality that kiwis are more familiar with – given our heritage of backyard living and open views and spaces. Intensification through in-filling and building upward centres on conserving the quality of land beyond the urban realm, where the existing character of rural, coastal and conservation land is retained. Arguments exist for and against each of these angles. The present government is in support of the extension of city boundaries to open up greenfields for further housing and development. Conversely, Auckland Council (2012) side with the benefits of prioritising intensification, stating, “In their submissions on the draft Auckland Plan, Aucklanders strongly supported the principle of a quality compact city”.

1.1

STORMWATER PRESENCE & CAUSES

With city growth comes an increased area of impervious surface. Additional roofs, driveways, footpaths and roads are constructed to cater to the growing population. As rainfall runs over these surfaces it picks up contaminants from everyday urban life, these subsequently carried with the water as it drains to waterways.

Structural and infrastructural issues also result through high stormwater runoff volumes. The integrity of the banks of local streams and waterways can be challenged during high-flow events where water volume and velocity far exceed normal levels. the resulting flooding and/or erosion can put both people and their property at risk.

Waterborne pollution occurs when runoff carries pollutants away from the road and into sensitive receiving environments. Road runoff is a concern throughout the country, influencing regional development policies and mitigation practices and bringing about a standardisation of highway treatment on a national level (NZTA, 2010). Car ownership rates in New Zealand continue to be one of the highest in the world (Conder, 2009), and runoff rates will only increase with additional roads and road users.

Aucklandâ&#x20AC;&#x2122;s wastewater pipes are designed to handle five times the normal flow, to accommodate stormwater input in high rainfall events (Watercare, 2010). Overflows occur when these events stretch volumes to beyond capacity. A number of overflow points exist across Auckland, with excess being discharged into local waterways. Watercare calculated in 2012 that over 2,200,000m3 of diluted wastewater is released into these environments from the Central Interceptor on an annual basis.

Adverse environmental effects resulting from stormwater run-off are recognised by the Auckland Council to be caused by the following contaminants: Sedimentation Oils & Greases

Heavy Metals and Toxic Substances Nutrients and Bacteria

NZTA tabulate the source of key vehicle-related contaminants. This is replicated in Table 1.0.

TABLE 1.1

Vehicle Related Contaminants

Contaminant

Contaminant Source

Particulates Lead Zinc Iron Copper Cadmium Chromium Nickel Manganese

Pavement wear, vehicles, atmospheric, maintenance works Tire wear (lead oxide filler), lubricating oil and grease, bearing wear Tire wear (filler material), motor oil (stabilising additive), grease Rust, steel highway structures, moving engine parts Metal plating, bearing and brushing wear, moving engine parts, brake lining wear Tire wear (filler material) Metal plating, moving engine parts Diesel fuel and petrol exhaust, lubricating oil, metal plating, brushing wear Moving engine parts

1.2

CONTAMINANTS & THEIR EFFECTS

As contaminants from vehicles and their emissions collect on the road surface, neighbouring and connected waterways are put at risk of contamination. Elston et al. (2015) recognise that three-quarters of New Zealand’s native freshwater fish, crayfish and mussel species are now listed as threatened with extinction, attributing stormwater runoff as a key contributor. Table 1.2 lists the adverse effects contaminant groups have on aquatic ecology, drawn from Technical Publication 10 (TP10) (Auckland Regional Council, 2003). Additional detrimental effects on ecological systems include changes to temperature, dissolved oxygen, pH and conductivity.

TABLE 1.2

Environmental Effects of Stormwater Contaminants

Contaminant

Adverse Effect

Suspended Sediment

Sediment in the water can reduce light levels for aquatic fauna and flora. It can also clog fish gills and obstruct feeding processes for filter feeding shellfish.

Organic Matter

Natural processes that biodegrade organic matter consume oxygen, reducing levels in the water required by aquatic organisms. Viruses and bacteria derived from sewerage overflow can cause disease. As solid or dissolved metals don’t decompose, they accumulate in sediments, filter-feeding animals and plants. General organism health is deteriorated, leading to symptoms such as decreased fertility levels, a higher mortality rate, and susceptibility to infection. Organisms avoid the affected area as levels build up, leading to reduced ecological diversity in the area. Oils form a layer on the surface of organisms, creating a barrier to respiration. PAH’s (polyaromatic hydrocarbons) are direct toxins to flora and fauna, poisoning organisms. High nitrogen and phosphorus levels in the water can stimulate excess plant and algal growth. This can block sunlight, as well as bringing about daily fluctuations in available oxygen. Largely aesthetic, gross pollution such as litter has little impact on ecological health.

Pathogens Metals

Hydrocarbons & Oils Toxic Organics Nutrients Litter

1.3

AUCKLAND GOING FORWARD

Auckland Council (2014) quote the Urban Water Management Transitions Framework (Brown et al, 2008) in terms of locating itself on the pathway of development toward a “Water Sensitive City”. With the ultimate goals of resilience to climate change and adaptive infrastructure that reinforces values of water sensitivity, Figure 1.1 locates Auckland City’s 2014 stormwater practice as symptomatic of a “Drained City”. While government bodies discourage water wastage and the discharging of hazardous substances into the city’s stormwater grid, stormwater remains largely channelled through gutters and pipes to the nearest waterway.

Yet Auckland is steadily moving toward the “Waterways City” ideal. Waterways are being recognised as less of a stormwater conveyance and flood mitigation means, and are increasingly appreciated for their aesthetic and ecological amenity. These values are establishing themselves in the minds of Aucklanders first with larger water bodies, moving subsequently upstream to smaller local streams and drainage channels.

CUMULATIVE SOCIO-POLITICAL DRIVERS Water supply access & security

Public health protection

Flood protection

Social amenity, environmental protection

Limits on natural resources

Intergenerational equity, resilience to climate change

WATER SUPPLY CITY

SEWERED CITY

DRAINED CITY

WATERWAYS CITY

WATER CYCLE CITY

WATER SENSITIVE CITY

Supply hydraulics

Separate sewerage schemes

Drainage, channelisation

Point & diffuse source pollution management

Diverse, fit-forAdaptive, multipurpose sources functional infrastructure and conservation, & urban design promoting waterway reinforcing water protection sensitive behaviours

SERVICE DELIVERY FUNCTIONS

Figure 1.1

Urban Water Management Transitions Framework - reproduced from Brown et al. (2008)

Tools are in place to standardise not just the reduction of harmful inputs into our waterways, but also the treatment of stormwater pre-release and the replenishing of poorly waterways. This yearâ&#x20AC;&#x2122;s release of Auckland Councilâ&#x20AC;&#x2122;s Water Sensitive Design for Stormwater (2015b) builds on the respected TP10, making water sensitive design concepts and guidelines more accessible for design practitioners. This adds to the important and respected role NZ Transport Agency (2010) has had in establishing benchmark requirements for trunk lines and their peripheral landscaping.

1.4

TE ARANGA PRINCIPLES

The respect and appreciation of the wider environmental health visible in seeking to move closer to the Waterways City comes from a similar vein to traditional Te Aranga Principles. Auckland Council (2015a) promotes these seven foundational tenets in the Auckland Design Manual, bringing them together as a package to inform design and development with affirmation to Māori cultural priorities. 1. MANA The status of iwi and hapu as mana whenua is recognised and respected 2. WHAKAPAPA

Māori names are celebrated

3. TAIAO The natural environment is protected, restored and/or enhanced 4.

MAURI TU

Wider environmental health is protected, restored and/or enhanced 5.

MAHI TOI

Iwi/hapū narratives are captured and expressed creatively and appropriately 6. TOHU Mana whenua significant sites and cultural landmarks are acknowledged.

AHI KĀ

Iwi/hapū have a living and enduring presence and are secure and valued within their rohe.

1.5

CASE STUDIES

New suburban development in Auckland and beyond represents positive steps toward bringing the waterways-centred view to a normative place. The application of WSD concepts across public space through subdivisions not only begins the stormwater treatment process at the roadside/footpath where contamination begins. It also makes visible the treatment process - an important element in promoting public education of waterways and their contributors.

HOBSONVILLE POINT, AUCKLAND As a high-density housing area, impervious surface area is correspondingly high. The effect of this on stormwater runoff volumes and contaminant is countered with narrower roads and large amounts of open grassed space surrounding the housing areas. Walking is encouraged in the area (cf. driving) by the convenient location of local amenities as well as pedestrian routes being open and appealing - much of these with pervious surfaces. Inherent drainage catchments have been the foundation of subdivision layout in Hobsonville Point. Natural troughs in the land have been utilised as stormwater treatment wetlands, and natural gradients direct the flow of water toward them. Wetands are set in park-like surrounds where function and aesthetic appeal converge. The use of available land is maximised for infiltration and phytobial processing, while open space is provided, enhancing value and livability of the area. Vegetated swales and rain gardens are in abundance through this subdivision, reducing conveyance volume by promoting infiltration at source.

FLAT BUSH, AUCKLAND The layout of Flat Bush is catchment-based in its greater form, paying homage to the areaâ&#x20AC;&#x2122;s natural drainage patterns. The township and precincts are designed around Otara Stream and its contributors. Buffer zones separate development from riparian areas, and grassed areas on the periphery of riparian planting perform a filter strip function to treat road runoff. Green fingers established by the location of natural hydrology influence the placement of playing fields and informal recreation spaces. Commercial and residential zoning is kept at a distance from Otara Stream, separated by open space and stormwater detention ponds. The urban catchment proposes 49 stormwater ponds to manage both quantity and quality of runoff before it is released into the waterway. Similar to Hobsonville Point, housing density here is high and residential zones bring together a great area of impervious surface. However, Auckland Regional Councilâ&#x20AC;&#x2122;s TP108 (1999) has been utilised to anticipate and plan appropriately for effective stormwater processing.

1.6

AUCKLAND’S INCLINED LAND

The Auckland Design Manual pushes for a more productive use of available space by promoting a higher housing density to afford the city’s increasing population. While a range of variables affect the ‘value’ of a site, including location, aspect, ground stability and accessibility, the gradient of the existing landscape plays an important role in determining the land’s usability and subsequent worth to a developer. This is particularly relevant on steep land which may be deemed unusable by some. Steep slopes can only be built on with much capital investment (Valuation Consultants, 2012), so are relegated as lowest priority for development. In addition, it is not uncommon for steep land to present issues of erosion

and poor accessibility. Regarded as having low functional value, these spaces are often retired with marginal riparian planting. Bruce Phythian of AVJennings (pers comm., July 29, 2015) advises that there is no gradient too steep for development. The proviso for this statement is, however, an adequate backing of funds and/or incentive. Eric Bohard (1995) categorises land developability in a generalised sense based on land gradient (see Figure 1.2). Bohard’s ‘Critical 1’ category labels land within the 25-40% slope range as very difficult to develop. This recognises that there is potential

SLIGHT

INCLINED

HILLSIDE

STEEP

<8%

8-15%

15-25%

25-40%

>40%

Industrial Buildable

Commercial Buildable

Residential Buildable

Critical 2

Critical 1

Ease of Development & Roading

Ease of Development

Challenge to Development

High Restriction to Development

Undevelopable

0% Slope

8% Slope

15% Slo

25%

Slope

40%

VERY STEEP

Slo

25-40% Slope

Target Land Gradient

Figure 1.2

Developable Land Categorisation - based on Bohard (1995)

for development with the right resources, though in most instances development of the land poses too great a challenge to deem worthwhile. Sites with gradients greater than 25% are similarly labelled as steep by the Environmental Protection Agency (EPA, 2014), suggesting these slopes be protected and retired. The mapping of 25-40% gradient land across the wider Auckland highlights the prioritising of flatter land in the city’s spread. Land allocated as ‘Future Urban’ in Auckland Council’s Proposed Unitary Plan (2015c) occupies low gradient pockets between the “undevelopable”. In a similar light to the demand for space, a growing population adds growing strain on existing infrastructure. Auckland Council’s promotion of water sensitive design prioritises a healthier ecology and landscape for the environment and resident. While gravity and gradient are essential for water movement and the effective operation of ‘conventional’ WSD, the practice occupies the flatter of developable land which has potential for recouping an otherwise better economic return. The abundance of high-gradient slopes presented by the region’s volcanic foundations appears at economic odds with the incorporation of water sensitive design in new subdivisions as well as its integration into existing urban space. The questions is thus raised: How can high gradient land be better utilised to treat stormwater runoff in the Auckland region?

2.0

HIGHER GRADE THINKING

APPROACHING WSD ON SLOPING LAND

A range of devices exist in the toolkit of WSD stormwater treatment. All delivery systems combine several devices in a “treatment train”, to bring together the varying benefits of the devices used. The choice of device/s to be utilised in the train depends on a range of factors, including microclimate, slope, soil type, depth of bedrock, contaminants to be addressed, volume of water and the frequency of inundation. These devices can be generalised into several categories: • Gross pollutant trapping • Sedimentation (ponds & basins) • Filtration (swales & filter strips) • Constructed wetlands (highly vegetated shallow pond) • Bioretention (rain gardens, tree pits and planter boxes) • Infiltration • Subsurface wetland NZTA (2010) highlights the efficiency of stormwater management and specific contaminant removal by each device type. This is shown in Table 2.1.

Rain garden and public space. Daldy St, Auckland.

TABLE 2.1

WSD Treatment Device and Stormwater Control

Device Type Pond (dry) Pond (wet) Wetlands Filter Systems Rain Gardens Infiltration Swales & Filter Strips

Water Quantity Peak Control Capability

Sediment

Lead

Water Quality Capability Copper Zinc

High

Moderate

Low

High

Moderate

Low

High

Low

High

Moderate

Low

High High

TPH Low

Low

High

Moderate

High

Low

High

Moderate

Vegetated swale capable of supporting high volume storm events. Lincoln, Canterbury.

2.1

PRECEDENT RESEARCH

NZTA (2010) has also considered the efficiency of treatment devices on a higher gradient site relating to the key contaminants of concern from a roading environment. This draws closer to the subject study, and is summarised in Table 2.2 below. TABLE 2.2 Device Type

Slope Limitations of WSD Devices

As the slope increases, the amount of cuts and/or fills increases. Ponds generally are not suitable on slopes > 10%.

Sand Filters

Sand filters can either be prefabricated units or constructed in place. For prefabricated units, generally live storage can be provided within the unit so slope is not a critical issue. For open systems, the slope problems are similar to ponds or rain gardens. Similar to ponds, live storage is a problem on steeper slopes. The surface of the rain garden must be level to ensure an even flow through the media.

Rain Gardens

Slope Limitation

Ponds/Wetlands

Infiltration

Infiltration practices are not recommended on steeper slopes or on fill slopes. There is a potential for slope instability, with seepage coming out on the slope below the practice or for lateral flow to occur at the natural ground/ fill interface. Infiltration should only be used when a geotechnical engineer certifies it as appropriate.

Swales/Filter Strips

Not suitable for slopes > 5% unless check dams are used to flatten overall slope.

In addition to the context of the site in contemplating what WSD device/s (and layout) might provide best fit for a sloping environment, factors inherent to working on a gradient are an important consideration. Soil Disturbance Soil structure is retained by minimal soil disturbance â&#x20AC;&#x201C; important in reducing erosion through maintained soil stability â&#x20AC;&#x201C; as well as avoiding the WSD processing being brought unnecessarily close to bedrock or other semi/impermeable horizons. Minimal cut and fill also achieves a light ecological footprint on the site. Weight Distribution One of the greatest limitations of working with water on a slope is the additional weight and stress placed on slope stability. Reduction and distribution of added weight minimises this pressure. Water Depth Increasing water depth on a site results in a reduced coverage of vegetation. This leads to a reduced contact surface area on which bioprocesses can work to deactivate contaminants in stormwater. Water Retention & Residence Time Holding stormwater on site maximises its infiltration and treatment time and helps mitigate pressure on outflow systems in a storm event. Flow Velocity Reducing the speed at which stormwater travels down a slope minimises the incidence of scouring and erosion of the soil. It also assists in maximising residence time and subsequent water treatment. High Soil/Plant Interface A high contact surface area between plants and soil achieves a high microbial degradation of stormwater contaminants, as well as ensuring maximum evapotranspiration.

Urban high-gradient vegetated slope surrounded by urban development. This slope in Newmarket Gully, Auckland, is an example of land potentially suitable for hosting a stormwater treatment device.

2.2

CASE STUDIES

PENNESSIERES, FRANCE Izembart & Le Boudec (2008) detail the function of the wastewater treatment plant in the rural township of Pennessieres, France. A vegetated gravity-driven steppedponding plant is utilised to process the municipalityâ&#x20AC;&#x2122;s wastewater. The process comprises a chain of several stages, wherein each stage achieves a separate function. Settlement, filtering, evapotranspiration and oxygenation are carried out within this sequence. The staging involves attention to holding and release to ensure the waste material spends an appropriate duration within each pond. Plant species vary within this regime depending on function of the stage. A key species is Schoenoplectus lacustris, a common clubrush. The intense growth of the rhizomes of this species, even through winter, favours the use of this plant. Rhizomes provide aeration to the waste matter, and also possess an antibacterial effect. Phenol contaminants within the subject matter are also dissipated through the action of this plantâ&#x20AC;&#x2122;s rhizomes. The operation of this treatment plant is independent of electricity. Its vegetative element requires maintenance once every several years. Much can be learned and translated from this plant for application to use in a stormwater environment. Attention to the interaction between contaminant, plant species and processing time is particularly relevant.

NEWMARKET PARK, AUCKLAND The re-assessment of stormwater flow in the Newmarket area has come about with the upgrade of the local rail crossing and the areaâ&#x20AC;&#x2122;s local connections. Combined sewerage and stormwater overflow into Newmarket Stream is a common occurrence, and the capacity of local catch-pits is insufficient. Among other things, Opus International (NZ) have been tasked with planning the intended stormwater processing on site. The two contending factors most relevant to this study are the high gradient of the subject sloping land, and the history of the slope as a capped landfill, leading to the existing situation of high instability and caution regarding toxins beneath ground level. Options were considered for a treatment device midway down the slope, as well as at the flatter land below the slope. The latest publication on the project by Opus (2015) verifies that a device will be placed at the top of the slope, where weight is better supported by reduced gradients. The use of a rain garden has been decided upon for the site, with confidence that ample soil engineering will ensure the added weight to the slope at capacity will not test the siteâ&#x20AC;&#x2122;s integrity. The rain garden will be lined to prevent the generation of leachate and avoid further destabilisation of the slope. A vegetated swale running from the east of Cowie St has been added to the concept, diverting a portion of the catchment away from the sensitive slope toward an ornamental pond within Newmarket Park. Were toxic material and unstable fill not a concern, there might have been opportunity for infiltration as a supporting function of the stormwater treatment. More stable land below the park presents opportunity, however, best practice advocates treating runoff as close to the source as is practical.

2.3

EXPRESSION TRIALS

In treating stormwater runoff within the 25-40% gradient constraints, several objectives were arrived at to assist in assessing potential designs. • High opportunity for infiltration of stormwater into the soil. • Low flow velocity, to avoid erosion potential, as well as assisting in increased infiltration rates through a longer residence time. • High contact surface area of soil/plant to maximise rates of adsorption and evapotranspiration. Interviews with Mark Lewis - Landscape Architect and Principal at Boffa Miskell (pers. comm., April 28, 2015) and Robert Gordon - Principal Environmental Engineer at Opus (pers. comm., July 31, 2015) agreed with concerns of the challenge. The two primary cautions were: - Maintaining integrity of slope, due to the increased weight of stormwater being retained for any period of time; and - Ensuring the site and device are reasonably accessible for periodic maintenance. A number of generalised expressions were assessed in the movement of water down a slope. Contaminant removal through varying functionalities was explored, with the key objectives and concerns in mind.

Stormwater treatment pond. Te Whariki subdivision, Lincoln, Canterbury.

1. POND ON SLOPE

A hillside pond provides high water retention and high sediment capture. Flow velocity is minimised in conjunction with a maximised residence time. In avoiding compaction of soil and additional gound engineering, Keller & Sherar (2003) recommend not exceeding a grade of 2:1 (26.57%) for side slopes. In tests at both ends of the subject grade extent, the following was surmised: •

Major soil disturbance is required, reducing soil structure and stability, and increasing proximity of water to bedrock.

•

Area of storage available for water in relation to volume of soil removed reduces as both hillslope or base area of pond increases.

•

Weight of water being held on the slope is large and has limited distrubution, promoting slope instability and the sliding of earth.

• Surface area of soil/vegetation in contact with water is low in relation to water volume, achieving low rates of evapotranspiration and microbial degradation.

*assumed pond specifications : 2:1 graded sides, 0.2m freeboard, mean depth 0.5m.

2. RILLS

When cut directly into the soil, rills provide a high surface area to water volume ratio. Water flow velocity can be reduced by ensuring rills are non-linear, also extending residence time of stormwater on site. Disadvantages are as follows: •

Narrow rills can easily be blocked by loose soil/ stone, roots and leaf litter

•

Opportunity for surface erosion is high during water movement.

•

Rills provide minimal water retention, as well as minimal volume of water treatment.

•

Low water volumes are supported

*assumed rill depth of 0.2-0.5m.

3. HOLES

Holes can be bored into the ground using a standard manual post-hole borer. Small channels in the soil assist water movement into and between adjacent holes. An extended stormwater residence time is achieved, as well as reducing flow velocity. Soil to water surface area ratio is high. Negatives include: • Holes can present a trap for fauna such as skinks and hedgehogs, leading to both faunal death and the souring of water held.

•

Weight of water presented by each hole is high and has limited distribution, promoting slope instability.

•

Water held in holes can become ecologically stale over time, reducing microbial activity at soil level.

•

Greater depth of hole leads to greater proximity to semi/impermeable strata, inhibiting infiltration.

4. FILTER STRIP

Essentially a highly vegetated slope, a filter strip can spread stormwater flow across the width of the slope. Slope stability is supported with native grass and shrubs. Scouring is avoided through dense and non-linear planting. Soil disturbance is minimal. Disadvantages of a filter strip include: •

Low water retention capacity on site.

•

Residence time is minimal.

• Surface area of soil/vegetation in contact with water is low in relation to water volume, achieving low rates of evapotranspiration and microbial degradation.

5. CORRUGATED FILTER STRIP

Similar to a filter strip, but with corrugations running crossslope. This has the advantage of increasing surface area of soil as well as reducing stormwater flow velocity. Negatives include:

•

Low water retention capacity on site.

•

Residence time is minimal.

•

Surface area of soil/vegetation in contact with water is still low in relation to water volume.

6. TERRACED VEGETATED SWALES In maintaining swale gradient at or below 5%, cross-slope swales have the advantage of slowing flow velocity by extending the passage of stormwater flow and subsequent residence time on site. The weight of the site’s water holding capacity is diffused. There is a high soil surface area to water volume ratio, and high opportunity for evapotranspiration. Negatives include: •

Distributed weight can still place pressure on slope integrity.

•

Site water retention capacity is low.

•

Some soil disturbance is required, though not to any great depth.

2.4

DEVICE DEVELOPMENT

In assessing the potential expressions of WSD treatment on a high-gradient slope, the terraced swale train presented as the best opportunity to achieve multiple objectives. •

A high distribution of water across the site allows for a maximised infiltration rate.

•

This high water distribution also brings the benefit of a high surface area of contact between the soil/plant interface and water volume.

•

If kept shallow, the swale train could extend residence time to well beyond the standard minimum requirement of 9 minutes.

•

Detailing could be carried out to retain water to some degree, further slowing the flow velocity and enhancing infiltration.

• Soil disturbance is low where swale trenches are shallow and suitably located to best-fit existing site contours. Potential benefits in addition to stormwater treatment include:

Supporting local and migrating wildlife both in habitat and temporal habitat (Meurk & Hall, 2006)

Presenting opportunity for educational interaction through the site

Slope stabilisation and erosion control through vegetation

Respecting mana whenua through adhering to Te Aranga principles

Reducing wastewater overflow incidents in high rainfall scenarios.

Enhancing adjacent land value through visual and recreational amenity

2.5

HYDROLOGY CALCULATION

A site’s physical, cultural and climatic context would influence layout and detail of such a device. NZTA (2010), unless otherwise attributed, recommend swale dimensioning with the following specifications: • Swale longitudinal gradient > 1% to ensure water movement. •

Where longitudinal slope > 5%, checks dams are to be provided to ensure effective slope < 5%.

•

Where longitudinal slope < 2%, perforated underdrains are to be provided.

•

Maximum flow velocity 0.8m/sec for a water quality storm.

•

Flow velocity <1.5m/sec unless erosion protection is provided.

•

Minimum length of 30m.

•

Water depth to not exceed vegetation height.

•

Maximum bottom width of 2m.

• Swale sides are recommended not to exceed 2:1 grade*. •

Where concentrated flows enter the swale from pipes, level spreaders are to be used at the head of the swale to disperse flow.

• Vegetation coverage minimum 80% in swale (Barrett, 2004).

* NZTA (2010) recommend a maximum side slope of 4:1 (14.04%), however this is assuming the swale is vegetated with grass and is provided as a guideline to assist in mowing practicality. Barrett (2004) advises that infiltration rates are positively influenced by higher contact of water with the side slopes. Fill slopes are suggested by Keller and Sherar (2003) to not exceed 1½:1 to assist in growth of vegetation and slope stability. It is surmised that the side slopes of the vegetated swale train are best treated in respect to the design and recommendations of a filter strip. A maximum 2:1 grade of side-slope will be used herein.

Grassed swales for capture, infiltration and movement of motorway runoff. Upper Harbour Motorway, Greenhithe, Auckland.

Stormwater treatment rain gardens at Wynyard Quarter, Auckland.

Swales with riprap to reduce stormwater flow velocity. Hobsonville Point, Auckland.

Grassed swales for infiltration and transport of stormwater. Hobsonville Point, Auckland.

Variation in swale gradient and supported water depth affect holding capacity, flow velocity and minimum swale length. Five depth variations were explored, including 200mm and 250mm where suitable swale floor vegetation could match this height. Three longitudinal swale gradients were included in calculations against these depths. The following equations, drawn from TP10 (Auckland Regional Council, 2003) were utilised in calculating diagnostics within the five variations, where: d b R Q L s

= = = = = =

depth of flow (m) T = top width of trapezoid shape (swale cross-section) bottom width of trapezoid (m) A = cross-sectional area (m2) hydraulic radius (m) n = Manningâ&#x20AC;&#x2122;s coefficient 3 design runoff flow rate (m /s) V = flow velocity (m/s) swale length (m) t = hydraulic residence time (mins) longitudinal slope as a ratio of vertical rise/horizontal run

Assumptions followed in hydrology calculations:

- Swale floor vegetation height would be at an average of 150mm.

- Swale side slope is at 2:1 (26.57%)

- Swale base width is at 500mm

Specifications delineated in TP10 in support of a successful swale design include:

- Flow velocity (V) cannot exceed 0.8m/s

- Hydraulic residence time (t) must be minimum 9 minutes, to achieve 80% TSS removal rates

- Slope grade must exceed 1%, and remain below 5%

freeboard

- Swale length (L) cannot fall below 30m

(Top width) T d (Depth) b (Base width)

FIGURE 2.1

Swale Components

e (Side slope width)

Manningâ&#x20AC;&#x2122;s coefficient (n) = =

0.153 d-0.33/(25s + 0.75) 0.013 d-1.2/(25s + 0.75)

Cross sectional area (A) Hydraulic radius (R) Flow velocity (V) Z

= = = =

where d < 60mm where d > 60mm

bd+Zd2 A / (b+2d(Z+1)1/2) Q/A e/d

Top width (T) Design runoff (Q) Swale length (L)

= = =

b+2dZ AR0.67s0.5/n Vt

Table 2.3 illustrates the ruling out of several configurations of swale dimensioning where either swale length or flow velocity parameters do not meeting performance requirements. Similarly, additional judgement calls might be made on several of the configurations where impractical swale lengths are called for to achieve the minimum nine-minute residence time.

TABLE 2.3

Swale Dimensioning through Hydrology Calculaton

50mm depth

100mm depth

150mm depth

200mm depth

250mm depth

Holding capacity (m3/m)

0.03m3

0.07m3

0.12m3

0.18m3

0.25m3

2% Q (m /s) V (m/s) slope

0.3289

0.1648

0.1013

0.0717

0.0549

0.0016

0.0113

0.0400

0.0999

0.2064

0.0535

0.1615

0.3329

0.5552

0.8258

L (m@9mins)

28.88 87.22 179.77 299.81 445.93

3% slope

0.2741

0.1374

0.0844

0.0598

0.0457

Q (m3/s)

0.0024

0.0166

0.0587

0.1469

0.3034

V (m/s)

0.0786

0.2374

0.4894

0.8160

1.2137

L (m@9mins)

42.44 128.18 264.26 440.62 655.38

0.2350

0.1177

0.0724

0.0512

0.0392

0.0031

0.0224

0.0791

0.1979

0.4087

0.1059

0.3198

0.6589

1.0992

1.6350

4% Q (m /s) V (m/s) slope 3

L (m@9mins)

57.18 172.69 355.81 593.58 882.89

2.6

DEVICE LAYOUT

FIGURE 2.2

Conceptual Slope Terracing and Stormwater Flow

To support manual access for maintenance, a base width of 500mm is recommended. Lesser than this may make swale floor access difficult during maintenance, while a greater base width would effect greater soil disturbance than necessary.

Where restricted space limits the lateral swale run or in situations of higher longitudinal gradient, weirs might be implemented to assist in passing water from one swale to the next. The use of riprap aprons is imperative in both situations to avoid surface erosion, as is its place where one swale passes to the next without weirs.

FIGURE 2.3

Conceptual Terrace Weirs

FIGURE 2.4

Slope Retention

Soil disturbance is a requirement within the given terraced swale train proposal, albeit minimal compared to many alternatives. Higher gradients within the range necessitate higher levels of soil disturbance in ensuring the maximum 2:1 side slope gradient. Higher gradients also push terraces further apart, resulting in less efficient implementation of the available land. Sections of retaining steps (see Figure 2.4) may be implemented in these scenarios, allowing swales to be brought closer together to overcome higher gradient effects. These may be expressed as device-length walls, or they may only be a small section in overcoming an isolated challenge in the topography. It is recommended that these are used sparingly, and only in overcoming small height differences. The use of retaining steps comes with the disadvantages of adding soil weight to a section of the slope, as well as incorporating an item that may add to monitoring and maintenance requirements of the overall device.

During high rainfall events, a highly vegetated terraced slope has the advantage of not only withstanding the erosive potential of water volumes flowing perpendicular to and over parallel swales. It also assists in slowing flow velocity. Swales give a corrugated effect to the slope when flows pass overtop in this direction. Resistance is added by rigid vegetation such as Phormium, Austroderia and Gahnia species that will hold up against heavy flow volumes.

FIGURE 2.5

Overland Flowpath in High Rainfall Events

2.7

THE LIVING ELEMENTS

SUPPORTED ECOLOGY The terraced swale train provides a unique ecological environment. Its requirement to be maintained as a non-regenerative low-vegetation site provides opportunity for rare and threatened local species to be nursed, strengthening their place on the isthmus. Meurk & Hall (2006) discuss the importance of corridors and patches for assisting ecological habitat, migration and breeding. Their study and spatial modelling was focused on birds and flighted insects, relating habitat patch size, proximity and connectivity to flight-paths, seed dispersal and the gene flow between plant and animal populations. The vegetated treatment slope might be seen in a similar light. The small area occupied, the high ground-floor light levels and the recommended scarcity of trees suggests that the patch may, however, be better envisioned as a habitat stepping stone for fauna based at ground level. Native amphibians and crawling insect populations can be easily supported in a minimallydisturbed urban environment such as this. Integration of such a swaled slope into a regionâ&#x20AC;&#x2122;s ecological mapping has the potential to broaden corridors and patches for a diversity of native fauna. Local ecology might be taken into account during the selection and placement of planted material, relative to local populations, microclimate and existing adjacent ecologies.

FLORAL SELECTION As the fundamental element in the phytobial treatment of stormwater, vegetation provides the important soil-root interface where microbial oganisms flourish and the adsorption and degradation of contaminants can take place. The penetrative capacity of roots assists in infltration of stormwater into the soil strata, and evapotranspiration through water taken into and passing through the plant can reduce overland flow volumes significantly. In the high gradient setting of the vegetated slope, the soil anchoring provided by roots is particularly crucial, assisting slope stability in stormwater flow and pedestrian access. The requirement for high root content within the soil as well as a minimum 80% vegetative coverage on the swale floor itself calls for high light levels at ground level and deliberate plant choice. While trees might provide deeper anchoring of the slope, they are generally to be avoided in this scenario, due to their shading influence. Exceptions are where thin-canopy species are used, and trees are planted sparsely. Three niches exist within the swale train environment. 1. Wider distribution species - for maximum soil root content and water absorption 2. Sparse-planted large species - deeper anchoring of the slope 3. Swale side and floor species - dense vegetation rising to minimum150mm high, tolerant of some shading from neighbouring plants

Recommended species for planting within the Auckland isthmus are listed on the following pages.

WIDER DISTRIBUTION SPECIES Apodasmia similis Oioi Arthropodium bifurcatum Rengarenga Lily Astelia fragrans Bush Lily Blechnum parrisiae Rasp Fern Carex virgata Swamp Sedge

Machaerina sinclairii Pepepe Phormium cookianum Mountain Flax

SPARSE-PLANTED LARGE SPECIES Austroderia fulvida Golden Plume Toetoe

Cyperus ustulatus Giant Umbrella Sedge Dianella latissima Strappy Blueberry Dianella nigra Turutu/Blueberry Ficinia nodosa Knobby Club Rush Juncus pallidus Giant Rush

Muehlenbeckia axillaris Creeping Pohuehue

Cordyline australis Cabbage Tree Gahnia setifolia Giant Gahnia Knightia excelsa Rewarewa Phormium tenax Harakeke Sophora chathamica Coastal Kowhai

SWALE SIDE & FLOOR SPECIES Acaena anserinifolia Bidibid Asplenium flabellifolium Necklace Fern Blechnum fluviatile Creek Fern Blechnum minus Swamp Kiokio Blechnum penna-marina Alpine hard Fern Carex uncinata Hook Grass Disphyma australe ssp australe NZ Ice plant Fuchsia procumbens Creeping Fuchsia Gunnera prorepens Gunnera Juncus caespiticius Grass-leaved Rush

Juncus prismatocarpus Branching Rush Leptinella squalida Creeping Button Daisy Lobelia angulata Panakenake / Pratia Mazus radicans Swamp Musk Microlaena avenacea Bush Rice Grass Nertera depressa Nertera Oplismenus hirtellus imbecillis Basket Grass Pellaea rotundifolia Button Fern Poa anceps Broad-leaved Poa Selliera radicans Remuremu Viola lyallii NZ Violet

3.0

APPLICATION THROUGH SITE TRIAL

THREE SITES, THREE CONTEXTS

Auckland hosts a diversity of volcanic and non-volcanic land form, and the development and roading configuration atop and around this varies similarly. The spectrum of settlement, development and infilling stages contributes to the cityâ&#x20AC;&#x2122;s growing and ever changing form. Situations of stormwater runoff from impervious surfaces across Auckland differ based on this landform and social construct, to name only two of the key contributors. The following pages detail the application of slope-based stormwater treatment on three sites within Auckland.

How can high gradient land be better utilised to treat storm water runoff in Auckland?

A number of sites across Aucklandâ&#x20AC;&#x2122;s developed and rural form were considered in assessing the potential for the testing of a terraced swale train treatment device. Catchment run, stormwater source, site gradient, proximity to waterways and surrounding context were analysed. Three sites of higher gradient land (25-40% slope) were selected to represent a snapshot of some of the typical flow-path scenarios in Auckland. Context was assessed both physically and culturally to identify the best-fit WSD approach for each site.

OMAHA

WHANGAPARAOA

A. B.

RIVERHEAD

RANGITOTO

BUCKLANDS BEACH

FLATBUSH

AUCKLAND AIRPORT

KARAKA

The selected sites are as follows:

A. Vaughans Road, Okura

The Okura area is one of rural land and lifestyle blocks. While on the outside of Auckland’s MUL, this land typifies real estate that urban Auckland might expand into through rezoning and subdivision as a response to a growing population. This area has been selected as an example where water sensitive design may be worked into its greater character as part of the local region’s development. Constraints of established housing and infrastructure are minimal, permitting a broader potential WSD impact.

B. Penguin Drive, Murray’s Bay

Located on the upper slopes of the Murrays Bay subcatchment, this site is adjacent to neither ecological space, nor a waterway where treated or excess waters might flow. The treated outflow from this site would feed directly back into the stormwater drainage system. Contaminant levels of the drain outlet in Murrays Bay would therein be reduced. This is an urban retrofit, but is set apart by its residential intensification and distance from a receiving waterway. The site was selected for its vacancy of established buldings, and occupies the space of a standard private section on the street.

C. Phyllis Reserve, Mt Albert

This site is located on the periphery of Phyllis Reserve, where the banks of Oakley Creek represent the transition space between urban developed land and natural (or naturalised) ecological habitat. Water that enters Oakley Creek is no longer in a path susceptible to further contamination, but at the same time has the potential to affect sensitive flora and fauna directly. The receiving environment that is Oakley Creek represents many waterway systems common across urban Auckland where stormwater flows from residential and roading surfaces, but has the potential be treated in an urban retrofit scenario. The Phyllis Reserve site was chosen over other local slopes adjacent to Oakley Creek because of its proximity to a key runstormwater outlet.

3.1

VAUGHANS ROAD

Okura is an area with strong pride in its local environment. The area hosts significant wildlife sanctuaries, including Long BayOkura Marine Reserve and Okura Estuary Scenic Reserve. These reserves contribute an important stepping stone in supporting the migration of birds across the route between Tiritiri Matangi Island in the east and the Waitakere Ranges to the west. Figure 3.1.1 overleaf highlights this pathway, known as the North-West Wildlink. Extensive areas of established and regenerating native bush significant to the local ecology are scattered beyond these reserves through farmland, urban and recreational areas. Long Bay Regional Park provides an important feeding and nesting ground for wading and shore birds, highlighting the importance of a diversity of habitat and not a reliance on the forested environment. Ecological strengthening along this route is encouraged by Auckland Council, alongside a number of local and national interest groups. Some of these include:

Friends of Okura Bush (FOOB)

Okura Residents Association

Depatment of Conservation

Forest & Bird NZ

Long Bay-Okura Great Park Society

Te Kawerau a Maki

Keep Okura Green

Plans are underway to create further links in what will be known as The Crimson Walkway, a pedestrian route linking Okura Bush Walkway to Long Bay Coastal Track, These plans seek also to extend the walkway north to connect to Orewa (Long Bay-Okura Great Park Society, n.d.). The increase in residents as a result of the Long Bay developments will bring additional attention to these attractions. The areaâ&#x20AC;&#x2122;s increasing green tourism appeal demonstrates the potential connection between environmental health and Okuraâ&#x20AC;&#x2122;s economy.

WAIWERA

RAROA RESERVE

TIRITIRI MATANGI ISLAND

OKURA ESTUARY SCENIC RESERVE

HELENSVILLE

LONG BAY-OKURA MARINE RESERVE

PAREMOREMOALBANY HEIGHTS WILDLIFE CORRIDOR

WAIHEKE ISLAND TUFF CRATER PROJECT TWIN STREAMS

STATE HIGHWAYS 16 & 18

AUCKLAND CBD

ARK IN THE PARK

WAITAKERE RANGES AUCKLAND AIRPORT

FIGURE 3.1.1 North-West Link

LONG BAY-OKURA MARINE RESERVE

OKURA BUSH

LONG BAY REGIONAL PARK

OKURA ESTUARY SCENIC RESERVE

GH VA U

AY RW TO

O A

OKURA TOWNSHIP

st oa

C st

GLENVAR RO

Ro ad

Vaughans Road marks the northern boundary of Aucklandâ&#x20AC;&#x2122;s Metropolitan Urban Limit (MUL). Road widening is planned within the next 10 years, including the addition of a 3m shared pedestrian/cycle path. Development of the Long Bay precinct will be very influential in affecting views, traffic and pressure for further development along Vaughans Road. The Long Bay development seeks to connect the eastern extent of Vaughans Road to Beach Road Extension within 10-15 years, adding access to Long Bayâ&#x20AC;&#x2122;s retail and residential zones. Having spoken with David Johnston of the Keeping Okura Green initiative (pers. comm., October 12, 2015), this connection has strong opposition from local residents hoping to maintain their peaceful country lifestyle and distance from urban bustle. Traffic is anticipated to increase from 500 vehicles per day to c.3,500 vehicles per day along Vaughans Road, and from 2,600 to 6,250 vehicles per day on Okura River Road (Auckland Transport, 2015a).

Rural production Residential Okura Bush Conservation land Ecologically significant land Okura/Long Bay Marine Reserve Other native bush Informal recreation Existing walkways Proposed walkways Selected site 0m

250m

500m

750m

1km

Long Bay development has paid great attention to mitigating the flow of stormwater and its contaminants through its Long Bay catchment. However the increase in traffic flow and subsequent stormwater runoff brought to Vaughans Road by this development has not been addressed - all runoff passing into the Okura catchment. and beyond.

FIGURE 3.1.2 Okura/Long Bay Regional Analysis

0km

1km

2km

3km

4km

5km

3.1.1

STORMWATER SOURCE & FLOW

The existing low level of development in the area in conjunction with narrow roading widths effects a low level of impervious surface. All runoff from land between Okura River Road and Vaughans Road is collected in the lowland trough, and runs out to Antrim Bay.

A large proportion of the land is high gradient (25-40%). Roads and driveways are long and winding as a side effect. Properties in this locale are host to large houses, often featuring tennis courts and parking areas for multiple vehicles. This combination leads to high impervious surface levels per head of population. The localeâ&#x20AC;&#x2122;s high-gradient land directs overland flow at a sharp gradient from Vaughans Road. Ground contouring delivers stormwater to the selected site along Vaughanâ&#x20AC;&#x2122;s Road of an approx. 830m long catchment from Okura River Road.

Generalised stormwater flow accumulation 25-40% gradient land Impervious surfaces Selected site

50m

100m

150m

200m

250m

FIGURE 3.1.3 Okura Area Stormwater Flow and Drainage

RO AD S AN GH VA U

Section A1

 AD

3.1.2

SITE LOCATION & OPPORTUNITY 0m

ROADING

WATER TREATMENT

RESIDENTIAL

LONG B

FIGURE 3.1.4 Cross-Section A1: Vaughans Road Site Analysis & Potential

Given the current low population density, the local area is not yet reticulated. Stormwater from roads and other impervious surfaces is captured in unlined road-side ditches, and channeled to neighbouring farmland where lower lying troughs in the land pass the flow toward estuarine tributaries. With the low current road use and the subsequent low levels of contaminant in runoff, this existing pathway allows ample time for infiltration as well as phytobial processing. The catchmentâ&#x20AC;&#x2122;s high proportion of steep land and the future increase in stormwater volume and contaminant levels identifies the locality as a prime opportunity for the use of slope-based stormwater treatment devices.

10m

20m

100m

WATERWAY

OKURA



50m

Section A2



SEDIMENT POND/WETLAND

VA U

TERRACED SWALE TRAIN

1. 0m

10m

N LI

20m

FIGURE 3.1.5

D OA

AL SW

50m

100m

Vaughans Rd Proposed Wider Treatment Train

3.1.3

WIDER TREATMENT TRAIN POTENTIAL

The early stages of development that help in defining the present character of Vaughanâ&#x20AC;&#x2122;s Road enable the implementation of water sensitive design principles to a far greater and more effective extent than in areas already developed and hosting much higher housing densities. With Vaughans Road looking to be resurfaced, widened, lengthened and paved surfaces planned for both sides, the integration of WSD in the path forward seems a logical conclusion. The mindset of residents affects their everyday decisions and activities that contribute to or detract from stormwater volume and contaminant load. As population increases in the area, it is important to maintain a prominent and visible front to promote ecological sensitivity. Maintaining a recreational and educational focus across the catchment and train of treatment assists in influencing this mindset toward water sensitivity.

for the integration of light-canopy trees, that can support the stability of the slope and add to the siteâ&#x20AC;&#x2122;s biodiversity without out-shading and reducing effectiveness of smaller species below. A number of variations on the design might have been implemented. Output might have been at a higher point and riprap used to prevent scouring of the lower slope from outflow. A higher longitudinal gradient might have been utilised, providing a shorter run, higher water velocity and reducing residence time for waters. Integrated roadside swales reduce the volume of water entering both the wetland and the subsequent slope train, and so the altering of these will also alter requirements for devices further along the treatment train. Treatment devices with greater than necessary capacity can cater to future growth of the area.

Figure 3.1.5 on the facing page illustrates a potential layout in the local vicinity to cater to the roading catchment flowing from south-west. Proposed road widening and walkway integration has been included. Proposed vegetated roadside swales capture runoff from the road surfaces and begin infiltration and treatment. Swales lead excess road runoff to the sediment wetland, where particulates and suspended solids are allowed to settle out. The subsequent terraced swale-train provides high plant contact for maximum absorption as water passes down the slope. Gradients above 40% are avoided, requiring excess soil disturbance and leaving the vicinity more unstable than is necessary. These highest grade areas provide an opportunity

Swale length within this context is relevant to the distance required for a 2% longitudinal gradient to traverse the 25m elevation difference on this terrain. While diagrammed distance exceeds 650m, this dimensioning can readily be varied through a number of factors. For this site, the elevation difference marks the distance between entry point and where gradient begins to shallow for appropriate flow release.

*Calculations provided are made on a swale design without retention weirs.

At the proposed c.650m of swale this slope provides, opportunity exists for variation of swale depth and gradient: 50mm depth

3-4% slope

19.5m3 holding capacity

100mm depth

2-4% slope

45.5m3 holding capacity

150mm depth

2-4% slope

78m3 holding capacity

200mm depth

4% slope

117m3 holding capacity

FIGURE 3.1.6 Cross-Section A2: Vaughans Road Proposed Treatment Slope - Sectional Analysis

OVERFLOW FILTER STRIP

VEGETATED SWALE

PEDESTRIAN BOARDWALK

VEGETATED SWALE

PEDESTRIAN BOARDWALK

BUFFER VEGETATION

SETTLEMENT WETLAND

BUFFER VEGETATION

ROADSIDE FOOTPATH

ROADSIDE SWALE

VAUGHANS ROAD

ROADSIDE SWALE

ROADSIDE FOOTPATH

Detailed catchment-based calculations with consideration of rainfall will provide a good estimate as to the required capacity for a device at this point on Vaughans Road. Soil analysis and proximity to underlying bedrock are important to ascertaining not only treatable volumes, but also the viability of this site for such a device and its inherent dimensioning.



RUNOF TO ANTRIM BAY

VEGETATED SWALE

OVERFLOW FILTER STRIP

VEGETATED SWALE

PEDESTRIAN BOARDWALK

OVERFLOW FILTER STRIP

VEGETATED SWALE

SETTLEMENT WETLAND

1.5m (w) BOARDWALK

FIGURE 3.1.7 Vaughans Road Treatment Slope - Perspective View

The very nature of WSD maintains the flow of water above ground, increasing visibiilty of the water and affected ecologies. The aesthetic value added by the inherent greening also enhances the areaâ&#x20AC;&#x2122;s livability. The amenity value of the wetland cannot be understated. Waterways and waterbodies are a magnet for passive recreation, wherein the local social construct becomes engaged with the site and process. This area adjacent to the road and footpath presents an inviting entrance to the boardwalk through the subsequent stage of the treatment train. The boardwalk, running directly off the roadside footpath leads the user through the area. Engagement in the concept and the local ecology is brought about, with attention brought to the land and water by the winding of the path with the slopeâ&#x20AC;&#x2122;s contours, and the passing over of the water run repeatedly.

VEGETATED SWALE

Taiorahi catchment Mixed housing urban zone Mixed housing suburban zone Ecologically significant vegetation

S WN

R BAY

Local reserves Pedestrian access Selected Site

20m

40m

60m

80m

100m

PENGUIN

EAST

COAS

T ROA

SPEEDYS BUSH

DRIVE

INT

3.2

PENGUIN DRIVE

Until recently, housing within the upper Murrays Bay area has been relatively low density. Much of the land occupies significant gradients and is host to patches of native bush. These factors have supported such properties with a recommended density of one dwelling per 700-1000m2. Guidelines for properties within a similar vicinity but without native bush or steep slopes recommended one dwelling per 350-700m2. The Proposed Auckland Unitary Plan roughly zones this same area as Mixed Housing Suburban (see Figure 3.2.1), tightening housing density with a blanket guideline of one dwelling per 300-400m2.. This comes with a maximum 60% impervious surface proviso. New Mixed Housing Urban zoning is allocated along East Coast Road, increasing housing density from one dwelling per 400-450m2 to one dwelling per 300-400m2. These zoning changes are likely to see increased impervious surface area volumes occupy the catchment. Alleyways provide connection between spaces in the area, yet are straight and long, with little visibility. Reserve walks also provide access, particularly in linking Sunrise Ave to Penguin Drive through Speedys Bush. Yet neither alleys nor bush walks appear safe and inviting here. Retail attractions are not within walking distance. In conjunction with the shortage of reserves, the greater Murrays Bay area is not pedestrian friendly or family friendly. It presents as a strictly residential zone, where driving (and supporting infrastructure) is an essential way of life.

FIGURE 3.2.1 Zoning for Taiorahi Catchment in the Proposed Unitary Plan

KNIGHTS ROAD

BA YR

OA D

E NU

VE SA ON LY



Section B1

IN PENGU

DRIVE

BA DLE

SAD

RIS

BELLBIRD RISE

RO IS

AV E

AC H

T AS CO

3.2.1

STORMWATER SOURCE & FLOW

The majority of the Taiorahi sub-catchment within what is commonly known as Murrays Bay has in recent times been assigned as a Stormwater Management Area. The sensitivity of Taiorahi Streamâ&#x20AC;&#x2122;s ecology is recognised here, in conjunction with the large proportion of impervious surface area threatening the waterway. This categorisation prioritises measures in the area to mitigate stormwater issues.

High levels of impervious surface also contribute to significant flooding problems, despite the susceptible zones not being particularly low-lying. Runoff volumes accumulate rapidly in high rainfall events, backing up in a minor and restricted waterway where high flow volumes are not suited. Figure 3.2.2 illustrates the extent to which roads, roofs and driveways occupy the space, preventing a large proportion of rainfall from permeating directly into the ground.

Generalised stormwater flow accumulation Stormwater Management Area (SMAF 2) Flood plain Impervious surfaces Selected Site

100m

200m

300m

400m

500m

FIGURE 3.2.2 Murrayâ&#x20AC;&#x2122;s Bay Wider Hydrology Analysis

Flow accumulation modelling demonstrates high overland flow volumes particularly along the extent of Penguin Drive. While this section of the catchment is no more densely housed that other parts, houses are larger, covering a greater area of each property. Similarly, driveway turning and parking space occupies a greater area in accompanying these houses. Rapid water accumulation and flooding of Taiorahi Stream occurs often due to these factors as well as a minimal area of local reserves. Speedys Bush captures a great volume of rainfall within the small gully it occupies, though this gully is host to little residential space so is not a great contributor to flooding issues. Water sensitive design is absent from the area, with all stormwater draining from roads to pipes to waterways. The most significant contaminant in this flow is erratic volume, with aquatic fauna and the stream habitat threatened with each flash flood event. The outlet of this sub-catchment runs beneath Beach Road as the flow opens out to Murrays Bay. Marine habitat is in turn threatened. The selected site sits high up within this catchment. Currently an empty residential property, it shows potential to carry and process some of the runoff from Browns Bay Road above. Much of the runoff from Browns Bay Road is presently collected at the intersection of Gannet Point and piped to contribute to Taiorahi catchment issues.

0km

1km

2km

3km

4km

5km

3.2.2

SITE LOCATION & OPPORTUNITY 0m

10m

20m

ROADING

RESIDENTIAL

WATER TREATMENT

Y NS BA

BROW

FIGURE 3.2.3 Cross-Section B1: Penguin Drive Site Analysis & Potential

Due to the rapid drop in elevation on the southern side of Browns Bay Road, footpaths on Browns Bay Road leave little room to move. There is an absence of street trees here as well as on Penguin Drive, potentially revealing an opportunity for ease of roadside swale installation. If stormwater from Browns Bay Road was to be diverted through the selected site, it would not have a waterway or ecological path to outflow to. Options for outflow appear to either be diversion into existing stormwater reticulation - creating a dilution of contaminants and reduced volume output - or the creation of a streetside flowpath along Penguin Drive to continue to a point further down the catchment. The latter option involves far greater disruption to existing services and residential patterns, and draws too much focus away from the high-gradient orientation of this project. As such, the first option will be approached.

RESIDENTIAL

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Little incentive exists for local residents to walk through their area, where they might become familiar with local ecological concerns. This presents the second challenge of creating pedestrian access on this steep site, facilitating community education in the process.

3.2.3

WIDER TREATMENT TRAIN POTENTIAL

The constraints of a developed and established area are ever present at this location. Browns Bay Road is narrow and busy, yet portions of grassed berm offer the opportunity of replacement with vegetated swale to assist in retaining some of the volume of water running past in this small catchment space. These swales also assist in capturing some of the initial particulate load prior to the water passing on the the treatment slope where access for maintenance is not as practical.

1. VEGETATED SWALES

The terraced swale train passes the flow of water across an elevation drop of c.26m. Carrying out this feat within the length of this property requires utilising a 4.5% longitudinal grade. Riprap is required at every â&#x20AC;&#x153;cornerâ&#x20AC;?. At the proposed length of c.190m, water depth would have to be less than 15mm for the desired minimum 9-min residence time to be achieved. Strictly speaking, as treated water from this device is not feeding directly out to an ecological environment but back into stormwater reticulation, any reduction in contaminant load is acceptable, albeit unsatisfying if below standard desirable treatment levels. The high gradient also necessitates the use of soil retention steps to minimise soil disturbance and stabilise the slope surface. Beyond the treatment slope, the flow follows a linear swale to Penguin Drive where it descends to underground reticulation.

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FIGURE 3.2.4 Penguin Drive Proposed Wider Treatment Trai

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FIGURE 3.2.5 Cross-Section B2: Penguin Drive Proposed Treatment Slope - Sectional Analysis

PEDESTRIAN BOARDWALK

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VEGETATED SWALE

PEDESTRIAN BOARDWALK

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ROADSIDE FOOTPATH

BROANS BAY ROAD

The incorporation of roadside vegetated swales on Penguin Drive assists with the high flow accumulation on this road. This also adds visibility of the area’s stormwater treatment, giving connection of theme to the slope and the treatment on Browns Bay Road.

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PENGUIN DRIVE

ROADSIDE FOOTPATH

RESIDENTIAL

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PEDESTRIAN BOARDWALK

VEGETATED SWALE

As an education point, the boardwalk provides the key feature of attraction. Being a linking accessway between Browns Bay Road and Penguin Drive, the boardwalk and its many steps provide an appealing destination and thoroughfare. CPTED principles are supported in the visibility of the boardwalk from adjacent properties and from across the road on Penguin Drive. The low planting also supports a feeling of safety. Like a landscape tapestry, the sloped site makes a significant visual impression. The siteâ&#x20AC;&#x2122;s low number of trees and focus on lowlying plants gives opportunity for mass planting with creative expression. The artful slope adds recreation and aesthetic value for local residents and all users. It can also increase government valuation of the area through the provision of open space and connection to adjacent areas. At the expense of increased soil disturbance, potential exists for the levelling of a central area of the site to host a parklet of sorts, giving the slope value as a pedestrian destination.

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FIGURE 3.2.6 Penguin Drive Treatment Slope - Perspective View

Phyllis Reserve

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Ecologically significant vegetation Oakley Creek Oakley Creek walkway Proposed Waterview Shared Path Selected Site

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3.3

PHYLLIS RESERVE

Sited on the western fringe of the suburb of Mount Albert, Phyllis Reserve is an informal recreation reserve established atop a covered domestic landfill. Prior to this, excavations on the site were made as part of a basalt quarry. The reserve hosts three soccer fields and two softball pitches, and is home to the Akarana Dog Training Club and Metro Mt Albert Sports Club.

The reserve has residential zoning to the east. Its northern extent touches the grounds of Unitec, while the western periphery hosts ecologically significant revegetation alongside Oakley Creek. Oakley Creek walkway runs through the reserve, connecting pedestrians with Harbutt Reserve to the south, as well as linking pedestrian access northward and across the creek to the Waterview.

Early 2016 will see the first stages of the Waterview Shared Path begin, which will provide further connection for cyclists and pedestrians (Auckland Transport, 2015b). This path will pass through Phyllis Reserve (as shown in Figure 3.3.1), bringing additional users to the park and its surrounds.

Parties interested in Phyllis Reserve and its contribution to the area include:

Ngāti Whātua Ōrākei

NZ Transport Agency

Auckland Council

Auckland Transport

Metro Mt Albert Sports Club

Akarana Dog Training Club

Friends of Oakley Creek

Campaign for Better Transport (NZ)

Cycle Action Auckland (CAA)

North Western Community Association

Unitec Institute of Technology

Mount Albert Residents Association

Albert-Eden Youth Board

Local residents

The site selected as part of this project is zoned for Conservation. This relates to the significance of Oakley Creek and surrounds, and the habitats of Motu Manawa Marine Reserve into which its waters flow.

FIGURE 3.3.1 Phyllis Reserve Contextual Analysis

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3.3.1

STORMWATER SOURCE & FLOW

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The catchment flowing into Oakley Creek through the Phyllis Reserve area is of a slight incline across its greater extent. Roads within this mapped area east of Oakley Creek are largely low-use residential traffic, aside from Woodward Road which is a busy thoroughfare, particularly at peak hours.

t Nor th Ro

aGdreat

The areaâ&#x20AC;&#x2122;s stormwater runoff is channeled directly into Oakley Creek, to the extent that Oakley Creek is mapped as a stormwater drain in regional mapping tools.

Grea

The majority of the areaâ&#x20AC;&#x2122;s stormwater is channeled through drains running under the soccer field at the northern end of Phyllis Street. Much of this water runs close to the selected Phyllis Reserve site. This suggests an opportunity for the diversion of some of the flow to allow its treatment by a slope-situated WSD device at this location. Similar sites and opportunities may exist at multiple locations along the extent of Oakley Creek. Blockhouse Bay Road

Generalised stormwater flow accumulation Oakley Creek Impervious surfaces Underground stormwater drains Selected Site

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FIGURE 3.3.2 Phyllis Reserve Stormwater Flow & Drainage

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3.3.2

SITE LOCATION & OPPORTUNITY

RESIDENTIAL AVONDALE

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FIGURE 3.3.3 Cross-Section C1: Phyllis Reserve/Oakley Creek Site Analysis & Potential

PHYLLIS RESERVE

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3.3.3

WIDER TREATMENT TRAIN POTENTIAL

The implementation of WSD in the area is not too challenging a scenario to envision. The wide, low-traffic streets accompanying a relatively low housing density provide opportunity for WSD implementation without great interruption of existing processes. The only significant thoroughfare in the catchment is Woodward Road, meaning WSD installation at the street level is likely to come with minimal traffic disruption. Figure 3.3.4 offers a potential framework for treatment train layout within the localised area.

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Among other functions, rain gardens established on and along Phyllis Street (1.) provide the element of sediment capture - important for road runoff. The temporary storage of significant runoff volume during high rainfall events is also a key reason for the placement of these lower in the catchment. The 72m linear swale (2.) links Phyllis Street to the treatment slope. This provides opportunity for further infiltration and contaminant capture, though has a primary function as a water transport channel. The third device in the treatment train is the terraced swaletrain itself (3.), where the impact of phytobial processing is focused. Further detail of this is overleaf.

FIGURE 3.3.4 Phyllis Reserve Proposed Wider Treatment Train

FIGURE 3.3.5 Cross-Section C2: Phyllis Reserve/Oakley Creel Proposed Treatment Slope - Sectional Analysis

85 VEGETATED SWALE

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At the proposed 313.5m of swale this slope provides when derived using a 2% longitudinal gradient, opportunity exists for variation of swale depth and gradient: 50mm depth

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The location of this site facilitates activation of the space through passive use of Phyllis Reserve. Throughout the year, the reserve’s appeal to walkers and runners, dog-walkers sports fans and the sporting themselves brings a regular flow of people through its connecting pathways. The incorporation of the treatment slope into the adjacent Oakley Creek Walkway encourages walkway users to appreciate the transitional space between the reserve’s highly kept fields and the streamside revegation at Oakley Creek.

An invitation to pass through the area is provided by low plantings in visible contrast to the high revegetation growth along the rest of the park’s western border. The incorporation of an foreign element such as a statue or a corten steel object enlivens the space and helps the user recognise its call.

With the upgrade of neighbouring streets to incorporate sensitivity to ecological processes, connection is made to this site for local residents and frequenters of the reserve and walkway. The upgrading process and subsequent appreciation of the sloping site and its connected treatment train support stronger local knowledge and affinity for water sensitivity in their lifestyle and surrounds.

FIGURE 3.3.6 Phyllis Reserve Treatment Slope - Perspective View

4.0

THE AUCKLAND POTENTIAL

MAPPING POSSIBILITIES

Great diversity of context exists across Aucklandâ&#x20AC;&#x2122;s land mass. Mapping of some of the key factors pertinent to stromwater treatment on steep slopes provides a greater understanding of the extent to where the possibilities may lie. GIS analysis on this scale makes generalisations that can subsequently be assessed at a scale smaller and a context more local. Two local boards were selected on which GIS analysis was carried out. For no reason other than these boards playing host to the three community-level context sites assessed in previous pages, these local boards are:

Hibiscus & Bays Local Board

Albert-Eden Local Board

4.1

HIBISCUS & EASTERN BAYS LOCAL BOARD

EXPLORATORY MAPPING

Figure 4.1 illustrates the boardâ&#x20AC;&#x2122;s significant land-based and marine ecological areas. These represent particular sensitivities on which greater value is placed for protection over other ecological zones. As in the North-West Link example given in Vaughans Road discussion, further research can identify linkages and opportunities for further connection between these. Significant land-based ecological zones Significant marine-based ecological zones

FIGURE 4.1

Hibiscus & Bays: Significant Ecological Areas

Informal recreation zoning Conservation zoning

Land zoning under the Proposed Auckland Unitary Plan is highlighted in Figure 4.2. Conservation and Informal Recreation zone allocations show the most potential to be supported by slope-based stormwater treatment. Smaller sites are of particular interest within these allocations, as the larger areas often donâ&#x20AC;&#x2122;t have a great degree of roading or other such impervious surface within. Hibiscus & Bays: Conservation & Informal Recreation Zoning

FIGURE 4.2

Figure 4.3 shows the five greater catchments within the Hibiscus & Bays board, including a small section of the Upper Waitemata Harbour catchment flowing south. In conjunction with other layers of data, the significance of potential within a catchment can be related to the water quality at outflow points and the local waters they inhabit. Waiwera catchment Whangaparaoa catchment Okura catchment Upper Waitemata Harbour catchment East Coast Bays catchment

FIGURE 4.3

Hibiscus & Bays: Catchment Analysis

25-40% sloping land

The identification of 25-40% steep slopes in Figure 4.4 recognises Aucklandâ&#x20AC;&#x2122;s volcanic layout and its influence on city spread. Comparison with other layers illustrates how proximity to urban settlement and flatness of land compete for development. Hibiscus & Bays: High-Gradient Land

FIGURE 4.4

EAVES RESERVE

SHAKESPEAR REGIONAL PARK

OKURA BUSH SCENIC RESERVE

Figure 4.5 brings to attention 25-40% sloping land within Conservation or Informal Recreation zones areas. while larger conglomerations of sloping land generally indicate an area coinciding with minimal impervious surface, often opportunities may exist on the periphery to cater to stormwater runoff from adjacent catchments.

FIGURE 4.5 CENTENNIAL PARK

Hibiscus & Bays: Steep Land X Conservation / Informal Recreation Zoning

4.2

ALBERT-EDEN LOCAL BOARD

EXPLORATORY MAPPING

The significant Motu Manawa Marine Reserve sits adjacent to the AlbertEden local board. Its waters and ecology are affected by stormwater from activities through a great extent of the board. Mount Eden, Mount Albert, Mount St John and Oakley Creek are demonstrated as the most sizeable land-based ecologies in the area. Significant land-based ecological zones Significant marine-based ecological zones

FIGURE 4.6

Albert-Eden: Significant Ecological Areas

Informal recreation zoning Conservation zoning

Figure 4.7 demonstrates the conservation status placed on the local ecological icons. Much of Oakley Creek has been rezoned to Conservation (from Informal Recreation) since the Unitary Planâ&#x20AC;&#x2122;s 2013 draft. Albert-Eden: Conservation & Informal Recreation Zoning

FIGURE 4.7

The majority of the ward is in the Auckland catchment, discharging into the eastern Waitemata Harbour. Motu Manawa Marine Reserve receives a great volume of its fresh waters from the subcatchments of Oakley Creek, highlighting the creekâ&#x20AC;&#x2122;s importance in local marine ecology. Auckland catchment Northern Manukau catchment

FIGURE 4.8

Albert-Eden: Catchment Analysis

25-40% sloping land

The enclosing banks of Oakley Creek create a visible channel to Waterview Junction, and local volcanic cones similarly are made clear.

Albert-Eden: Conservation & Informal Recreation Zoning

FIGURE 4.9

MT EDEN

OAKLEY CREEK

MT ST JOHN

MT ALBERT

FIGURE 4.10 Albert-Eden: Steep Land X Conservation / Informal Recreation Zoning

In contrast to the Hibiscus & Bays local board, Albert-Eden topography is much flatter, with exception given to the volcanic peaks. Opportunity for stormwater treatment slopes is less scattered herein. A more exploratory analysis might be offered on the lower slopes of volcanic cones, as well as around inflow points to Oakley and Meola Creeks.

5.0

REFLECTION

RETROSPECTIVE PLANNING

The constraints imposed by Aucklandâ&#x20AC;&#x2122;s abundance of steep land imposes pressures on the location of not only development, but also stormwater treatment sites within the city. The potential application of stormwater treatment to sloping land in Auckland presents an alternative means for reducing stormwater runoff pressures on the environment, while freeing up flatter land that might otherwise be occupied by conventional water sensitive design functions. The intent of this project has been to approach conventional water sensitive design with a fresh perspective, assessing alternatives that might be integrated into the existing Drained City framework. This study has revealed recommendations on the form this might take, as well as bringing clarity to stormwater treatment functions that are better fulfilled on flatter land. Directions for further analysis and consideration are highlighted.

Retrofitting The retrofitting of existing infrastructure brings with it the challenge of disrupting existing land uses, as well as a capital investment that regulatory policy doesn’t necessitate. As such, design has evolved to consider more localised stormwater treatment options with potential for independence from the larger catchment. The opportunity for peripheral integration of WSD devices in the Drained City was highlighted in the Phyllis Reserve site study, addressing contaminant runoff by extension of the ecological zone inward toward development. This blurs the boundary between human and natural habitat edging the meniscus between “dirty” and “cleaned” flow further inland as territory of contaminant-free flow is reclaimed. The gradient of banks siding Oakley Creek through much of its length provides opportunity for multiple treatment slopes. Each added WSD device and every instance of reduction in impervious surface in the catchment contributes to this boundary shift. The Penguin Drive site study presented quite a difference, where an “ecological zone” is absent from the flow other than at the outlet to Murrays Bay. Location and design herein provided a “treatment island” that achieved a reduction and dilution of stormwater entering underground reticulation. These site studies are both examples of treatment independence, where integration of WSD across the greater area was not necessary in achieving stormwater treatment. While a minor treatment train is presented in each scenario, the idea that each element adds effect to the greater picture

must not be lost. Treatment devices, when used in isolation, can have a positive impact on stormwater runoff issues. The addition of subsequent devices in a catchment over time can be carried out as development grows/changes, or as opportune sites become available. Device The use of a swale train concept for steep slopes brings together existing thought around filter strips and swales. Rain garden theory is added when weirs are utilised in the design to prevent continuous flow-on. With slope stability as the key risk, device specifications given adhere to commonly accepted guidelines. The device’s design was approached with the idea that compaction of this site was to be avoided, and consequently gradients of cut slopes are kept shallow. Shallower swale are preferred for higher gradient slopes due to their requirement for reduced soil disturbance, as well as the lighter carrying load they apply to the slope. Collaboration with geotechnicians will be pivotal for thorough soil and bedrock analysis. Recommendations from professionals based on soil chemistry, organics and physical parameters will likely contribute to location and dimensioning within each site, as well as soil health for microbial processing. Variation of context will affect dimensions and feasibility of the concept, with few constants remaining between sites. Correspondingly, the size and context of the site will influence capacity volumes as well as the extent to which contaminants are removed.

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Challenges to Present Thinking 1. The terraced swale train concept brings a mind shift to “naturalisation” and the revegetation that such sites might normally host. Unless in a formalised garden setting, the ecologically-minded individual is familiar with native planting being encouraged to establish and succeed through natural regeneration to a more mature vegetative state. The WSD concept generated in this project requires a high fibrous root content to maximise both phytobial processing and slope integrity. The vigour of monocotyledonous perennials and their supporting fibrous root system can be compromised by the shade of shrubs and trees. As such, high light levels reaching the ground plane are important in maintaining the slope’s treatment function. Larger vegetation can leave the soil immediately below the canopy vulnerable to erosion through overland flow when grasses and groundcovers are shaded out. 2. Contemporary thinking favours the cleansing of stormwater prior to its release into the natural environment. After treatment has been carried out, it is not standard practice to run the water back existing reticulation as the case has been presented in the Penguin Drive site study. Negatives associated with this practice include adding to reticulated stormwater volumes and associated issues, and the diversion of waters away from source where a contribution to groundwater recharge might have been made. This variance to the norm is a positive option where an immediate or appropriate ecological zone is not close. The benefit herein is the dilution of contaminated reticulating waters, assisting in seeing outflow contaminant concentrations at Murrays Bay reduced. While this might

have not been the ideal stormwater route, it brings preference over the higher contaminant and volume contributions that might have been made were the treatment slope not a significant element in the flow path. This concept might be replicated several or many times within a catchment, reducing the load on other devices and processing systems. Future Direction The concept of slope-based stormwater treatment requires professional development, refining and testing, as it has the potential to offer solutions in the high gradient niche where previously solutions didn’t exist. Direction forward can begin with assessing contributions drawn from existing theory in similar fields, and further questions asked. Headley and Tanner (2008) discuss the importance of the biofilm that forms around plant roots, providing a surface area that is biologically active and hosts biochemical processes.

•

What factors promote the presence and growth of this biofilm layer?

• How can they be encouraged in a sloped treatment setting? •

How can contact between stormwater and this biofilm layer be enhanced?

Levin & Mehring (2015) promote natural biodiversity theory in bioretention systems, suggesting optimised function can be achieved through sampling effect, complementarity, insurance hypotheses and facilitation. These concepts of creating a resilient, healthy and dynamic environment of the

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most appropriate species might be applied to treatment slopes in Auckland, as well as across the wider spectrum of devices within the WSD toolkit. A number of studies have also been carried out on the effectiveness of different plant species in stormwater treatment. The case study in Pannessieres (Izembart & Le Boudec, 2008) highlighted the benefit individual species can have within the range of ecosystem services required in the remediation process. Auckland Motorway Alliance (NZTA, 2011) compared benefits of Cyperus ustulatus, Ficinia nodosa and Apodasmia similis in a motorway swale environment, but no stormwater treatment comparison was made.

• Which native plant species have the highest rates of evapotranspiration? •

Which native plant species are best for stormwater contaminant removal?

•

What correlations can be made between native plant species and specific contaminant removal?

Investigation into land valuation differences could shed additional light into positive and negative economic attributes of a treatment slope. Comparative analysis of the wider opportunity costs of the placement of stormwater treatment on 25-40% steep land over locating treatment on flat land may assist in bringing further objectivity to the table.

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Future Auckland Responsibility lies with municipal bodies to initiate water sensitive runoff treatment at a greater frequency in established urban areas, where devices may be independent of or in conjunction with a wider treatment train. The motorway system that is a source of large volumes of runoff features many opportunities for slope-based stormwater treatment. At off-ramps and on-ramps, particularly across North Shore and Manukau, grassed slopes exist downslope of large motorway catchments. Auckland Transport is host to these highly visible points of potential runoff treatment. Similarly, Auckland Council’s daylighting of some streams and the naturalising of previously culverted streams is occurring in isolated instances. However, water sensitive design is being kept largely to areas of new development, while underground reticulation systems in established areas age and maintenance costs increase. Auckland’s many and widely distributed instances of high-gradient land demonstrates profuse opportunity for integration of treatment slopes into our urban fabric. The value of an area to its residents and to the wider audience is enhanced when stormwater resilience is improved. By making stormwater treatment visible, the Drained City mindset is challenged and water sensitive alternatives become the preferred. As such, the provision of recreational opportunities around WSD events become educational and change-making opportunities.

REFERENCES Auckland Council. (2012). The Auckland plan. Retrieved on October 6, 2015 from http://www.theaucklandplan.govt.nz Auckland Council. (2014). Technical Report 2014/007: Benchmarking Auckland’s stormwater practices against the Water Sensitive Cities framework. Auckland Council, New Zealand. Auckland Council. (2015). Auckland design manual. Retrieved on 24-March 2015 from http://www. aucklanddesignmanual.co.nz/ Auckland Council. (2015). Guideline Document 2015/004 (GD004): Water sensitive design for stormwater. Auckland Council, New Zealand. Auckland Council. (2015). Proposed Auckland unitary plan. Retrieved on 26-March 2015 from http:// unitaryplan.aucklandcouncil.govt.nz/Pages/Plan/Book. aspx?exhibit=PAUPSept13 Auckland Regional Council. (1999). Technical Publication 108 (TP108): Guidelines for stormwater runoff modeling in the Auckland region. Environment Division, Auckland Regional Council, New Zealand. Auckland Regional Council (2003). Technical Publication 10 (TP10): Stormwater management devices – design guidelines manual. Environment Division, Auckland Regional Council, New Zealand. Auckland Transport (2015). Okura River & Vaughans Road. Retrieved on October 6, 2015 from: https://at.govt.nz/ projects-roadworks/long-bay-transport-programme/okurariver-vaughans-roads/ Auckland Transport (2015). Waterview shared path. Retrieved on October 6, 2015 from: https://at.govt.nz/projectsroadworks/waterview-shared-path/

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Barrett, M. (2004). Performance and design of vegetated BMPs in the highway environment. Critical Transitions in Water and Environmental Resources Management: pp. 1-10. Bohard, E. (1995). Determining Developable Vacant Lands Through A Growth Management Process. Paper presented at the ESRI International Users Conference, 1995. Clark County, Box 5000, Vancouver, WA 98666-5000. Brown, R., Keath, N. and Wong, T. (2008). Transitioning to water sensitive cities: historical, current and future transition states. In Proceedings of the 11th International Conference on Urban Drainage, Edinburgh, Scotland, 2008. Conder, T. (2009). Development and application of a New Zealand car ownership and traffic forecasting model. NZ Transport Agency Research Report 394. Elston, E., Anderson-Ledere, R., Death, R.G. and Joy, M.K. (August 2015). The plight of New Zealand’s freshwater diversity. In: Society for Conservation Biology: Conservation Science Statement No. 1. Environmental Protection Agency. (2014). Addressing green infrastructure design challenges in the Pittsburgh region: steep slopes. 2012 Green Infrastructure Technical Assistance Program. Pittsburgh, Pennsylvania. Headley, T.R. and Tanner, C.C. (2008). Floating treatment wetlands: an innovative option for stormwater quality applications. In Proceedings of the 11th International Conference on Wetland Systems for Water Pollution Control. November 1-7, Indore, India. Izembart, H. and Le Boudec, B. (2008). Land&ScapeSeries: Waterscapes. Using Plant Systems to Treat Wastewater. Barcelona: Gustavo Gili.

Keller, G. and Sherar, J. (2003). Low-volume roads engineering – best management practices field guide. USDA Forest Service/ USAID. Virginia, USA: Virginia Polytechnic Institute and State University. Levin, L.A and Mehring, A.S. (2015). Optimization of bioretention systems through application of ecological theory. WIREs Water 2015, 2: 259-270. doi: 10.1002/wat2.1072. Retrieved on April 28, 2015 from http://wires.wiley.com/ WileyCDA/WiresArticle/wisId-WAT21072.html Long Bay-Okura Great Park Society. (n.d.). Crimson walkway. Retrieved on October 6, 2015 from http://longbaypark.org. nz/crimson-walkway/ Meurk, C.D. and Hall, G.M.J. (2006). Options for enhancing forest biodiversity across New Zealand’s managed landscapes based on ecosystem modeling and spatial design. New Zealand Journal of Ecology, 30(1): 131-146. NZ Transport Agency. (2010). Stormwater treatment standard for state highway infrastructure. Wellington, NZ. NZ Transport Agency. (2011). Stormwater swale planting improvements. Retrieved on August 13, 2015 from https:// www.nzta.govt.nz/assets/resources/stormwater-management/ docs/swale-planting-improvements-2011.pdf Opus International Consultants. (2015). Newmarket level crossing project: stormwater management – assessment of environmental effects. Compiled for Auckland Transport, Auckland, New Zealand. Valuation Consultants. (2012). Risk matrix. Retrieved on 24-March, 2015 from http://vcnz.co.nz/risk-matrix/ Watercare. (2010). Wastewater overflows. Retrieved on October 17, 2015 from: https://www.watercare.co.nz/ common-content/wastewater-overflows/Pages/default.aspx

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Reuben McPeak 2015 Bachelor of Landscape Architecture Unitec Institute of Technology