THE ECONOMICS OF LOW IMPACT STORMWATER MANAGEMENT

GLENCOURT PLACE

Éva-Terézia Vesely

Jan Heijs

Chris Stumbles

David Kettle

In this presentation …

• Context – Background information

• Methodology – LCC

• Results

• Discussion

Urban Stream Health

Compromising factors:• Modified flow• Poor water quality• Lack of physical habitat• Lack of riparian vegetation• Barriers to migration of fish and other biota• Exotic plants and animals• Channelisation, erosion and sedimentation

Framework for Stream Restoration

Low impact urban design approaches that reduce drainage connection are the most effective management solution to the protection of stream biota in urban catchments.

Christopher J. Walsh – Cooperative Research Centre for Freshwater Ecology, Monash University, AU:

Windows of Opportunity for Low Impact Design

STORMWATER INFRASTRUCTURE

Age Distribution

Typical Lifetime

REPLACEMENTinfrastructure inertia

lock in effects / path dependency

windows of opportunityto explore alternative design

Background information

• Location: Windy Ridge, North Shore City

• Sub-catchment size: 2.6 ha

• Historical disposal: soakage (soakpits)

• The problem: serious overland flow problems

Two approaches …

1. The conventional approach

reticulating the area with stormwater pipes and/or flowpaths

2. The low impact approach

using an engineered system of gravel trenches, contoured flowpaths and minimal piping backed up with raintanks retrofitted to existing properties

Photo by N

adine Wakim

Methodology

• Compare the Life Cycle Costs of the two approaches

• Reveal aspects that impose higher costs on the low impact approach

• Highlight changes and potential for future cost savings

Life Cycle Costing

Australian and New Zealand Standard (AS/NZS 4536:1999)

Life cycle costing is the process of assessing the cost of a product over its life cycle or portion thereof.

Life cycle cost is the sum of acquisition and ownership costs of an asset over its life cycle from design stage through manufacturing, use, maintenance and disposal.

Life Cycle Costing

Assumptions for the LCC

• Timeframe: 50 to 100 years

• Type of cost: real costs with 2005 as base year

• Discount rates: 3.5% and 10% real discount rates

• Water savings: 125$/raintank/year

Acquisition Costs

• Conventional

- based on the pipe layout used for the initial engineering costing

- new costing data from recent contracts

• Low Impact

- cost data from the construction contracts

- NSCC accounting data

- costs occurred before 2005 2% inflation factor

Renewal Costs

• Pumps replaced every 10 years at NZ$600

• Raintanks replaced every 25 years at NZ$2350

Operation and Maintenance Costs

• Conventional

- maintenance pipe: NZ$ 1/linear meter pipe

• Low Impact

- maintenance pipe: NZ$ 1/linear meter pipe

- operation of one pump: NZ$ 10/year

- maintenance of one pump: NZ$ 20/year

- maintenance of one raintank: NZ$ 80/year

- raintank registry and inspection: NZ$ 10/raintank/year

- maintenance gravel trench and channel: -

Not quantified

• Decomissioning costs

• Cost of land

• Risk costs

Photo by N

adine Wakim

Results

Results

-50000

-40000

-30000

-20000

-10000

0

1 7 13 19 25 31 37 43 49

Private Household Cost

NSCC Cost

Private Household Benefit

Results

73%

3%

12%

12%

a) life cycle cost (base year 2005, real cost) NZD 823,000

85%

2%

6%7%

b) life cycle cost (base year 2005, 3.5% real discount rate), NZD 703,000

94%

1%

2%3%

NSCC Acquisition

NSCC Maintenance andInspection

Private HouseholdAcquisition

Private HouseholdOperation andMaintenance

c) life cycle cost (base year 2005, 10% real discount rate), NZD 639,000

Lessons

LCC a useful analytical tool for:

• comparing different options

• potentially improve design

• highlight the importance of pre-acquisition costs

• budgeting

• cost sharing

• assess financial sustainability

Looking behind the numbers …

In favour:

- Staff interest / commitment

- Lack of conventional infrastructure in place

- Commitment to low impact at strategic level (NSCC Stormwater Strategy 2004)

- Funding opportunities (Infrastructure Auckland funding)

Looking behind the numbers …

Disadvantages / Changes:

-Expertise and experience with the conventional approach

- Lack of expertise and experience with the low impact approachExperience building up (pilot projects)

- Public perceptions Experience building up with the consultation process

- Technical uncertainties Technical flexibility exposed

Looking behind the numbers …

Disadvantages / Changes:

-Standard restrictions Restrictions removed

- Lack of legal arrangements Legal arrangements sorted

- High perceived risk associated with assuring continuous operation and maintenance Stormwater Policy: Responsibilities for Stormwater Infrastructure

Investment in eco-innovation

• The novelty of the project translated into higher costs for design, project management and consultation.

• These costs are expected to drop in the future.

• The externalities remained out of the scope of this analysis.

• Market distortions will impact such analysis.

Dynamic framework

• Increased data availability on cost elements• Changing public attitudes• Future water prices

• Criteria

• Performance against these criteriaare changing.

Performance against the COST MINIMISATION criterion

Acknowledgements

The authors are grateful to Nadine Wakim, Barry Carter, Frank Tian, Viv Eyberg and Ban Aldin for insightful

discussions.