Modeling water supply networks in New Zealand

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Only a few years ago the use of network models for water distribution in New Zealand was mostly limited to basic planning and investigation work. However the introduction of several key pieces of legislation is fundamentally changing the role of network modeling throughout the country. This has resulted in a drive towards all-mains models built to high standards with modern GIS and modeling software packages. The influence of experienced modelers drawn from overseas to work in New Zealand is also evident, as model building and calibration standards are now matching or in some cases exceeding the best international standards.

In this article, Dan Stevens, a Principal Environmental Engineer with New Zealand based Opus International Consultants, describes the key legislation and other influences driving this change, and looks at how models are being used to help the water suppliers to comply with legislation and improve levels of service.

Water Supply in New Zealand

Water in New Zealand is supplied by 74 Territorial Local Authorities (TLAs), with two bulk suppliers providing water to the cities of Auckland and Wellington. Peak water consumption in some areas is over 660 gallons/property/day (2500 liters).

People pay for their water to the local council as part of their rates, and in international terms the cost of water is low, making it an under-valued resource. However with many areas experiencing population growth and relocation and an ever increasing demand for water, some areas are already experiencing water shortages through falling aquifer and river levels, so this is a situation that is bound to change. Evidence of this change is already being seen as many TLAs consider moving to develop domestic metering programs and “water-wise” education campaigns. There is also a growing awareness of the need to manage unaccounted-for–water (UFW) and initiatives are being developed to help benchmark and reduce UFW in many of the TLAs.

Key legislation

Whilst network modeling has traditionally been used in New Zealand for developer services enquiries and basic pipeline sizing, three key new pieces of legislation are encouraging the TLAs to make more use of hydraulic models as key decision-making tools.

These pieces of legislation are:

  • the Local Government Act (LGA 2002);
  • the Civil Defence Emergency Management Act; and
  • the Drinking Water Standards New Zealand 2005.

The New Zealand Fire Service Fire Fighting Water Supply Code of Practice is also a key driver for many communities, and models are increasingly being used to assess compliance with the code and help design system upgrades.

Local Government Act (LGA 2002)

The LGA 2002 is perhaps the principal driver for extending the use of hydraulic models. It introduces Long Term Council Community Plans (LTCCPs) These require councils to take a long term view and consult with the community, which often means using models for strategic planning for five, ten and up to twenty or fifty years.

To be able to achieve this, the councils need to better understand the demand from likely development, how their water distribution systems operate, and how to assess the condition of the network and plan their capital expenditure.

Capital expenditure has to be targeted to meet key performance indicators and community needs. Councils have to prepare asset management and activity plans, and need to justify expenditure to the community – and increasingly to Audit NZ, the government's spending watchdog.

Drinking water standards New Zealand (2005)

In addition to the Local Government Act 2002, new drinking water standards are being introduced that will require considerable investment throughout New Zealand. The water supplied to many communities will not meet these standards, and TLAs will need to find the ways to supply drinking water that satisfies the standards.

In New Zealand, there are two main water quality problems: groundwater often has little or no treatment before it is distributed to users, and many of the existing treatment works are out of date and have limited capabilities. Because of this, the TLAs are increasingly using models to help find solutions, either by combining systems, upgrading a single works, or building new treatment plants.

The Taupo Water Treatment Works model

Models are also being used to assess the infrastructure improvements required and seek the most economical technically sound long-term solution. Projects exemplifying the new emphasis on a modeling approach in response to changing regulatory and land use pressures include the Taupo water treatment works upgrade. Taupo is a leading tourist resort and a growing community on the shores of Lake Taupo, situated in the central North Island.

Currently the water supplied from the lake has only a minimal treatment before it is supplied to the community. The water treatment works requires upgrading to comply with the new Drinking Water Standards and the sites of the two existing water treatment plants have a small footprint and insufficient space for upgrading.

Opus built a detailed model (including all mains and system valves) to help locate the new water treatment plant and designed system upgrades to meet the current and future demand extending to year 2033. The model demonstrated that the capital and operational costs associated with the eight possible treatment plant locations under consideration varied by as much has $3.8m USD ($5m NZD). These savings more than justified the investment made in the model, and the council now has a very powerful operational and planning tool for the future.

Civil Defence Emergency Management Act 2002

New Zealand sits on the boundary between the Pacific and Australasian tectonic plates, making it very tectonically active. Earthquakes are a known risk and the TLAs need to plan for the resulting emergency situations.

Under the Civil Defence Emergency Act 2002, TLAs have to prepare detailed action plans for submission to the Director of the Civil Defence Emergency Management Group, highlighting how water supplies will be maintained and/or restored in the event of an emergency. The new National Civil Defence Emergency Management Plan will be effective from 1 July 2006.

It is early days, but TLAs are becoming increasingly aware of the potential use of network models for emergency planning and management. Throughout New Zealand models are starting to be used to establish contingency plans and help provide action plans for operators in the event that an earthquake causes the failure of a key supply main, reservoir or treatment plant.

New Zealand Fire Service Water Supply Fire Fighting Code of Practice

The Fire Fighting Code of Practice sets flow, pressure and system storage criteria required to fight fire based on the types of properties in the area such as domestic, industrial, residential care, hotels and hospitals. It also sets a maximum distance between hydrants and distance from properties.

In the past models have been used mostly in the larger urban areas to assess fire fighting capability, but as more and more models are built it is evident that many small communities in particular are looking to improve their systems to comply with the new Code of Practice.

InfoWorks WS has been used recently to help such communities to evaluate fire fighting capabilities for their existing systems and to design new systems that will meet the requirements set by the Code of Practice. For example, Automatic Fire Hydrant simulation can evaluate flow and pressure requirements on all hydrants in the network within a single simulation run.

Population growth, relocation and changing land use patterns

Many areas of New Zealand are experiencing rapid population growth or re-location. This is putting a strain on a number of systems, and models are being used to investigate possible new sources of water and to design system upgrades to meet these needs. Also, as in many parts of the world, investment backlog is resulting in aging and often under-sized infrastructure.

Rapid change in land use is another driving force behind the increase in hydraulic and water quality modeling of supply networks. Changes in land use, from forestry to farming or from sheep farming to dairy farming, significantly change the water use in systems, often stressing them beyond their capacity.

In New Zealand, there has also been a general trend towards developing rural ‘lifestyle blocks' – enabling urban residents to move to rural areas to achieve a better quality of life. Many rural supply systems were designed essentially for stock use and have little or no water treatment prior to use and are often very limited in capacity.

With the aid of detailed hydraulic models, the TLAs can analyze how changes in land use will affect the system and develop strategies to meet future water needs.

The Ashburton Town ship model

The Ashburton township in the South Island is an area experiencing growth in both the domestic and commercial sectors. Water was historically supplied to Ashburton from a river source to the north of the town using a gravity system. The gravity main is old, in poor condition and subject to high leakage.

A detailed model of the system was developed and used to help design three new borehole sources within the town (drawing from secure groundwater sources) and the required system upgrades to improve the robustness of the system. User Programmable Controls were used to model the proposed PLCs (programmable logic controllers) and to optimize pump settings. The model has also been used to assess critical mains and to help to recommend ways to improve security of supply.

The Owaka township model

A good example of how models have been used to meet the needs of small communities as well as the larger urban centers is a project recently undertaken to assess the potential for upgrading the Owaka township water supply .

The township of Owaka (approximately 175 properties) currently has a restricted supply, fed by an untreated groundwater source. Currently the system does not meet either the new Drinking Water Standards for New Zealand 2005 or the New Zealand Fire Fighting Water Supply Code of Practice.

In addition, the township wants to grow and a number of new motels are being built to cater for an increasing number of tourists attracted to this scenic area.

The community has limited funding available due to the low rate base and wanted to gain the maximum benefit for the system for the funds available. An all mains model of the system was developed and used to study ways to improve the system and provide budget costs. Its work included:

  • Investigating possible water treatment options and necessary system modifications;
  • Considering the upgrades required to move from a restricted to an on-demand system; and
  • Designing system upgrades to meet the fire fighting code of practice

UV treatment was identified as the preferred short-term treatment solution with future additional treatment processes to be phased in over time, and the system will be upgraded for fire flow, but the costs of upgrading to an on-demand system were outside the current budget.

Modeling forums and software specific user conferences

As awareness of the potential use of hydraulic modeling has increased over the last few years, the role of modeling forums (such as those coordinated through the New Zealand Water and Wastes Association) and the software specific user conferences (such as the InfoWorks WS User Conference) have also developed. These forums have provided a means by which modelers can come together to learn about the latest developments in modeling software and share ideas on how to apply these new tools in the New Zealand environment.

As a result of the growing desire to meet international best practice, the New Zealand Water and Wastes Association – Modeling Specialist Interest Group has developed comprehensive Modeling Guidelines* to help modelers to achieve higher standards of model construction and calibration.

Summing up

Water modeling in New Zealand is coming of age. Key legislation and other drivers are encouraging the TLAs to expand their knowledge and use of network modeling throughout the country. This increase in modeling work is also attracting experienced hydraulic modelers from overseas, particularly from the UK, South Africa and Canada. These Engineers have seen that they can build on what they have learned over the years in their own countries and apply this knowledge in a whole new environment. Network modeling in New Zealand is now meeting and often exceeding best international modeling practice, driven by legislation and community needs and aided by an increasingly skilled modeling community.

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