Creating efficient wastewater and stormwater systems requires careful planning to ensure that the large sums of money needed are invested in the most effective way. Before we could design wastewater improvement programmes we needed a good understanding of how our sewer network performs.

In 1998 we began using a computer model (called SEWCOM) to give us a clear picture of how the wastewater system was performing and where it needed improvement. Our engineers mapped the performance of North Shore City’s trunk sewers using computer modelling techniques to reconstruct events over the past 17 years, based on meteorological records and sewer flow information gathered across the city.

By the middle of 1999, SEWCOM had enabled us to identify those sewers that needed improvement and served as a ‘baseline’ for ongoing improvement programmes. For the first time we had credible information about the location and volumes of overflows during rainfall, and a better understanding of the major issues.

The computer model was also used to identify the sewer upgrades needed to meet the city’s growth requirements to 2050.

We identified the parts of North Shore City with greatest flow defects. At the same time we started investigating the most appropriate options for building storage and extra capacity within the network to better manage peak flows.

Initial SEWCOM modelling revealed the volumes of wastewater overflows and towards the end of 1999 further modelling gave us a picture of actual bacterial pollution at beaches in North Shore City. We found that while the condition of beach water on an annual basis is healthy, wet weather is the time when problems occur, due to heavy stormwater flows and wastewater overflows.

Computer modelling and the ‘finding’ stage of Project CARE allowed us to test proposed solutions to the city’s wastewater problems by simulating any change to the sewer network. Our detailed investigations gave us the confidence to carefully plan repairs and improvements to the network and ensure that the large sums of money needed are invested in the most effective way.

August 1999

• Accelerated options in East Coast Bays and Albany/Oteha Valley (277KB) - after 18 months of investigation into our sewerage system, we could accurately measure the level of wet weather overflows and pinpoint some problem areas.
   

• Impacts of growth in East Coast Bays and Oteha Valley (133KB) - computer modelling revealed that growth in East Coast Bays and Oteha Valley to 2005 would trigger more frequent wet weather overflows that would be of greater volumes than before.
   

• Existing sewer system performance report (738KB) - results of the computerised trunk sewer model (SEWCOM), which represents approximately 10 per cent of the total public sewer system length in North Shore City.

December 1999

• Water quality report (678KB) - for the first time we had a picture of actual bacterial pollution on eastern beaches in North Shore City.
   

• Peer review (346KB) - an internationally recognised water quality expert, Karl Iver Dahl-Madsen of Denmark, peer reviewed Project CARE for North Shore City Council.

Improvement options

This page outlines the recommended option and alternative options we proposed to upgrade the sewer network and improve beach water quality in North Shore City. The network improvements were designed to cater for the city’s wastewater needs through to 2050. Allowances were made for future maintenance of the system and improvements.

Even with no Project CARE improvements (no reduction in wet weather overflows) the costs are high to fund Project Rosedale (the sewage treatment plant upgrade) and to meet city growth needs and other cost increases. To operate a reliable wastewater service, the base sewage charge for every household will rise progressively from $381 per annum in 2000/2001, to about $1000 per annum in 2010/2011. This equates to an average annual sewage charge of about $700 over the 10-year period. The present wastewater charge for every household for 2003/2004 is $402, the same as 2002/2003.

Using the SEWCOM computer model we calculated the cost of Project CARE improvements to beach water quality by upgrading the sewer network. The model showed clearly that the level of improvement achieved – for each dollar spent – diminishes significantly between a 65 per cent and an 80 per cent improvement. (An 80 per cent improvement in beach water quality is considered the maximum achievable by reducing sewer overflows).

We recommended targeting a 65 per cent improvement in beach water quality with a programme of works extending over a 20-year period, to 2021. A 65 per cent improvement would decrease wet weather overflow events from an average 12 per year down to two per year. The average annual sewage charge would be approximately $740 per year over the first 10-year period. Further increases would be required for years 11 to 20.

Other options we considered:

1. To target a smaller improvement in beach water quality of 50 per cent (four wet weather overflow events per year) the average annual sewage charge would be approximately $730 over the first 10 years with a programme of works extending over a 20-year period.

2. To target improving beach water quality further to 70 per cent (one wet weather overflow event per year) the average annual sewage charge would be approximately $750 over the first 10 years with a programme of works extending over a 20-year period.

3. To complete most of the work for 65 per cent improvement within 10 years (rather than 20 years) the average annual sewage charge would be approximately $770 over the 10-year period.

Please note that Project CARE consultation occurred early 2001 and that the implications on rates that were used in this consultation programme were based on the 1st draft of North Shore City Council’s Long Term Financial Strategy (LTFS) (December 2000) and changed significantly with the subsequent draft LTFS.