By Paul Patrick, Yarra Valley Water
Fast growing urban areas present a challenge for water utilities. Often located on the city fringe, they can be remote from existing infrastructure. Yarra Valley Water outlines the future planning they undertake to ensure they keep up with this growth.
Background
The existing townships of Mernda and Doreen are located approximately 30 kilometres northeast of the Melbourne CBD. Currently the area contains approximately 8,000 properties and is expected to reach an ultimate figure of approximately 22,000 properties by 2040.
The area was identified for development in the late 1990’s and construction commenced on the first sub-divisional development in the early 2000’s.
Following the identification of the area for development, Yarra Valley Water (YVW) commenced planning works for the provision of sewerage services to the area. The initial servicing investigation identified the following issues for YVW in providing initial and long term sewerage services to the area.
• The area was remote from the existing sewerage system, with the nearest sewers located approximately 8kms away in the two neighbouring catchments of Eltham/Lower Plenty and Darebin Creek North.
• The Eltham/Lower Plenty and Darebin Creek North catchments did not have sufficient capacity to accommodate the ultimate flows from Mernda/Doreen, which would require that flows be attenuated in the Mernda/Doreen catchment during wet weather events.
• Of the two adjacent catchments, the Eltham/Lower Plenty catchment had the greatest available spare capacity, estimated to be in the order of 425L/s of available capacity in the Eltham Main Sewer.
To service the initial developments, the Painted Hills Sewage Pump Station (SPS) and associated 13km DN250 transfer main was commissioned in 2001 to transfer flows into the Eltham Main Sewer.
The Painted Hills SPS solely serviced development in the area up to 2008, when the following assets were commissioned.
• Mernda South Sewage Pump Station (SPS) and Flow Control Facility (SFC)
• Mernda North Sewage Pump Station and Flow Control Facility
• Doreen Sewage Pump Station and Flow Control Facility
The current day-to-day operation of the system is complex, with interlocks in place to control the operation of all the pump stations and flow control facilities. As development has increased, the operation of the system is becoming more difficult, and issues are becoming evident.
Issues
The issues that were identified arising in the system with the increase in growth were:
• The transfer system has limited capacity, inhibiting the ability to transfer flows out of the catchment.
• The limited transfer system capacity has resulted in the flow control facilities operating during some peak dry weather events, and increased times to pump down the system following wet weather events
• The complex interlock system between the pumps and flow control facilities is restricting operational flexibility in peak flow events
A hydraulic assessment was undertaken on the system utilising a calibrated InfoWorks CS model to determine the cause of these issues and identify potential solutions. Following is a brief outline of the assessment and outcome of the of each of the issues.
Transfer System
The original transfer system comprised of the Painted Hills SPS and DN250 transfer main, which was upgraded in 2008 to include connection of the Doreen SPS. Upon connection of the Doreen SPS, the operation of Painted Hills SPS and Doreen SPS was interlocked.
The transfer main comprises of a rising section and a falling section of pressure main, and the hydraulic assessment determined that its transfer capacity was impacted by two factors, being:
• The falling section of the pressure main had a maximum capacity of approximately 80L/s
• The interlock between the Doreen and Painted Hills pump stations resulted in periods where neither pump operated, whilst the pumps changed over
Ultimately, the solution would be to upgrade the transfer system to increase its capacity via the construction of a larger transfer main. To facilitate this would require significant capital expenditure, not available in the current Water Plan, so a short term solution was required to increase the transfer capacity.
The short-term solution identified was to remove the interlock between the Doreen and Painted Hills pump stations and to divert flows from Painted Hills SPS into the Doreen SPS catchment. Doreen SPS is the larger of the two pump stations with a capacity of 85L/s, and removal of the interlock would remove the no flow issue during pump change over.
Assessment of the sewerage network identified the diversion of flows from Painted Hills SPS into the Doreen SPS catchment would be relatively simple and require the construction of a short section of DN225 sewer.
In addition to these works, it was also found that during dry weather periods it would be beneficial if flows from Mernda North SPS and/or Mernda South SPS to Doreen SPS could be reduced.
Assessment of the system determined that reducing flows could be achieved by either increasing the capacity of the flow control facilities at both Mernda North and Mernda South, or transferring flows to an adjacent catchment.
The preferred solution identified was to transfer dry weather flows from Mernda South SPS into the adjacent Darebin Creek North sewer catchment. Growth within the Darebin Creek North catchment has resulted in the sewerage network expanding to within a few kilometres of the Mernda/Doreen catchment. Transfer of flows from Mernda South to the Darebin Creek North catchment can therefore be achieved via the construction of a 3km link main.
Installing this dry weather diversion would reduce the volume of flow being sent to the Doreen SPS and modelling indicated this would result in the flow control facilities no longer operating during peak dry weather periods.
Interlocks
The current interlock system is complex with multiple interlocks existing between the pump stations and flow control facilities. The interlocks serve two main functions, being:
• Interlocking pumps to inhibit concurrent operation
• Interlocking the flow control facilities to maintain each facility within a set volume of each other.
The hydraulic modelling determined that as flows had increased, these interlocks were limiting the flexibility of the system, and it would be ideal if the number of interlocks could be reduced.
These improvements had the net effect of removing the interlocks that Doreen SPS had with Painted Hills SPS and Mernda South SPS.
Modelling indicated that with Doreen SPS no longer being interlocked with Painted Hills SPS and having unrestricted access to the transfer main, combined with Mernda South transferring dry weather flows to an adjacent catchment, the requirement to maintain the flow control facilities within a set volume of each other was no longer necessary. This allowed the interlock between Mernda North SPS and Doreen SPS to be removed.
Future Growth
As the Mernda and Doreen areas continue to grow, significant upgrade works will be required, and current modelling of the system indicates that by 2040:
• Painted Hills SPS will have doubled its current hydraulic capacity and have a flow control facility.
• Doreen SPS will have five times its current hydraulic capacity.
• Mernda North SPS and SFC will have an increased hydraulic capacity and have tripled their flow control facility capacity.
• Mernda South SFC will have tripled its flow control capacity.
Conclusion
The servicing of the Mernda and Doreen areas highlights the ongoing problem Water Authorities encounter when providing sewerage services to the ever expanding growth areas. The existing sewerage system is often remote from the development area and/or not sized to accommodate the proposed growth. The servicing of the initial development is problematic due to low flow rates, which often require construction of temporary assets or staging of asset construction.
Often complex solutions are needed to service new developments in the short to medium term, often at a significant cost to the Water Authority.
