Assessment and improvement planning for the Rochdale South sewerage system


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Rochdale South is a predominantly residential suburb south of the Queensland capital city of Brisbane. It has a total population of 10,680 and a sewerage system catchment area of 580ha. The sewerage system includes five pumping stations which forward sewage flows to a downstream treatment plant. Rochdale South comes under the jurisdiction of Logan City Council, and Logan Water is the body that administers water supply and sewerage services for this Council.

Engineering consultancy firm Earth Tech was commissioned by Logan Water to undertake a planning study of the Rochdale South sewerage system. This study was undertaken according to Logan Water’s Sewerage System Assessment and Improvement Plan or SSAIP specification. This specification requires the sewerage system to be assessed for Logan Water’s Desired Service Standards (DSS) as well as for Critical Design Storm events for existing and future development conditions.

As part of this assessment, the relative impact of infiltration and inflow (I/I) was assessed and an ’optioneering’ exercise was undertaken to assess the best means of addressing any hydraulic deficiencies identified in the system. The preferred options were then put forward for consideration in Logan Water’s future capital improvement programs. Wallingford Software’s InfoWorks CS modeling software was used to conduct the system performance assessments required and for developing improvement works.

This study is an example of the benefits of dynamic modeling, which allows for the time-varying effects of rainfall on sewerage systems to be properly considered. It also allows for improvement options such as operational control and real time control to be considered as viable alternatives or additions to capital works.

The client needed to understand hydraulic deficiencies when the Rochdale South system was subjected to flows at the DSS.Their SSAIP specification also requires the impact of one in five year average recurrence interval (ARI) rain events to be assessed. These assessments needed to be made for existing development as well as the 2025 planning horizon.

Desired Service Standard (DSS) Assessment
Logan Water’s DSS specifies a design flow of five times average dry weather flow (ADWF) for design of sewers and pump stations. Since the calibration was undertaken during extreme drought conditions and while water restrictions were enforced, it was not considered appropriate to use the return to sewer discharge figures observed during the flow monitoring period to conduct this assessment. Therefore, in consultation with Logan Water, the observed dry weather contributions per person were changed to 220L/EP/day for all of the system performance assessments.

The DSS also specifies operational and emergency storage requirements. Emergency storage has to provide four hours’ protection at ADWF. Pump capacity has to be greater than the peak flow at five times ADWF. Pipes are allowed to surcharge provided that the peak hydraulic grade line is at least 1m below manhole cover level at five times ADWF.

The assessment of the system at five times ADWF revealed no manhole overflows or sewage spills at this design standard. 18 pipes were identified to be surcharged by depth, and 25 were surcharged by flow. The pumping station assessment showed that at all stations the available dual pump capacity was comfortably higher than the DSS requirement of five times ADWF.The emergency storage assessment highlighted deficiencies at one pumping station only where the available protection was under half that required by the DSS.

In summary, when the system was tested against the DSS, there were no manhole overflows or spills through emergency structures, all existing pump capacities were greater than the DSS requirement, and emergency storage was adequate for all but one of the pumping stations. The conclusion was that overall the system complied with the DSS.

Critical Design Storm Assessment
The application of the one in five year ARI critical design storm to the calibrated dynamic model produced a very different system performance result. For this assessment, one in five year ARI design storms of 30, 60, 90 and 120 minute duration were applied to the model. The critical design storm was found to be the 60-minute event as it was this that produced the most deficiencies in the system.

The critical design storm assessment identified eight manhole overflows, three sewage spills from overflow structures, 183 pipes surcharged by depth and 79 surcharged by flow.

The disparity between the outcomes of the DSS assessment and the Critical Design Storm assessment is due to the assessment of storm water inflow and infiltration (I/I) into the system. The DSS assessment makes an allowance for this by ensuring spare capacity in the system. However, the Critical Design Storm assessment is based on a calibrated hydraulic model where the volume of rainfall entering the system has been measured for observed storm events and used to set the runoff parameters in the model. Thus the Critical Design Storm assessment produces a realistic estimate of the additional flows likely to enter the system for the design event.

The findings of the Critical Design Storm assessment were consistent with field observations with historical records revealing frequent surcharging of sewers and evidence of sewer overflows. Logan Water had suspected for some time that they had a serious I/I problem in the Rochdale South catchment.

In order to assess the relative severity of I/I in the catchment, an additional assessment using the modeling results was undertaken. This assessment looked at the additional flows entering the system during the Critical Design Storm as a function of sewer length as well as total rainfall volume. This assessment revealed that I/I levels were not considered unusually high. For most subcatchments less than 1% of the total rainfall for the event was observed to enter the sewer. From Earth Tech’s experience of modeling other sewerage networks, it was considered that I/I abatement works would not be likely to improve on this figure.

Earth Tech therefore proceeded to develop improvement options for capital works that will prevent sewage spills from the system for the Critical Design Storm events. 

This project demonstrated how different planning outcomes resulted when the Rochdale South hydraulic model was tested against Logan Water’s DSS and critical design storm events of the order of one in five year ARI. This outcome has prompted a review of the design standard that should be applied to Logan Water’s sewerage systems.

Earth Tech would like to thank Logan Water for their assistance with the undertaking of this study and their support of the preparation of this paper. The assessment could not have been completed without the active participation of Logan Water’s planning and operation staff.  Earth Tech would especially like to thank Logan Water’s Senior Water Planning Engineer John Bretz and Water Planning Engineers Marco Bonotto and Joshua Canaris.

This article is based on a presentation by Kithsiri Tennakoon and Stephen D’Agata of Earth Tech Engineering to the Wallingford Software InfoWorks CS user group conference in Sydney on 29 November 2007.

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