11.7 Stormwater ingress
This line item refers to an estimate of the total ingress of water into the wastewater collection system. The volume was calculated by the Bureau of Meteorology using data obtained from ActewAGL. For the 2011 Account the volume was 6,176 ML.
No estimation of ingress of stormwater was calculated into the wastewater network managed by Queanbeyan City Council (QCC). Dynamic wastewater modelling of the Queanbeyan wastewater network is not currently undertaken.
This volume is calculated by assuming a weekly baseflow of wastewater into Lower Molonglo Water Quality Control (LMWQCC) for the year. An inflow into LMWQCC during a typical dry week flow could not be selected in 2010–2011 as it was an exceptionally wet year; instead the dry week 9–15 November 2009 was taken from the previous year. The inflow for this week (555.7 ML) was then multiplied by 52 to give an annual volume for wastewater collected.
The baseflow volume of the inflow to the LMWQCC was calculated by assuming, in a typical dry week, the flow into LMWQCC was the same as the volume of sewerage of collected An inflow into LMWQCC during a typical dry week flow could not be selected in 2010–2011 as it was an exceptionally wet year, instead the dry week 9–15 November 2009 was taken from the previous year. The inflow for this week (555.7 ML) was then multiplied by 52 to give an annual volume for the total baseflow inflows to LMWQCC for the reporting year. The resulting value was then subtracted from the total inflow to LMWQCC to give an estimate of the ingress of stormwater to the sewer (including any groundwater infiltration where the water table has risen due to wet weather).
|Inflows using a typical week with no rainfall||555.7|
|multiply||Total weeks of the year||52|
|Total inflows to the LMWQCC||35,072|
|less||Base-flow inflows over year||28,896|
Assumptions, Limitations, Caveats and Approximations
- The reported volume relates to the ingress into the Canberra wastewater system only. Ingress into the Queanbeyan sewerage system is unknown.
- This volume is the net volume of infiltration minus leakage and based on board assumptions. It does not take into account groundwater ingress during dry weather, or discharge to groundwater as part of the baseflows.
The uncertainty estimate was not quantified.
The value reported as comparative year corresponds to the following line item reported in the National Water Account 2010: 19.6.1 Infiltration from landscape into the urban wastewater system (1,297 ML).