Assessments of Stormflows in Sewerage Systems

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Assessments of Stormflows in Sewerage Systems

Report no. UWRAA 133

March 1998



The adequate design of sewage conveyance and treatment facilities requires the accurate estimation of peak flows that occur in the sewers after storms. In this study, the stormwater allowance for sewerage design was examined in light of analysis of existing sewerage catchments throughout Australia.

In current Australian sewerage design practices, stormwater allowances are usually expressed either as a function of a single catchment parameter, such as sewered area or tenements, or by adopting ratios of peak wet weather flow (PWWF) to peak dry weather flow (PDWF). The design stormwater allowances vary from 1 to15 times average dry weather flow (ADWF) depending on the sewered area and population.

In consideration of these issues the authors recognised that a significant quantity of sewer flow data was stored by various wastewater authorities, much of which has not been analysed for quantification of stormflows under various conditions.

Direction of Work

This study attempts to review the available sewer flow measurement data in separate sewerage catchments in country Australia leading to the development of a strategy for the development of future storm allowances in sewer system design.

The main direction of the work was toward:

  • Comparison of rainfall data to measured flows in selected sewerage catchments
  • Examination of relationships between the calculated stormflows arising from this analysis and various catchment parameters.
  • Development of a strategy for determining the stormwater allowance for sewer design.

The components of the work were broken into the following main tasks:

  • Review current design practices of other authorities and organisations;
  • Seek suitable nominated sewerage catchments from various authorities, select catchments for study and collect relevant data;
  • Collate and analyse data;
  • Develop strategy for achieving revised sewer criteria.

The results obtained from the analysis were assessed and the value of the approaches taken considered in the development of a preferred strategy for moving forward in sewer design practices.


Based on returned questionnaires from water authorities across Australia, 13 catchments were selected for this study and detailed data was obtained. The dry day data and the wet day data was differentiated between. Storm flows were then assessed and their relationships with the antecedent rainfall analysed.

The first approach examined involved the development of data envelopes for the stormflow/rainfall relationship, by considering the average best fit, the 95% confidence limits and the upper boundary of results.

The second approach examined was based on the calculation of storm flow from a sewerage catchment area and relating this volume of storm flow to the amount of rainfall runoff entering the catchment.

The correlations between stormflow (also termed excess flow or inflow/ infiltration (I/I) and rainfall in relation to catchment parameters were performed in two approaches: single parameter and multiple parameters analysis. Different forms of I/I were adopted, i.e., sewage flow, excess flow, unit excess flow (flow per EP, per hectare etc.), ratio of (I/I)/ADDF (I/I is inflow/infiltration, ADDF is average dry day flow). The different forms of I/I are related to each parameter in terms of different average recurrence interval (ARI) rainfall values.


Correlation of Inflow/Infiltration Flows and Rainfall

  • Reasonable correlations between excess flows and rainfall were observed for some of the 13catchments analysed;
  • The values of Flow factor RF = (I/I)/ADDF varied significantly among the catchments and coastal areas generally had higher RF values than inland areas;
  • The RF values increased as rainfall increased;
  • Unit excess flows related to equivalent population (L/s/EP) or catchment area (L/s/ha)varied among catchments, and increased with increasing rainfall.

This approach was found to be statistically valid and worthy of further pursuit.

Correlation of Inflow/Infiltration Coefficient and Rainfall

  • I/Coefficients (CI/I) varied significantly within data points for each catchment, and no significant correlation between CI/I and rainfall was determined;
  • I/Coefficients (CI/I) did not display significant change with rainfall ARI variation. This implies that the amount of excess flow may change with rainfall intensity but not as much with the product of rainfall intensity and area.

This approach was not found to be statistically valid for the data analysed and caution should be employed if this approach is to be pursued further.

Inflow/Infiltration and Rainfall in Relation to Catchment Parameters

  • Inflow/infiltration flow rates were found to have a discernible relationship to the magnitude of equivalent population and sewered area. The order of significant contribution was: equivalent population (EP), catchment area (A), and density (EP/A). In terms of unit excess flow (flow/EP or flow/ha), there was no clear relationship noted;
  • Some positive correlations were found between I/I and surface runoff coefficients (C10).

Most of the analysis of catchment parameters was limited by the availability of data and the method of categorising complex parameters into simple categories.

From this study it has been determined that there is merit in processing actual sewer flow data for a particular catchment. Expected peak storm flows can be calculated in relation to storm recurrence intervals and other catchment parameters. It is expected that such information could be categorised for different regions and conditions across Australia and that sewer design guidelines should move in this direction in the future.

This study has not acquired adequate data, both in terms of precision and support to a framework, to allow the relationship to be examined against the catchment parameters in detail. The preliminary identification of equivalent population, catchment area and development density as main parameters should continue to be recognised.

The report proposes a methodology for estimating the stormwater allowances in sewerage systems.

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