Assessmentof Stormflows in Sewerage Systems

ReportNo WSAA 133

March 1998






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


In currentAustralian sewerage design practices, stormwater allowances are usuallyexpressed either as a function of a single catchment parameter, such as seweredarea or tenements, or by adopting ratios of peak wet weather flow (PWWF) topeak dry weather flow (PDWF). The design stormwater allowances vary from 1 to15 times average dry weather flow (ADWF) depending on the sewered area andpopulation.


In considerationof these issues the authors recognised that a significant quantity of sewerflow data was stored by various wastewater authorities, much of which has notbeen analysed for quantification of stormflows under various conditions.


Directionof Work


This studyattempts to review the available sewer flow measurement data in separatesewerage catchments in country Australia leading to the development of astrategy for the development of future storm allowances in sewer system design.


The maindirection of the work was toward:


        Comparisonof rainfall data to measured flows in selected sewerage catchments


        Examinationof relationships between the calculated stormflows arising from this analysisand various catchment parameters.


        Developmentof a strategy for determining the stormwater allowance for sewer design.


The componentsof the work were broken into the following main tasks:


        Reviewcurrent design practices of other authorities and organisations;


        Seek suitablenominated sewerage catchments from various authorities, select catchments forstudy and collect relevant data;


        Collate andanalyse data;


        Developstrategy for achieving revised sewer criteria.


The resultsobtained from the analysis were assessed and the value of the approaches takenconsidered in the development of a preferred strategy for moving forward insewer design practices.




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


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


The secondapproach examined was based on the calculation of storm flow from a seweragecatchment area and relating this volume of storm flow to the amount of rainfallrunoff entering the catchment.


The correlationsbetween stormflow (also termed excess flow or inflow/ infiltration (I/I) andrainfall in relation to catchment parameters were performed in two approaches:single parameter and multiple parameters analysis. Different forms of I/I wereadopted, ie, sewage flow, excess flow, unit excess flow (flow per EP, perhectare etc.), ratio of (I/I)/ADDF (I/I is inflow/infiltration, ADDF is averagedry day flow). The different forms of I/I are related to each parameter interms of different average recurrence interval (ARI) rainfall values.




Correlation ofInflow/Infiltration Flows and Rainfall


        Reasonablecorrelations between excess flows and rainfall were observed for some of the 13catchments analysed;


        The valuesof Flow factor RF = (I/I)/ADDF varied significantly among thecatchments and coastal areas generally had higher RF values thaninland areas;


        The RFvalues increased as rainfall increased;


        Unit excessflows related to equivalent population (L/s/EP) or catchment area (L/s/ha)varied among catchments, and increased with increasing rainfall.


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


Correlation of Inflow/InfiltrationCoefficient and Rainfall


        I/Icoefficients (CI/I) varied significantly within data points for eachcatchment, and no significant correlation between CI/I and rainfallwas determined;


        I/Icoefficients (CI/I) did not display significant change with rainfallARI variation. This implies that the amount of excess flow may change withrainfall intensity but not as much with the product of rainfall intensity andarea.


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


Inflow/Infiltration and Rainfall inRelation to Catchment Parameters


        Inflow/infiltrationflow rates were found to have a discernible relationship to the magnitude ofequivalent population and sewered area. The order of significant contributionwas: equivalent population (EP), catchment area (A), and density (EP/A). Interms of unit excess flow (flow/EP or flow/ha), there was no clear relationshipnoted;


        Somepositive correlations were found between I/I and surface runoff coefficients (C10).


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


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


This study hasnot acquired adequate data, both in terms of precision and support to aframework, to allow the relationship to be examined against the catchmentparameters in detail. The preliminary identification of equivalent population,catchment area and development density as main parameters should continue to berecognised.



The reportproposes a methodology for estimating the stormwater allowances in seweragesystems.


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