Seasonal variation in sewers, how it can affect your perception of system performance

In a typical sewer master planning project it is common to undertake monitoring and hydraulic modelling to determine optimised strategies and capital works programmes. In most cases short term rainfall and flow monitoring (8-12 weeks) is used to give a snapshot of how a wastewater system performs during a single wet season. Although this generally provides a sound basis to build and calibrate a network model it can sometimes skew our sense of system performance and ultimately the preferred set of options.

In this paper we will examine how long term monitoring significantly changed the model calibration and outlook on how the system performs. We will use the Whangarei sewer catchment as our case study which highlights dramatic seasonal differences in dry weather base flow and consequently inflow and infiltration estimates. These seasonal flow differences were unseen with the short term flow monitoring effort and this paper highlights the risk of using only a short snapshot of monitoring data to project long term trends and develop long term options.