The sewer system is not static: it is constantly challenged by all sorts of events. Blockages can occur by things that are not supposed to be in the sewer. Also, the sewers can be damaged by ground works and the roots of trees.
The current sewer monitoring system is not very effective. Visual inspections are expensive, labor intensive and are therefore done only once every five to ten years. Some systematic observations are done, but only in a limited number of places. Therefore, the knowledge about the state of the system is always incomplete.
Monitoring the sewer system can be done much cheaper and more effectively. Together with engineering company Witteveen+Bos, VORtech demonstrated a proof-of-concept using a model of the sewer system to identify changes in the sewer system from the available observations. First, a model of a just-cleaned sewer system is calibrated so that its predictions closely match the real situation. Then, a re-calibration is done at regular intervals, which will show where things have changed in the sewer system. Also, by using a Kalman filter, the development of blockages can be followed over time.
VORtech made an implementation of this method using the Sobek model of Deltares and the open source data-assimilation toolkit OpenDA. The method has been shown to work remarkably well as sewer monitoring system.