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Drainage system still not fully developed

March 15, 2001

Drainage systems can be far better designed. This is one of the conclusions that Professor Fran'§ois Clemens comes to in his thesis, for which he received his PhD on Tuesday 13 March. “The models that are used for the development of drainage systems are not realistic,” says Clemens, “That means to many or to few measures are taken during the design. The consequences are that costs are too high or a small environmental disaster is created.”

Drainage systems can be seen as one of the critical infrastructures in our society. Almost all Dutch households (97%) are connected to a sewerage purification plant. “The average investment is 80 billion Guilders, which is equal to that of the electricity networks. That is something most people don’t realise,” says Clemens.
A part of these investments are not properly used at the moment, because there is only a limited knowledge of many important factors needed in the calculation-models used for the construction of these systems. Clemens: “The calibration of existing models can also be improved on.” This is no wonder according to Clemens. “The design problem is like a plumb pudding being squeezed in a vice, it wants to go every which way. Moreover a number of sensors that are supposed to provide input for the models do not survive in the harsh chemical climate of the sewerage system.”

In his thesis, Clemens describes, among other things, a case study in Deventer. “In this study I searched for sources of errors in current models.” Clemens found a large number of parameters that can be very important. “It is for example very difficult to estimate exact water drainage when taking into account the puddles left on roads.” Clemens especially wanted to know what the acceptable margin of error was in current models. “Where do the models really go wrong? We have also discovered that the number of decimals of many numbers used in models can be of crucial importance.” In Deventer, Clemens was able to make a saving of 700 to 800 thousand Guilders in this way. “It could also have gone the other way, because we are dealing with the difference between a model and reality, and that can go both ways.”

As a new professor, Clemens is supervising the follow up research project himself. A PhD student is for example going to make a study of the question: is it possible to take not only the quantity but also the quality of the water into account in models? Other questions are: what influence do chemical reactions have? What effect does solid matter have? Clemens: “I completed my own study doing research on this subject. It is not easy, but it’s definitely interesting. It not only answers social questions but also fundamental scientific ones. The sewerage system is more interesting than one may think.”

Delft University of Technology