Modeling and monitoring sewerage systems

(RIONED-reeks nr. 2)

Monitoring is an essential step in getting a good understanding of the functioning of an urban drainage system. This is true under normal circumstances in which the wastewater is transported to a treatment plant as well as under more extreme conditions in which the system is overloaded and wastewater spills over into surface waters. Over the years the demands put on these systems have grown more and more stringent. Understanding the (hydraulic) behavior is of utmost importance in designing measures for improving the system in terms of protection against flooding or environmental damage. This leads to the necessity of stepwise, systematic methods to improve the reliability of models used in this field of engineering.

The activities as described in the Dutch guidelines (Leidraad Riolering module C2100) with respect to hydrodynamic calculation, aim at the acquisition of data and information by which the necessary understanding can be obtained using a hydrodynamic model. The present generation of hydrodynamic models has hardly ever been verified with ground truth. Research has shown that substantial difference can be present between theoretical (model) and actual (measured) hydraulic behavior. The possible causes are errors in or incompleteness of the database applied, limitations in the modelling concept, or incorrect assumptions with respect to the model parameters applied.

In practice, monitoring and systematic checking of monitoring data is not common practice. One of the causes for this is a lack of knowledge and possibly the unawareness of practitioners with methods to obtain more reliable models. Due to increasing uncertainty on the effects of climatic changes, increase of paved area, and implementation of new technologies on the functioning of urban drainage systems, there is an increasing demand for methods to improve the models used in this field of engineering. The Dutch guidelines (Leidraad Riolering module C2300) describe monitoring methods, but there is no link between monitoring and modeling in these guidelines. In order to fill this gap, RIONED foundation has taken the initiative to start a project of which the results are presented in this report describing the application of monitoring data for improving hydrodynamic models in urban drainage.

Several methods are presented, based on the assumption that a detailed model (describing the urban drainage system on the level of conduits and manholes) is applied as is described in the Dutch Guidelines. This puts very strict demands on the quality of database. In this report control-procedures, calibration methods, monitoring set-ups, measuring devices are described and illustrated using cases. Some issues are dealt with in some detail by examples. The reader is offered the possibility to use the methods described in practical applications. The methods are described for the situation of a combined sewer system and quantative monitoring only, otherwise the report gives the main point of interest with respect to interactions between (sub)systems (e.g. interaction between surface waters and sewer system during overflows). The exact methods for monitoring these and other possible interactions between urban drainage systems and other (sub) systems is not described in this report.

The recommended methods in this report start with a check of the database (describing the structure of the system and the geometry of the components) and end with the check of theoretical (model) results against measured results (monitoring program). Combining monitoring and modeling can lead to identification of the causes of differences between them. Calibration of model parameters can be applied on several moments in a project to improve the accuracy of the model at specific point. The working method described in this report holds 4 distinct steps:

  1. Check of the database
  2. Monitoring
  3. Analysis of the functioning of the system under study
  4. Parameter calibration

Step 1 is (mainly) a desk-study in which possible additional information is acquired by field check. The check aims at improving the consistency, completeness and accuracy of the database and is as such a primary requirement for the next steps like model verification and calibration.

Step 2 describes the monitoring in terms of the monitoring locations, measuring frequency, measuring period and measuring techniques applied. The accuracy and resolution in time and space is mainly determined by the main objective of the monitoring program. In some cases an estimation will do, in other cases very detailed and accurate measuring results are required. The methods described in step 2 give clues how to obtain a tailor made solution.

Step 3 discusses the methods by which the results of a monitoring program as well as the theoretical results (model) are checked using a water-balance. The objective is to obtain understanding of the possible causes of differences between theory and practice and to get estimations for some model parameters.

Step 4 describes methods to calibrate the (improved) model by means of parameter fitting and to get rid of ‘noise’ in order to obtain more ‘practical’ values for model parameters.

The water balance is chosen as the main landmark in the methods described, deviations in the water-balance can lead to clues with respect to the possible causes of these deviations. Under specific conditions some flows in the water-balance are known to be zero, thus offering the possibility to obtain explicit information on one particular parameter. The following three situations are distinguished:

  • • Dry Weather Run off
  • • Precipitation, no overflows, internal nor external
  • • Precipitation with in- and/or external overflows.

One of the main requirements when using the methods described in this report is that the monitoring data and the used database model relate to the same system. This implies that using ‘old’ monitoring data can only be done when the database model describes the structure and geometry of the system at the time of the acquisition of the monitoring data. The minimal measuring set-up for an analysis of the water balanced under Dry Weather conditions, contains a rain gauge and a measuring device for the discharge out of the system (mostly by a pumping station). Extension of this set-up by a water level gauge in the direct vicinity of the spill over with the lowest weir level and possibly at some other locations in the system allows a check of the water balance under storm conditions, for storms that do not initiate an external spill over. A further extension is possible to allow for the check of the water-balance under conditions in which a spill over functions during a storm, in this case the overflow coefficient of the individual weir has to be known either by model calibration or by the measuring of the Q-h relation of the overflows in separate field experiments. Application of a hydrodynamic model of an urban drainage system that is verified with ground truth leads to a more consistent and trustworthy basis for investments in measures for improving the system.