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.