Influence and modelling of urban runoff on the peak flows in rivers.
Surface waters and urban drainage systems are usually studied separately. However there are important interactions between both systems. Urban drainage systems can have an important impact on the surface waters, mainly at combined sewer overflows. On the other hand during periods of high water levels in a river, the runoff from the urban drainage system can be significantly influenced by backwater, which increases the probability of flooding in is not obvious, because the modelling tools for both systems are often hard to combine properly. To properly assess the probability of flooding for this kind of integrated water systems, different submodels are needed for both subsystems. In practice often one single model is used to describe the runoff to rivers despite the presence of urban catchments. The main objective of this study is to show the limits of this simplified approach. Furthermore, it is necessary to use continuous long term simulations, because of the differences in runoff behaviour. Detailed hydrodynamic models do not really fit for this purpose because of long simulation times and high demands in memory and disk space. Therefore simplified conceptual models are more useful.
The use of integrated modelling in an urban water system and its influence on flood estimation
In an urban catchment there are several systems present: a sewer system (a combined or separate 
system), a receiving water body and surface flood pathways created during extreme events. 
Regulating urban flooding is very complex because of the interactions between the sewer system and 
the surface water system. Presently each of the policies regulates a different part of the urban water 
system, but there are no policies that consider the urban water system as a whole. In practice this 
can easily lead to problems. If the water levels of the receiving water body are too high there can be 
no free discharge of combined sewer overflow, resulting in backwater in the sewer system. These 
backwater effects can cause flooding in the urban area if the storage of the sewer system is filled by 
a rainfall event. Furthermore combined sewer overflows can have a significant effect on the peak 
discharge of the receiving water body. To determine the impact of flooding models are used which 
calculate a scenario that occurs on average once every few years, the so called design-event. If 
flooding is caused by the interaction of the sewer system and the receiving water system it is unclear 
which design-event should be used to assess the flood risk and the return period of the event, 
because the flooding can be caused by multiple types of events. 
Although it is possible that interactions occur between the subsystems in an urban water catchment, 
water systems in the urban area are often modelled without little cross-reference to each other, 
although in more complicated situations this may be required. To determine whether integrated 
modelling should influence the design of the urban water system a case study was performed. The 
study area is the industrial area De Vosdonk in Etten-Leur, a town in the Dutch province Noord- 
Brabant. Near the Vosdonk are frequent problems with flooding due to the limited discharge capacity 
of the receiving water body and the insufficient capacity of the sewer system. 
The objective of this study is to compare different methods of integrated modelling and determine 
the influence of integrated modelling on return periods of flooding of the urban water system near 
the Vosdonk. 
There are different methods of integrated modelling available, each with their own strengths and 
weaknesses: 
 The models can be run separately and allow for interaction using level hydrographs, this 
method is easy to set up, but because a number of iterations are required to increase the 
accuracy of this method, the calculation times are higher than with the other methods. 
 Coupling of existing models using a modelling framework. A relatively new modelling 
framework is the Open Modelling Interface (OpenMI). At the moment, only a few case 
studies have been done using OpenMI and none of them used the models that are used in 
the Netherlands. 
 Fully integrated models. Fully integrated models are difficult to create because of software 
limitations, but once they are finished they are easy to use. The main difficulty in creating 
fully integrated models is that current models often cannot be used because they were not 
built with integration in mind. 
To determine which integrated modelling method is preferable to determine the flood risk of the 
urban water system, the different methods of integrated modelling are evaluated on a number of 
criteria. Since the calculation times for the method using level hydrographs are too high and the fully 
integrated model gave memory errors for long simulation periods, the model framework OpenMI 
was the preferred method in this study. 
The influence of integrated modelling on the calculated number of floods is determined using a timeseries 
of precipitation data of measurement stations near the study area. Using fifteen years of 
precipitation data, two types of calculations were performed. The first one is a stand-alone 
calculation of the sewer system and the receiving water body and the second one a calculation using 
the integrated model in OpenMI. The number of floods in the sewer system that were calculated in 
both methods was compared and there was a small, but no significant, increase in the number of 
floods calculated with the integrated model. Although the number of floods did not change much 
using integrated modelling, the flood depth increased compared to the stand-alone calculations. The 
“stand-alone” model of the receiving water body already uses a simple schematization of the urban 
area and this proved a good estimation of the water levels in the receiving water body. 
From the results of the integrated model it can be concluded that integrated modelling is of no 
added value for the sewer system in this case when the policy is only based on the frequency of 
flooding, since there is no significant increase in the number of flooding events. However, when the 
policy is based on flood risk, integrated modelling is of importance, since the calculated flood depths 
increase when using the integrated model. The added value of integrated modelling for the receiving 
water body is limited since the stand-alone models often incorporate a simple schematization of the 
sewer system which can give a good insight in the outflow from the sewer system to the receiving 
water body. 
Policy makers which incorporate flood risk in their plans should determine the need for integrated 
modelling in their plans. A first insight whether it is necessary to create an integrated model is to 
determine how sensitive the urban water system is for interactions between the subsystems. Since 
this study showed that integrated modelling influences the total flood risk, this will be likely the case 
in other urban areas as well, especially those with similar characteristics as the Vosdonk. Future 
research should focus on whether there is an effect of integrated modelling on the flood risk in the 
urban water system for urban areas with other characteristics, such as a smaller urban catchment 
compared to the total catchment size, as well.
Characterization Of Sewer Systems WithStorage/throughflow-relationships
Ideally, for emission calculations long term dynamic simulations should be performed, but this requires long calculation times. Simplifications are consequently necessary. Due to the non-linear behaviour of sewer systems, it is difficult to find a direct relationship between the input (rainfall) and the output (overflows) of a combined sewer system. Since continuity always has to be fulfilled, the sewer system storage is the key parameter. Simplified models make use of storage/throughflow-relationships to characterize sewer systems. For a number of real sewer systems storage/throughflow-relationships were determined under different rainfall conditions. Although the rainfall conditions varied considerably, the storage/throughfiow-relationships seem to remain quite similar. This provides a good argument to build faster, simplified models for long term simulations, where only the storage/throughflow-relationship and the continuity equation are necessary. By fitting a piecewise linear relationship to the determined storage/throughflow-relationship, it is readily implemented in a linear reservoir model. For such model the solution can be analytically obtained. This results in very low calculation times, of the order of 10* to 10* times smaller than for the dynamic simulation.