 |
Why does flooding occurs in urban drainage systems? (Patrick Willems, Raf Bouteligier) |
 |
|
When flooding occurs, combined sewer systems are often immediately blamed. However, drainage systems are complex systems with large uncertainties on the processes and a highly variable input. The main causes of the failure of the drainage systems (for extreme runoff) can be divided into the following categories : extreme rainfall input, insufficient maintenance and a design with an insufficient safety.
It is clear that nowadays more flooding with damage occurs. It is however doubtful that this is caused by an increase in rainfall. Mainly the human impact on land use and drainage systems are leading to more flooding problems. Large progress is made in recent years with respect to knowledge acquisition on drainage systems and modelling tools. The practical implementation of it is however a huge effort. It is obvious that all sewer system owners have to take their responsibility as well for the design and maintenance as in case of failure of the drainage system. A sustainable improvement can only be obtained using source control measures, which includes technical measures as well as a harmonisation with urban planning.
<top> |
|
Influence
of rainfall input and model simplification on the design of
combined sewer systems (Patrick Willems, Raf Bouteligier) |
|
|
A wide range of computer models and
different types of rainfall input are used in practice for
combined sewer system calculations. Driven by rapidly improving
computer technology, modelling techniques, tools and possibilities
are changing equally fast. The question can arise whether
there is an unambiguous relationship between more detailed
models and more accurate modelling results. Therefore, models
and rainfall input were investigated and compared in order
to balance their degree of detail with the accuracy of the
results. Two types of rainfall simplifications are studied
in detail and the simulation results are compared with the
results obtained with the original historical rainfall series.
First, the most extreme simplification into mean rainfall
(i.e. IDF-relationships) is discussed, together with the
single design storms (i.e. composite storms) which are derived
from these IDF-relationships. Secondly, the selection of
critical short time series from the historical rainfall
series is discussed. Concerning the model simplifications,
a rather extreme simplification into a reservoir model has
been studied. With this approach the minimum requirements
can be addressed, which are necessary to obtain results
within a specific range of accuracy. These different types
of simplification are then applied to different types of
calculations : design of combined sewer systems versus impact
calculations.
POSTER 'Rainfall input requirements for
urban drainage modelling' (2000)
Full paper 'The effect of changing technology
on combined sewer system design' (1999), 8th International
conference on Urban Storm Drainage, Sydney, Australia.
<top> |
|
Guidelines for the design
of sewer systems (Raf Bouteligier, Patrick Willems) |
|
|
Currently the Flemish administration
(Aminal Water) has asked the Hydraulics Laboratory to rewrite
the Flemish guidelines for urban drainage. This project
is carried out togheter with the Ecosystem Management Research
Group of the University of Antwerp.
More on guidelines for the design of Dry
Weather Flow systems.
<top> |
|
Urban drainage software evaluation
(Raf Bouteligier) |
|
|
The Hdronaut users group is the Flemish user group of sewer system modellers and member of the Integrated Modelling User Group (IMUG).
New software is tested by the Hydraulics Laboratory and Aquafin before they are used in Flanders.
<top> |
|
|
Urban flood modelling (Raf
Bouteligier) |
|
|
Combined sewer systems and storm
water sewers are designed to guarantee the rainfall runoff
with a certain design return period. At extreme rainfall
conditions flooding can occur at specific locations because
of the insufficient capacity of the sewers. In order to
assess the flood risk the currently available modelling
practice fails, because in the current modelling software
it is assumed that the water, that raises above the ground
level and causes flooding, is remaining at the specific
location where it leaves the sewer system. The local topography
is neglected. This does not correspond to the reality. It
is not only a problem of correct estimation of the local
water levels, but in practice it can occur that the water
flows over the streets to lower locations and leads to damage
at completely other locations than predicted with this kind
of models. This paper describes how the street runoff can
be incorporated in the model as a second layer using existing
commercially available hydrodynamic software and what the
effect might be.
Full paper 'Modelling of floods caused by urban drainage
systems' (2004) at 6th international conference on Urban
Drainage Modelling, Dresden, Germany.
<top> |
|
|
Guidelines for the design
of waste water sewers (Raf Bouteligier) |
|
|
In the past combined sewer systems were
considered as the only economically feasible urban drainage
system. In the mid ‘90's this started gradually
to turn towards separate systems. However, up till now
there were no specific guidelines for Dry Weather Flow
(DWF) sewers. In 2002 the Flemish administration Aminal
Water asked the Hydraulics Laboratory to write new
guidelines for DWF sewers.
Different types of DWF systems were considered : gravitary
as well as pressurised systems. For the gravitary systems
improvements were investigated using intermediate pumps,
pre-sedimentation tanks and flushing with a limited
upstream rain water inflow. However, each system must
be tailor-made, taking into account the hydraulics of
peak flow and the minimum self-cleansing ability. Probably,
hybrid systems will often offer the optimal solution.
<top> |
|
