Flood Defence Systems Jan T.L. Yap - AguaJaring Integrated Flood Management
Flood Embankment in Action
Flooded Flood Plain
Failed Flood Embankment
Piping through Embankment
Strengthening Flood Embankment
Good Performanced Embankment
Failed First-line Embankment
Emergency Intervention - Blow-up
Flood Defence Systems Philosophy of Risk Management Failure Mechanisms Failure Tree Probabilistic Design
Overview design methods of embankments
Initialy: pure based on experiences . Crest level established with reference to the highest known water level;
In the 50s: applying statistical techniques to determine the “Design Water Level” -> Deterministic design method using fixed design values for the various parameters;
Since the 70s – 80s: Probabilistic Design Method -> using stochastic character of the various load and strength parameters -> based on an analysis of failure mechanism and their probabilities.
Review deterministic design methods
Crest level only one of the design parameters of embankment profile. Slope, crest width & berms: stability, maintenance, construction and usage of crest;
Hydraulic conditions & soil mechanics parameters essential for embankment stability -> limited degree of accuracy -> Safety factors applied based on experience, tradition and intuitive judgement;
Only determined per embankment stretch or section, while it is in fact part of a flood defence system -> many stretches of different characteristics, and many structures like sluices & water gates and …….. protecting different regions: number of inhabitants, invested capital, strategic utilities, future land use, etc.
Components of a flood defence system
Shortcomings deterministic design methods
Various embankment sections in a defence system do not provide same degree of safety;
It is not known which failure mechanism gives the greatest contribution to the probability of failure of an embankment section;
The overall length of the embankment system has no influence upon the design of the embankment section;
Magnitude of damage or loss has no influence on the design;
The actual probability of inundation (flooding) of the protected area is not known
Principle of Risk Analysis
Risk = Probability x Consequence
Probability of an undesirable event : explosion, inundation, etc.
Consequence of the occurrence of that event : -> economic loss, damage, loss of life, etc.
Risk Analysis: the whole set of activities asimed at
the probability of an undesirable event ..and .. the consequence of the occurrence of that event .
Advantages of Probabilistic Approaches
The flood system is analysed and described as a whole;
Components of the system can be better inter-related, i.e. less over- and under-designing;
Various uncertainties are rationally incorporated in the assessment of the safety of the system;
Better insight into the sensitivity of probability of failure of the system related to the various uncertainties;
Better insight into the priority for improving the flood defence system (maintenance).