1.2 Safety: General Principles#
This section introduces some basic concepts regarding the safety of flood defences and these will be further elaborated in later chapters of these lecture notes.
The main function of a flood defence is to provide safety, i.e. to retain water and prevent flooding. The definition of failure for flood defences is the loss of the water-retaining function. This usually implies the initiation or development of a breach or excessive amounts of water passing the defence line (e.g. by overtopping that leads to flooding in the projected area).
As in other engineering disciplines, various failure mechanisms (or failure modes) need to be considered for flood defences. That means that all known (or imaginable) and relevant ways a dike or flood defence structure could fail to fulfil its water-retaining function need to be considered. Well-known failure mechanisms of dikes are failure due to wave overtopping leading to consequent erosion of the inner slope, or instability of the inner slope.
In order to determine whether failure occurs, the load on the defence and its resistance needs to be considered for every failure mechanism. A so-called limit state function \(Z\) can be defined, so that:
Failure occurs if the load is greater than the resistance (\(R<S\)), so more generally when the limit state has a negative value (Z<0). For every failure mechanism a model can be developed to consider the physical failure phenomenon and the associated loads and resistance. For example, if only overflow over the dike crest is considered, the dike height would be the resistance parameter and the water level would be the load. For most of the failure mechanisms, the most relevant loads include hydraulic loads, such as the water level and waves. Other loads could lead to failure as well, e.g. earthquakes can result in failure due to instability and structures can fail due to ship collision. Relevant resistance properties include geometric properties of a defence (elevation, slope) as well as soil properties, e.g. erodibility of the grass cover or the shear strength of the clay in the dike.
Several approaches are available to assess the safety of a flood defence system as part of the design or the assessment of an existing system. These considerations are often based on the concept of risk. In a general sense, risk is determined by the probability of an undesired event and its consequences (see Chapter(Ch:Floodriskanalysis)) . In a risk-based approach the failure probability, the consequences and the costs and effects of risk-reducing interventions are considered.
Consequences of flooding could encompass various types. Economic damage and loss of life are often quantified, but also other consequences such as societal disruption and environmental impacts can also occur.
The safety of the flood defence itself can be quantified by means of the probability of failure (\(P_{f}\)). This is the probability that the load is greater than the resistance, i.e. \(P_{f} = P(Z<0)\); and it is generally expressed per unit of time (often per year). A probabilistic (or reliability) analysis needs to be made to assess the probability of failure for failure mechanisms, and for various elements of a flood defence system (e.g. different dike sections, gates and dunes). In such an analysis loads and resistances and model uncertainties are characterized by means of probabilistic probability distributions. Probabilistic techniques (. reliability methods such as Monte Carlo simulation or FORM – first order reliability method) are used to determine the failure probability. The new safety standards that are used in the Netherlands since the year 2017 are formulated in terms of an acceptable probability of failure of a flood defence system.
Since a full probabilistic analysis requires expert knowledge and can be time consuming, a simplified approach is often used in practice. Applying design values of loads and resistance. This approach is called “semi-probabilistic” as design values of loads and resistance have been derived in such a way that the failure probability is sufficiently small. Examples of these types of criteria, are the design loads for flood defences that are used in various countries. In many parts of the world, hydraulic loads with a “100 year return period” are used for design. These criteria refer to the return period (= 1 / probability of exceedance) of the hydraulic loads that the flood defence must be able to withstand safely. In addition, factors of safety for strength parameters are often given in guidelines to ensure sufficient resistance of the defence. Until 2017, the safety standards in the Netherlands were expressed in this way, and safety standards for various areas ranged from 1/250 to 1/10,000 per year. For example, the defences of South Holland were required to be able to safely withstand hydraulic loads that are expected to occur (on average) once in 10,000 years. Since these standards are coupled with criteria for a critical overtopping discharge, the approach is often referred to as the overload-approach (overbelastingsbenadering in Dutch). Worldwide, the overload-approach is very common. Therefore, the overload approach will be described and applied in several parts of these lecture notes.
The safety of flood defences has to be considered in different phases of the lifetime. Of course, ensuring the required the safety during the envisaged service life is the essential task of flood defence design and safety assessment. But also the safety during construction and repair requires special attention as flood defences are typically more vulnerable in these phases.
Several parts of these lecture notes will provide more information on the concepts mentioned in this paragraph. Chapter 2 will provide more information on failure mechanisms. Chapter 3 will introduce flood risk analysis and the basics reliability analysis for flood defences. Chapter [10]CH:Saf_stand_ass_design will give more information on reliability-based design and safety assessment of flood defences.