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turbidity and siltation." (Gravity)

Natural Hazards

The coast is exposed to a number of natural hazards. They include beach erosion, storm surge, hurricane, and tsunami. If we analyze it a bit closer, the storm surge is high water caused by wind and low pressure (and perhaps combined with high tide). Hurricanes cause damage by bringing a storm surge, as well as strong winds, high waves, and rain that causes inland inundations. The storm surge owes a substantial part of its destructive ability to the waves propagated on it. Since beach erosion is caused by waves, and tsunamis are waves, most coastal hazards are thus associated with waves.

In other environments we find a range of natural hazards associated with gravity: Running water in rivers and overland, mudflows, landslides, rock falls, turbidity currents, underwater screes, and the movement of glacial ice. Although it is convenient to classify these processes, in reality many of them appear in a continuum with one another (for instance: rock fall - mudflow - river flow - turbidity current, in a continuous sequence from the mountain top to the ocean floor). This means that for the purpose of modeling and calculations, one must seek parameters that are as universal as possible through all the classes (for instance, solid and liquid discharge can be used to quantify everything from rock fall to turbidity current).

Natural hazards are caused by natural processes. Natural disasters happen when man or his property gets in the way of those processes. For instance, it is part of the normal geomorphologic processes that some storms erode certain beaches so that whatever is on the beach ends up in the water. However, if a number of houses with occupants are destroyed, it becomes a disaster. This makes it clear that there are (theoretically) two fundamentally different ways to avoid the disaster: To prevent that the beach erodes, and to make sure that there are no houses or people on the beach when it erodes. In some cases the disaster can also be mitigated by making sure that the houses can survive the event, so that they again are usable if the beach gets restored to its previous shape after the storm.

Cost - Benefit

The choice of action should ideally be optimized so that the most benefit is achieved at the lowest cost. This requires a comparison of risk (a possible cost) with the costs of prevention, mitigation, and so on. Note that voluntary insurance is not a valid option, since from a society point of view it does nothing to prevent or mitigate a natural disaster (although from a government perspective a disaster recovery fund may be useful to prevent the natural disaster from spilling over into an economic problem). For insurance to work as a preventive tool the cost of it must be proportional to the risk, and it must be compulsory. From a society point of view it is essential to spend the money wisely, not just in terms of the optimal strategy for a specific risk, but also on the risk where the effort pays off the most. There are thus several reasons why it is essential to quantify the risk cost mathematically.

The following analysis has been adopted with some minor changes from GIS for Natural Hazard Mitigation, from the conference GIS Planet 2005, Portugal. It was originally developed as part of a natural hazard mitigation project in Nicaragua.

Definition of terms
Disaster An event that directly or indirectly causes the loss of human life, or exceeds the capacity for limiting the damages or commencing the recovery in the affected community
Event An incidence of a peak magnitude of a stochastic process
Threat A potential event capable of causing a disaster
Hazard A threat with probability applied
Vulnerability The loss caused if a threat materializes
Risk Vulnerability multiplied by probability

With these definitions the terms can be quantified, so the hazards, risks, and vulnerabilities can be used quantitatively to minimize the total risk. The following table may help to illustrate how.

Mathematical relationship between terms related to natural disasters
Sphere The hypothetical, p = irrelevant The possible, 0 < p < 1 The occurred, p = 1
Nature Threat (magnitude, process) Hazard [ = threat / T ]* Event (magnitude, process)
Society Vulnerability [ cost = f (threat) ] Risk [ = vulnerability / T ] Disaster [ cost = f (event) ]

*The return period (T) is the inverse of probability (p).

In other words, threat and event are functions of process and magnitude. Some examples: an inundation with a water depth of 2 m; a river discharge of 1000 m3·s-1; an earthquake with peak acceleration 5 m·s-2; a wind velocity of 48 m·s-1; or a mudflow with a discharge of 100 m3·s-1.

The key is to code that physically quantifiable parameter that directly causes the damage to society. For instance, one must quantify the wind velocity and the inundation, rather than the hurricane category. If the threat is that an earthquake-induced landslide buries buildings, one should quantify the landslide and not the earthquake (the earthquake hazard shall instead be used as input when estimating the landslide hazard).

Hazard would be translated as "danger" in many other languages. One can best understand the distinction between threat and hazard/danger by considering one person threatening another with an unloaded gun. The threatened person is never in danger from the gun since the probability is zero that it will go off. Thus, a threat with zero probability corresponds to zero hazard.

The definition of hazard is fuzzy in general speech, and not apt for use in a mathematical context. In this system it has now been formally described as analogous to risk, i.e., "Hazard = Threat / Return Period". This creates an attractive symmetry between vulnerability and disaster, the cost of each being a function of threat and event, respectively. It also makes hazard and risk direct parallels.

The risk is the key value that needs to be minimized, or be kept at an acceptably low level. Risk is vulnerability multiplied by a hazard's probability. To return to the example with the falling cliff, the vulnerability represents the value of the houses, and the number of people on the cliff. The probability refers to the likelihood that the cliff will fall.


To be able to prioritize where to spend the resources most effectively on mitigation, the following data are needed:
- Potential processes (threats)
- Probability as a function of magnitude for each process (hazards)
- Potential damage "cost" as a function of threat (vulnerabilities)
Given these values on can calculate the risk, which can be compared to the mitigation cost.


Mitigation refers to the long-term efforts to decrease the risk before something happens. For most natural hazards it is mainly the vulnerability we can affect through mitigation. It is much harder to stop the powerful natural processes causing these events. In the case of an eroding beach, stopping it from eroding would require damping the waves before they reach the beach. It may be technically possible, but to justify it one has to compare the cost with the cost of moving the infrastructure, and with the risk cost as defined above.

In many cases the vulnerability can be decreased by using some kind of early warning system. When it comes to coastal inlet sedimentation, siltation, scour, and beach erosion, a monitoring instrument like the SediMeter can be used to continuously supervise key points. It may also be used for gathering background data on the frequency-magnitude relationsship of the processes.

For the data to become of full benefit to society they can be incorporated in a decision support system, preferably a GIS based one, such as HazMit, a hazard mitigation system developed for the government of Nicaragua.

© Ulf Erlingsson, 2007 - 2010

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