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 apart from strong winds, high waves, and rain that brings inland inundations as well. 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, all coastal hazards are thus associated with waves.

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 a storm erodes a beach cliff so it falls into the water. However, if a number of houses with occupants fell with the cliff, it becomes a disaster. This makes it clear that there are two fundamentally different ways to avoid the disaster: To prevent that the cliff falls, and to prevent that there are houses and people on it when it falls.

The following analysis has been adopted with some minor changes from GIS for Natural Hazard Mitigation, from the conference GIS Planet 2005, Portugal.

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.

Conceptual 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)

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

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.

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 the falling cliff, stopping it from happening would probably require stopping the waves from reaching the foot of the cliff. Even if it would be technically possible with the Wave Engulfer™, it is hard to see how the costs could be justified since it is so much less expensive to move a few houses. Unless we have a good justification, we should probably let nature run its course.