As discussed elsewhere, tsunamis cause losses by a wide variety of direct and indirect processes, some of which produce losses well outside the inundation zone. This section considers the physical forces exerted by tsunamis as they extend over the inundation area and then retreat, and also in the near shore area where the water retreats as the wave troughs arrive at the coast.

		A surge wave drove a plank into a truck tyre at Whittier, Alaska in 1964. Whittier incurred $10 million in property damage (1964 dollars)

The nature of the forces exerted, particularly in the onshore inundation area, depends strongly on the character of the tsunami waves as they impact the coast. As noted above, a number of types of wave behaviour have been observed, often within the same tsunami event as successive waves may differ markedly from one another. The characteristics of the main types of wave can be summarized as:

Non-breaking waves (tide-like)

• rare (Alaska 1964, Nicaragua 1992)
  
• usually first wave in wave train, without precursory withdrawal
  
• rise and fall over periods up to tens of minutes
  
• runup equal to height of wave
  
• largest inundation distances (less drag)
  
• turbulent scouring and damage by drag forces limited (often only in last phases of withdrawal of wave)

• all shorter-period waves, and those with precursory withdrawals (propagate over backwash)
  
• break close to shore when particle velocity within wave exceeds wave velocity
  
• develop into fast-moving, turbulent surges
  
• surge momentum adds to runup height (2-5 times wave height at shoreline)
  
• internal turbulence and high velocity limit inundation distance

• long-period waves which develop steep, breaking fronts ahead of fast-moving bodies
  
• most destructive and energetic relative to height
  
• commonly develop in constricted channels (river mouths and beds, harbours, fjords and inlets)

Forces exerted by all tsunamis (including tide-like tsunamis):

Floating of buoyant objects

Many objects encountered by tsunami waves, even ordinary wooden houses, will retain a high degree of watertightness for short periods. These objects, which also include such things as gas and oil storage tanks (especially when empty) as well as boats and ships, will therefore float. Objects that are free to move are liable to be shifted and deposited in inconvenient locations: for example, the 1868 Chile and 1883 Krakatau tsunamis both transported small ships inland where, although largely intact, they were hopelessly stranded. Smaller vessels such as fishing boats and harbour craft have suffered similar fates in more recent tsunamis.

Objects that are fixed, such as fuel storage tanks, are more severely damaged as they are ripped out of their foundations by buoyancy forces: fires from ruptured fuel tanks have been a major cause of secondary damage in a number of tsunamis.

A number of post-disaster reports (for example, USGS Professional Paper 542, dealing with the 1964 Alaska Tsunami) have examined the depth of inundation at which buoyancy forces become a significant cause of damage, and have found that relatively weak structures such as wooden houses are displaced and damaged when inundation depths exceed only 1 metre.

Hydrostatic forces

Conversely, an object or building that is fixed in place and surrounded by significant depths of water as a tsunami wave comes onshore will be subject to hydrostatic forces exerted by the weight of water on the outside of the house or other building. These may cause it to collapse in on itself. The depth of water necessary will be very dependent upon details of construction and no good data exists for these effects.

Anomalous currents

Even the most tranquil, tide-like tsunamis involve large movements of water in the nearshore zone and in semi-enclosed bodies of water such as inlets and harbours. These give rise to anomalous currents that may persist for hours or even days after the main tsunami waves have dispersed. Without careful navigation, the result may be stranding and wrecking of ships under way. The most recent example is that of the Chilean ship Santiago, which survived the 22 May 1960 Chile tsunami only to be dragged off course and wrecked a day later. A less certain example involves a flotilla of American destroyers that ran aground on the coast of California in thick fog in early September 1923: it was argued at the subsequent court - martial that the ships might have been deflected off course by currents associated with the Tokyo earthquake a week earlier. Whilst modern navigation methods make a repetition of such an event less likely (although the Santiago was physically dragged onto rocks in good weather) these events may have implications for application of proximate cause, liability and hours clauses, especially in the hypothetical case of explosion or pollution damage arising from a ship dragged off course and wrecked in the aftermath of a tsunami.

Fast-moving tsunamis (breaking waves, surges and bores):

The effects of the fast-moving, often turbulent masses of water in these types of tsunami waves upon engineered structures are discussed in detail by Camfield (1980). In addition to these effects, tsunami waves may erode soil, loose sediment and even rock surfaces with economically damaging consequences, for example in loss of farmland, blocking of roads with debris, and undermining of buildings and coastal defences.

Surge forces

These are associated with rapid rise/fall of water on either side of obstacles, even where gaps are present through which water can pass. The imbalance of forces between one side of a raised embankment or wall and the other can physically push it over or displace it off its foundations.

Hydrodynamic forces (turbulence)

The rapid accelerations and decelerations associated with turbulent flow in fast moving tsunami bores and surges are amongst the most destructive natural forces owing to the high density of the flowing material and the high velocities involved. Observed flow velocities in historical tsunamis have been inferred to be of the order 10 m/s to 30 m/s, consistent with the approximate relationship:

(Keuleugan, 1950)

where d is the depth of water in metres and g is the acceleration due to gravity (10 ms-2)

The dependence of flow velocity in this equation upon depth implies that even higher velocities can be expected in the bores or surges developed in giant tsunamis generated by oceanic island collapses or impacts, since these are likely to be tens or even several tens of metres deep. Flow velocities close to one hundred metres per second (over two hundred miles per hour) can be expected in such events.

The destructive power of a tsunami surge or bore is very much greater than that of a breaking storm wave of comparable size; furthermore, the larger volume of the wave means that it will penetrate further inland. Nott (1997) shows that boulders with masses of around 200 tons (or about 5 metres across) can be displaced by tsunami surges only 10 metres deep, whereas short period storm waves with heights of 100 to 150 metres (beyond the bounds of credible wave heights by a factor of 2 or 3) are required to produce the same movements. The very largest wave - displaced boulders recorded are in a suspected tsunami deposit in the Bahamas, some 123 000 years old. They have masses of about 2000 tons (Hearty 1997), implying tsunami surges 30 to 40 metres deep: they are found on ridges up to 40 metres above contemporary sea level in the Bahamas, consistent with this depth. The forces involved in such events are such that all but the most massive structures (on the scale of major dams or barrages, for example) are unlikely to survive.

An important scientific consequence of these very high bore and surge flow velocities is that over most of the area of inundation the larger tsunamis are likely to be erosional rather than depositional events. Identification of their occurrence in the geological record is therefore likely to be more difficult than the identification of smaller tsunamis, which produce highly characteristic sheet sand deposits (see geological records of tsunami events). From the practical point of view of expected damage, it also means that the zone of destruction in these larger fast - moving tsunami surge or bore events is likely to be more or less coincident with the zone of inundation, with only structures at the periphery surviving. For the purposes of constructing probable maximum loss scenarios in particular, it can reasonably assumed that the percentage loss in the zone of inundation, once this is defined, is close to 100%.

The depths of erosion of the substrate beneath a tsunami flow have not proved amenable to precise analysis, because of the complexities of the materials involved, and the importance of the duration as well as the velocities of the flows. Even quite moderate tsunamis such as the 1996 Java tsunami have been found to produce up to 2 metres of erosion of beaches and soils along large stretches of coastline. Boulders and blocks removed in this way, together with fragments of trees, buildings, boats and other objects, contribute substantially to impact damage associated with tsunamis.

Impacts between objects carried in wave, and with fixed objects

The turbulent nature of tsunami bores and surges means that objects within them often collide violently, as well as with projections sticking up from the surfaces over which the tsunamis are flowing. Impacts commonly produce the most spectacular pictures of tsunami damage (see, for example, images at http://www.tsunami.org/archivespics.htm and http://www.pmel.noaa.gov/~ann/tsunami_damage_slide_sets.htm).

Impact damage is most widespread and severe in ports and built-up areas where there are both concentrations of values at risk and of moveable objects. Small boats in particular are a major potential hazard to waterfront areas and it is primarily for this reason that the evacuation orders issued in Pacific Ocean ports of the USA (on receipt of official tsunami warnings from the Pacific Tsunami Warning Centre - see Tsunami Warning) are legally enforced by the US Coast Guard.

Owing to the widely varying nature and abundance of potential debris it is not possible to make generalizations about controls on the intensity of the impact hazard, except that it increases with the size of the tsunami (hence the inundation area and the flow velocities) and with the amount of potential debris available.