Tsunami dampening mechanisms Encouraged by the zeitgeist let me ask the following:
Is it feasible (now or in the future) to build systems a certain distance of a vulnerable coastline which can serve to dampen a tsunami before it reaches the coast itself?
The following picture comes to mind. We have some device which is small enough to be mounted on a buoy. An array of such buoys can be anchored some distance off the coastline, controlled by computers which are linked into the global tsunami warning network. This setup is quite feasible given what a cell phone can do today.
On the detection of an oceanic earthquake or other tsunami generating event (such as a piece of the Rock of Gibraltar falling into the sea) and given present environmental conditions one can calculate the expected path of the tsunami and its intensity. If a TDS (Tsunami Dampening System) of the type I describe above happens to be deployed in range of the tsunami, all relevant information is conveyed to the TDS controllers by the global tsunami warning system. All this happen in a matter of moments.
This leaves plenty of time (25min - 2hr) for the devices in the TDS array to be activated and to do their job. The question in this case would be - what form could such devices take? Perhaps mini-turbines which could dissipate the incoming energy into the ocean by first converting it to electrical energy.
Any such undertaking involves physical estimates for the size and shape of such an array, the precise capabilities of each device, etc. Many of these concerns involve simple physical questions.
Can you come up with the details of such a system or alternatively outline your own design? If you think that this idea would never work in the first place - for technological, physical or some other reasons - please explain what you think would be the prime obstacle and why?
 A: Active systems seem like a engineering nightmare to me: enormously expensive to build and maintain, they would be rarely used and need to be capable of handling huge power loads (or if you over-engineer some continuous use system like a wave power installation, the excess capacity would be rarely tested). 
A passive system is plausible.
A series of artificially shallow regions off-shore would dissipate some of the energy, by causing the wave to partially break several times on the way in.
Obviously this is a huge civil engineering project, and it may have unforeseen consequences such as


*

*Changing beach erosion patterns. Though "shallow" depends on the wavelength of the wave, and could be set deep enough to not effect day-to-day wave activity. 

*Effects (positive or negative) on the marine eco-system and local fisheries.

*Is "shallow" deep enough for shipping to pass? If not, how does the added shipping risk (ecological, human, and financial) compare to that posed by the big waves?

*...


Of course, what you get out of this isn't "no tsunami" it's "less tsunami", so you are still looking at building sea walls, paying more for insurance near the shore and running toward high ground if you see the ocean pull back.
A: The cheap (in more than one sense :)) and obvious answer is building restrictions on populated coastlines. Strategic seawalls would also help. These have both been mentioned, but I have framed this as an answer because other methods would be ineffective (predicting long term location of earthquakes is extremely imprecise) and prohibitively expensive. The problem with a tsunami is that until it hits shallow water, it is tiny and the speed is very fast. Our technology is no where near sophisticated enough to stop a tsunami given its spread size and energy.
A: Harbors have solved the problem of waves for centuries. It is walls. The same is true of the Netherlands who live below sea level. Solution is walls. A series of artificial islands/walls could be geo-engineered, at great cost, where the tsunami waves could break.
The permanent solution is to forbid any buildings below the height of the maximum recorded tsunamis in the region. Also costly. It is speculated that the minoan civilization disappeared from a tsunami from the explosion of the volcano in Thera/Santorini 3500 years ago. Long time constants. If you are in Greece do not miss a visit to Santorini. It is a breathtaking island with an active caldera still.
A: I doubt it makes economic sense. Because tsunamis extend to great depths surface structures to absorb the energy won't be very effective -except perhaps in shallow water. Also they are pretty rare ocurrances, maybe once per decade in the most favorable locations. Various sorts of breakwalls can be made, but like in the case of river levies, if you get an event larger than designed for, they can be overtopped, and because of the false sense of security this might make things worse.
As others have said. The key is not to build things in vulnerable areas. I would modify this. You need to take into account the risk. Some structures can be sacrificed to a once a century event, and warning systems can be used for timely evacuation. Also structures can be built to withstand a tsunami, or to minimize damage. The danger needs to be quantified, and the mitigative measures analyzed to determine the most sensible strategies.
A: I agree particularly with the response from anna v.
Just want to add a possible measure of last resort that can be tested to protect a specific location, for example, the entry of a river.
You can cause interference with the tsunami wave, picking so that destructive interference is aligned with the entrance of the River.
The effect could possibly be achieved using one or more explosions (nuclear?) underwater.
The accuracy of location and timing of the blasts would be key to success. Would have to be adequately studied the cost / benefit on the environment.
A: Most of the energy of a tsunami wave is gravitational energy. It is the rise in sea level, and the retreat of the surge, which is most devastating. Our cities are vulnerable because we build right to the edge of the sea. If we left a reasonable buffer of tidal flats, marshes, or sand berms between ourselves  and the ocean, the energy of tsunami or storm waves would dissipate itself traveling across the contours of these natural terrains. We would then also be able to enjoy the rich biological diversity of these environments.
There is evidence that mangroves and other natural barriers are critical components in the overall resilience of coastal areas to threats posed by tsunamis, hurricanes, and other natural disasters (Adger et al. 2005).
