Tag Archives: predictability

Physics and Engineering of Breaking Dams, Glaciers, and Tides

The title “Bombing Hitler’s Dams” fascinated me when I saw it featured on my TiVo last night. The story told was that of a group led by a smart, creative, and somewhat crazy Cambridge University engineer Dr. Hugh Hunt trying to recreate all the engineering elements needed to blow up a dam with a bomb that skips along the surface of the water the same way that a flat stone skips over a calm pond. While we all know intuitively how to throw a flat stone at the right speed at the correct angle, imagine to do it from an aircraft dropping a bomb to skip a few hundred yards over the surface of a reservoir, kiss the dam, sink, then blow it up via a depth charge. After many elaborate tests starting with 3 foot baby’s pool 5 inches deep, the show concluded with the real blowing up of real dam in northern Canada by a much enlarged group of engineers, construction workers, students, pilots, contractors, etc.

Photograph of the breached Möhne Dam taken by Flying Officer Jerry Fray of No. 542 Squadron from his Spitfire PR IX, six Barrage balloons are above the dam

If you think this is hard to do, it is. On May 16/17, 1943 a British bombing raid called “Operation Chastise” deployed cylindrical drums filled with explosives like skipping stones to hop over torpedo nets meant to protect the dams from subsurface mines. Two of three targeted dams were blown up, 53 airmen perished, German heavy industry in the Rhine-Ruhr valley was disrupted for 4 months, and over 1500 civilians drowned in the flood wave created by the breaking dams:

The story reminds my of my adviser, Dr. Richard Garvine (an aeronautical engineer by training) whom I met in 1986 while a physics student from Germany studying physical oceanography for a year in Bangor, Wales. Rather than returning to Germany, I went to the United States for graduate school where I met my sweetheart. One of my first graduate assignment was to study the “breaking dam” problem (Stoker, 1948: “The Formation of Breakers and Bores”, Communications of Pure and Applied Mathematics, 1, 1-87). The “breaking dam” problem is now a classical problem in fluid dynamics that relates to breaking waves, tsunamis, tides, as well as the discharge of fresh water from cooling plants, estuaries, and glaciers. I applied it to tides in the Conway Estuary in North-Wales for my MS thesis that the Swedish Royal Society saw fit to publish as my first contribution to science.

Conwy Estuary at its mouth near high tide.

I stole the above photo of the Conway Estuary, North Wales from a set of beautiful travelogues of an area where I camped for 6 weeks to guard instruments that measured currents along the 50 km tidal reach of this beautiful estuary. The tides rush in like the waves of a breaking dam, yet, they do not break (no bore forms, why?). To model the physics, I needed to study the work done by American, British, and French engineers who labored hard to defeat Nazi Germany by blowing up actual dams developing new and applying old ideas in physics and engineering along the way. Studying their work, I got my answer, too.

Addendum: A review of the aftermath and devastation of the 1943 flood wave from a German perspective is posted here with original photos from both British and German sources.

Uncertainty in the Physics and Philosophy of Climate Change

I wrote this post last year for the National Journal, but it also relates to the way I think about Petermann Glacier’s ice islands. There are now at least 4 larger ice islands that formed from last year’s single calving: one is the tourist attraction off Labrador and Newfoundland, a second has left Petermann Fjord last week, a third was grounded off Ellesmere Island for much of the year and is now where #1 was Nov.-2010, while the fourth … I do not know. Last I heart, it was grounded off central Baffin Island. With this much variation of where pieces of the ice island went, how can we possibly claim any skill in predicting anything?

Neither climate nor weather is linear, but this neither makes them unpredictable nor chaotic. The simple harmonic pendulum is the essence of a linear system with clear cause and effect relations. Oscillations are predictable as long as the initial forcing is small. Furthermore, a linear trend will show the pendulum to slow down due to friction. Corrections are straightforward.

Unfortunately, climate is not a simple, harmonic, or linear system. While this does not make it unpredictable or chaotic, it means that our “common sense” and loose talk of “totality of events” can easily fool us. We know that CO2 emissions for the last 150 years changed global temperatures. We also know that our current climate system has been very stable over the last 10,000 years. What we do not yet know is how small or how large a perturbations the last 150 years have been. If the pendulum is forced too much, if the spring is stretched too far, the system will find another stable state by breaking. Climate dynamics can find an adjustment less tuned to the areas where people presently live. This is what “tipping points” are about. Only numerical experimentation with the best physics and models will suggest how close to a different stable climate state we are. The IPCC process is one way to do so.

Ice cores from Greenland contain air bubbles 100,000 years old, which clearly demonstrate that our present climate state is the “anomaly of quiet” in terms of temperature fluctuations. The absence of large fluctuations for about 10,000 years made agriculture and advanced civilizations possible. The ice cores show that abrupt climate change has happened and may happen again, not this election cycle, but it is one possibility perhaps as likely as the possibility that climate change is mundane, linear, and follows trends that we can easily correct or mitigate later. Both are excellent hypotheses.

For scientists, these are exciting times as we conduct a massive, global experiment to see how much CO2 we can add to the atmosphere to perhaps find a different climate state. Dr. Terry Joyce, Senior Scientist at Woods Hole Oceanographic Institution once said: “I’m in the dark as to how close to an edge or transition to a new ocean and climate regime we might be. But I know which way we are walking. We are walking toward the cliff.” I agree with this sentiment, but add that we do not know if this cliff is a 1000 feet fall or a 2 feet step. Can we affort to wait until we know for sure? As a scientist I do not care. As a citizen, however, I think the time to act responsibly is now.