Thursday, August 23, 2007

Storm Tide Redux

It is amazing that for all the science we have today, we still can’t figure out how ocean waves work. I will spare the reader a lot of technical voodoo, but ocean waves are themselves chaotic events, and while we can record them we still do not understand them. Here is my feeble attempt.

At first I was interested in storm surge, which usually comes along with a hurricane. Previous media releases said Hurricane Dean could produce an 18-foot storm surge with 12-foot waves on top of that. That would add up to a massive 30-foot tall column of water. Indeed, some of the largest storm tides are about 30 feet high, so this was not unrealistic. However, it turns out that what we call “waves” and “surges” are incredibly complex. Consider the following graphic:

What this says is that wave height includes the trough, which can be roughly half the size of a standing ocean wave. Indeed some of the largest waves ever recorded, possibly 100 feet tall, have some of the deepest troughs. To avoid much of the problem with figuring out storm surge versus wave size, most authorities now simply record “storm tide,” which takes into account many things at once including the tide, rainfall, barometric pressure, ocean waves, and the large blob of water being pushed by a hurricane. Conceptually it might be presented as in the following diagram, but is expressed as a single number in terms of feet or meters above mean average tide datum.

This is not the greatest depiction because the storm tide should include some wave action, maybe half, and there is a great deal of evidence that waves actually get smaller when up on the beach because rotational energy has been spent. More about wave set-up and swash run-up later, as this stuff is only beginning to be understood and modeled correctly – or shall we say more elegantly.

From a conceptual view, not all storm tides are created equal, since a hurricane spins counter-clockwise. This means that most of the hurricane surge will be on the right-hand side of the cyclone. This is not to say that there is no hurricane surge surrounding and following the eye on both sides, but that maximums will be located as indicated below, on the front right quadrant.

Depending on storm direction, angle of attack, severity, and fetch (duration over water) the left or weak “subsidence” side of the storm may have a large storm tide, such as we saw when Katrina hit New Orleans, or have the unusual effect of sucking all the water out of the bays until they are nearly dry. Implications for South Padre Island could be that under certain conditions, a glancing blow offshore could cause Laguna Madre to create a storm tide that could flood the island from the bayside.

Most conventional models used by emergency planners these days only estimate storm surge and are notoriously very crude. They do not take into consideration the effects of diminishing wave heights near-shore, or the effects of wave set-up and swash run-up. Most wave heights are recorded offshore where they are at their maximum; as they run into the five or six sandbars that parallel our coast, they tend to break, thus releasing their energy. That is why 15-foot waves offshore might only be 5 feet tall when in the beach zone. This effect is called “wave set-up” and depending on the area can cause diminished waves onshore or in other places generate huge monsters (e.g., waves off Australia). Once the ocean wave finally breaks for the last time and travels up the beach that is called “swash run-up.” The swash is what causes most of the erosion, damage, and flooding here on South Padre Island.

The picture shows wave swash on a fairly steep beach, and not only increases storm tide height but also can have a horizontal distribution inland as well. After the swash expends its force, water rushes backwards to the ocean like a giant vacuum cleaner, called swash return. This is the main reason why we lose sand off our beaches during major cyclones.

In conclusion, I do not claim to be on the forefront of science, as all I wanted to do was to introduce some new concepts and go onto say that since Katrina, scientists have renewed their efforts to understand what we thought were very simple things. I found several hundred academic articles written between late 2005 and the present. Scientists are still baffled by it all, such as how the storm could blow giant tumble-weed balls of trash 50 miles inland, and why we would find debris from 50 miles inland way out in the ocean, seemingly all at once. The fact is, we simply don’t understand waves.

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