Surfing the tidal bore in Turnagin Arm Alaska. Photo courtesy of Scott Dickerson.
I’ve been paying so little attention to the news since getting getting back from my latest conference that I sort of missed all the hype about the supermoon this weekend. But then, I realized the supermoon meant one really exciting thing: SUPER TIDES!!!! (insert witchlike cackle here)
The supermoon occurs because the moon is the closest to the earth it will be all year long. Because of the moon’s proximity, its gravitational pull on the water in the oceans is also stronger. This causes really low low-tides and really high high-tides. The former is great if you are into exploring sea beasties that are uncovered below the normal range of the intertidal zone. The later is only a problem if you left your beach chair and belongings at the normal high tide line on the beach (it is so sad when a cooler full of beer and snacks is washed out to sea). This also means that tidally-generated internal waves (waves inside the ocean) are also bigger than normal, which in my research world is really cool.
But there is another phenomena which supertides can cause, tidal bores. Tidal bores are solitary waves formed as the tide rises in specific regions, such as river inlets and long narrow bays. Unlike Tsunamis, these are a true ‘tidal wave.’ These waves can be over 30 feet tall, travel for miles and can even be surfed! There only about 100 locations worldwide where bores occur, some famous examples include the Trent River in England, Turnagin Arm in Alaska, “pororoca” on the Amazon River, and the world’s largest on the Qiantang River in China. It is almost 30 feet tall and travels up to 40 mph! Tidal bores can also be dangerous, sinking unwitting ships and drowning foolhardy swimmers.
But tidal bores don’t happen all the time. They need a combination of a really really tide followed by a really really high tide. The difference in height, and therefore volume, between the really low and really high tide means a lot of water flows into the bay really fast in the 6 hours between the two extremes. As all this water is filling the bay, water near the bottom slowing down due to friction with the seafloor. But the water near the surface is still traveling at the same speed, now outrunning the water near the bottom. The surface water piles up, forming the crest of the wave. Eventually the face of the wave gets so steep, it topples over, breaking like a wave at the beach. The resulting turbulent whitewater is now a classic tidal bore.
Tidal bores can also be important for marine ecosystems. They mix up sediment and nutrients: uncovering creatures that were previously buried and attracting all sorts of opportunistic feeders, including piranhas in the Amazon. Belugas have played in them. Moose have unsuccessfully tried to outrun them.
There are a few proposals to harness the power of tidal bores to generate electricity. One small power plant is in operation in the Bay of Fundy, but has had some problems such as trapping whales.
This Piranha only wants you to surf the Pororoca in Brazil if you are tasty.
For my PhD, I studied deep tidally-generated bores that occur in the ocean near the base of the continental slope, over 1 mile beneath the surface. These babies can be huge, over 600 ft tall! They dwarf the largest surface tidal bores observed in China, but are much slower, traveling at less than 1 mph. While surface tidal bores may be rare, there is evidence that these bores are much more widespread occurring all over abrupt bathymetric features like continental slopes, seamounts, mid-ocean ridges, etc. They are also turbulent like their surface counterparts. Therefore they may be an important mechanism that exchanges water and nutrients between the deep and shallow oceans.
So the supermoon is not only a really awesome astronomical spectacle, it has consequences here on earth too. And supermoon will cause insanity and hysteria, but only for the surfers that will be jumping into freezing or piranha-infested waters to surf the tidal bore this weekend!!