An Analysis of Alaska’s Turnagain Arm Tidal Bore

Matthew Drews

Physical Oceanography

Fall, 2014

Analysis of The Turnagain Arm Bore

What is a tidal bore?

• A tidal bore is a pronounced wave of water that travels upstream from the mouth of a tidal river as the tide rises.

• Due to it requiring very specific conditions for wave propagation, tidal bores have only been observed in roughly 40-50 waterways throughout the entire world.

• While rare in number of qualifying waterways, they can be predicted rather easily given tidal and lunar conditions in the places that have them.

Analysis of The Turnagain Arm Bore

Characteristics

• Bores can range in height from just a few inches to over 30 feet high and travel up to 20 mph.

• Highly energetic, with high values of TKE, stress, and shear– Tke ~ 100 J/m3

• Occurs just after low tide as the water begins to return– Can be one large wave or several medium-sized waves

• The water continues to rise after the initial bore has passed through

• Very finicky topographically – dredging a river’s channel upstream has been found to reduce or even stop a river’s tidal bore

Analysis of The Turnagain Arm Bore

Tidal Bore Requirements:

• Located in an area with large tidal fluctuations, > 6 meters typically

• The river’s depth must decrease rapidly as it moves upstream from the mouth

• The river must also very quickly narrow beyond its mouth

Analysis of The Turnagain Arm Bore

Difficulty in Studying Tidal Bores

• While tidal bores have been observed for centuries, there is still a lot of unknowns in their physics

• This is primarily due to their rarity, but also their strength. Many field projects have been hampered by equipment malfunctions due to the strength of these bore waves.

• More recent attempts to measure their dynamics, TKE, and other parameters have involved computer modeling rather than field observations.

Analysis of The Turnagain Arm Bore

Analysis of The Turnagain Arm Bore

Analysis of The Turnagain Arm Bore

The Turnagain Arm Bore

• One of the more noteworthy bores in the United States

• Capable of being 6-10 ft tall and travel at over 20 mph

• It’s the only bore found in a highly mountainous region

Analysis of The Turnagain Arm Bore

For example…

Analysis of The Turnagain Arm Bore

Thanks for Listening!

Questions?

References

“Alaska Bore Tide”, Alaska.org, <http://www.alaska.org/advice/alaska-bore-tide>

Liu, Huaxing, et al. "SPH Modeling of Tidal Bore Scenarios." Journal of Natural Hazards (2014): 1-24. Print.

Simpson, J. H., N. R. Fisher, and P. Wiles. "Reynolds Stress and TKE Production in an Estuary with a Tidal Bore." Journal of Estuarine, Coastal, and Shelf Science 60.4 (2004): 619-27. Print.

“Paddle-boarding the Turnagain Arm Tidal Bore” Youtube user Harry Bland, url: <https://www.youtube.com/watch?v=WEvhmu2kx3A&src_vid=aU12PVQUdR0&feature=iv&annotation_id=annotation_938463647>

“Tidal Bore” – Wikipedia, <http://en.wikipedia.org/wiki/Tidal_bore>