Tidal Bores as AN Analog for Tsunamis?

A tidal bore is a phenomenon in which the leading edge of the incoming tide forms a wave (or waves) that travel up tidal channels or rivers against the direction of the current. Tidal bores occur in relatively few locations worldwide. Areas with a large tidal range where an incoming tide is funneled into a shallow, narrowing channel via a broad bay. The height of a tidal bore increases with the range of the tide and may vary in height from just a ripple to several meters. We studied the tidal bore at Turnagain Arm near Anchorage, Alaska during the first week of July, 2004 with full moon tides having a range of approximately 10 meters. Many tsunamis approach the coast as a bore or a breaking wave so tidal bores may provide a useful analog. Generally tidal bores propagate up a channel with the wave front running parallel to the shoreline. In certain cases, where the channel geometry refracts the wave out of the main channel path the bore will run directly up a bank or beach. A suitable site was selected based on earlier informal studies of the bore at Turnagain Arm. The site was located on the south side of Turnagain Arm near Hope, Alaska. The study site is known to refract the tidal bore, so that the approach is head-on to the shore. This characteristic yields the possibility of studying the bore during the runup phase, which likely mimics tsunami inundation closely. We observed complex runup patterns and shoreline interaction using photography and videography. We also studied sediment transport and deposition by the bore. This was attempted by introducing a pile of coarse grained sand on the muddy (silt and fine sand) tidal flat at low tide and anchoring sediment traps to the tidal flat inland from the pile of sand. We also carried larger particles as boulders (> 25 cm in diameter) and cobbles onto the tidal flat. The incoming waves varied from 0.3-1 m in height. None of the boulders or sediment traps were retrieved at the next low tide. They were probably eroded and moved by the bore and the incoming tide behind the bore. They could also have been transported by the currents during the next outgoing tide. The main difficulty of using a tidal bore for a tsunami analog in sediment experiments is that the incoming tide behind the bore might confound the experimental results, as it did in our case. The height of the bore is difficult to predict and the tidal cycle only provides limited time to set up experiments. However, we still believe that using a tidal bore as a tsunami analog can be very useful and provides horizontal scales that are impossible to obtain in a lab.