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Open-File Report O-20-08, Coos Bay tsunami modeling: Toward improved maritime planning response,
by Jonathan C. Allan, Joseph Zhang, Fletcher E. O’Brien, and Laura L. S. Gabel; 78 p. report.

This open-file report provides technical background for Maritime Tsunami Response Guidance (MTRG) publication MTRG-2020-OR-01 - Port of Coos Bay

WHAT'S IN THIS REPORT?

This study evaluates new tsunami modeling results completed for both distant and local tsunamis for the Coos estuary. The goal is to examine the interaction of tsunamis with fluctuating (dynamic) tides (as opposed to modeling using a fixed tidal elevation such as mean higher high water), different riverine flow regimes, and friction to provide an improved understanding of tsunami effects at Coos Bay. These data are then used to develop maritime tsunami guidance to assist ships, commercial and recreational vessels operating offshore the mouth of Coos Bay and within the estuary.

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EXECUTIVE SUMMARY
Recent tsunamis affecting the West Coast of the United States have resulted in significant damage to ports and harbors as well as to recreational and commercial vessels attempting to escape the tsunami. Although local tsunamis will strike the coast within minutes after the start of earthquake shaking, providing little response time to evacuate, distant tsunamis are expected to arrive some 4 to 12 hours after the event, providing more time to respond. This study evaluates new tsunami modeling results completed for both distant and local tsunamis for the Coos estuary. The goal is to examine the interaction of tsunamis with dynamic tides (as opposed to modeling using a fixed tidal elevation such as mean higher high water), different riverine flow regimes, and friction to provide an improved understanding of tsunami effects on maritime traffic operating offshore the mouth of Coos Bay (MCB) and within the estuary. This was accomplished by evaluating a suite of tsunami simulations (19 in total) for Coos Bay focused on two distant earthquake scenarios: the 1964 Anchorage, Alaska (AK64) earthquake and a maximum considered eastern Aleutian Island (AKMax) earthquake, and two local Cascadia subduction zone (CSZ) scenarios: Large1 (L1) and Extra-extra-large1 (XXL1).

Our modeling indicates that for a maximum considered eastern Aleutian Island (AKMax) earthquake, the tsunami would arrive at the MCB ~4 hours after the start of the earthquake. From the mouth of Coos Bay the tsunami takes an additional 13 minutes to reach its maxima inundating the community of Barview; 15 minutes to reach Empire; 27 minutes to reach Jordon Point; and ~41 minutes to reach the town of Coos Bay; total travel time to the town of Coos Bay is 4 hours 41 minutes. The largest tsunami waves are concentrated at the MCB, where the AKMax tsunami reaches ~5.6 m (19 ft) in height. Maximum water levels remain high for much of the channel before decreasing substantially upriver of Jordon Point, where the tsunami waves expand out into the broader upper Coos estuary, and where bathymetric shallowing effectively disburses much of the energy. Strongest currents are observed at the MCB, while large parts of the estuary would be affected by currents >2.0 m/s [>4 knots], which are capable of causing significant damage to facilities located adjacent to the ports and harbors as well as to any vessels that may be moored.

For a maximum considered distant tsunami, we recommend that vessels seaward of the MCB proceed to a staging area greater than 46 m (25 fathoms/150 ft) (located ~2.5 nautical miles northwest of the mouth of Coos Bay [2.4 nautical miles north of the Cape Arago lighthouse]). Dangerous currents > 2.6 m/s [5 knots] are expected to occur at depths shallower than 27 m (15 fathoms/90 ft). Offshore maritime evacuation may be feasible for some vessels operating out of Charleston harbor, or in the navigation channel downstream of Jordon Point. Seaward evacuation for vessels in the upper Coos estuary is not advised because those vessels might be transiting the mouth at the time when a tsunami arrives.

For a maximum considered locally generated CSZ tsunami, we find that the tsunami reaches the MCB in as little as 7 minutes and takes an additional 18 minutes to reach Jordon Point; the XXL1 local tsunami arrives at the town of Coos Bay ~39 minutes after the start of earthquake shaking. Maximum water levels exceeding 17 m (56 ft) will be observed at the MCB, decreasing to 10 to 13 m (~33 to 43 ft) in the navigation channel downriver of Jordon Point. Extreme currents exceeding 6 m/s [12 knots] will be observed across the entire estuary. Damage is expected to be devastating for ports and harbors in the lower estuary.

Due to the speed at which a CSZ tsunami reaches the MCB, there is insufficient time for mariners in ports to respond to this event other than to evacuate by foot to high ground. Vessels operating on the ocean west of the MCB should immediately evacuate toward deeper water. We recommend a Coos Bay maritime evacuation zone for a local tsunami hazard zone beginning at ~128 m depth (70 fathoms) and extending westward to depths > 274 m (150 fathoms). Mariners should prepare to remain offshore for potentially days as the MCB is unlikely to be navigable following a CSZ tsunami. As a result, plans to evacuate to potentially safe ports located south of Cape Mendocino on the California coast should be developed. For vessels in the Coos estuary, the only course of action is to head vessels toward the nearest point of high ground and evacuate uphill out of the tsunami inundation zone.