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Open-File Report O-22-07, Umpqua River Tsunami Modeling: Toward Improved Maritime Planning Response 
by Jonathan C. Allan, Joseph Zhang, Fletcher E. O’Brien, and Laura L. S. Gabel; 76 p. report.

WHAT'S IN THIS REPORT?

This study evaluates new tsunami modeling results completed for both distant and local tsunamis for the Umpqua River 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), average riverine flow, and friction to provide an improved understanding of tsunami effects along the river and in the Ports of Winchester Bay and Reedsport. These data are then used to develop maritime tsunami guidance to assist all vessels operating offshore of the mouth of the Umpqua River and within the estuary.

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• Report (76 p., 11.4 MB PDF)
  
  

EXECUTIVE SUMMARY
Distant tsunamis affecting the West Coast of the United States over the past two decades 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 time to evacuate, distant tsunamis are expected to arrive some four to 12 hours after the event, providing time to respond. This study evaluates new tsunami modeling results completed for both distant and local tsunamis impacting the Umpqua Estuary, including the towns of Winchester Bay and Reedsport. Previous tsunami modeling used a fixed tide level — defined as mean higher high water (MHHW) — and no river flow or friction. The purpose of this study is to examine the interaction of local and distant tsunamis with dynamic tides (as opposed to a fixed tidal elevation such as MHHW), average river flow, and frictional effects to provide an improved understanding of tsunami effects on maritime operations offshore the Umpqua River and within the estuary. This was accomplished by evaluating a suite of tsunami simulations (15 in total) for the Umpqua Estuary 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 river mouth ~3 hours, 50 minutes after the start of the earthquake. The tsunami takes an additional 8 minutes to travel from the estuary mouth to Winchester Bay, and ~36 minutes to reach the town of Reedsport. Total travel time to Reedsport is 4 hours, 26 minutes. The largest tsunami waves are concentrated at the estuary mouth, where the AKMax tsunami reaches ~3.8 m (19 ft) in height. Water levels remain high between the mouth (river mile; RM-1) and Winchester Bay (RM1), before decreasing substantially upriver toward The Point (RM7). These changes are due to a combination of factors including bathymetric shallowing that effectively disperses much of the energy and morphological controls such as the shape and width of the estuary. The strongest currents are observed at the estuary mouth, whereas much of the lower estuary (RM-1 to RM4) would be affected by currents >2.0 m/s (>4 knots). These currents can cause damage to marina facilities such as those located in Winchester Bay, and vessels that may be moored in the marina. In contrast, our simulations indicate that a distant tsunami is unlikely to have an adverse effect on vessels and port facilities at Reedsport. This is because the distant tsunami loses significant energy by the time it reaches Reedsport.

For a distant tsunami, we recommend two maritime evacuation options:

1) Offshore: Seaward of the Umpqua River mouth, proceed to a staging area located ~1.8 km (1 nm) west of the mouth, where water depths are greater than 18 m (10 fathoms; 60 ft). Dangerous currents > 2.6 m/s (5 knots) are expected to occur at depths shallower than 18 m (10 fathoms; 60 ft). Offshore maritime evacuation may be feasible for some vessels operating out of Winchester Bay, or in the navigation channel downstream of The Point (RM7). Vessel operators need to assess if there is sufficient time to reach the staging area ahead of the tsunami.

For a maximum-considered locally generated CSZ tsunami, we find that the tsunami reaches the estuary mouth within minutes following the earthquake. The tsunami reaches Winchester Bay in ~21 minutes (peak wave at 25 minutes) and will reach Reedsport ~42 minutes after the start of earthquake shaking. Maximum water levels exceeding 14 m (46 ft) are observed at the estuary mouth, decreasing to ~10 to 12 m (~33 to 39 ft) in the navigation channel between RM2 and RM3. Extreme currents exceeding 6 m/s (12 knots) will be experienced in the lower estuary between RM-1 and RM4. Damage to Winchester Bay is expected to be devastating. Although modeling of the L1 Cascadia scenario indicates smaller tsunami waves throughout the Umpqua Estuary when compared to XXL1, the effects from an L1 event will remain damaging for infrastructure located in the tsunami inundation zone.

Due to the speed at which a CSZ tsunami reaches the Umpqua River, there is insufficient time for mariners in Winchester Bay to respond to this event other than to evacuate by foot to high ground. Vessels operating on the ocean west of the mouth should immediately evacuate toward deeper water. We recommend an Umpqua River maritime evacuation zone for a local tsunami hazard beginning at water depths of ~118 m (65 fathoms) and extending westward to depths >182 m (100 fathoms). Mariners should prepare to remain offshore for potentially days as the estuary 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 Umpqua 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. No time can be spared in parking the boat at a designated site; the priority must be reaching high ground on foot.