Open-File Report O-24-03, Brookings Tsunami Modeling: Toward Improved Maritime Planning Response

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 tsunamis. 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 time to respond. This study evaluates new tsunami modeling results completed for both distant and local tsunamis for the Brookings-Harbor area, including the Chetco River and marina. Previous tsunami modeling used a fixed tide level defined as mean higher high water (MHHW), with no friction included. The purpose of this study is to examine the interaction of local and distant tsunamis with dynamic tides (as opposed to modeling using a fixed tidal elevation such as MHHW) and frictional effects to provide an improved understanding of tsunami effects on maritime operations offshore the Chetco River and within the estuary and marina. Due to the generally low river discharge experienced on the Chetco River, we chose not to simulate a tsunami interacting with river flow. Modeling was accomplished by evaluating a suite of tsunami simulations (17 in total) for the region focused on two distant earthquake scenarios: the 1964 Anchorage, Alaska (AK64) earthquake and a maximum-considered eastern Aleutian Island (AKmax) earthquake, and three local Cascadia Subduction Zone (CSZ) scenarios: Medium1 (M1), Large1 (L1) and Extra-extra-large1 (XXL1). 

Our modeling indicates that for an AKmax earthquake, the tsunami would arrive at the river mouth ~4 hours after the start of the earthquake. From the river mouth to the Port of Brookings Harbor marina the tsunami takes an additional 2 minutes; inundation of the marina occurs ~12 minutes after wave arrival at the mouth. The largest tsunami waves are concentrated at the estuary mouth, where the AKmax tsunami reaches ~2.9–5.3 m (~9.5–17.4 ft) in height. Water levels remain high between the mouth and the Chetco River Bridge, before decreasing substantially upriver. 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 river channel. Strong currents that range from 3 to 4.5 m/s (~6-9 knots) are prevalent at the Chetco River mouth, navigation channel and within the entrances to both boat basins. These currents are capable of causing significant damage to marina facilities as well as to any vessels that may be moored in the boat basins.

For a maximum-considered distant tsunami, we recommend that vessels located seaward of the Chetco River mouth, proceed to a staging area greater than 55 m deep (30 fathoms/180 ft) located ~8–10 km (4–5 nautical miles) west of the mouth. 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 the Port of Brookings Harbor marina, or in the navigation channel. However, vessel operators need to assess if there is sufficient time to reach the staging area ahead of the tsunami, and if the marine conditions would allow for safe passage to the staging area.

For a locally generated CSZ tsunami, we find that the tsunami reaches the estuary mouth within minutes following the earthquake. The tsunami reaches the river mouth in ~15 minutes, while the peak wave occurs several minutes later; total time is ~20 minutes for the peak wave to inundate the marina. Maximum water levels for an XXL1 tsunami were found to reach ~18 m (59 ft) at the river mouth, decreasing to ~10 m (~33 ft) at the Chetco River Bridge; an L1 event produces tsunami waves ~10.8 m (~35 ft) at the river mouth, while a smaller M1 event results in an 8.2 m (26.9 ft) tsunami. Extreme currents exceeding 6 m/s (12 knots) will be experienced throughout the entire area. Damage to the marina and all facilities within the inundation zone is expected to be catastrophic. Although modeling of the L1/M1 Cascadia scenario indicates smaller tsunami waves throughout the estuary when compared to XXL1, the effects from an L1/M1 event will remain damaging for infrastructure located in the tsunami inundation zone.

Due to the speed at which a CSZ tsunami reaches the Chetco River, there is insufficient time for mariners in the Port of Brookings Harbor marina to respond to CSZ event other than to evacuate by foot to high ground. Vessels operating on the ocean west of the river mouth should immediately evacuate toward deeper water. We recommend a Chetco River maritime evacuation zone for a local tsunami hazard beginning at 91 m depth (50 fathoms/300 ft) and extending westward to depths > 128 m (70 fathoms/420 ft); increased safety will be achieved at even greater depths ~182 m (100 fathoms/60 ft). Mariners should prepare to remain offshore for potentially multiple 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 Chetco River navigation channel, 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.