Local Hazards

Tsunami

A tsunami is a strong, fast-moving wave or series of waves. They are generated when geologic events, such as earthquakes, landslides, submarine landslides, or volcanic activity, cause large, rapid movements in the sea floor that displace the water column above. In Puget Sound, tsunamis can be caused by local landslides from surrounding bluffs. The displacement of water creates a series of high-energy waves that radiate outward like pond ripples. Offshore tsunamis can strike adjacent shorelines in minutes and cross the ocean at speeds as great as 600 miles per hour to strike distant shores. Kitsap County is at risk from tsunamis of both local and distant origin. These destructive waves are most commonly caused by submarine earthquakes. Our current technology gives us adequate warning for tsunamis produced by distant quakes; however, an earthquake on the Seattle fault would have near immediate impacts.

Bainbridge Island is located in the tsunami inundation zone and current modeling indicates a tsunami resulting from a Seattle Fault and/or Cascadia Fault earthquake could produce a 1-2 meter tsunami (relative to land elevation) in certain low-lying waterfront areas. We are also at risk for a landslide-induced tsunami. This type of tsunami has the potential to be slightly larger, but more localized.

The Department of Natural Resources (DNR) has produced models indicating the tsunami inundation resulting from a 9.0 Cascadia Fault rupture and a 7.6 Seattle Fault rupture. The models can be viewed by clicking the links below. 

Tsunami Modeling: 

DNR has recently published two sets of tsunami data; one based on a 9.0 Cascadia subduction zone earthquake and one based on a 7.5 Seattle fault earthquake.

Cascadia Model

• Tsunami Bainbridge Map Cascadia SZ

Seattle Fault Model

• Tsunami Bainbridge Submersion Map Seattle Fault
• Tsunami Bainbridge Currents Map Seattle Fault

Seiche

Seiche is type of vertical wave that occurs in a confined body of water. It is similar to water sloshing around in a bathtub. Seiche is normally experienced long distances from a seismic event. Puget Sound is vulnerable to seiches. 

_Sources: 

_{Kitsap County Risk Report, Kitsap County Multi-Hazard Mitigation Plan, City of Seattle Office of Emergency Management}

Bainbridge Island is subject to landslides and soil erosion due to wind, water, and flooding anytime throughout the year. Landslides can cause deaths, significant damage to properties and infrastructure, and in some cases, losses of the use of land for many years due to the extensive cost of restoration. Earthquakes also have the potential to trigger landslides.

Landslide occurrences on the island have been concentrated along its coastal bluffs. Several landslides were triggered by storms throughout the winter of 1996-1997. One of these storm events caused a landslide in the Rolling Bay area, which forced a house off its foundation and down a hill into Puget Sound. Several other landslides severely damage homes, roads, and utilities along the bluffs of Bainbridge Island. One of these events resulted in the death of a family of four and destroyed millions of dollars of both public and private property. This deadly landslide demonstrates the unpredictability and destructiveness of landslides on Bainbridge Island. The scar of the slide was nearly 15 meters wide and 15 to 20 meters high. While assessing the damage from this incident, the USGS also observed numerous other scars from many previous landslides that occurred on the steep bluffs of Bainbridge Island.

Bainbridge Island contains a significant number of buildings that are exposed to the effects of landslides.

The most current landslide risk data for Bainbridge Island can be downloaded here. 

Image depicting 1997 Rolling Bay area landslide. 

_Sources:

_{Kitsap County Risk Report, Kitsap County Multi-Hazard Mitigation Plan, City of Seattle Office of Emergency Management}

Flooding is the most common hazard occurring in Kitsap County. Heavy, prolonged rain in the fall, winter or spring months often result in saturated ground and high stream flows. Due to ground saturation, structures on Bainbridge Island located in low-lying areas tend to flood during prolonged periods of rain. Wind-driven tidal flooding is also possible along the inland waters. Flooding is due to runoff, ground saturation, or tidal flooding. Floods may result in loss of life as well as damage to residences, business establishments, public buildings, roads and bridges, utilities, agricultural land, fish and shellfish habitats, stream banks and flood control structures.

King Tides may also cause flooding on Bainbridge Island. King tides bring unusually high-water levels, and they can cause local tidal flooding. Over time, sea level rise is raising the height of tidal systems. Average daily water levels are rising along with the oceans. As a result, high tides are reaching higher and extending further inland than in the past. King tides preview how sea level rise will affect coastal places. As time goes by, the water level reached now during a king tide will be the water level reached at high tide on an average day.

Flooding can obstruct access to roadways. If possible, move vehicles to higher ground before the King Tides begin and wash your vehicle down with fresh water after driving through salt water. Elevate anything at home, such as paint cans, automotive supplies, lawn chemicals, etc. that could float away in the water and cause water quality degradation.

Additionally, follow these safety precautions:

• Do not walk through flood water if possible. It is a health hazard.
• If you do need to walk through flood waters follow good hygiene practices and wash your hands, clothes, and pets after.
• Do not drive through flooded areas - it is dangerous and can damage your vehicle. Find an alternative route. Remember: Turn Around, Don’t Drown.
• Boaters should be aware that high tides cause lower clearance under bridges - check the tide before leaving the dock.

Preparedness Resources

• https://www.floodsmart.gov/first-prepare-flooding
• https://www.redcross.org/get-help/how-to-prepare-for-emergencies/types-of-emergencies/flood.html
• https://www.fema.gov/sites/default/files/2020-07/how2009_anchor_tanks_cylinders.pdf
• https://www.kitsapdem.com/kc-coastal-flooding/
• https://kitsappublichealth.org/information/files/kphdflooding_storms_factsheet.pdf

Bainbridge Island has the largest number of buildings in the SFHA (1-percent-annual-chance flood zone) and has the highest loss ratio which compares the losses due to flooding to the overall building value within Kitsap County. Not all of the buildings within the floodplain experience damage due to level of flooding as well as current floodplain regulations. 

The most current 1 percent (formerly called 100 year) flood data is available here.