Coastal Flooding: An Overview

In Washington, thousands of acres of coastal lands sit less than four feet above the average high tides. During extreme events when multiple factors combine, such as storm surge with wind-driven waves, these lands can be flooded with seawater. As sea levels continue to rise, these areas will be flooded more frequently.

If you live in a coastal community, coastal flooding may damage your house, vehicle, business or other private property. Even if your possessions are out of harm’s way, coastal flooding can damage infrastructure that you and many others rely on: wastewater treatment plants, stormwater outfalls, ferry terminals, coastal roads, rail transportation and more. Coastal flooding can also cause increased erosion, exacerbating the effects of flooding.

Coastal flooding is influenced by many factors. Some of these factors are more likely to occur seasonally, such as wintertime storm surge, king tides, and increased river flows. Others occur over a long period of time, such as sea level rise, or occur instantaneously regardless of seasonality or predictability, such as tsunamis.

Disregarding tsunamis, the likelihood of flooding in your area can be described by “recurrence intervals.” Expressed as percentages, recurrence intervals describe the likelihood that water levels will reach a specific magnitude during a specific time range. These percentages are sometimes referred to in terms of years, such as the term “100-year flood.” This term means that there is a 1-in-100 (1%) chance that an extreme flood of this magnitude will happen in any given year. A “20-year flood” means that there is a 1-in-20 (5%) chance, and a “5-year flood” means that there is a 1-in-5 (20%) chance. You can visualize these numbers on a map by using NOAA’s Sea Level Rise Viewer. While this app is meant for sea level rise, the slider can portray local increases in water level due to many factors.

As sea levels rise, extreme coastal water levels will occur more often. This means that a “100-year flood” may become a “20-year flood.” With sea level rise, today’s extreme flood event could become tomorrow’s high tide. Storms will combine with these frequent high tides, causing extreme high waters to occur more often. This will result in coastal flooding that lasts longer, has higher water levels, and has impacts further inland. For more information see our Sea Level Rise Introduction.

A tsunami will result in coastal flooding to a larger degree than discussed here. For more information on tsunamis, see our Tsunami Introduction.

 

Extreme Still Water Level Magnitudes (feet above MHHW, 2019)
recurrence interval	1-year	5-year	20-year	50-year	100-year
annual exceedance probability	100%	20%	5%	2%	1%
magnitude: Puget Sound	1.2	2.6	2.9	3.1	3.2
magnitude: Pacific coast	2	3.3	3.7	3.9	4

This chart describes recurrence intervals or annual exceedance probabilities, and the associated water levels for Puget Sound and Pacific coast shorelines. The water levels are expressed in terms of feet above mean higher high water (MHHW), or the average high tide. “Still water level” is the coastal water elevation due to average high tide plus storm surge, or everything except for waves and wave run-up, described in detail in Miller et al. (2019).

There are many strategies that can help keep your community safe from coastal flooding. Private landowners or neighborhood groups may be able to implement some strategies themselves. However, comprehensive community-wide strategies are often identified through local planning meetings, which are organized by local planning commissions or jurisdictions. Planning can take a long time, and large projects are expensive, so the time to act is now! With wise and proactive planning, communities can develop strategies to address specific local issues.

Wise planning can conserve and restore coastal ecosystems, ensuring that these protective buffers are not lost to flood-prone development. By considering which areas are susceptible to flooding, coastal communities can develop a diversified set of strategies to adapt to high waters such as: structural flood protection measures (e.g., dikes and levees), early warning systems, risk-informed land planning, nature-based solutions (e.g., natural floodplains, wetlands, and oyster reefs), and social protection and risk financing instruments (e.g., flood insurance). These strategies can make your community more resilient to flooding.

 

Educate yourself:

• Pay attention to high waters in your area to see what normal water levels may look like in the future. For example, you can attend a local King Tides Viewing Party, sponsored by Washington Sea Grant.
• Attend Shoreline and Coastal Planners Group meetings or a Coastal Training Program course.

Get involved in local coastal planning:

• Contact local planners, planning commissions or Marine Resource Committees to share experiences, voice concerns, recommend that public projects account for sea level rise or attend a public meeting.
• Engage with local Shoreline Master Program or Comprehensive Plan (Growth Management Act) updates to encourage wise planning for the future.
• Download the MyCoast app to document storm surge, king tides, changes to your local shoreline, and more. This information is used to better understand coastal hazards and their impacts, while raising awareness among decision-makers and stakeholders.

Adapt your project to coastal flooding:

• Consider sea level rise early in a project, when new permits are required, during renovations or during repairs.
• Visit the Washington Coastal Hazards Risk Reduction Project Mapper to see how other people in Washington have adapted to coastal hazards or connect with an expert via the Washington Coastal Hazards Resilience Network.

See our Coastal Flooding: Research and Tools page for real-time water level monitoring, interactive maps, historic shorelines, and more tools to assist with coastal flooding.

See our Coastal Hazards Risk Reduction Project Mapper for case studies of projects adapting to coastal flooding and other coastal hazards.

See our Planning Assistance page for resources related to jurisdictional planning for coastal flooding.

See Washington Sea Grant’s King Tide Program page for more informaiton and resources related to King Tides and the MyCoast App. 

Related hazards introductions:

• Riverine Flooding
• Sea Level Rise
• Tsunami
• Erosion and Deposition