TCS – Cascadia Chapter Website
Coastal Erosion and Flooding
Hugh Shipman
Trends and Threats: Coastal Erosion and Flooding
(for Cascadia Chapter of the Coastal Society)
Trends
Erosion, landslides, and floods have occurred for millennia along the coast of Cascadia and
continue to shape the shoreline today. Although these geologic processes only affect a narrow
band of coastline, it is the same narrow band in which increasing numbers of people wish to
live, work, and play.
High cliffs and coastal bluffs, small estuary floodplains, and exposed beaches, when combined
with severe Pacific storms and periodic earthquakes, form a natural setting for geologic hazards
in the Northwest. Storms and waves erode beaches, redistribute sediment, and undermine
bluffs. Heavy rainstorms weaken slopes and send torrents down coastal watersheds.
Earthquakes trigger landslides and tsunamis and may lead to significant changes in coastal
elevations.
WA Dept of Ecology
Rapid population growth in the region has greatly increased development pressure in coastal
areas. Rising real estate values and greater affluence have led to increasing numbers of more
expensive homes along the region’s shorelines. Tourism brings more people and more demand
for high density development and for amenities, while at the same time placing a premium on
natural shorelines, beach access, and on recreational experiences. Increasing investment in
hazardous shoreline areas leads to greater losses when disaster strikes, more elaborate and
expensive efforts to reduce risks, and more pressure on politicians to respond to short-term
crises.
During the last decade, public awareness of the risks and consequences of development along
dynamic shorelines has increased. Not only is there more understanding of the natural hazards
inherent in coastal areas but there is also recognition that traditional engineering approaches to
erosion and flooding may have unanticipated consequences in the longer term, reducing
recreational access and harming biological resources.
Issues
Northern California. The region of Mendocino, Humboldt and Del Norte Counties contain some
of the most rugged and secluded portions of the California coast. The landscape strongly
influences the major industries, with timbering in the forest areas, dairy in the valleys, fisheries
in the streams, rivers and bays, and tourism throughout the region. Erosion in this region
varies depending on the coastal landscape. Issues include bluff retreat from rock falls,
1
TCS – Cascadia Chapter Website
Coastal Erosion and Flooding
Hugh Shipman
landslides and groundwater induced slope failures; severe erosion from a combination of wave
action and landslides; very high littoral transport rates; dune deflation due to vehicle and foot
traffic; and tsunami risk. Some areas, such as Big Lagoon, north of Trinidad, have experienced
20 or 30 feet of bluff retreat from a single storm. Despite erosion and flooding, there is high
development pressure.
Oregon Coast. The Oregon Coast is characterized by high rocky headlands, broad beaches, and
large river-mouth estuaries. Beach erosion, driven by severe storms and intensified during
periodic El Ninos, affects development on sand spits, on the marine bluffs, and adjacent to tidal
inlets. Landslides, ranging from small to very large, are common along the coast, affecting
residences, communities, and Highway 101.
OR DLCD
Southwest Washington. Prior to the 1990s, most communities along the broad sandy beaches
of Washington’s southwest coast were more concerned about rapid beach accretion than
erosion, but this situation has changed. Dams on the Columbia River and large jetties at both
the mouth of the Columbia and at Grays Harbor have altered patterns of sediment movement
and deposition along the coast and numerous erosion problems have emerged. Recent erosion
at Ocean Shores, Westport, Cape Shoalwater, and Fort Canby State Park has led to
controversial engineering fixes and vigorous public debate.
Puget Sound. Puget Sound is an inland sea with over 2100 miles of shoreline, much of it high
bluffs subject to periodic erosion and landsliding. Heavy rainfalls in 1996 and 1997 led to
widespread landslide damage and two federal disaster declarations. In addition, floods,
erosion, and storm damage frequently impact residential communities built on sand spits and
other low lying shorelines. Problems within the region are exacerbated by rapid population
growth in the Tacoma-Seattle-Everett urban corridor and by growing recognition of the need to
balance the needs to protect private property from erosion with a desire to maintain beaches
and a biologically rich nearshore environment.
2
TCS – Cascadia Chapter Website
Coastal Erosion and Flooding
Hugh Shipman
British Columbia. British Columbia has witnessed enormous population growth in the lower
Fraser valley and on southern Vancouver Island in recent years. Erosion and flooding are
significant issues along the shoreline of the Strait of Georgia, not unlike the geologically similar
Puget Sound region to the south. Erosion, flooding, and earthquake hazards are major
concerns on the Fraser River delta, where much of the Vancouver area’s industrial base is
located.
Solutions
Finding better ways to address erosion and flooding problems requires good science, informed
coastal management, and an educated public. Each of these components is reflected in efforts
currently underway in the Pacific Northwest.
Oregon. A decade of commitment by Oregon Sea Grant, the state’s Coastal Management
Program and Department of Geology and Mineral Industries has greatly improved
understanding of coastal processes and natural hazards. Local communities and the general
public have been actively engaged in the discussion and have begun to develop planning tools
for managing development in erosion and flood-prone coastal areas. Research at Oregon State
University and elsewhere has complemented these efforts with important studies of geologic
and oceanographic processes.
Southwest Washington. For over six years, the Department of Ecology and the USGS have
carried out detailed studies of the beaches of southwest Washington, with the goal of better
understanding the complex relationships between storms, Columbia River sediment supply,
dredging and jetty construction, and beach erosion and accretion. This work has begun to
inform discussions at the state and local levels about long term approaches to this dynamic
segment of coastline.
3
TCS – Cascadia Chapter Website
Coastal Erosion and Flooding
Hugh Shipman
Puget Sound. Under the state’s Shoreline Management and Growth Management Acts, local
governments are developing improved policies, based on good science, for guiding development
of hazardous coastal areas. The Washington Coastal Planner’s Group, jointly led by Washington
Sea Grant and the Department of Ecology, provides a popular forum for state and local
shoreline officials to discuss a wide variety of issues, including erosion and flood hazards. The
U.S. Geological Survey and the University of Washington are beginning to critical shoreline data
to support scientific understanding of coastal geological processes.
Links
Oregon Coastal Management Program
http://www.lcd.state.or.us/coast/hazards.html
Oregon Department of Geology and Mineral Industries
http://www.oregongeology.com/earthquakes/Coastal/CoastalHazardsMain.htm
United States Geological Survey – El Nino
http://coastal.er.usgs.gov/response/
Washington Coast
http://www.ecy.wa.gov/programs/sea/coast/index.html
Puget Sound Landslides
http://www.ecy.wa.gov/programs/sea/landslides/index.html
British Columbia
http://sts.gsc.nrcan.gc.ca/geoscape/vancouver/home.asp
Washington Division of Geology and Earth Resources
http://www.wa.gov/dnr/htdocs/ger/landslid.htm
4