Beach Morphology Monitoring Program
- The seasonal exchange of sediment between onshore and offshore is large within the Columbia River littoral cell. The CRLC beaches lower
approximately 0.5 m (1.6 ft) during the winter season and retreat horizontally between 20 and 30 m (65.6 and 98.4 ft). This seasonal
change is primarily due to the large winter wave climate and seasonal variability of wave direction and water levels in the Pacific
Northwest. During the high wave conditions of the winter season, sediment is transported northward and offshore while during the low
wave conditions of the summer season, sediment is transported back onshore and southward. As a result, the net change over the full
annual cycle is small relative to the seasonal variability. However, the interannual variability within the CRLC is still quite large (many
beaches changed by more than 10 m/yr (32.8 ft/yr) during the first three years of the monitoring program), and can mask short- to
medium-term shoreline change trends.
- The 1997/1998 El Niņo was one of the strongest on record and had a regional effect on the CRLC beaches. Associated with the El Niņo were
higher than typical water levels and wave heights and waves approaching from a more acute angle from the south. The impact of this
event was evident in the annual response of the beaches, as El Niņo conditions resulted in higher net northward sediment transport and
accumulation in each sub-cell. Some of the highest net beach accretion rates during 1997-1998 occurred at the northern-most section in each
sub-cell (Clatsop Spit, Leadbetter Point, Westport, and Point Grenville).
- Approximately 85 percent of the beach profile sites, and all of the 16 surface map sites, experienced erosion during the winter of
1997/1998, averaging 25 m (82 ft) of shoreline recession. By summer 1998, approximately 55 percent of the beach profiles had not recovered
beyond the 1997 baseline, resulting in an average recession rate of 1.2 m/yr (3.9 ft/yr), as measured by the retreat of the 2.0-m contour.
- The La Niņa of 1998/1999 also had a strong effect on the region’s beaches. While water levels were not as high as during the 1997/1998
El Niņo, wave heights were more extreme, with monthly averages more than 1.0 m above normal during the winter months. By summer 1999, the
end of the 2nd year of the monitoring program, 62 percent of the beach profiles had experienced net recession, and the average
rate of change during this two-year period for all profiles was approximately 1.8 m/yr (5.9 ft/yr) of shoreline recession.
- The largest wave event on record in the Pacific Northwest occurred during the La Niņa winter, (3 March 1999), with deep-water
significant wave heights measuring over 10 m (32.8 ft) and an associated storm surge measuring approximately 1.4 m (4.6 ft). This
major storm caused widespread erosion and flooding throughout the CRLC and destroyed a restroom facility in the City of Ocean Shores, WA.
- By summer 2000, the end of the 3rd year of the monitoring program, 64 percent of the beach profiles had experienced net
accretion, and the average rate of change during this three-year period for all profiles was approximately 4.5 m/yr (14.8 ft/yr) of
shoreline progradation. After these 3 years 63 percent of the beach surface maps had also experienced accretion averaging close to 2.3
m/yr (7.5 ft/yr) of progradation. As compared to the El Niņo year of 1997/1998 and the La Niņa year of 1998/1999 the winter of 1999/2000
was relatively mild with water levels, wave heights and wave periods being very close to the long-term averages. Although the majority of
beaches reveal a trend of accretion, several sites did exhibit an erosion trend during the three years of observations. The Grayland
Plains eroded at a rate of over 8 m/yr (26.2 ft/yr) and the southern Long Beach Peninsula eroded at 10 m/yr (32.8 ft/yr).
- Nearshore morphology among the four sub-cells of the CRLC is strikingly different. Nearshore morphologic features (such as
sandbars) along Long Beach were large and three-dimensional in 1999 while along the Grayland Plains the bars were more linear and much
smaller in magnitude. Both the North Beach sub-cell and the Clatsop Plains have substantial nearshore morphologic variability.
- Some of the largest sandbars ever observed were found along the Long Beach Peninsula during summer 1999. The maximum observed sandbar
height was 6.0 m (19.7 ft) as measured from crest to trough. The mean height of the outer sandbar was approximately 4.0 m (13.1 ft) over an
alongshore distance of 20-km (12.4 miles). Initial evidence suggests that there may be a connection between sandbar characteristics, such
as size and distance from the shoreline, and the susceptibility of the shoreline to erosion.
- Initial analysis of nearshore bathymetry data collected in summer 2000 suggests a clear lowering and migration (either onshore or the
bars have migrated offshore and disappeared) of the majority of outer bars in the littoral cell. This behaviour appears to be linked with
the beach progradation observed throughout the cell between 1999 and 2000.
- The offshore trending bars observed along the Long Beach sub-cell in 1999 and 2000 spatially (and possibly temporally) mimics the net
offshore bar migration that has been observed on a variety of other coasts. Net offshore bar migration is thought to follow a three-stage
process; bar generation near the shoreline, seaward migration, and bar degeneration in the outer nearshore.
- Monitoring data is helping to refine and verify process-based shoreline change models as well as to assess the flooding potential of
upland properties. Shoreline variance is being used to define the zone of possible shorelines that bracket predictions.
- An observation with implications for shoreline change predictive modeling is that the annual limit of measurable cross-shore profile
change from 1998 to 2000 ranges from 6 to 12 m (19.7 to 39.4 ft) (NAVD 88).
- The mean beach sand grain size in the CRLC is approximately 0.18 mm with median grain sizes ranging from 0.1 to 1.0 mm throughout the
littoral cell. There is a regional gradient in sediment size with grain sizes decreasing (finer sand) with increasing distance from the
Columbia River. This trend is interrupted near the mouth of Grays Harbor, where coarse sediment deposits exist on the beach.
- The sand size gradient is roughly mirrored by a gradient in beach slope with slopes decreasing with distance from the Columbia River.
The northern portion of the North Beach sub-cell exhibits the lowest sloping beaches in the CRLC. The mean beach slope, measured between
the 1.0 m and 3.0 m elevation contours (NAVD 88), in the CRLC is approximately 0.022 (1:45) and ranges from 0.008 (1:125) to 0.1
- The highest sand dunes in the CRLC can be found in the Clatsop Plains sub-cell, with dunes measuring as high as 15 m (49.2 ft). The
smallest dunes in the littoral cell are between Moclips and Point Grenville, WA, where small incipient dunes have formed in front of the
backing sea cliffs and bluffs.
- Monitoring program data is currently being used by a variety of resource agencies for permitting, project review and long-range
planning. Monitoring data is also being used by coastal engineers for project design.
Beach monitoring program summary plots
- Beach profile - vertical change
- Beach profile - horizontal change
- Surface map contour change
- Surface map seasonal change
- Nearshore beach profiles
- Beach state parameters