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During late summer 1997, NASA's Wallops Flight Facility, NOAA's Coastal Services
Center, and the USGS Coastal & Marine Program formulated a plan to determine the
magnitude, spatial patterns, and causative processes of El Niño-induced change along the
west coast of the United States. A key element of the plan was to survey 1200 km of
representative reaches of the west coast both prior to and following the El Niño winter
storms using scanning airborne laser altimetry, a technology which has only recently been
applied to coastal change research.
Figure 1: Extent of airborne scanning laser (lidar) surveys along the west coast
of the United States during October 1997 and April 1998.
A plan was formulated to address the following questions:
To provide magnitudes and spatial patterns of beach and coastal cliff
response to the El Niño storms, NASA's Airborne Topographic Mapper
(ATM) was used to survey three reaches of coast in October 1997, prior to
the onset of the El Niño storms, and again in April 1998, immediately
following the winter season. (Fig. 1)
- What were the magnitudes and spatial patterns of beach and coastal
cliff changes along representative reaches characteristic of the entire west
coast of the United States? How were these magnitudes and patterns
different than what normally occurs during non-El Niņo winters?
- To what extent were elevated sea levels responsible for beach changes
and ultimately cliff erosion?
- To what extent were the observed beach and cliff changes a result of
changes in the wave climate during El Niño years?
NASA's Airborne Topographic Mapper
Figure 2: Diagram showing the elliptical scan pattern of NASA's ATM operated
from a NOAA Twin Otter.
The ATM can survey beach topography along hundreds of kilometers of coast
in a single day with data densities that cannot be achieved with traditional
survey technologies (Fig. 2 ). For each pass along the coast, the ATM lidar
scanned a 375 m wide swath along the aircraft flight line. For most of the
study area, four overlapping passes were flown yielding a typical surveyed
swath ~700 m wide with laser spot elevations every 3
m2. The aircraft pitch,
roll, and heading were obtained with an inertial navigation system and the
positioning of the aircraft was determined using kinematic Global Positioning
System (GPS) techniques. The twin-engine turboprop aircraft, a De Havilland
Twin Otter, was provided and operated by NOAA's Aircraft Operations Center,
McDill Air Force Base, Tampa, FL.
Figure 3: A. Coastal topography near Pacifica, CA derived from lidar data
obtained in April 1998. B. Blowup of part of the lidar map showing the
Esplanade Dr. area of Pacifica. The image is of gridded lidar data at 2 m
resolution and shaded according to elevation. C. Photograph of the cliff
area near Pacifica in early January, 1998 (looking north, see Fig. 3B). D.
Photograph of the same area taken during February 1998, following a series of
severe storm events (photographs by Monty Hampton, USGS).
Sea cliffs in the Pacifica, CA area (Fig. 1) were severely impacted
during the El Niņo winter, threatening a number of homes. The local
topography of the area was derived from ATM data acquired in October
1997 prior to the onset of the winter storms (Fig. 3A). Note the sea
cliff which ranges from greater than 35 m high in the north near Mussel
Rock to about 15 m high in the central part of the map. Adjacent to the
cliff, the lidar data show the shape and dimensions of houses and buildings
(Fig. 3B). Pacifica is the area which attracted a great deal of media
interest during the winter of 1997-98 as the houses on the brink of
the cliff (shown in Fig. 3B) were threatened by erosion. Ultimately,
12 houses were condemned as unsafe and seven were razed before they
were claimed by the sea.
At the camera location (indicated on Fig. 3B), oblique photographs were
taken prior to and after a series of severe winter storms in early 1998
(Figs. 3C & D). Comparison of the photographs clearly shows evidence
of landward retreat of the cliff which is composed of loosely consolidated
sands and gravel.
Beach and cliff changes are quantitatively shown by comparing
cross-shore profiles consisting of individual laser spot elevations
(Fig. 4A & B). Locations of these profiles are shown in respect to
lidar topography in Figure 3B. First note that in comparing pre- and
post-El Niño surveys, profiles over homes and buildings are very close
in shape and position. This provides visual confirmation that the ATM
lidar data is sufficiently accurate to resolve the magnitudes of beach
and cliff changes observed here. In Figure 4A, about 100 m north of
the camera location, the crest of the cliff retreated 13 m landward
while the toe of the cliff retreated about 10 m. Interestingly, the
long term average erosion rate for cliffs in this general area is
roughly .2 m/year (Lajoie & Mathieson, 1985). Hence, the 1997-98 El
Niño accounted for about 50 years of cliff erosion at this location.
However, along the coast, the cliff did not retreat landward uniformly;
in fact, there was considerable spatial variability in the responses of
both beaches and cliffs. For example, consider the stable cliff in Fig.
4B which was located only several hundred meters north of the eroding
cliff (Fig. 4A). Further, the beach below the stable cliff accreted during
the El Niño winter about 2 m vertically. In contrast, the beach below
the eroding cliff eroded about 2 m vertically. These dramatic spatial
changes in response may be similar to what Komar (1985) visually
observed during 1982-83 El Niño on Oregon pocket beaches. The more
southerly wave approach during an El Niño winter may have transported
sand from the southern profile to the northern location protecting the
more northerly cliffs from wave erosion.
Figure 4: Shore-normal profiles derived from lidar data of October
1997 and April 1998 of the cliff area near Pacifica. Each profile shows
individual laser spot elevations that fall within a 2m wide strip oriented
approximately shore normal. A. Example profiles showing erosion of the
sea cliff and adjacent beach located at the southern profile location in
Fig. 3B. B. Example profiles showing a stable cliff and accreting beach
located at the northern profile location in Fig. 3B.|
For more info, please contact:
Coastal and Marine Program >
St. Petersburg Coastal and Marine Science Center >
Research by Theme >
Hurricane & Extreme Storm Impact Studies >
Coastal and Nearshore Lidar Mapping >
Coastal Erosion Along the U.S. West Coast During the 1997-98 El Niņo:
Expectations and Observations
U.S. Department of the Interior,
U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center
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Updated May 06, 2013 @ 09:24 AM