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Coastal Erosion Along the U.S. West Coast During the 1997-98 El Niño:
Expectations and Observations

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Figure 1: Extent of airborne scanning laser (lidar) surveys along the west coast of the United States during October 1997 and April 1998.
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.

Research Plan

A plan was formulated to address the following questions:

  • 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?
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)


Figure 2: Diagram showing the elliptical scan pattern of NASA's ATM operated from a NOAA Twin Otter.
NASA's Airborne Topographic Mapper

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).

Initial Results

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.

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.
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.

For more info, please contact:

Hilary Stockdon
USGS St. Petersburg Coastal and Marine Science Center
St. Petersburg, FL 33701

hstockdon@usgs.gov
Bill Krabill
NASA Wallops Island Flight Facility
Wallops Island, VA 23337

krabill@aol5.wff.nasa.gov
NOAA Coastal Services Center
Charleston, SC 29405

csc@csc.noaa.gov

Coastal and Marine Program > St. Petersburg Coastal and Marine Science Center > Research by Theme > Hazards > 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 (THF)