Coastal Erosion Along the U.S. West Coast During the 1997-98 El Nino: Expectations and Observations
- Abby Salenger
United States Geological Survey (USGS)
- Bill Krabill
National Aeronautics and Space Administration (NASA)
- Dave Eslinger
National Oceanic and Atmospheric Administration (NOAA)
End Date: April 30, 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 Niņo-induced change along the west coast of the United States. A key element of the plan was to survey 1200 km of representative areas of the west coast both prior to and following the El Niņo winter storms using scanning airborne laser altimetry, a technology which has only recently been applied to coastal change research.
Summary to DateTo provide magnitudes and spatial patterns of beach and coastal cliff response to the El Niņo 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 Niņo storms, and again in April 1998, immediately following the winter season. 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.
- Landslides - look for circular or oval areas of erosion on steep slopes indicating the scar caused by a landslide. Commonly, a depositional area occurs seaward, or downslope, of the erosional area where the failed material came to rest. Sea cliffs in the Pacifica, CA area were severely impacted during the El Nino 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. Adjacent to the cliff, the lidar data show the shape and dimensions of houses and buildings. 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 were threatened by erosion. Ultimately, 12 houses were condemned as unsafe and seven were razed before they were claimed by the sea.
- Beach and cliff changes are quantitatively shown by comparing cross-shore profiles consisting of individual laser spot elevations. Locations of these profiles are shown in respect to lidar topography. First note that in comparing pre- and post-El Nino 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.
- Evidence of longshore sediment transport can be seen in the lidar imagery. Look for a pattern where an erosional area is adjacent (in a longshore direction) to an accretional area, suggesting transport direction. These patterns are also pervasive at manmade structures designed for navigation and property protection, but also occur adjacent to natural irregularities in the coast, such as headlands.
- Evidence of overwash may also be seen in the imagery. Look for a pattern where a depositional area occurs immediately landward of an erosional area, suggesting overtopping of the beach by wave runup. This results in erosion of the beach and net sediment transport (and deposition) landward. This process occurs more extensively on the low-lying barrier islands of the East and Gulf of Mexico coasts, but there are many examples of overwash on the West coast during the 1997-98 El Nino winter as well.
Study MethodsFor each pass along the coast, the Airborne Topographic Mapping 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 square meters. 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.
Elevation differences between lidar surveys obtained at the beginning (October 1997) and end (April 1998) of the El Niņo winter were superimposed on the passive data. To calculate the change, the elevation of each data point in one survey is differenced from all data that occur within 1 m horizontal in the other survey.
Passive Data - a "panchromatic" digital image of the coast that is obtained simultaneously with the laser elevation (lidar) measurements made by the NASA Airborne Topographic Mapper (ATM). The passive data are geo-referenced in a mosaic that appears as a 'strip' image of the coast. The post-El Nino (April 1998) passive data are shown in a strip map roughly 700 m wide.
Profile comparisons at selected locations shore-normal transects were defined. Profiles along these transects were determined from lidar surveys obtained in October 1997 and April 1998. Each data point shown represents a laser spot elevation. All spot elevations within 1 meter of the shore-normal transect were used to construct the profiles.