Oblique Aerial Photography - Coastal Erosion from El Nino Winter Storms
- Abby Salenger
United States Geological Survey (USGS)
End Date: April 30, 1998
The U.S. Geological Survey as part of its Coastal and Marine Program uses aerial photographs to assess coastal erosion from severe storms. Our mission was to acquire precision-located oblique still and video photography before and after storm events to document storm-related changes to the coastline.
USGS acquired baseline coverage of over 1000 km of coastline from the west coast of the US in October, 1997, in anticipation of storms generated by the El-Niņo warming of the Pacific Ocean. One of the targeted areas was central California, from Point Arena to Point Sur. A follow-up mission was completed in April, 1998 after the storm season.
One hundred aerial photographs - 50 matched pairs are available - show examples of different types of coastal changes occurring during the 1997/98 winter. The photographs were taken from a NOAA Twin Otter aircraft flying at 500 feet and 130 knots. The photos in each area can be accessed at:
The pre- and post-storm oblique aerial photography has provided visual documentation of coastal morphological change. Landslides, longshore sediment transport, overwash, and dune and cliff erosion are evident in the photographs.
Summary to DateNASA's Airborne Topographic Mapper (ATM)
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. 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.
Sea cliffs in the Pacifica, CA area (south of San Francisco) were severely impacted during the 1997-98 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. 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.
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 can be quantitatively compared via cross-shore profiles based on individual laser spot elevations. When comparing pre- and post-El Niņo surveys, profiles over homes and were 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. The crest of one cliff retreated 13 m landward while the toe of the cliff retreated about 10 m.
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, a stable cliff, which was located only several hundred meters north of the eroding cliff, accreted during the El Niņo 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 Niņo on Oregon pocket beaches. The more southerly wave approach during an El Niņo winter may have transported sand from the southern profile to the northern location protecting the more northerly cliffs from wave erosion.
- The long-term average erosion rate for cliffs in the Pacifica area is roughly 0.2 m/year (Lajoie & Mathieson, 1985). Hence, the 1997-98 El Niņo accounted for about 50 years of cliff erosion at this location (since the cliff retreated 10 m during a single winter).
- Geological characterization