QUANTIFYING EROSION RATES FOR AN ACTIVELY FAILING CLIFFED COASTLINE

The Beverly Beach littoral cell, located just north of Newport, Oregon, is a sediment-starved cell extending from Yaquina Head in the south to Cape Foulweather in the north. The beaches receive a majority of their sand from the chronic failure of the coastal cliffs that back the beaches along the entire length of the cell. The cliffs are composed of seaward dipping siltstone and sandstone units that commonly fail during the winter when large waves remove basal material and heavy rains saturate the cliffs. In addition to the hazards resulting from failure of the coastal cliffs, this stretch of coastline also contains many large, translational landslides that frequently damage coastal highway 101, along with houses and other community infrastructure. Previous studies by the Oregon Department of Geology have documented the erosion rates of the coastal cliffs by mapping the long-term cliff edge retreat on 2-dimensional orthophotographs. The results showed virtually no measureable long-term retreat of the cliffs for several reasons: 1) the true position of the edge of a cliff in an area with substantial vertical relief (30-40 m) is difficult to resolve on a 2-dimensional map, and 2) as the coastal cliffs erode landward, the landmass itself moves seaward as translational blocks, resulting in no net change in the position of the cliff. In a cooperative effort involving federal, state, county and academic partners we have undertaken a study to quantify the long-term erosion hazards in the Beverly Beach littoral cell using digital stereo photogrammetry. Aerial photographs from 1939 and 1993 are converted to high-resolution digital format, and the images are then processed using surveyed ground control points. With this method, we delineate the edge of the cliff while viewing in stereo, thus assuring positional accuracy, as the true slope break can clearly be visualized. In addition, the processing of the data to obtain a stereo model requires the creation of a digital terrain model (DTM) that provides a 3-dimensional representation of both the historical and recent land surface. These are subtracted from one another to quantify the long-term volumetric change within the littoral cell, thus providing a net volume change from the cliff erosion regardless of the movement of the translational landslide blocks.