2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 10
Presentation Time: 10:30 AM


SCHULZ, William H., Denver, CO 80225, wschulz@usgs.gov

Landslides have posed a significant hazard in Seattle, Wash. since at least the late 1800s. Seattle landslides usually occur on hillslopes above coastlines and drainages and are triggered by heavy rainfall. LIDAR was used to map Seattle landslides, which are otherwise obscured by dense vegetation. LIDAR aided identification of 173 landslides, which is nearly 400% more than identified by previous field and aerial photograph mapping efforts. Comparison with previous maps and historical landslide inventories indicates that most of the LIDAR-mapped landslides are complexes consisting of multiple smaller landslides. In addition, many of the historical landslides described in a comprehensive city-wide inventory occurred within LIDAR-mapped landslide boundaries (57%); when human-caused landslides are discounted, this number increases to 66%. Nearly all of the LIDAR-mapped (93%) and historical (95%) landslides occupy hillslopes whose toes have been subjected to surface-water erosion, suggesting that undercutting of slopes by surface water is the primary cause of Seattle landslides. Groundwater, stratigraphic conditions, and precipitation are secondary causes. This LIDAR study also indicates that most Seattle landslides occur above coastlines, which should be expected because sea-level rise since withdrawal of glacial ice from the area has resulted in significant erosion and retreat of coastlines. Coastal retreat has driven recession of the hillslopes above, primarily through landslide activity, again suggesting that slope-toe erosion is the primary cause of Seattle landslides. Evaluation of historical landslide activity indicates that coastal protective structures constructed during the last hundred years have not reduced landslide activity on adjacent hillslopes. However, protected hillslopes above some coastlines uplifted by tectonic activity during the past several thousand years exhibit shallower slope angles and reduced landslide activity, suggesting that they may be nearing equilibrium. It follows that landslide activity may continue for perhaps thousands of years on hillslopes above coastal protective structures. More than previous landslide maps, the LIDAR landslide map clearly identifies those areas most likely prone to future landslide activity.