CLASSIFYING MASS MOVEMENT SCARS IN PUGET GULCH, TACOMA, WASHINGTON STATE

The Puget Sound Region is marked by many postglacial mass movement scars, including those of modern origin, due to the poorly consolidated glacial and interglacial deposits that characterize the local stratigraphy. LiDAR maps of Puget Gulch, a gulch formed by downcutting as the level of Puget Sound lowered during deglaciation, reveal the topography of mass movement scars with incredible detail. Puget Gulch, an urban park surrounded by residential neighborhoods in North Tacoma, Washington State, is three quarters of a mile long with a base vertical relief of 256 feet. Slides were grouped by location, elevation, and origin. Some side gullies were first carved by streams and later modified by slides, while others were created by an initial landslide, creating a new path for runoff to take. By observing the uphill or perpendicular direction at which small side gullies enter the main gulch, landslide or stream origin can be determined. Gullies created by streams tend to mirror the shape of traditional stream tributaries. A series of patterns in slide geometry were observed. Certain areas within the gulch show a strong correlation between width and depth, while other areas seem to be completely random. The relative degree of slope development of each slide was also estimated using a depth to breadth ratio, assuming that this ratio decreases with age, and age was incorporated into previously mentioned location, elevation, and origin measurements to look for patterns in age within those categories. These data may help to predict the geometry of future slides in the region.