RESULTS OF A FORENSIC LANDSLIDE INVENTORY WITHIN FRESHWATER CREEK AND ELK RIVER WATERSHEDS, 2002-2003 STORM EVENT

Landslide area-volume relations are developed from field observations and are utilized to estimate the magnitude of landsliding that occurred on privately managed coastal redwood timberlands. This study is a response to record rainfall totals that triggered landsliding in northern coastal California during the 2002-2003 winter season. Collection of site specific landslide information detailing the geology, geomorphology, and land management history of landslide initiation sites are compiled in a forensic landslide inventory (FLI) database. The FLI is one of several efforts in progress that are working towards the creation of a single database of all mass-wasting that has occurred across the timberland holdings. This information is incorporated into a watershed monitoring program employed by the timberland owner. The FLI provides preliminary conclusions regarding landscape response to a series of storm cells that culminated in the December 2002 storm event. The latter of the December storm cells produced nearly 9 inches of rainfall in a 48-hour period and was centered on the Elk River and Freshwater Creek watersheds.

We observed about 320 landslides initiating or reactivating across about 200,000 acres of managed timberlands between December, 2002 and June, 2003. Just over 100 locations were ground-truthed, and 93 landslides are included in the FLI database. From the 93 sites measured in the field, it is estimated that ~171, 548 cubic yards of hillslope material was displaced, and ~ 41, 872 cubic yards of material delivered to stream networks throughout the Eel River, Van Duzen River, Elk River, and Freshwater Creek watersheds. 53 percent of landslides included in the FLI were reactivations of older landslide scars and 85 percent were associated with roads.

Our study focuses on the center of the storm cell within the Elk River and Freshwater Creek watersheds near Eureka, CA. About 1/3rd of the mapped landslides initiated in the two watersheds. We compared area-volume relationships sorted by lithology, watershed, and road failure association. Within the Freshwater Creek and Elk River watersheds, area-volume relationships were developed to allow accurate sediment budgets to be estimated from visited and unvisited landslides.