GIS Data Catalog

No simple metric was identified in the literature that could be used to screen lowland basins and identify those most likely to be inundated by such debris flows.

A review of sites with known recent debris flows or debris floods in the Puget Lowland areas of King County suggests that they share some characteristics in common: (1) all of the recent incidents reviewed entailed discharge from ravines with steep side slopes, typically > 60 percent slopes; (2) all discharged onto well-defined, preexisting fans, and these flows were often initiated by a sudden release of water, and in a number of recent cases this has been the result of failure of an upstream.

beaver dam; (3) where the channel gradient above the fan was steep, in excess of a 15 percent slope, the discharge was in the form of a debris flow; and (4) where the channel gradient was less than a 15 percent slope, the discharge took the form of a debris flood.

These observations are based on a small number of recent incidents, which is not enough to develop robust screening criteria.

Based on the characteristics above, DNRP geologists used LiDAR imagery and closely spaced contour lines to map depositional fans in the Puget Lowland areas.