Introduction to the special issue on debris flows initiated by runoff, erosion, and sediment entrainment in western North America

Debris flows are one of the most hazardous types of landslides in mountainous areas of western North America because they are fast moving and can occur with little warning. Field observations and existing literature indicate that three primary types of nonvolcanic debris flows affect areas of western North America (Fig. 1): 1) debris flows that mobilize from landslides (i.e., “slides” using the Varnes, 1978 classification) and travel over the surface of the hillslope, often flattening vegetation and leaving a thin veneer of deposits (e.g., Ellen et al., 1988), 2) debris flows that mobilize from slides that then erode and entrain hillslope and channel materials (e.g., Jakob et al., 2000), and 3) debris flows that initiate from surfacewater runoff that erodes and entrains hillslope and channel materials (e.g., Larsen et al., 2006). A variation of the third type of debris flow results from glacial outburst floods (e.g., Jackson et al., 1989) or failures of dams (e.g., O'Connor et al., 2001). Seven of the eight papers in this special issue focus on the third type of debris flow, and one (Hungr et al.) focuses on the second type. As reflected by the papers in this issue, the third type of debris flow has been documented in areas burned by wildfire, and in semi-arid to arid regions with sparse vegetation and an abundance of exposed bedrock, including parts of the intermountain west and southern California. In these settings, overland flow is an important part of the hydrological response of hillslope materials to rain storms. The first and second types of debris flows (i.e., those that initiate primarily from slides) are frequently described in wetter, more temperate regions with abundant vegetation, where rainfall occurs at low-to-moderate intensities over long durations, and overland flow is rare (e.g., the Coast Ranges of California, Oregon, Washington, and British Columbia). The initiation processes and hazard implications for debris flows that are generated through runoff and