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An assessment of hazards from rain-induced debris flows on Mount Rainier

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Author(s): Nicholas T. Legg

Document Type: Unpublished Internal Report
Publisher: Mount Rainier National Park
Published Year: 2015
Pages: 30
DOI Identifier:
ISBN Identifier:

Rain-induced debris flows initiated from Mount Rainier's upper flanks can inflict major damage to downstream areas, and therefore need to be incorporated into short- and long-term management decisions. This work provides tools that allow the National Park Service to assess the hazards from debris flow initiation in space and time. Hazard mapping provides tools to assess debris flow potential spatially, and a storm characterization and simple decision tree allow forecasting of debris flow hazards in time.

Hazard mapping involved separately mapping and then overlaying debris flow initiation potential and sediment availability. Initiation potential was mapped across drainage networks of Mount Rainier using measured slope and drainage area. Slope-drainage thresholds for debris flow initiation found for the November 2006 storm were used to identify segments of the drainage network with high initiation potential. Once initiated, debris flows can incorporate large volumes of sediment on Mount Rainier’s upper slopes which in turn give debris flows greater potential to travel and inflict damage far downstream. Sediment availability is therefore an important factor in debris flow hazards. Geomorphic mapping roughly characterized sediment availability on Mount Rainier’s upper slopes, so that areas with high initiation potential and sediment availability could be overlaid to develop hazard maps. A simple watershed-based hazard rating identifies relative debris flow hazards by major streams and rivers in the park. Maps can therefore be used to assess hazards from the scale of individual gullies to entire watersheds.

The storm characterization involved a simple accounting of storm conditions in past debris flow storms to in turn classify debris flow hazards in future storms. Precipitation, temperature, and snowpack measurements taken from 1980-2014 the Paradise SNOTEL station were used to characterize conditions during 11 known debris flow producing storms. All storms had less than 5 inches snow water equivalent (SWE), suggesting minimal antecedent snowpack is a requirement for debris flow initiation. Measured temperatures indicated that freezing levels were at or above the elevation range of high hazard areas mapped in the first section of the report, supporting the simple idea that rain needs to fall in order to produce runoff necessary for debris flow initiation. Finally, storm precipitation and antecedent precipitation were characterized using cumulative precipitation measured over 3 and 15 day periods. These periods allow comparison of debris flow producing storms to an existing landslide threshold for Seattle, Washington (Chleborad et al., 2006). Eight of the eleven storms exceeded that threshold. The seasonality of high hazard storms (those with minimal snow, high freezing levels, and rainfall exceeding the Seattle threshold) exclusively have occurred in late summer and fall months. The resultant forecasting methodology gives park officials a tool to forecast debris flow hazards three days in advance using weather forecasts for the same time period.

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Suggested Citations:
In Text Citation:
Legg (2015) or (Legg, 2015)

References Citation:
Legg, N.T., 2015, An assessment of hazards from rain-induced debris flows on Mount Rainier: Unpublished Internal Report, Mount Rainier National Park, 30 p..