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Effect of rainfall intensity and duration on debris flows in central Santa Cruz Mountains, California

Gerald F. Wieczorek
Effect of rainfall intensity and duration on debris flows in central Santa Cruz Mountains, California (in Debris flows/avalanches; process, recognition, and mitigation, John E. Costa (editor) and Gerald F. Wieczorek (editor))
Reviews in Engineering Geology (1987) 7: 93-104


Rainfall intensity and duration of storms has been shown to influence the triggering of debris flows. After examining storm records of the San Francisco Bay region, documenting when debris flows occurred, and measuring piezometric levels in shallow hillside soils, continuous high-intensity rainfall was found to play a key role in building pore-water pressures that trigger debris flows. Debris flows in 10 storms between 1975 and 1984 in a 10-km (super 2) area near La Honda, California, were examined, and their rainfall records compared to the records of other storms to determine the antecedent conditions and the levels of continuous, high-intensity rainfall necessary for triggering debris flows. No flows were triggered before 28 cm of rainfall had accumulated each season, which suggests that prestorm soil-moisture conditions are important. After this sufficient antecedent rainfall, a threshold of rainfall duration and intensity--which accounted for triggering at least one debris flow per storm within the study area--was identified. The number of debris flows increased in storms with intensity and duration characteristics significantly above this threshold. By studying where debris flows initiated in storms of different intensity and duration, debris flow susceptibility was found to depend on soil thickness and hillside concavity and steepness. Moderate intensity storms of long duration triggered complex soil slump/debris flows in thick soils on concave slopes below large drainage areas, whereas high-intensity storms of short duration caused complex soil slide/debris flows in thinner soils without respect to size of drainage area. From these observations, an empirical model based on geology, hydrology, and topography is proposed to account for the triggering of debris flows at selective sites by storms with different combinations of intensity and duration once the antecedent and intensity-duration thresholds are exceeded.

ISSN: 0080-2018
EISSN: 2169-799X
Coden: GAEGA4
Serial Title: Reviews in Engineering Geology
Serial Volume: 7
Title: Effect of rainfall intensity and duration on debris flows in central Santa Cruz Mountains, California
Title: Debris flows/avalanches; process, recognition, and mitigation
Author(s): Wieczorek, Gerald F.
Author(s): Costa, John E.editor
Author(s): Wieczorek, Gerald F.editor
Affiliation: U. S. Geol. Surv., Menlo Park, CA, United States
Pages: 93-104
Published: 1987
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Engineering Geology Division and the Quaternary Geology and Geomorphology Division of the Geological Society of America
Meeting location: Reno, NV, USA, United States
Meeting date: 19841105Nov. 5, 1984
References: 26
Accession Number: 1988-018900
Categories: Engineering geology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, sketch map
N32°30'00" - N42°00'00", W124°30'00" - W114°15'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 1988
Program Name: USGSOPNon-USGS publications with USGS authors
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