JONES CREEK - 5000 YEARS OF DEBRIS FLOW HISTORY

Creek fans whose volume consists of low frequency – high magnitude events are prone to catastrophic loss if the hazard and risk are not properly quantified and mitigated against. Jones Creek is a tributary of the South Fork Nooksack River located about 20 miles east of Bellingham, Washington located some 30 km east of Bellingham in Washington State. A debris flow occurred in 1984 causing little damage. Realizing that the 1984 event may have been much smaller than the design event with a 500-year return period, Whatcom County retained Kerr Wood Leidal Associates (KWL) to carry out a comprehensive debris flow hazard and risk analysis and to propose measures to mitigate against the 500-year return period debris flow. Presently, approximately 250 people reside in 99 houses in the town of Acme which is located on the fan of Jones Creek. The Jones Creek watershed has an area of 6.8 km² and is typical for other watersheds along the western foothills of the northern Cascades. The lower watershed is underlain by the Darrington Phyllite which is a very brittle, easily deforming and quickly weathering metamorphic rock. At several locations along the lower channel, deep seated complex landslides reach the creek. It is hypothesized that at times sections of these landslides fail and impound the creek for up to several days. Although never witnessed, catastrophic outbursts of these landslide dams will lead to debris flow development by entrainment of the landslide debris and sediments stored in the channel below the landslide dam. To establish a debris flow frequency-magnitude relationship, 18 test holes were excavated on Jones Creek fan to depth between 3 and 6 m. Nine radiometric dates and 14 AMS dates were obtained from organic material in debris flows deposits. The radiocarbon data suggests an average debris flow frequency of 240 years. Using the deposit depth and associated age it was possible to reconstruct debris flow volumes. The average volume was 25,000 m³ while the design debris flow had a volume of 80,000 m³. An event of this size would spread over approximately 40% of the fan surface causing severe damage or destruction to multiple dwellings with the associated risk of loss of life. A combination of debris flow basin and deflection berm is proposed to prevent future damages to the town of Acme.