This report documents research on the causes, magnitude and frequency of landsliding and debris flooding in the Boulder Creek drainage basin, and the actions the Washington State Department of Transportation can take to mitigate the impacts of these geologic hazards on State Road 542. The research approach was divided into two phases: 1) compilation of precipitation, temperature and streamflow data analyses of eight historic debris floods in Boulder Creek; and 2) investigation of the progression of erosion and sedimentation since mid-century. A terrain evaluation procedure was employed to evaluate the present level of hillslope and channel instability and to determine future landslide hazard.
Debris-flood generation is highly dependent on rain-on-snowmelt conditions during moderate-to-high frequency storms. Since the 1940's, an eighteen-fold increase in area of landsliding along a 2.5 mile stretch of the main channel has produced much of the debris for these floods. This dramatic increase in landsliding is a highly-interdependent function of the local geology, hydrology and timber-harvest activities. The probability of a new landslide forming in the next 2 years is 74%, and increases to 97% for the next 5 years. The high risk of future landsliding, combined with profuse sediment storage in the main channel, assures the continued frequency of debris flooding in the vicinity of the Boulder Creek bridge for at least the next 20 years.
The capacity of the Boulder Creek bridge is no match for the present-day sediment discharge of Boulder Creek. Strategies for short-term and long-term hazard assessment, as well as hazard-avoidance planning have been designed. Preliminary steps for the development of a flood warning system are provided through a determination of the severity of weather conditions and a comparison with historic flood events. Weighted landslide hazard rankings have been applied to separate reaches of the main channel of Boulder Creek. Several mitigative options ranging from methods of protection to avoidance of the area are outlined; evaluation of the feasibility of a new bridge and/or highway alignment is recommended. Participation in the management of the drainage basin resources is encouraged. Visual communication is a highly effective way to increase awareness of the drainage basin conditions. Two documents auxiliary to this technical report have been prepared for this purpose: The Atlas of Lower Stream Reaches and Photographic Folio of the Boulder Creek Drainage Basin.