title: The Mechanisms of Landslide Initiation and Flood Generation in the Boulder Creek Basin, Whatcom County, Washington
Creators: name: Gowan, Monica E.
Publisher: name: Western Washington University
Year: 1989

description: <p>Over the past fifty years, the frequency of landsliding and flooding in the Boulder Creek basin, Whatcom County, Washington, has increased. This trend is examined through analysis of the landslides and their mechanisms of initiation, the conditions for generation of runoff during eight debris-laden floods, and long-term precipitation and land-use patterns. Thirteen active landslides are identified along the main channel of Boulder Creek, located within the shear zone of a late Eocene high-angle fault. Landslides are classified as debris slides and slump-earthflows. Average landslide volume is 8,700 cubic meters. The largest slide observed is 36,700 cubic meters. Slope angles range from 25 to 53 degrees. Slump-earthflows continue to supply sediment to the channel even after the wet winter season; the surface velocity of one failure at 32 cm/day in late May. Landslides are supplying more debris than the creek can normally transport. Within the channel, twenty-two debris jams serve as temporary sediment-storage sites, retaining an average volume of 300 cubic meters of material per jam. These debris jams increase channel instability and the risk of damaging floods.</p>
<p>The principal factors controlling the susceptibility of hillslopes to failure are identified losing the variables of the modified Mohr-Coulomb equation and are ranked in order of decreasing importance: 1) low apparent cohesion, 2) low artificial cohesion, and 3) redistribution of total normal stress. Steep slopes with thin glacial and colluvial soils are underlain by highly fractured, jointed, and sheared bedrock, providing evidence of low apparent cohesion. Low artificial cohesion is attributed to root decay following forest removal. Road construction has changed the weight distribution of hillslopes and concentrated water flow. At sites where one or more of these unstable conditions are present, increased pore-water pressure due to precipitation is the most-likely triggering mechanism for failure.</p>
<p>Results of precipitation, temperature, and streamflow data analyses for eight flooding events show that floods are caused by the following hydrometeorological conditions, listed in order of decreasing frequency: 1) rainfall accompanied by snowmelt, 2) rainfall onto deeply frozen, but thawing ground, and 3) moderate to heavy rainfall. Recurrence intervals for associated 24-hour storms range from 0.2 to 38.8 years, with precipitation intensities ranging from 37 to 145 mm. The basin altitude ranges from 183-1671 meters, and thus the Boulder Creek basin is situated directly within the transient snow zone of the North Cascades. Estimates of snowmelt-generated runoff for the eight floods show an 8 to100 percent volumetric increase over runoff from rain only, and suggest that snowmelt may depress the magnitude and frequency of precipitation necessary for flood generation.</p>
<p>Slightly increasing trends in mean annual and winter precipitation over the fifty years of record are not convincing explanations for the accelerated frequency of landsliding and flooding. In addition, the high year-to-year variability of precipitation eliminates the possibility of using the trends as reliable predictors. Rather, the accelerated frequency of landslides and debris-laden floods is caused by the synergetic effects of geologic, hydrologic, and vegetative factors.</p>