Debris-flow simulations on Cheekye River, British Columbia

Cheekye River fan is the best-studied fan complex in Canada. The desire to develop portions of the fan with urban housing triggered a series of studies to estimate debris-flow risk to future residents. A recent study (Jakob and Friele 2010) provided debris-flow frequency-volume and frequency-discharge data, spanning 20-year to 10,000-year return periods that form the basis for modeling of debris flows on Cheekye River. The numerical computer model FLO-2D was chosen as a modelling tool to predict likely flow paths and to estimate debris-flow intensities for a spectrum of debris-flow return periods. The model is calibrated with the so-called Garbage Dump debris flow that occurred some 900 years ago. Field evidence suggests that the Garbage Dump debris flow has a viscous flow phase that deposited a steep-sided debris plug high in organics in centre fan, which then deflected a low-viscosity afterflow that travelled to Squamish River with slowly diminishing flow depths. The realization of a two-phase flow led to a modelling approach in which the debris-flow hydrograph was split into a high viscosity and low viscosity phase that were modelled in chronologic sequence as two separate and independent modelling runs. A perfect simulation of the Garbage Dump debris flow with modelling is not possible because the exact topography at the time of the event is, to some degree, speculative. However, runout distance, debris deposition and deposit thickness are well known and serve as a good basis for calibration. Predictive analyses using the calibrated model parameters suggest that, under existing conditions, debris flows exceeding a 50-year return period are likely to avulse onto the southern fan sector, thereby damaging existing development and infrastructure. Debris flows of several thousand years return period would inundate large portions of the fan, sever Highway 99, CN Rail, and the Squamish Valley road and would impact existing housing development on the fan. These observations suggest a need for debris-flow mitigation for existing and future development alike.