Progress of a sediment wave along the Lillooet River, British Columbia following a large debris flow

Abstract:

We report field data and preliminary numerical modeling results documenting the transit of a sediment wave along the Lillooet River following a large debris flow in 2010. The Lillooet drains a midsize (~4000 km²) glacially-influenced alpine catchment. Our study reach lies 70 km between the Mt. Meager Volcanic Complex (MMVC) and Lillooet Lake, which traps over 90% of incoming sediment. The channel transitions from braided and cobble-bedded to straight and sand-bedded, and the downstream half is dyked. Sediment supply is dominated by frequent debris flows originating in the MMVC.

In 2010, a landslide on the MMVC released 48.5 x 10⁶ m³ of material and triggered a debris flow that extended 2.5 km down the Lillooet Valley, briefly damming the river and causing an outburst flood. 1500 valley residents were evacuated temporarily, and the event led to ~$10 M CAD of damage to commercial forests and roads. The subsequent sediment wave is expected to lead to channel aggradation, putting pressure on the Lillooet’s dyking system.

We conducted bulk sampling and Wolman pebble counts along the river in 2014. Grainsize distributions from channel bars along the reach are compared to those of cutbanks to track the extent of the wave and the mobility of individual grainsize classes. We use the Morphodynamics and Sediment Tracers in 1-Dimension (MAST-1D) model to simulate the Lillooet’s response to the debris flow on decadal timescales and to predict its recovery time. Sediment from the debris flow is traced in the model to estimate the proportion of the sediment wave that will remain in permanent storage within the valley fill. Future work will focus on quantifying the role of floodplain-channel interactions on sediment wave evolution for decadal and longer timescales.