Stability of the Fraser River Delta front

The distributary front deposits of the delta consist mainly of interlayered sand and silt and lie in one of the most seismically active zones in Canada. Subsurface deposits at the north tend to be siltier than those at the south. Sediments on the surface of the delta slope range from clayey silts at the north to pure sands at the south. Slope angles vary from 23° at the head of the slope to 1–2° within 2 km beyond the tidal flats.Hydraulic bedforms observed on the sandy slopes southeast of the main channel gradually increase in size from small ripples to large asymmetric sand waves 2–3 m high and approximately 30 m long. They are generated primarily by flood tidal currents which scour the seabed to a water depth in excess of 100 m.Formation of gullies which crease the delta slope probably is initiated principally by failure of oversteepened deposits at a channel mouth. Previous studies have suggested that the large hummocks or ridges at the base of the slope off the Main Channel have been formed by the compressional folding of failed deposits. Recent reflection seismic surveys suggest that such a process has recurred in this area during the growth of the delta.Interpretation of SPT data, in the light of recent research findings, suggests that previous analyses have overestimated failure potential. It is estimated that the slope may withstand an earthquake with a duration corresponding to approximately 15 significant cycles of motion and a peak acceleration in the range of 11–13% G. Keywords: stability, Fraser River Delta front, mass wasting, erosion, liquefaction potential, cyclic mobility.