Prediction of the dynamic behaviour and migration rates of sand waves in the Monterey Canyon System of California

This research focus on the sand wave field along the canyon axis in the upper 4 km of the Monterey Submarine Canyon revealed by high-resolution multibeam bathymetry collected by MBARI/Mapping Lab of the California State University (http://seafloor.csumb.edu/), spanning six years period (spring 2003 to fall 2008). The goal of the research is to understand how erosion, gravitative processes, sediment liquefaction by storm events, sediment transport by tidal currents, and deposition of the reworked sediment are connected in distal basins with quasi-horizontal floors. Morphometric analysis has been conducted on the sand wave on the canyon head and main axes comparing the waves shape of nearest surveys by means of smoothing filtering and geostatistical techniques. The analysis has allowed to classify the canyon floor in upstream migration zones, downstream migration zones, and completely reworked zones. In the first two zones a sand migration velocity has been inferred, and in the last ones, where the wave field is completely reworked between each surveys, a minimum admissible migration velocity has been deducted. A simple mathematical model has then permitted to reproduce the main features of sand wave inception and growth. In particular the model focus on the prediction of the migration rates that sand waves undergo because of tidal currents. The model output has been compared versus the morphometric analysis results and match and mismatch are discussed. Results of the research show that the sand waves migrate in a predominantly up-canyon direction with tidal and internal tidal currents, despite different behaviour along the canyon. However the research shows that these are not the dominating flows within the canyon. Seismic profiles interpretation, other morphometric analysis results like local channel widening causing lateral erosion of older channel and extension of gully head on canyon walls and rim, point out high velocity transport processes in the canyon main axes. This mechanism can be related to storm events and gravitative processes, possibly triggered by seismic events, as suggested by the slope stability analysis. Moreover, the liquefaction potential analysis of the poorly consolidated sediments characterising the canyon’s head suggests that pore fluid overpressures may develop during earthquake shaking (i.e. earthquakes with peak ground acceleration (PGA) > 0.18 - about 50% of the PGA related to the 1989 M 7.1 Loma Prieta earthquake), further contributing to the destabilization of the canyon slope.