How do hyperpycnal flows travel so far down submarine canyons? entrainment, detrainment and salinification.
Hyperpycnal flows occur where fluvial discharge to the ocean contains very high suspended sediment loads. They are common in areas affected by frequent typhoons, such as offshore Taiwan. Laboratory experiments and numerical models suggest that such flows should experience rapid buoyancy reversal and lofting due to sediment deposition. However, evidence from Gaoping Canyon shows that some such flows travel for hundreds of kilometres along the canyon and onto the floor of the Manila Trench, breaking telecommunications cables en route, and clearly maintaining their integrity as underflows. We suggest that several factors contribute to this long run-out. Hyperpycnal flows in submarine canyons are most likely auto-suspending rather than depositional, and thus do not experience buoyancy reversal. During down-canyon transit, three mechanisms may contribute to transformation of the flow into a normal turbidity current with saltwater interstitial fluid, thus increasing run-out. The first is simple entrainment of ambient fluid and its turbulent diffusion through the current, which also dilutes the sediment concentration field. The second is by double diffusion whereby salt diffuses from entrained enclaves of ambient fluid into the interstitial fluid. The third is by detrainment of fresh water as plumes from the top of the current, carrying fine sediment and heat.
Session 1: Canyon processes in space and time (formation, evolution, circulation)