Abstract WPC 2010 a


Hyperpycnal Current-Sensitive Continental Margins

James P.M. Syvitski, Albert J. Kettner, Eric W.H. Hutton

Community Surface Dynamics Modeling System (CSDMS), Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder CO, USA.

Hyperpycnal (HP) flows generated by rivers discharging into marine continental shelves have received limited study. Theory is well advanced of field observations that remain limited and very hard to obtain. The geophysical importance of HP events has also been speculative, particularly on the nature of whether such flows are erosive or depositional upon leaving the coastal zone, and whether sediment can be carried directly into the deep ocean. Numerical models, calibrated on flume experiments, have provided perhaps the most insight into the frequency and magnitude of HP events, and which margins are likely to be greatly impacted by these events. Theory suggests hyperpycnal flows begin with coastal concentrations of tens of grams of muddy sediment per liter, with flow velocities of <3 m/s. The flows are long body quasi-steady flows lasting hours to weeks. HP discharge into the world’s ocean is mostly limited to small and medium-sized rivers that drain mountainous terrain capable of generating hyper-elevated sediment concentrations during high-energy floods. HP sensitive continental margins include:

  1. Taiwan and New Zealand, where the generating conditions include rain-intense cyclones, set up by their earthquake sensitive landscape, and in the New Zealand case major deforestation;
  2. California, where ENSO + PDO conditions align to produce high intensity frontal rainfall, again in a tectonically active setting set up with deforestation activities; and
  3. mountainous Mediterranean and Black Sea rivers where high intensity convective rainfall produces extremely short lived yet turbid flood events.

In all of these site examples, the critical threshold to overcome the buoyancy set up of coastal-ocean salinity has either been documented through in-situ observations, or with established hydrological rating curves, or hydrological modeling. When these flows enter the ocean, the immediate slope of the seafloor becomes the controlling factor. If the slope is gentle, model simulations suggest that the flow enters a depositional state immediately. Modeled deposit characteristics are very similar to a deposit from deposition under a surface plume. If the seafloor slope is steep enough, erosion of a near shore channel may take place. The transit across the continental shelf is then controlled by the strength of along-shelf currents in competition with the slope of the shelf. Hyperpycnal flows, at least in models, may sometimes not complete their transit across the continental shelf, where the rate of deposition is high, and the transit length is long, and flows dilute through entrainment. Here the role of wave support of the deposition rate becomes important, and thus the coherency of an ocean storm with the river flood. Flows may move down a continental slope, at least in models, as a line source, if along shelf currents are strong.