Effect of flocculation on turbidity currents

Abstract

Flocculation between inorganic sediment, salt ions and microscopic organic
matter present in the marine environment might play an important role in the
dynamics of turbidity currents. The ability to predict, understand, and potentially
leverage the effect of flocculation on turbidity currents will help to minimize the
impact of human interventions such as dredging, trenching, and deep-sea
mining. To better characterize the effect of flocculation on the benthic turbidity
currents generated by these activities, a series of laboratory experiments were
performed. Turbidity currents were created by means of lock exchange
experiments. The present work focuses on the flocculation of clays that are
representative for abyssal regions where deep-sea mining is performed, but
most of the conclusions of this work are generic and can be applied to other
types of benthic flows, occuring in harbours and channels. The effect of salt and
organic material as flocculant agent was investigated. Various concentrations of
clay and organic flocculant were tested. Video analysis was used to determine
the head velocity of the plume. Samples at different run-out lengths were
collected at the end of the lock exchange experiments for particle size and
settling velocity measurements. The velocities of the turbidity currents in fresh
and saline water (when no organic matter was present) were found to be similar,
which was expected considering the timescales of salt-induced flocculation
(about 30 min or more compared to the duration of lock exchange
experiment <60 s). It was however demonstrated that, in presence of
organic matter, flocculation occurred during the short time (30–60 s) of the
experiment, leading to a reduced current propagation and a significant change
in floc sizes (from 20 to 1,000 μm) and settling velocities (from 1 to 60mms−1).
Salt ions contributed to flocculation in the sense that flocculation with organic
matter was improved in the presence of salt.