Fettweis, M.

Lee, B.J.

Toorman, E.

Wang, J.

Open Marine Archive

 - 45 -
MULTIMODALITY OF A PARTICLE SIZE DISTRIBUTION OF MARINE 
SUSPENDED SEDIMENTS: SIGNIFICANCE AND APPLICATION 
Lee Byung Joon1, Michael Fettweis2, Erik A. Toorman1 and Jian Wang1 
1 Hydraulics Laboratory, KU Leuven, Kasteelpark Arenberg 40, 3001 Heverlee, Belgium 
E-mail: joon.lee@bwk.kuleuven.be  
² Management Unit of the North Sea Mathematical Models, Royal Belgian Institute of Natural Science 
Science, Gulledelle 100, 1200 Brussels, Belgium 
 
Particle Size Distributions (PSDs) of suspended particulate matter are log-normal and multi-modal in 
general. The log-normality describes a more or less skewed distribution toward small size particles, 
and the multimodality describes a distribution consisting of multiple peaks. In a coastal 
environment suspended sediments consist often of flocs. The multimodality is caused by mixing of 
several floc classes which are different in size and physicochemical property. Those floc classes 
have undergone different fates in marine environments, and so are able to show their record of floc 
dynamics under flocculation and transport. Therefore, this research was purposed to develop and 
test a new experimental method of decomposing multimodal PSDs of suspended particulate matter 
and tracking decomposed floc classes for investigating floc dynamics. A time series of multimodal 
PSDs and flow characteristics (velocity and turbulence) measured near Zeebrugge were selected for 
testing the new method. An automated peak decomposing software (DistFit, Chimera Technologies 
Inc.) decomposed a measured multimodal PSD to three unimodal PSDs, consisting of clay particle (2 
~5μm), microflocs (size varying), and macroflocs (300~400μm), and estimated the mass fractions of 
the constituent particles and flocs.  
 
Among many findings from decomposing a multimodal PSD and tracking decomposed PSDs, it is 
important to note that the volume fraction of macroflocs was maximized at the slack of a tidal cycle 
due to flocculation. This agrees with the previous finding in that floc size becomes a maximum 
under aggregation at the lowest turbulence but a minimum under breakage at the highest 
turbulence (Winterwerp, 2002). Furthermore, noteworthy is that the volume fraction of macroflocs 
consistently appeared during the ebb period of a tidal cycle but disappeared during the flood 
period. This is explained by the general lower ebb currents resulting in more efficient flocculation 
and the stronger flood currents limiting the growth of flocs. During maximum flood currents the 
PSDs indicate local resuspension of non-cohesive sand-sized particles, which agrees with finding of 
Baeye et al. (2010).  
 
In conclusion, the new method of decomposing a multimodal PSD and tracking decomposed PSDs 
was proven to be a useful tool for investigating floc dynamics in a turbulent flow field of a marine 
environment. Deposition or erosion of a mud layer, which can be either beneficial or detrimental for 
a local marine environment, could be estimated by applying the new PSD-tracking method in a long-
term and systematic way. Also, simulating interactions between two or three decomposed PSDs (e.g. 
aggregation and breakage) and re-superposing PSDs can be an easier numerical method to simulate 
flocculation of a fully continuous multimodal PSD in a large-scale marine system.  
 
References 
Baeye M., M. Fettweis, G. Voulgaris and V. Van Lancker. Sediment mobility in response to tidal and wind-
driven flows along the Belgian inner shelf, southern North Sea. Ocean Dynamics (in press). 
Winterwerp J. 2002. On the flocculation and settling velocity of estuarine mud. Continental Shelf Research 
22:1339-1360.  


Multimodality of a particle size distribution of marine suspended sediments: significance and application

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