The effect of bed age and shear stress on the particle
morphology of eroded cohesive river sediment in an
annular flume
Micheal Stone
a,
*, Bommanna G. Krishnappan
b
, Monica B. Emelko
c
a
School of Planning and Department of Geography, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
b
Aquatic Ecosystems Impacts Research Division, National Water Research Institute, Burlington, Ontario, Canada L7R 4A6
c
Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1
article info
Article history:
Received 28 February 2008
Received in revised form
10 June 2008
Accepted 17 June 2008
Available online 27 June 2008
Keywords:
Cohesive river sediment
Flocs
Fractals
Biofilm
Annular flume
abstract
Erosion experiments were conducted in an annular flume to determine the effect of bed
age and shear stress on the particle morphology (fractal dimensions D, D
1
, and D
k
)of
eroded cohesive river sediment. Sediment beds were deposited under low shear and left
to consolidate for one, two and seven days. Fractal data and photomicrographs show
particle morphology changed with shear stress and bed age. During the one-day experi-
ment, flocs were highly branched and particle geometry became more complex with
increasing shear. Microflocs present in suspension at low shear, formed larger more loosely
bound flocs at moderate shear due to flocculation. At higher shear, larger flocs were less
prevalent due to particle breakup. As bed age increased, less sediment was eroded and
particles appeared less porous and more angular in shape for a given shear stress. Changes
in floc morphology and eroded sediment mass at various shear stresses may be related to
bed age-associated biostabilization of bed deposits.
ª 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The settling of fine-grained suspended solids in aquatic
systems promotes the formation of cohesive sediment beds
referred to as surficial fine-grained lamina (SFGL) (Droppo
and Stone, 1994). Quantitative data regarding the critical shear
stress for erosion (s
crit
) and the rates of SFGL erosion as a func-
tion of shear stress and changing environmental conditions
are necessary to improve models designed to predict the
source, transport and fate of sediment-bound contaminants
often present in these deposits (Stone and Droppo, 1994).
The erosion and deposition behaviours of SFGL are complex
due to flocculation processes and are therefore difficult to
represent in numerical models. Consequently, parameters
such as critical shear stress for bed erosion and the erosion
rate of SFGL with differing physical and geochemical proper-
ties must be determined from direct measurements of sedi-
ment concentration and applied shear stress (Kuijper et al.,
1989; Droppo et al., 2001).
Several recent studies of fine-grained sediment transport
in laboratory flumes have advanced knowledge regarding
the formation and erosive behaviour of SFGL. For example,
erosion experiments conducted in an annular flume using
commercially available kaolinite and contaminated bed sedi-
ment from Hamilton Harbour in Ontario, Canada demon-
strated that bed strength (erodibility) is dependent on both
the degree of biostabilization and flow conditions under
which the bed is deposited (Droppo et al., 2001; Gerbersdorf
et al., 2008). Lau and Droppo (2000) investigated the influence
of antecedent conditions on bed erosion and demonstrated
* Corresponding author. Tel.: þ1 519 888 4567x33067; fax: þ1 519 725 2827.
E-mail address: mstone@fes.uwaterloo.ca (M. Stone).
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.watres.2008.06.019
water research 42 (2008) 4179–4187