Nira L. Salant, Marwan A. Hassan

1
Physical effects of streambed periphyton on
particle deposition and flow hydraulics Paper
Number H-----
Nira L. Salant, Marwan A. Hassan Department of
Geography, University of British Columbia
Hydraulics Velocity and shear stress profiles
Deposition Inorganic content of surface samples
Introduction What is periphyton? Periphyton is
a complex matrix of algae, bacteria and
associated polysaccharide secretions found in the
hyporheic and benthic zones of streambeds.
Periphyton assemblages dominated by green algae
are typically thick and filamentous, while diatom
and bacteria are usually low-profile and
mucilaginous.
Closed BEeff BEp
Open BEeff lt BEp
BEp
BEeff BEp
BEeff
What does periphyton do? Periphyton provides
food and habitat for many aquatic organisms, but
some evidence suggests that periphyton
assemblages may indirectly influence stream
organisms by altering the geomorphic and
hydraulic conditions of their local environment.
Recent research suggests that periphyton can
enhance particle deposition via adhesion and may
alter hydraulic conditions above and within
streambeds. Like large-scale macrophytic
canopies, periphyton mats may significantly alter
flow and sedimentation patterns according to
their thickness, structure and density.
0.6
Periphyton-Open
Periphyton-Closed
0.6
None
None
0.5
Field studies and flume experiments
0.5
0.4
Slight acceleration
0.4
Acceleration
0.3
z/H
Reduced near-bed velocities
z/H
0.3
www.frw.ca/rouge.php?ID38
0.2
0.2
0.1
0.1
BEp
0
BEp
0
0
10
20
30
40
50
0
10
20
30
40
50
Flume experiments
Ux (cm/s)
Ux (cm/s)
Reduced shear stresses
Hypothesis Streambed periphyton influences the
deposition of fine particles directly by adhesion
or trapping and indirectly by changes to nearbed
hydraulics the magnitude and nature of change
depends on periphyton density and
structure Predictions Hydraulics 1) Low density,
low-profile forms will not significantly alter
near-bed flow patterns, but low-density, rigid
filamentous algae will increase surface
roughness, reducing near-bed flow velocity and
turbulence intensity. 2) High density and/or
flexible filaments shielded from the flow will
cause flow constriction and accelerated
velocities throughout the flow depth. Deposition
1) Sticky exopolysaccharides of mucilaginous
biofilm (e.g. diatoms and bacteria) will enhance
particle adhesion and deposition relative to
filamentous forms (e.g. green algae). 2) Particle
deposition will increase with biofilm density,
but will plateau as mats become saturated with
material.
BEp
BEp
Conclusions
Table Values are averages (SE) of all trials in
one of six categories based on flow level
(HFhigh, LFlow) and type of periphyton mat
(Cclosed, Oopen, or Nnone). Bed shear
stresses (tRe0) and turbulence intensities (TKE0)
were determined from 3-D velocity fluctuations of
the near-bed measurement. Ux, Umax, and U0 are
the mean, maximum, and nearbed horizontal
velocity. Volumetric density (AFDM/h) of each mat
was determined from the areal density (measured
as AFDM) and the measured height of the filaments
(h).