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| Titel |
Exploring flocculation of suspended burned sediment using an annular flume |
| VerfasserIn |
W. H. Blake, P. Clarke, A. J. Manning, M. F. Fitzsimons |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043712
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| Zusammenfassung |
| The frequency and severity of wildfire events are predicted to increase in many
fire-prone areas of the world with implications for erosion, sediment transport and
sedimentation. While cohesive suspended sediment is known to be transported primarily as
flocculated material in river channels, with important implications for catchment
nutrient and contaminant fluxes, there has been little work to date to explore the
effect of burning on suspended sediment flocculation processes. Since heating has
profound effects on surface soil biogeochemistry, it can be hypothesised that in-channel
flocculation processes may also be affected as burned eroded material is transported
through the catchment system. Using an annular flume and LISST-ST (Laser in Situ
Scatter and Transmissometry with Settling Tube) particle size analyser, short-term
suspended sediment flocculation dynamics were examined in burned and unburned
sediment collected from a wildfire-impacted catchment, Southern Peloponnese,
Greece.
Fine sediment (< 63 μm) samples were introduced to a small annular flume (45 L
capacity) which was operated at range of turbulent shear stresses (0.1, 0.3, 0.6 and 0.9 Pa).
Experiments were undertaken for a range of suspended sediment concentrations
(111, 222 and 333 mg l-1) of burned and unburned material. For each shear and
sediment concentration scenario, the flume was operated for 30 minutes to induce a
theoretical equilibrium between flocs and fluid shear stress after which 5 replicate
subsamples were collected and analysed for effective particle size using the LISST-ST.
Material was also analysed for absolute particle size following chemical and ultrasonic
dispersion.
At the two higher sediment concentrations, the effective particle size distribution of
unburned material notably coarsened at shear stresses of 0.1-0.3 Pa in comparison to the
absolute particle size distribution. This is reflected in a reduction of the percentage of 250 μm) e.g. from 14.4 ± 4.1
% to 5.9 ± 2.0 % at the highest sediment concentration. While similar increases in
effective particle size were seen at the lower sediment concentration, there was no
apparent break down of flocs at higher shear stresses. In burned sediment a similar
increase in effective particle size was observed but only when shear stress reached
0.3 Pa. At 0.1 Pa the particle size distribution of the lowest and highest sediment
concentrations was similar to that of the disaggregated material. There was no apparent
breakdown of flocs at higher shear stresses. Data suggest that while burning leads to
the formation of more robust sediment flocs, greater particle interaction (at higher
turbulent shear stress) is required to initiate formation. Further replication with a
greater range of mixing times is required to confirm these preliminary observations. |
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