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| Titel |
Flocculation and resuspension of suspended matter in a tidal freshwater river |
| VerfasserIn |
Maximiliano Sassi, Ton Hoitink |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089210
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| Publikation (Nr.) |
 EGU/EGU2014-3406.pdf |
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| Zusammenfassung |
| Knowledge about the delivery of suspended sediments to the coastal zone is essential for land
and water management in estuaries and river deltas. Sediment discharge estimates, that
largely rely on in situ measurements and model efforts, can vary over relatively short
time-spans by a countless number of processes whose underlying mechanisms are
of somewhat different nature. We present observations in a large river system of
two distinct processes of flocculation and resuspension, sharing common features.
We discuss the implications for long-term suspended sediment monitoring and
modeling.
An instrumented frame was deployed at the bottom of the River Mahakam (Indonesia)
and located at the entrance of a mild bend, approximately at 50 m from the riverbank and at a
mean depth of about 15 m. Observations took place during the wet and dry seasons, when
typical mean river discharges exceeded the 5000 m3 s-1 and remained well-below 1000 m3
s-1, respectively. In both seasons, salinity intrusion was negligible and the river reach under
consideration remained a tidal freshwater river. The flow at this location is characterized by a
streamwise velocity component strongly modulated by the semidiurnal tide and by
moderate secondary circulation cells. Secondary circulations develop during ebb
in the wet season and both during ebb and flood in the dry season. Slack flow is
observed only during the dry season with flood flows of the same magnitude as ebb
flows.
The distribution of volume concentration measured by a LISST instrument shows a clear
co-evolution with the semidiurnal tide and a strong shift of the distribution towards the coarse
size class when secondary circulation is absent. Spectra of size distribution per volume
generally show two main modes with a peak at nearly Ï = 3 and Ï = 1.5, respectively. An
objective classification analysis based on information theory yields two groups of spectra
having distinct characteristics: group 1 is characterized by the same distribution in
observations, both in the dry and the wet seasons, and group 2 exhibits two slightly different
distributions among the surveys. Estimates of the flow strength confirm the spectral
classification objectively splits the observations in periods corresponding with high and low
shear stress, for both surveys. For high shear-stress observations (group 1), the median
particle size remains nearly independent of the flow strength. For low shear-stress
observations (group 2) a strong negative correlation between particle size and flow strength is
observed in the wet season.
These observations suggest turbulence controls the processes of aggregation and
disaggregation of suspended matter during the wet season (as mean particle size decreases
with flow strength and vice versa) and the resuspension of coarse material from the bed
during the dry season. These two seemingly unrelated physical processes share common
features in time as they are bounded by the characteristic period of the semidiurnal tide,
however, they differ significantly both in terms of spatial scales and transport characteristics.
Our results may have consequences for the interpretation of long-term temporal
analysis of suspended sediment transport based on observations and modeling. |
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