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| Titel |
Floc Size and Settling Velocity Observations From Three Contrastingly Different Natural Environments in the USA |
| VerfasserIn |
Andrew Manning, David Schoellhamer, Ashish Mehta, Daniel Nover, Geoffrey Schladow |
| 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 |
250037792
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| Zusammenfassung |
| Environmentally, monitoring the movement of suspended cohesive sediments is highly
desirable in both estuaries and lakes. When modelling cohesive sediment transport and mass
settling fluxes, the settling speed of the suspended matter is a key parameter. In contrast to
purely non-cohesive sandy sediments, mud can flocculate and this poses a serious
complication to the modelling of sediment pathways. As flocs grow in size they become more
porous and significantly less dense, but their settling speeds continue to rise due to a Stokes’
Law relationship.
Much research has been conducted on the flocculation characteristics of suspended
muddy sediments in saline/brackish tidal conditions, where electrostatic particle bonding can
occur. However very little is known about freshwater floc dynamics. This is primarily due to
flocs being extremely delicate entities and are thus very difficult to observe in situ. This paper
primarily describes a recently developed, portable, low intrusive instrument INSSEV_LF,
which permits the direct, in situ measurement of both floc size (D) and settling velocity (Ws),
simultaneously.
Examples of floc spectra observed from three different environments within the USA are
presented and compared. The first site was the turbidity maximum zone in San Francisco Bay,
where the suspended solids concentration (SSC) was 170 mg.l-1 and many low
density macroflocs up to 400 μm in diameter, settling at speeds of 4-8 mm.s-1 were
observed.
The second location was the shallow (1.7 m mean depth), freshwater environment of Lake
Apopka in Florida. It is highly eutrophic, and demonstrates a turbid SSC of 750 mg.l-1
within a benthic suspension layer. These conditions resulted in D from 45 μm up
to 1,875 μm; 80% of the floc were > 160 μm (i.e. macroflocs). Present theories
for the settling of flocs rely on fractal theory of self-similarity, but this does not
appear to be applicable to the Lake Apopka flocs because they do not possess any
basic geometric unit that is the building block of higher order fractal structures.
Bioflocculation is deemed extremely important in freshwater environments such as lakes.
The Lake Apopka macroflocs (D > 160 μm) encompassed 92% of the floc mass,
and demonstrating a Wsmacro of 1.7 mm.s-1 (twice as fast as the Wsmicro), this
translated into the macroflocs contributing 96.4% of the total mass settling flux (1.5
g.m-2s-1).
Lake Tahoe, which crosses California and Nevada, was the final study location. With a
maximum depth of 501 m, it is the second deepest freshwater lake in the USA. However it
significantly less turbid than Lake Apopka, with SSC rarely exceeding 10 mg.l-1
during the floc surveys. At depths of both 5 m and 450 m, the SSC was 9.6 and 3
mg.l-1, respectively. Flocs at both depths exhibited Ws of 2-5 mm.s-1. Whereas at a
depth of 35 m (Fig. 3c), the SCC was 6.8 mg.l-1 and the flocs fell comparatively
slowly (Ws of 0.03-2.4 mm s-1) which suggest that the floc population at 35 m will
have a long residence time in the water column, thus impairing long term light
penetration. Interestingly, the MSF at 450 m was 10.8 mg.m-2s-1, which was
double the flux measured at 35 m, even though the deep water was only half as
turbid.
This comparison indicates that the size and settling velocity of individual flocs can be
measured simultaneously with the video INSSEV_LF instrument within SSC of several
g.l-1. This floc data is of great importance for accurate numerical sediment transport model
calibration in estuaries (e.g. San Francisco Bay), whilst also equally valuable for addressing
environmental problems (e.g. water quality issues) in freshwater environments such as Lake
Tahoe and Lake Apopka. |
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