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Contacts:
Caroline Ulses
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Suspended
sediment transport in the Gulf of Lions (NW Mediterranean):
Impact of extreme storms and floods
C.
Ulses, C.
Estournel, X. Durrieu de Madron and
A. Palanques, (CSR 2008)
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In
situ observations were combined with 3D modeling to gain understanding
of and to quantify the suspended sediment transport in the Gulf of
Lions (NW Mediterranean Sea). The outputs of a hydrodynamic –
sediment transport coupled model were compared to near-bottom current
and suspended sediment concentration measurements collected at the
head of seven submarine canyons and at a shallow shelf site, over
a 6-month period (November 2003 – May 2004). The comparisons
provide a reasonable validation of the model that reproduces the observed
spatial and time variations. The study period was marked by an unusual
occurrence of marine storms and high river inputs. The major water
and sediment discharges were supplied by the Rhone, the largest Mediterranean
river, during an exceptional flood accompanying a severe marine storm
in early December 2003. A second major storm, with moderate flooding,
occurred in February 2004. The estimate of river input during the
studied period was 5.9 Mt. Our study reveals (i) that most of the
particulate matter delivered by the Rhone was entrapped on the prodelta,
and (ii) that marine storms played a crucial role on the sediment
dispersal on the shelf and the off-shelf export. The marine storms
occurring in early December 2003 and late February 2004 resuspended
a very large amount of shelf sediment (> 8 Mt). Erosion was controlled
by waves on the inner shelf and by energetic currents on the outer
shelf. Sediment deposition took place in the middle part of the shelf,
between 50 and 100 m depth. Resuspended sediments and river-borne
particles were transported to the southwestern end of the shelf by
a cyclonic circulation induced by these onshore winds and exported
towards the Catalan shelf and into the Cap de Creus Canyon which incises
the slope close to the shore. Export taking place mostly during marine
storms was estimated to reach 9.1 Mt during the study period.
http://dx.doi.org/10.1016/j.csr.2008.01.015
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Effect
of stratification due to suspended sand on velocity and concentration
distribution in unidirectional flows
Herrmann,
M. J., Madsen,
O. S., (JGR 2007)
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Sediment-induced
stratification effects on velocity profiles and sediment concentration
distribution in a steady, uniform turbulent flow are examined in this
paper. The early work concerning sediment stratification relates this
to the von Karman constant's variability. Subsequent attempts to account
for stratification were based on the stratified flow analogy, introducing
the parameters a and b,
whose values were assumed to be those obtained for thermally stratified
flows. Following these investigators, we assume stratification effects
to be expressed through these parameters. We solve the governing equations
for velocity and sediment concentration for a parabolic neutral eddy
viscosity model. Analytically closed-form solutions are obtained.
We run our model against experimental data to obtain the optimal set
[a, b]. For
neutral conditions, b= 0 by definition,
and we obtain a = 1. For stratified conditions
we obtain [a = 0.8, b=
4.0]. This is the first time both a and
b have been obtained from sediment-laden
flow observations. Accounting for stratification improves the prediction
of velocity and concentration. For predictive purposes, we need to
know the movable bed roughness and the reference concentration. Analyses
of experimental data sets provide predictive relationships for these
in terms of sediment and flow parameters. doi:10.1029/2006JC003569 |
