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| Titel |
Vertical distribution of dissolved oxygen in gravel-bed streams: impacts of sand transport |
| VerfasserIn |
Rui M. L. Ferreira, Lara M. Ferreira, Ana Margarida Ricardo, Mário J. Franca |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056116
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| Zusammenfassung |
| Increased sediment production resulting from the erosion of unprotected soil in a
catchment, following deforestation or forest fires, may result in important impacts in the
morphodynamics and hydrodynamics of mountain rivers. Sediment overfeeding
can lead to the degradation of salmonids’ spawning sites, both directly and in the
short-term, reducing the porosity of spawning sites, and indirectly and in the long-term,
disturbing fluxes of oxygen and nutrients to the egg pockets and reducing the drainage
efficiency.
The objective of this study is to evaluate the impacts of sand transport on flow
hydrodynamics and on the vertical profile of dissolved oxygen (DO) in gravel-bed
rivers.
To accomplish the proposed objective, conditions similar to those found in nature, in what
concerns flow and bed material, were reproduced in a laboratory flume and a theoretical
model of the vertical distribution of DO was developed. Three laboratory tests simulated (i)
an undisturbed openwork gravel bed; (ii) a framework-supported gravel bed with a sand
matrix and, (iii) a framework-supported gravel bed with imposed sand transport at
near-capacity conditions.
Instantaneous velocity maps were obtained with Particle Image Velocimetry (PIV).
Velocity data was sampled into instantaneous velocity profiles distributed over a measuring
area larger than the characteristic wavelength of the bed topography variations. Remaining
flow variables were derived from the instantaneous vertical and horizontal velocities. Due to
high spatial variability of the flow characteristics, double-averaged – in time and space –
methods (DAM) were applied for the definition and quantification of mean flow variables.
Special attention was given to the laboratory characterization of the mean (time- and
space-averaged) flow variables relevant to quantify the turbulent mass diffusivity and the
vertical fluxes of DO, namely vertical and longitudinal velocities and Reynolds shear
stresses.
No measurements of DO were taken in the laboratory. Instead, a new model for the
vertical distribution of DO was developed and solved numerically. The model features
conservation equations for DO, dissolved nitrogen, gaseous oxygen, gaseous nitrogen and
number density of gas bubbles. Closure equations for the flux of oxygen and nitrogen, for
saturation concentrations of dissolved species and for biochemical oxygen demand were
adapted from the available literature. The results of the numerical simulations allowed for the
calculation of vertical profiles of the conserved variables, namely DO but also dissolved
nitrogen, gaseous oxygen and nitrogen and number-density of bubbles. The profiles obtained
for the reference situation (openwork gravel) and for the disturbed beds (with sand transport)
were then compared and discussed.
It was found that mean Reynolds stresses were unaffected by reduced bed porosity or
sediment transport even in the near-bed region. Sediment transport affected the vertical
distribution of the longitudinal velocity and eddy viscosity was found to decrease in most of
the flow depth. Numerical simulations of the vertical distribution of DO and of gaseous
species were carried out under the hydrodynamic and sedimentological conditions of the
flume work. A sensitivity analysis to the values of the spatial mass diffusivity was carried out.
The results of the numerical simulations show that, in the presence of sand transport,
near-bed concentrations of DO are strongly reduced, in some cases down to 2 mg/l,
mostly due to the reduction of downward diffusive fluxes of gaseous oxygen and
DO. This may represent an important negative impact to salmonid species in the
long-term.
The study was partly funded by the Portuguese Foundation for Science and Technology,
project FCT/PTDC/12040/2006. |
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