l]I. Sediment Transport
69
2.6.2
Reference Level in Case of Bedforms
Basically, the flow over a boundary covered with bedforms (dunes) is nonuniform
showing acceleration effects at the upsloping sections and deceleration effects at
the downsloping sections. The bed-load particles are transported close to the bed
by rolling, sliding and saltating at the upsloping sections of the dunes. Arriving
at the top of the dune, the majority of the bed- load particles jump over the edge
and roll down the leeside slope of the dune towards the dune trough where they
are buried waiting for a new [transport] cycle. The suspended load particles are
entrained from the bed-load layer developing at the upsloping section. The
entrainment rate is maximum at the dune top (large velocities). Above the dune
trough the suspended particles are transported further upwards by turbulence
mixing produced in the shear layers of the vortex generated in the dune trough. A
detailed mathematical representation of the afore-mentioned processes requires
the application of a sophisticated model, e.g. the K-E model of Rodi (1980), in
combination with a convection-diffusion model for the suspended sediment
particles. The horizontal grid size should be much smaller than the dune length
(Ax <~ L), while the reference level should be applied at the upper edge of the bedload layer (a (< A). To get a first understanding of the phenomena involved, a
relatively simple two-dimensional vertical model of the Delft Hydraulics
Laboratory (Van Rijn, 1985) was used to compute the concentration profiles for
the flow over dunes.
1.0
o7
\
oot |
o
05
~,
"JO °J
2
4
, I
'II
if!
8"~ "z
2
4
6 810 *~
2
4
6 810 °
1. dlrnQn$1onl(zSs C~C~ntr~.l~,~.
Ca
Fig. 9 : Influence of errors in hydraulic parameters on concentration profile.
69
2.6.2
Reference Level in Case of Bedforms
Basically, the flow over a boundary covered with bedforms (dunes) is nonuniform
showing acceleration effects at the upsloping sections and deceleration effects at
the downsloping sections. The bed-load particles are transported close to the bed
by rolling, sliding and saltating at the upsloping sections of the dunes. Arriving
at the top of the dune, the majority of the bed- load particles jump over the edge
and roll down the leeside slope of the dune towards the dune trough where they
are buried waiting for a new [transport] cycle. The suspended load particles are
entrained from the bed-load layer developing at the upsloping section. The
entrainment rate is maximum at the dune top (large velocities). Above the dune
trough the suspended particles are transported further upwards by turbulence
mixing produced in the shear layers of the vortex generated in the dune trough. A
detailed mathematical representation of the afore-mentioned processes requires
the application of a sophisticated model, e.g. the K-E model of Rodi (1980), in
combination with a convection-diffusion model for the suspended sediment
particles. The horizontal grid size should be much smaller than the dune length
(Ax <~ L), while the reference level should be applied at the upper edge of the bedload layer (a (< A). To get a first understanding of the phenomena involved, a
relatively simple two-dimensional vertical model of the Delft Hydraulics
Laboratory (Van Rijn, 1985) was used to compute the concentration profiles for
the flow over dunes.
1.0
o7
\
oot |
o
05
~,
"JO °J
2
4
, I
'II
if!
8"~ "z
2
4
6 810 *~
2
4
6 810 °
1. dlrnQn$1onl(zSs C~C~ntr~.l~,~.
Ca
Fig. 9 : Influence of errors in hydraulic parameters on concentration profile.