Geoscience Reference
In-Depth Information
2.1 Turbulent Fluid Motions and a Viscous-Fluid Flume
As turbulence is ubiquitous in natural water flows, it is reasonable to assume
that turbulence is closely associated with fluvial bedforms (e.g. Velikanov
1955
;
Kondrat'ev et al.
1959
; Jackson
1976
). Yalin (
1992
) ascribes the formation of
alluvial dunes and bars to bursting processes associated with the turbulent nature
of the flow. Raudkivi (
1997
) also proposes that the initiation of ripples can probably
be ascribed to turbulent bursting processes that exhibit certain orders. Yalin and da
Silva (
2001
) note that no periodic bedforms, including ripples and dunes, occur in
laminar flows, thereby inferring the crucial role of turbulence in bedform origina-
tion. Challenges to the concept that bedforms are turbulence generated include that
the principally random nature of turbulent events in time and space contrasts with
the highly structured nature of bedforms (e.g. Liu
1957
). Furthermore, turbulence-
based theories do not satisfactorily explain the observed scaling of the sand
waves initially formed on the bed principally with sediment size rather than flow
characteristics (e.g. Coleman and Melville
1996
).
The view that ripples and dunes can form only in turbulent flow is attributed by
Yalin (
1972
) primarily to the experimental work of Tison (
1949
), who carried out a
series of experiments to determine whether ripplemarks can be generated in
uniform laminar flow. Yalin comments, however, that the experiments of Tison
(
1949
) cannot be regarded as exhaustive. Johnson (
1916
) also notes the work of de
Candolle (
1883
), who produced ripple marks artificially by experimenting, not only
with sand and various substances in powdered form covered by water, but also with
liquids of varying viscosity, covered with water and other liquids. De Candolle
(
1883
) was able to make ripples in sand with a variety of fluids, but with olive oil it
was found to be impossible (Darwin
1883
).
In order to test whether bedform-generation is driven by flow turbulence, it was
decided to investigate whether bedforms could be generated for laminar flows over
planar sediment beds. To achieve this goal, a glass-walled tilting viscous-fluid
channel was constructed measuring 0.3 m
2.55 m (long) with
a header tank at the inlet and a collection bay at the outlet (Fig.
1a
). The centrifugal
pump circulating the flow was controlled using a variable speed drive, with vertical
sluice gates installed at the upstream and downstream ends of the channel to aid
control of flow within the flume (Coleman et al.
1998
; Coleman and Eling
2000
). A
recess in the wooden channel base measuring 0.025 m
0.1 m (wide)
1.3 m
(long) was filled with sediment to create an erodible bed section. Sediment was not
recirculated. The fluid used in the experiments was Shell Tellus Grade 32 hydraulic
oil, with the bed comprised of respective uniform sediments of median sizes
d
0.1 m (wide)
0.28-1.6 mm. For the temperature range of the experiments, the fluid density
could be taken to be essentially constant at
¼
870 kg/m
3
. The kinematic viscos-
r
¼
ity,
, of the oil was available in chart form as a function of temperature, with
n
¼
0.8-1
10
4
m
2
/s for the experiments undertaken. The particular challenge in
the experiments lay in ensuring that the flows would entrain and move the sedi-
ments whilst retaining a laminar nature.
n
