Geoscience Reference
In-Depth Information
clarification of concepts such as bursting cycle or coherent structure (cf. e.g.,
Nelson et al.
1995
). A thorough account of the significant achievements on
this field can be found in the work of Nezu and Nakagawa (
1993
), Chaps.
7
,
8
,
9
,
10
,
11
, and
12
.
Nevertheless, and despite the conceptual progresses on the structure of tur-
bulence, the discourse about near-bed sediment transport remained primarily
dominated by considerations on mean bed shear stress and bed shear stress
distribution.
Answers to fundamental interrogations about the nature of turbulent flows
over mobile beds are yet to be obtained. Do these flows have the same turbulent
structures and follow the same scaling laws as the flows over fixed beds? Are there
important feedback mechanisms between the organization of turbulence and
the quality and quantity of the sediments moving in the bed? If so, can we observe the
echoes of such interactions on macroscopic parameters such as the mean bed shear
stress?
In this section, a comparison between these sets of data with mobile and
immobile (armored) beds is offered, in an attempt to support answers to the
previous questions. In accordance to this strategy, the differences and similarities
identified on the statistics of the bursting cycle are pointed out and discussed. It
should be highlighted that, in order to illustrate the point that the same shear
velocity,
u
*
, might be associated with different organized turbulence, each set
of mobile bed data is compared with a set of fixed bed data with the same
u
*
.
3.2
Instrumentation and Experimental Procedure
The experiments were performed on a 12 m long and 40 cm wide recirculating and
tilting flume. The flume floor was covered with a 6 cm deep sediment layer over
its entire extension except the initial 120 cm and the final 60 cm, on which tiles
witharoughupperfaceandthesamedepth were placed. The side-walls are
made of glass, enabling visualization and laser measurements. In order to achieve
and maintain uniform flow conditions, a manually operated vane was placed at
the outlet. A smooth inlet transition constitutes the upstream end. The water is
recirculated from a reservoir by means of a centrifugal pump. The sediment is
collected at the downstream end, in a 250
m
m wire-mesh cone, placed inside
the main water reservoir, and recirculated by an independent centrifugal pump.
The sediment circuit can be set to trap and dispose the sediment while still
recirculating water.
The study required five kinds of measurements: (1) instantaneous velocity; (2)
water depth and bed elevation; (3) bed texture; (4) water and sediment discharge;
and (5) grain-size distribution of bed-load and of bed-surface composition.
Two orthogonal components of the instantaneous flow velocity were measured
with a DANTEC 55X Modular Laser Doppler Anemometer composed of a
