Geoscience Reference
In-Depth Information
2.4 PIV Measurements and Granular Transport
Seed-wave generation via a scour-deposition wave arising with boundary-layer
redevelopment downstream of a bed perturbation is the most widely considered
mechanism of wave generation due to granular motions (e.g. Cornish
1901
; Inglis
1949
; Raudkivi
1963
,
1966
; Smith
1970
; McLean and Smith
1986
; Venditti et al.
2005a
). Such an initial bed perturbation could arise through several means, includ-
ing a random sediment pileup, the impacts of turbulent events on the bed (e.g.
Williams and Kemp
1971
; Gyr and Schmid
1989
; Best
1992
), or shear-wave
interactions (Gyr and Kinzelbach
2004
). The above-described PIV measurements
of bedwave formation were able to be used to test this hypothesised generation
mechanism, where edge-detection of the bed surface in recorded PIV images can be
used to provide high-frequency estimates of bed levels at sub-grain-size accuracy
simultaneously across the measurement window (e.g. Schlicke et al.
2005
; Coleman
et al.
2008b
; Younkin and Hill
2009
).
The PIV results suggest that the seed waves from which both ripples and dunes
develop are generated on planar mobile sediment beds in a two-stage process
(Coleman and Nikora
2009
). The first stage comprises the motion of random
sediment patches. In the second stage (Fig.
7
), interactions of the moving patches
result in a bed disturbance that exceeds a critical height
H
c
and interrupts the bed-
load layer, with this disturbance then stabilising and growing and generating quasi-
regular seed waves successively downstream via a scour-deposition wave. The
stage-one sediment patches reflect the passage of sediment-transport events caused
by attached eddies. These eddy-transport events propagate at speeds that are
proportional to their size and less than overhead eddy convection velocities, but
potentially larger than local average fluid and sediment velocities. The second-stage
scour-deposition wave arises from the requirement of sediment mass conservation
and the sediment-transport and bed-stress (
t
b
) distributions downstream of a bed
perturbation. This proposed two-stage generation mechanism is potentially valid for
fully turbulent, hydraulically smooth, and rough bed flows of small to large sediment
transport rates. It is furthermore postulated to be valid for laminar flows, although
the critical disturbances leading to seed-wave generation arise through bed discon-
tinuities, and not eddy-based sediment-transport events. The identified generation
mechanism can explain the observed similar scaling of alluvial, closed-conduit,
lightweight-sediment, and laminar-flow seed waves, with preferred lengths
O
ΒΌ
(130) grain diameters (Coleman and Nikora
2009
).
3 Bed-Surface Characterisation
Although identifying and quantifying the parameters that adequately characterise
a field of bedforms is fundamental to analyses of bed topography and its inter-
relations with flow and sediment transport, even for the visually regular bedform
