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Smith 1970 ; Reynolds 1976 ; Engelund and Fredsøe 1982 ; de Jong 1983 , 1989 ;
Sumer and Bakioglu 1984 ; McLean 1990 ). Typically, the continuity equation for
the sediment phase is coupled with a suitable flow model to give predictions of
unstable regions in the parameter space for the linearised system, together with the
most-unstable sinusoidal-perturbation scales that are then predicted to appear in the
sediment bed. Flow models that have been adopted generally include a free water
surface and are based on potential flow (e.g. Kennedy 1963 , 1969 ; Coleman and
Fenton 2000 ), shallow-water flow (e.g. Gradowczyk 1970 ) or rotational flow (e.g.
Engelund 1970 ; Fredsøe 1974 ; Richards 1980 ; and Colombini 2004 ).
Challenges to the concept that bedforms are due to instability of the fluid-
sediment flow system include the degree to which such potential instabilities are
likely to be disrupted by highly turbulent near-bed flow layers. Yalin ( 1992 ) further
queries why the initial assumed perturbation for these approaches necessarily
extends infinitely in the direction of flow without any change in amplitude or
period. In general, these approaches also do not satisfactorily explain the observed
scaling of initial seed waves principally with sediment size rather than flow
characteristics (e.g. Coleman and Melville 1996 ).
Although instability theories primarily involve a free surface, Engelund and
Fredsøe ( 1982 ) conjecture that at low flow velocities dunes occur for closed conduits
as for open channels. Experimental investigations of bedforms in closed conduits
are presented by Ismail ( 1952 ), who describes observations of 'dunes' and larger
sand 'waves', and Nakagawa and Tsujimoto ( 1984 ), who made bed-profile mea-
surements at stages when the flow was stopped and the conduit soffit removed.
In order to investigate whether a free fluid surface acts to control subaqueous
bedform generation and growth, tests using a plexiglass mobile-bed water tunnel
were undertaken at The University of Illinois. The tunnel (Fig. 1b ) has a head tank
at one end and a pneumatic butterfly valve at the other end. The head tank maintains
pressures on the order of 2 m head along the test bed, while the valve control
enables flow to be rapidly changed to a desired rate. Water is supplied to the head
tank by laboratory pumps and returned to the laboratory sump via a downstream
tank facilitating settlement of transported sediments. The rectangular tunnel cross
section measures 0.1 m high and 0.3 m wide, where the upper surface of a 0.13-
m-thick sediment bed formed the invert of a 6-m-long test section of the tunnel for
the testing described herein. Pressure gradients along the tunnel can be monitored
using manometers attached to pressure taps on the tunnel soffit. A specially
designed mount on the soffit further enables an acoustic Doppler velocimeter
(ADV) to be used to measure the centreline velocity profile.
Twelve experiments were carried out over a range of flows for two uniform
quartz sediments of median sizes d
0.11 and 0.87 mm, respectively. For each
experiment, the 6 m length of bed was initially screeded to a constant level. The
tunnel soffit was then replaced, and the measuring systems were reinstalled prior
to the head tank and tunnel being filled with water. The downstream valve was
closed at this stage, with inflow diverted over an overflow weir in the head tank.
Pressure lines were then bled and correct operation of the measuring systems was
confirmed. To commence a run, flow was initiated over a period of seconds using
¼
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