Abstract:
Although rates of bed-form growth for steady flows (Nikora and Hicks 1997), have been clarified, practical
implications and models accessible to the engineer remain to be elaborated. For example, how and at what rates
bed forms change for increasing and decreasing flows remains to be quantified. There has not been much
progress since Julien and Klaassen 1995 in defining a relationship between flow and bed-form characteristics.
Sediment transport engineers in the current era have a very good idea of the size and shape of a particular dune
at various discharges and sediment types. But very little is known about the time it takes to change the dune
when the flow was to experience an increase or decrease. Previous research in this specific area was done by
(J.R.L. Allen, 1976) who did an earlier model for dune time-lag in periodically varying unidirectional flows.
The research undertook measurements of a river and data over discharge and dune wavelength over the year.
This data was then computed on a monthly basis. Model showed that hydrograph shape could substantially
influence dune behaviour in unsteady flows. For the same flow period and extreme discharge values, a reduction
in the relative duration of the high-water stages causes an increase in the phase differences between dune
dimensions and flow, and an increase in the dimensions averaged over the flow cycle as compared with the
similarly averaged dimensions given no lag.
The relative range of dimensions over the flow cycle is little affected. This research is mainly about how the bed
form reacts to the change in flow and specifically the time it requires for a specific bed form to adopt its new
bed form in regards to increase/decrease in flow. This research takes an experimental form to develop a
stochastic modal for the time required for the change in bed-form morphology in relation to the change in flow.
This includes the dune shape and height. The experimental analysis is in a flume with controlled sediment
type/density/size, water depth and also the flow rate. The analysis is for the flow in a uni-direction. The depth of
the dunes, shape and the velocity of the flow is measured by an ADV, and analyzed later using matlab to include
a 3D representation and analysis. Through which the temporal and special changes in bed form due to change in
flow is made clear and presented herein.
