The shape of the artificial bed roughness (Fig. 1,b) was determined 
by experiments. A large quantity of the artificial plastic sediment used 
in concentration measurements was put into the smooth-bottomed flume. 
The flume was oscillated at various amplitudes and periods covering the 
range of flow conditions to be studied. After the flume was oscillated 
at a constant rate for a period long enough to establish a natural bed 
shape, the flume was stopped and the bed dunes were measured. The bed 
shape was found to be approximately sinusoidal in the cross section under 
all flow conditions. The mean wavelength of the bed shape was 5.5 inches 
with a range of 4.5 to 8.0 inches; the mean wavelength-to-depth ratio was 
8.0 with a range of 5.5 to 12. The fixed artificial roughness used approx- 
imates this shape. The artificial dunes were constructed of wood and 
fastened to a flexible sheet of plastic. Natural sediment with a mean 
diameter of 0.3 millimeter was glued to both the dunes and plastic. The 
plastic sheet was fixed to the flume bottom, covering the central 6.33 
Hee OLe ther anc. 
The asymmetric end roughnesses shown in Figure 1(c) were used to 
eliminate secondary currents in the central measuring section of the 
flume. Proper placement of the asymmetric roughness elements depends on 
the flow conditions of the flume. The optimum placement of the roughness 
elements for a given flow condition was determined by dropping potassium 
dichromate crystals through holes in the horizontal wave suppressent board 
and observing the movement of the dye streaks. When no transverse move- 
ment of the dye streaks in the central part of the flume were observed, 
the roughness elements were considered to be in the optimum location. 
A lightweight, black plastic material with specific gravity of 1.25, 
which had been crushed and sieved, was used as artificial sediment in the 
experiments. The grain diameter of the sediments was uniform, bracketed 
by two consecutive sieve sizes of approximately the same diameter as the 
natural sediment glued to the flume bed. Only a small quantity of the 
sediment was used in the flume during an experiment in order to limit 
the deposition of sediment which would alter the flume bottom geometry. 
When the sediment was first put into the flume it was found that air 
bubbles adhered to the sediment particles, thereby changing its settling 
velocity. To eliminate this buoyancy effect, deaerated water was used. 
The water was deaerated in a 60-cubic foot-capacity tank located on the 
wall of the laboratory at an elevation above the swing flume, heated by 
a 5-kilowatt immersion heater to a temperature of 90° Fahrenheit, and 
then cooled to room temperature. The deaerated water was transported to 
the swing flume by gravity through a hose to reduce air entrainment. 
The optical concentration meter used in the experiments was developed 
by Das (1971) for measuring tm sttu concentrations in laboratory flumes. 
The equipment consists of a light source, a beam collimator, a receiving 
unit, and necessary recording units. A collimated beam of light, 8.5- 
millimeter average diameter, is projected through the glass walls of the 
flume to a duo-photodiode mounted on the opposite side of the flume. 
