SEDIMENT SUSPENSION AND TURBULENCE IN AN OSCILLATING FLUME 
by 
Thomas C. MacDonald 
I. INTRODUCTION 
Sediment transport by waves approaching the shore has been analyzed 
in two ways. Inshore of the breaker zone, the extremely complex flow 
patterns and turbulence resulting from the breaking waves have necessi- 
tated only a qualitative approach to sediment transport with quantita- 
tive estimates based on field measurements. Offshore of the breaker 
zone in relatively deep water, the problem of sediment transport can be 
approached in a more theoretical manner. In this zone, sediment trans- 
port studies are simplified because there is no turbulence in the flow 
field from breaking waves and the flow condition near the ocean bottom 
can be better estimated from linear wave theory. This report concerns 
sediment transport offshore of the breaker zone. 
Laboratory and field observations indicate that, as in unidirection- 
al flow, sediment transport at the ocean bottom offshore of the breaker 
zone is of two different types--bedload and suspended load. The dis- 
tinguishing feature between these two types of transport is that in 
suspended transport the entire weight of the sediment is continuously 
supported by the fluid; whereas, in bedload transport the sediment rolls, 
skips, and jumps along the bed and therefore its weight is partially 
supported by the stationary bed. For moving sediment to be supported 
by the bed means that the regime of bedload transport is contained in a 
thin layer adjacent to the stationary bed, two-grain diameters thick as 
proposed by Einstein (1950). The majority of the sediment in motion in 
this area is bedload and thus most research has concentrated on the 
turbulent boundary layer and the oscillatory bedload rate due to wave 
action (Li, 1954; Manohar, 1955; Kalkanis, 1957, 1964; Abou-Seida, 
1965). Sufficient advances in the theory of bedload movement in an 
oscillating flow have warranted studying the suspended load to deter- 
mine: (a) Approximately, what percentage of offshore movement is due 
to suspended load, i.e., a second-order approximation to total trans- 
port; and (b) if any of the now-predicted bedload is partially suspended 
load. 
This investigation develops, from an empirical approach, a method 
for predicting the distribution of suspended-sediment concentration 
based on the hydraulic flow conditions; i.e., surface wave amplitude 
and period, depth, sediment characteristics, and bottom roughness con- 
ditions. Although only one bottom roughness was studied, the resulting 
method is general enough to be extended to other roughness conditions 
by additional experimentation. The suspended distributions, when used 
in conjunction with the bedload function of Kalkanis (1964), should 
give a better approximation of the total sediment transport. 
