Figure 41-1, from (38)
The mechanisms include:
The parameter z is related to the roughness of the bed. From the shear velocity, the stress can be calculated.
The other relevant relationship is not as well constrain but expresses the flux of sediment in the direction of flow as a function of the excess stress.
This figure depicts the relationship of erosion to applied stress in the absence of organisms and the ways in which the transport is modified by the four processes listed above. Implicit in these plots is that as applied stress becomes very large, biological effects play a relatively less important role. An obvious limitation of this approach is that it is static. For example one can imagine a dynamic balance between the rate of track-making by organisms and the rate of track destruction due to applied stress--effects that can not be incorporated in the structure of this figure.
Within this general framework, we can consider some specific examples from HEBBLE-sponsored experimental work conducted in the Friday Harbor flume. A flume is an experimental appartus in which the flow of fluid through a channel can be very precisely controlled. In it instrumented so as to characterize the velocity profile (thus u* and z are determined). During experimental runs, the motion of the bed on the floor of the flume is visually monitored.
The Nowell et al. paper (34) describes three kinds of experiments:
Figure 41-3, from (34). The individual tracks are ~2 mm wide and have vertical relief of ~1 mm.
The critical shear stress is then again determined. Comparing the two runs:
Figure 41-4, from (34)
These are the two velocity profiles, the control run and the run post-modification, just at the onset of motion.
From the data for the untracked run: z is about 0.6 µm) and u* is about 1.7 cm/sec. The value of z is consistent with the bed being hydrodynamically smooth (HSF).
From the run post-tracking: the bed is somewhat rougher but the flow is still HSF. The shear velocity required to initiate motion is about 1.4 cm/sec. Note that the roughness is not uniform--the visual protrusions are about 1 mm high but widely spaced; the effect on the roughness scale is to increase it from .018 cm to about .036 cm.
Figure 41-5. The mound is ~4 mm high, the individual fecal pellets making up the mound, ~100 µm.
The conditions of initial motion are then determined. In this situation "motion" is an imprecise term; Nowell et al. distinguish rocking of the mound, collapse of the mounds and removal of mounds in terms of required shear velocity:
|64 µm silt||121 µm sand|
|rocking||1.20 cm/sec||1.52 cm/sec|
|collapse||1.60 cm/sec||1.93 cm/sec|
|removal||1.87 cm/sec||2.18 cm/sec|
Figure 41-6, from (34). The tracks are approximately 1 mm wide; the pellet at the right hand end of the track is ~9 mm long.
At 1 cm/sec shear velocity these travel about 10 cm "on the fly". Pellets lying still on the bed go into motion at a shear velocity of 1.8 cm/sec; however at lower shear velocity newly ejected particles will remain in motion. The net effect is to create tracks which are more easily eroded than for the untracked sediment.
Figure 41-7, from (39)
Figure 41-8, from (39)
Figure 41-9, from (39)
Three regions are seen in the stress map:
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