Biological Effects on Sediment Transport

Figure 41-1, from (38)

The mechanisms include:

• the wake zone of protrusions can create zone of increased and decreased stress and concomitant changes in the stability of the bed
• burrowing organisms can expose particles to the flow (and secondarily give them momentum)
• fecal pellets repackage sediment in forms with increased adhesion. These pellets also affect the roughness of the bed.
• some organisms directly effect particle momentum

Eq 41-1:

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.

Eq 41-2:

Figure 41-2

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:

1. entrainment of tracked sediments. The sediment is of 118 µm grain size, wet sieved to remove macrofauna and animal tubes. The relief is smoothed to less than 0.05 cm relief. Fluid flow through the flume is increased until motion occurs to establish the control. Fluid flow is reduced to zero, a small motile bivalve (transenella tantilla) is added to the sediment and the organism modifies the bed for 15 hours
   

   

   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.

2. fecal material from/on a coarse bed. As an example of these experiments, one run was conducted with Hobsonia florida, a polychaete, in two different sediments, a fine sand and a sandy silt (both non-cohesive in the absence of the polychaete). In these experiments the bed is smoothed, then the polychaete is introduced and allowed to feed and defecate for 8 hours. Some of the typical fecal mounds produced:
   

   

   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:

   

   Shear Velocity Required to Induce Mound Movement

   	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

   

3. fecal pellets ejected on a cohesive sediment (18.2 µm grain size) of high shear strength. A polychaete Amphicteis scaphobranchiata ejects pellets approximately 1 mm diameter x 9 mm long:
   

   

   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:

• A: an area of enhanced erosion due to applied stress, extended 1-5 tube diameters around the tube
• B: upstream of the tube, an area of reduced erosion in the vicinity of the divergence in flow
• C: downstream of the tube in two parallel bands, depositional ridges