ACTORES EXTERNOS

The calculated incidence of suspensions in particle tracks was much lower than would have been expected from observations. Many tracks showed no suspended trajectories, while in those that did only 1 or 2 saltations might be suspended out of a total of 40-50 trajectories, occupying a much lower percentage of the time than that observed. However the presence of some suspensions, Figure 5.7, and the variable time to initial motion, Figure 5.8, showed that velocity fluctuations could modify particle tracks, but that this did not occur very often. Examination of the calculated records of particle movements show that the number of times particles leave an eddy during a trajectory is low, which limits the possible number of suspended trajectories. To increase the possible number of suspensions the number of eddies that a particle moved through would have to be increased.

Particles might stay in eddies for too long for two reasons, the scale of the eddy was wrong, that is it was too large, or the speed with which the eddy was advected by

the flow was too low. The horizontal and vertical sizes of the eddies were taken to be the corresponding Eulerian length scales, described in Section 4.2.2.4. Measurements of the vertical length scale and its variation through depth are scarce and the value of this parameter was set using the best available data, that from the River Severn (Heslop et al., 1993). More data would enable a better description of this parameter to be derived. The effect of varying this parameter can easily be examined by using a fraction of the original length scale in the calculations, as described below.

The speed with which eddies are advected by the flow might also be too low. Examination of the calculated records of particle movement also show that a particle most often moves from eddy to eddy while close to the bed. The effect of this is that one or more trajectories will occur within an eddy before the particle moves into another eddy, also when in contact with the bed. If this happens there is no mechanism for suspension of trajectories to occur. Another result of the particle moving from eddy to eddy close to the bed is that they are advected by relatively slow flows close to the bed, even if, as here, it is assumed that eddies less than half their vertical length scale from the bed are advected by the mean flow at half their vertical length scale from the bed. This is particularly significant when the importance of the burst-sweep cycle in the bedload transport of sediment is considered. In the burst-sweep cycle fluid is ejected from the region near the bed outward and sweeps in toward the bed from outer regions. The use of fluid initially coincident with the sediment particle in the region of the bed might stop these types of events occurring. Methods of altering the speed with which fluid and sediment particle diverge, in order to examine the effect this has on particle behaviour are less obvious, so no calculations trying to produce this effect were performed.

Calculations were performed with the horizontal length scale set at various fractions of the original value (see Section 4.2.2.4). This decreased the size of all the other turbulence scales. The results of these calculations are shown in Figure 5.19, in

1.0—i 0.8- 4 is 0.8 o fl o 3 0 . 4 - v 4 & 0.2- 0.0 _{• I I I I I I--- 1--- 1--- 1} 0 -1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 0 . 9 1 .0

Fraction of original length scale

I I I I I I I I l l

0 .1 0 . 2 0 . 3 0 . 4 0 . 6 0 . 6 0 . 7 0 . 8 0 . 9 1 .0

Fraction of original length scale

6-J fl o 1_V4 - 3 5 3 - § 3 ’S2- eo s_{fl .} 8 1_ i. V 0u 0.7 1.0 0.6 0.8 0.9 0 . 4 0 . 5 F r a c t i o n o f o r i g i n a l l e n g t h s c a l e 0.3 0.2 0.1 0.0 F i g u r e 5 .1 9 E f f e c t s o f v a r y i n g l e n g t h s c a l e o f t u r b u l e n c e

terms of the fraction of tracks with suspensions, length of suspended trajectories and the percentage of time spent in suspension. The fraction of tracks containing

suspended trajectories increased with decreasing length scales, that is as the eddies became smaller. The time in suspension increased with decreasing length scales and the length of suspended trajectories decreased as shorter trajectories began to contain suspensions.

Reducing the turbulent length scales, increased the number of eddies through which particles passed and increased the number of suspended trajectories. Whether the reason for the low number of suspended trajectories was incorrect length scales or problems related to using the particle tracking method close to boundaries, this shows that the technique can be modified to produce results closer to those observed.