6. Discussion
It has been found that the multi-aerator simulation cannot be expected to converge much better than 1 maximum relative error of velocity. The inability to
converge may be associated with chaotic interactions between so many jets of high velocity water. Certainly convergence would have improved if the simulation mesh
had not been restricted by finite computer resources. In a single paddlewheel simulation it was found that 27-node quadratic elements provide faster and better
Table 11 Near bottom observations compared with the simulation
a
Simulation Observed
Difference Stake
Direction Simulation
Discrepancy °
° ms
° ms
ms 0.028
0.063 0.035
− 121
− 233
− 112
South of 1 −
84 −
233 −
150 −
0.011 1
0.032 0.043
− 0.002
South of 2 −
82 0.107
− 78
4 0.105
0.148 0.091
2 13
− 0.058
− 69
− 81
0.090 −
102 South of 3
5 −
106 −
0.003 0.087
0.042 −
99 −
111 3
− 12
0.125 0.167
0.141 0.104
0.037 −
115 55
170 East of 4
0.086 0.152
0.066 −
102 −
193 −
91 4
0.069 −
17 West of 6
− 19
0.095 −
2 0.027
6 −
19 −
32 −
13 0.032
0.082 0.115
0.002 −
54 −
34 7
20 0.072
0.074 0.023
− 47
− 33
West of 7 13
0.073 0.096
− 24
− 78
− 54
− 0.068
North of 8 0.022
0.091 0.010
0.050 −
0.040 −
44 −
105 −
61 8
9 −
5 −
165 −
160 0.052
0.084 0.032
− 170
− 167
3 0.086
0.145 0.058
West of 9 West of 11
0.071 0.104
− 22
− 14
7 0.176
11 0.101
0.168 0.067
− 15
− 16
− 21
− 34
− 13
0.022 0.056
West of 12 0.035
0.003 −
31 −
37 12
− 7
0.025 0.028
0.010 0.006
0.004 −
122 −
95 26
13 0.011
0.008 0.003
− 101
− 145
− 44
East of 13 −
17 −
185 −
167 −
0.005 West of 14
0.088 0.093
0.165 0.123
0.041 172
174 2
14 0.100
− 16
16 −
12 0.098
4 −
0.003 West of 16
− 7
− 7
0.091 0.127
0.218 0.004
13 −
20 West of 17
− 33
0.035 0.039
17 0.034
0.006 8
− 21
− 29
0.028 19
0.101 0.154
0.053 164
170 7
177 170
− 6
0.004 0.097
West of 19 0.093
− 0.056
31 −
3 21
− 34
0.087 0.031
0.045 0.135
− 0.090
7 −
12 −
18 West of 21
− 0.022
56 West of 22
33 0.048
− 23
0.027 22
0.030 −
19 42
61 0.037
− 0.007
a
Many observations were taken about 5 m away from the fixed survey stakes. These are denoted with a prefix indicating the direction of offset, i.e. ‘South of 1’.
Fig. 16. Near bottom speed correlation of simulations and observations.
convergence, and apparently more accurate results with nearly the same memory usage as the nominal simulation scheme.
In spite of imperfect convergence the nominal linear brick meshing scheme resulted in simulated velocities comparable to those observed 200 mm above the
bottom of Pond X. This largely verifies the vertical gradient at the bottom, which is essential to the estimation of benthic shear stress.
The ‘real-world’ simulation of shear stress and the observed particle size form a sensible pattern when conjugate-pairs are plotted on the non-dimensional Shields
diagram. Material sampled from locations known to experience scouring plot directly on the incipient motion curve, while samples from sites subject to sediment
deposition plot far below the Shields curve. The shear stress and particle size from the sampling location closest to the centre of the pond quantify a state of sediment
condition which is further below the curve than any other place in the pond. The deepest drifts of anaerobic sludge flocs were observed to form at the same location
with each production cycle of the pond. Sedimentation was generally greater at sites where the benthic shear stress and particle size plot farther below the Shields curve.
It is expected that an advection – diffusion analysis would confirm the spatial variability detected with the sediment traps described in Peterson 1999a, although
that is beyond the scope of the present research.
Perhaps the most important application of the present research is to identify where benthic shear stress exceeds 0.03 Nm
2
, where silt particles would be scoured.
It has now been demonstrated that the extent of silt and sand scouring may be determined with more confidence than lower stress zones. Portions of pond bottom
found to experience such high shear stress should be physically armored to prevent scouring of silt and sand. Scouring of these materials would otherwise be trans-
ported via a ‘conveyor-belt’ process, to fall out on surfaces experiencing less shear stress. Sedimentation forms an anaerobic sludge as organic matter is trapped in soil
interstices.
7. Conclusion
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