Geoscience Reference
In-Depth Information
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.01
0.1
1
y
/
h
(-)
E1
E0
E2
E3
E1D
E2D
E3D
Fig. 26 Variation of the ratio
M
IV
/
Mv
II
The interpretation of Fig.
28
proves itself ambiguous since mobile bed data does
not go as near the boundary as fixed bed data. However, it appears that the decrease
of the flux near the wall is more prominent in the fixed bed experimental tests. If this
interpretation is correct, the presence of sediments moving as bed load decreases
the skewness of
v
, by damping its negative fluctuations, as in the case of smoother
boundaries (Krogstad and Antonia
1999
). Although it seems that the differences in
the
uv
max
plots (cf. Fig.
27
) are reflected by the third order moments, the exact
mechanism by which the energy flux is changed remains to be explained.
3.5 Discussion and Conclusion of the Characterization of the
Bursting Cycle Over Mobile and Immobile Boundaries
During the presentation of the experimental tests, it was observed that the coarser
size fractions in Tests E1 to E3 should not appear on the bed load since the
predicted ratio t
*84
/t
*c84
was below 1.0 (0.67, 0.75 and 0.89 for Tests E1, E2
and E3, respectively). Yet, particles coarser than the
d
84
of the initial mixture
were found in nonnegligible quantities. This fact alone could be interpreted as the
result of the application of an inappropriate hiding-exposure factor and that the
mobility of the larger grains was underestimated. If this interpretation was true, an
armored bed would be difficult to attain given the high mobility of the largest
particles.
It was observed that an armored bed was indeed attainable. The final values of
the ratio t
*84
/t
*c84
were 0.66, 0.70, and 0.89, the same order of magnitude as for
the mobile bed tests. It can be concluded that the grain mobility of the largest
grains did not remain constant during the armoring process. A similar finding was
presented by Macauley and Pender (
1999
) and observed by many other research-
ers. The introduction of a fine sediment mixture on an armored bed induces the
