tongues in coupes logged in 1987/88.
The results of the measurement of the sediment tongues generated on these tracks are shown in Appendix 3.4 and summarized in Table 3.7.
R A IN F A L L SIZ D IM E N T M O V E M E N T
DATE Rainfall Max. daily Max. rain No.of new Total Max.length Tongue Total
tongues days * rainfall ^ intensity tongues sediment sediment extension extended
(inm/hr)^ generated^ tongues tongue (m)~ ( max.) 30.1.89 13 Water bars built on snig track number (1).
13.2.89 9 Water bars built on snig track number (2).
22.2.89 6 31 mm 27.8 Nil Nil Nil Nil Nil
17.3.89 7 26 mm 21.9 Nil Nil Nil Nil Nil
7.4.89 8 245 mm 66.6 2 2 11.4 Nil Nil
29.4.89 5 73 mm 48.3 5 7 11.0 6 m 2
17.6.89 15 50 mm 36.5 Nil 7 Nil Nil Nil
25.10.89 41 46 mm 26.8 Nil 7 Nil Nil Nil
1) Number of days between inspection and during which rain fell. 2) Maximum daily rainfall at Timbillica between inspections . 3) Maximum rainfall intensity at catchment 2.
4) The number of tongues generated at water bars between inspections. 5) The maximum length of sediment tongue generated between inspections.
6) Maximum extension of sediment tongues.____________________________________________________ Table 3.7. The summary of inspection date, storm events and sediment tongue generation on two snig tracks.
Six inspections were made. Sediment tongues were generated at seven of twenty two snig track drains.
At the First and second inspections, after notification of a rainfall of 31 mm (27.8 mm/hr) and 25.5 mm (21.86 mm/hr) respectively in one day at Timbillica, it was observed that there was some sediment deposition at the water bar but there were no sediment tongues. At the third inspection after rainfall readings of 245 mm (66.6 mm/hr) at Timbillica,
two sediment tongues, 8 and 11.4 metres in length, had been generated . At the fourth inspection after daily rainfall readings of 73 mm (48.3 mm/hr) at Timbillica, two sediment tongues measured at the third inspection had reached a drainage line with 'cut off extensions of 5 metres and 6 metres respectively. Five new sediment tongues were noted at this inspection. There was no extension of sediment tongues at the fifth inspections. The last inspection was on 25.10.89 and observations indicated that snig tracks had stabilized in terms of sediment production after eight months. Therefore in support of the finding in monitoring the extension of snig track an intensity of about 50 mm/ hr causes the formation of sediment tongues and further extension of formed tongues.
3.5.5. Particle size distribution of sediment tongues
The particle size distributions along the three randomly selected tongues are shown in Figures 3.12, 3.13 and 3.14.
100 Distance 2.6 m 5.2 m 7.8 m 10.4 m 13 m Retainer
500 urn 250 urn 125 urn 63 urn
1.0 mm
Sieve num ber
100 - |
Distance
O) 40 -
1.0 mm 500 urn 250 urn 125 urn 63 urn Retainer S ie v e n u m b e r
Figure 3.13. Particle size distribution along a sediment tongue in coupe 1/537 (Figure 1.1).
100 -I
Distance
ö) 40 -
1.0 mm 500 urn 250 urn 125 urn 63 urn Retainer S ie v e n u m b e r
The material in the sediment tongues was mostly coarse material, that is retained on the 1.00 mm fraction sieve.
It was hypothesized that sediment tongues would increase in length as a consequence of succeeding intense rainfall events, as finer particles rather than coarser moved down the tongues. It would follow that there would be a higher percentage of coarse material at the beginning of the sediment tongue than at the tip.
A student "t" test analysis on the particle size distribution of the five samples taken along each sediment tongue showed that the particle size distribution between samples was insignificant for all three sediment tongues.
It is inferred that sediment deposition in the sediment tongues does not follow an ideal pattern of coarser material dropping out first as the water velocity reduces with fine material carried furthest. It is suggested that scientific studies of deposition at obstructions could be undertaken in a laboratory flume with controlled flow and simulated rainfall. It would be postulated that obstructions reduce the velocity of the flow as they act as dams, with the effect of sediment deposition at successive obstructions with succeeding storm events. It would also be postulated that there would be sediment deposition between obstructions, as the water velocity is reduced up stream of obstructions, as a consequence of spreading of the flow and reduced hydraulic gradients due to the damming. It would be tested that water flowing over the obstruction would carry the full range of particle sizes to the next obstructions. These postulates imply that whenever erosion occurs on a snig track, existing tongues would widen and elongate until the tip reached the next obstruction.
The particle size distribution is seen as having important relevance in relating the results of this study to other logging areas, for example on other than granite soils and in pine plantations where water velocities may need to be reduced more to effect deposition and where obstructions may be very different in both size and frequency of occurrence. It is suggested therefore that the methodology developed for this study, that is inspections of the perimeters of logging coupes, be extended to such other logging areas.
3.5.6. V olum e o f sedim ent tongues
The width of the tongue and the depth of deposition at the centre of the tongues were measured at six points, as shown in Figure 3.15, along seven tongues selected randomly from the 30 sediment tongues.
Tongue tip Tongue begining
Figure 3.15. Position of depth measurement points along the sediment tongue.
The end-area formula was used to calculate the volume, that is for example.