The total length of the perimeters of the sixteen coupes inspected, Figure 1.1, was about 44 kilometres, 24 kilometres in unbumt and 20 kilometres in burnt.
Eighteen sediment tongues reached unbuffered drainage. The frequency distributions of the length of these sediment tongues in the unburnt and the burnt coupes are shown in Figures 3.2 and 3.3 respectively. The total length of unbuffered drainage line in the unburnt coupes was 13.4 km and in the burnt coupes 16.5 km.
There are too few tongues to distinguish statistically the tongues in the burnt from those in the unburnt. It is emphasized that these sediment tongues are cut off at the drainage line and would have been longer but for this.
Number of tongues = 8
Length of drainage line = 13.4 km
Figure 3.2. Frequency distribution of the lengths of 'cutoff sediment tongues reaching unbuffered drainage lines in the unbumt coupes.
Number of tongues = 10
Length of drainage line = 16.5 km
5 7 9 11 13 15 17 19 21
Length of 'cut off' sed im en t to n g u e (m )
Figure 3.3. Frequency distribution of the lengths of 'cutoff sediment tongues reaching unbuffered drainage lines in the burnt coupes.
There were no sediment tongues reaching streams or drainage lines through a buffer strip. Only six sediment tongues reached a buffer strip. The length of the sections of these sediment tongues, within the buffer strip, were 3, 5, 6, 6, 6 and 8 metres.
The frequency distributions of the lengths of sediment tongues measured at the boundary of the coupes and not reaching streams or drainage lines are shown in Figure 3.4 and 3.5 for the unbumt and the burnt coupes respectively. The total number of sediment tongues represented is 68. The eighteen sediment tongues reaching unbuffered drainage lines are not included because they were cut off at the drainage lines.
2 -
o
Number of tongues = 26 Length of drainage line = 24 km
to
3 5 7 9 11 13 15 17 19 21 23
Length of sedim ent to n g u e (m )
Figure 3.4. Frequency distribution o f the lengths o f sediment tongues at the boundary o f unbumt coupes and not reaching drainage lines.
CO
Number of tongues = 42 Length of drainage line = 20 km
L e n g th o f s e d im e n t to n g u e (m )
Figure 3.5. Frequency distribution of the lengths of sediment tongues at the boundary of the burnt coupes but not reaching drainage lines.
There are 16 more sediment tongues in the burnt coupes than in the unburnt although the length of drainage is 4 km (16%) less.
Sources of the sediment tongues are listed in Table 3.5.
Water bar on snig track
Feeder road Machine disturbance
Log rut TOTAL
BURNT 29 3 3 7 42
IJNBIJRNT 18 2 6 - 26
Table 3.5. Sources of sediment tongues.
29 sediment tongues were generated from water bars in the burnt coupes and 18 in the unburnt. It is concluded that the greater number of tongues in the burnt catchments is a consequence of operational procedures rather than propensity for erosion. There are simply more water bars at the perimeter of the burnt than the unburnt coupes. The occurrence of sediment tongues, at the ruts caused by snigging logs to the snig track, in only the burnt coupes indicates strongly that there is an increased risk of erosion from this source as a consequence of
burning. It is accepted here but not subsequently verified by going back to the sites that the reason is removal by burning of some ground cover.
The frequency distribution for the lengths of sediment tongues reaching unbuffered drainage lines for both burnt and unburnt coupes is shown in Figure 3.6 in which the width of protective strip, 5 m, is compared to the measured lengths of sediment tongues. The measured lengths of these 'cut off tongues are of course all over 5 metres length. Half are over ten metres. The number of tongues reaching these unbuffered drainage lines is seen as small relative to the length, 30 km, an average of one sediment tongues reached an unbuffered drainage line for each 1.6 km.
Two questions arise immediately from these observations.
The first in respect of the amount of sediment that reached the unbuffered drainage lines. In the field at the time of inspections the answer to this question was 'shovel full' rather than 'barrow loads' at each of tongue and probably a few barrow loads in total. Nevertheless this question was one reason for measuring the volumes of sediment in selected tongues and this question is raised again in relation to the volumes of sediment in tongues, in section 3.5.6.
The second in respect of forest practices that would further mitigate the number of tongues reaching unbuffered lines. The difficulty in applying the data in this context is of course that the tongues are cut off. Clearly widening the protective strip could reduce the number but quantitative deductions cannot be made. It is also clear that if the sources of these sediment tongues mainly water bars and log ruts were further from the drainage lines then fewer would reach the drainage line. Again quantitative deductions cannot be made. Given that sediment tongues did not pass through buffer strips, Figure 3.4 and 3.5, extension of buffer strips up the drainage line would also reduce the number of sediment tongues reaching the drainage lines. This of course would reduce the volume of wood produced and raises in turn the question of the relative efficiency of buffer and protective strips. This is not discussed here but later in the context of the results for sediment tongues at buffered drainage lines.
22 20 H c/> W 18 H fa “ 16H JC % 14 H (0 Q> 12 H 10 - 8 - 6 - 4 - 2 - 0
Width of protective strip for unbuffered drainage line = 5 m Number of tongues = 1 8
Length of unbuffered drainage line = 29.9 km
3 5 7 9 11 13 15 17 19 21
Length of 'cut off' sediment tongue (m)
Figure 3.6. Frequency distribution of the lengths of sediment tongues reaching unbuffered drainage lines (combination of burnt and unbumt).
Buffer strip width at Buffer strip width at Unbuffered drainage line 18 degree slope >18 degree slope
1
1________ I
L e n g t h o f s e d i m e n t t o n g u e ( m )
Figure 3.7. Frequency distribution of the lengths of sediment tongues at the perimeter of the coupes and not reaching drainage lines(combination of burnt and unbumt).
Figure 3.7, a combination of the lengths of sediment tongues in the burnt and in the unburnt coupes, compares the lengths of tongues with the width of buffer strip provided and the width of protective vegetation provided on unbuffered drainage lines.
Figure 3.7 enables assessment of the probability for sediment tongues to enter the stream courses. In that context it should be noted that only rarely do snig tracks reach buffer strips. For example in the inspections undertaken in this study there were no snig track drains at the boundaries of buffer strips. That is the source of sediment tongues would only rarely be at the edge of a filter strips. With hindsight it is very desirable that the position of the last drain on snig tracks be located with respect to buffer strips and unbuffered drainage lines.The data in Figure 3.7 shows
1) That none of the observed sediment tongues would reach a drainage line through a buffer strip 30 metres wide.
2) Only two out of the 68 tongues (3%) would reach a drainage line through a buffer strip 20 metres wide.
3) Only four out of 68 tongues (6%) would reach a drainage line through a buffer strip 15 metres wide.
These probabilities assume of course that the mechanics of deposition of sediment within buffer strips is the same as on logged areas at the perimeter of coupes. This may or may not be a correct assumption. The data from the field observations in this study do not provide any evidence for the comparative deposition lengths of sediment tongues on logged areas and in buffer strips. On the one hand there is more vegetation in the buffer strips than on logged areas, particularly immediately after post logging burning which generally does not penetrate into the buffer strips. On the other hand buffer strips are much more likely to become wetted areas during rainfall events that would cause erosion and eroded material may therefore be carried by overland flow in the wetted areas.
more effective as a filter than the ground cover left on logged areas. It is suggested this be
confirmed in future studies because the data presented here clearly show that the position of the
end of the snig track and the performance of the drain at the end of the snig track may be more
critical than the width of the buffer strip in mitigating the ingress to the drainage line of eroded
material.
The findings of this study support generally the report of Chalmers (1979). He
found that eroded materials in the Eden Woodchip Area did not move in excess of 30 metres
from drain outlets.
The literature reviewed in Chapter two indicated that filter strip width was often
related to slope, for example Packer (1967). In the Eden W oodchip Area buffer strips are
provided with widths in accordance with slope, see Figure 1.2 in Chapter One. In the first
instance the slopes of sediment tongues measured with an Abney level were compared with
slopes obtained from the compartment map. The position of the sediment tongue number was
located in the field during the field inspections and marked on the compartment map. The slope
at this map location was calculated from the contours. The field measurements and the map
determinations of slope for each sediment tongues are shown in Appendix 3.3.
The comparison of the field measurements and map determinations of slope are
y = - 3.2855 + 1.2546X R = 0.97
□ □
Field slope (degree)
Figure 3.8. Testing for correlation between map slopes and field slopes of 'cut off sediment tongues.
y = - 1.359+ 1.1175X R = 0.95
□ □
Field slope (degree)
There is good agreement between the field and map measurement and it was concluded that estimates of slope from maps could be used in practice for estimating length of sediment tongues. However such estimates, that is lengths in relation to slopes are not helpful for as reported below the measured lengths of sediment tongues are not correlated with measured slopes.
The length of sediment tongues and their slopes are shown in Figure 3.10 and 3.11 for cut off and full length respectively.
There is no correlation and it is concluded that at the boundaries of logging coupes on granite soils in the Eden Woodchip Area the lengths of sediment tongues are not strongly related to slope up to slopes of 20 . This was a surprising result, particularly in relation to the results reported in the literature.
The result is in conflict with some reported results of studies of logging operations in relation to the influence of slope on sediment deposition. Trimble and Sartz (1957), op cit page 34, found that downslope gradient was the key in determining the distance sediment moved from forest roads but they did not test other influences. On the other hand Haupt (1959), op cit page 35, studied distance sediment moved from roads and concluded that lower side slope gradient was not significant as a control over sediment movement.
Melmoth (1976) studied the distance sediment moved from culvert outlets in the Cotter catchment in the A.C.T. and also found that this distance was not related to slope. It is important to note in relation to Melmoth's study that the observations were made at culvert outlets along roads constructed more than ten years previously. The results are therefore related to sediment deposition in drainage lines from culvert outlets. Because of this they are not seen as directly related to sediment deposition at the perimeter of logged areas. Nevertheless much of the evidence for filter strip width is based on such observations.
y= 11.48 + 0.203X R = 0.22
Length of 'cut off' sediment tongue
Figure 3.10. Testing for correlation between map slopes and length of 'cut o f f ' sediment tongues.
y = 12.8 - 0.0417X R = 0.06
□ □
□ @ i
Length of 'full length' sediment tongue
In theoretical terms it must be accepted that sediment deposition would be related to slope. Certainly this was accepted when undertaking the literature review at the time of the planning the study and during the field inspections. It is necessary therefore to reconsider the mechanics of sediment deposition in the logged areas.