There are two main soil types in the Eden Woodchip Area, granites and shales. The granite soils are more extensive and more erodible ( Forestry Commission of N.S.W, 1982 ) and this study concentrated on those soils.
Before deciding which coupes on these more erodible soils should be included in this study, rainfall records were examined and related to logging history.
The months October 1987 to April 1988 were relatively dry at Eden. Significant storm events occurred in May and June 1988 with erosion on recently logged areas Thus, but subject to investigation, it was initially accepted that coupes logged from October 1987 to April 1988 may have been subject to erosion only from the storm events during May and June 1988.
A " burning study" is in progress in the Eden Woodchip Area, (Bridges and Ridley, personal communications). In that study logging coupes are classified as burnt and unburnt. The coupes associated with the 'burning study' and logged in October 1987 to April
1988 were incorporated into the study although the standard practice is to burn all coupes after logging.
With the assistance of officers from the Eden District of the Forestry Commission of New South Wales, records of logging operations were studied in June 1988 to extract information on coupes logged in the period October 1987 to April 1988. The intention was to make field inspections in coupes logged in the 'dry period' October 1987 to April 1988 to assess erosion that occurred in May and June 1988.
3.4.2. On filter strips and wetted areas
During a storm event the area of a catchment contributing immediately to surface runoff is the area which includes and is adjacent to the stream channel. It is often called the initial wetted area. This area is dependent on antecedent conditions and tends to increase during the storm with the increases dependent on storm intensity and duration. In forested catchments the 'roaded' parts of the catchment may become wetted depending on the relative values of the infiltration rate and rainfall intensity at a point in the catchment at any time. In general terms for example road pavements would become wetted before compacted logged dumps which in turn would become wetted before snig tracks and in turn the relatively undisturbed forest floor.
The wetted areas may be sources of erosion depending on the soil erodibility and the velocity of the overland flow generated on the wetted area. The distance eroded soil particles are moved before deposition is of course a very significant factor in determining the severity of erosion but also relates to the width of filter strip needed to protect the stream from the ingress of eroded material.
It was argued therefore that the performance of filter strips must be considered in terms of sediment deposition both inside the filter strip and on the adjoining logged areas and the amount of material reaching the streams.
catchment 2 in Yambulla State Forest, was inspected in June, 1988. The objective was to assess the feasibility of measuring sediment tongues deposited proximate to the coupe boundary and a consequence of erosion in the logged area. The observations made in this the first field inspection associated with the study were critical to the development of the study. The length of the coupe boundary inspected was 3.7 km. Eight sediment tongues were found at the boundary of the coupe, none passed through the buffer strips. The length of tongues ranged from 8 to 12 metres.
These inspections confirmed that sediment tongues could be measured and indicated that sediment deposition in filter strips may be relatively rare. The initial observations also indicated that the position of snig track drains, relative to the coupe boundary, would be critical in determining if sediment tongues reached the protective filter strip and in turn the streams and drainage lines. Sediment tongues were usually associated with snig track cross drains or water bars.
It was concluded that visual inspection of sediment, together with measurements of the deposited material, would provide a useful basis for assessing widths of filter strips. The next step was to select coupes for systematic inspections.
3.4.3. Inspections on burnt and unburnt coupes logged in 1987/88
Eight coupes were randomly selected from those logged and then burnt between October 1987 and April 1988. Eight coupes were randomly selected from those logged in the same period but not burnt. To facilitate comparison the burnt and unbumt coupes were paired mainly on the basis of time of logging. The intention was to ensure, as far as was possible, that paired coupes had been subject to the same storm events. The selected coupes were inspected by pairs, again to ensure that paired coupes were subject to the same storm events for it was foreseen, anxiously, that a major rainfall event could occur between inspections of paired coupes. The paired selected coupes are as shown in Table 3.1.
No. Paired selected coupes to be inspected
Logging period Unbumt/Bumt
1. 4/439 4.3.88 - 1.4.88 Unbumt 1/423 1.3.88 - 25.3.88 Burnt 2. 4/485 16.10.87 - 1 .4.88 Unbumt 3/437 9.10.87 -18.12.87 Burnt 3. 4/514 18.12.87 - 25 .3.88 Unbumt 3/420 20.11.87 - 18.12.87 Burnt 4. 2/531 4.9.87 - 18.9.87 Unbumt 2/398 18.9.87 - 25.3.88 Burnt 5. 2/539 5.11.87 - 31.12.87 Unbumt 2/418 20.11.87 - 25. 3.88 Burnt 6. 1/537 4.9.87 - 18.10.87 Unbumt 4/371 4.9.87 - 16.10.87 Burnt 7. 1/532 25.9.87 - 23.10.87 Unbumt 2/421 20.11.87 - 18.12.87 Burnt 8. 1/433 4. 9.87 - 18.12.87 Unbumt 4/417 20.11.87 - 18.12.87 Burnt
Table 3.1. List of paired selected coupes.
The boundaries of the selected coupes were inspected comprehensively by walking around the boundaries and making observations. This work was assisted by compartment maps provided by the Eden Forestry District office. A illustrated example of a compartment map is shown in Figure 3.1. Observations included the lengths of sediment tongues, source of sediment, slope where sediment tongues were found and the intrusion of sediment into filter strips and unbuffered drainage lines. The lengths of the sediment tongues were measured with a tape and the slope at the sediment tongues with an Abney level. An example of observations recorded in each coupe is shown in Table 3.2.