S.C. Boucher
School of Geography and
Environmental Science
Monash University, Victoria 3800
This article has been reviewed by Dr. Ian Sargeant.
Introduction | Rainfall and tunnelflow | Particle size of eroded material | Composition of suspended sediment | Conclusions | References
Introduction
Photo: Tunnelflow carrying suspended sediment into a tributary gully near Costerfield. Photograph: Stuart Boucher | An overview of tunnel erosion in Victoria is provided on this website. Whereas most research into tunnelling in Australia has focussed on the properties that make a soil susceptible to erosion, this article is concerned with how rainfall from a single storm event moves through a tunnel network. The mechanisms of tunnel hydrology have not been previously studied in Australia apart from the work conducted by the author (e.g. Fitzpatrick et al. 1994, Boucher 1995, 2002). The research site in question, Costerfield, is located in the south-west area of the Goulburn Broken region. Some of the main results are as follows:
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Rainfall and Tunnelflow: 12 May 1992 | The diagram below illustrates the rainfall and tunnelflow characteristics of the storm. The storm commenced at 0940 hours and ceased at 1035 hours. Flow monitoring was discontinued when discharge reached the low rate of 1.05 litres per minute (i.e. L/min) during the recession. The discharge measured for the first hydrograph peak was 12.2 L/min whilst that for the second peak was 7.5 L/min. The time from peak rainfall to peak runoff for the first peak was only 10 minutes. This result is significant when one considers that the rainfall has to enter the soil, as opposed to becoming part of surface runoff, and move vertically into the subsoil prior to reaching the weir. Therefore, flow must have passed through established pathways in the soil such as large macropores and macropores. By way of comparison, flow through the soil matrix would produce much slower flow speeds. |
The second hydrograph peak appeared unrelated to rainfall and was probably a delayed contribution from another tunnel branch upslope. The ‘'plateau-like'’, which followed the steep falling limb of the second peak, commenced at 1115 hours when discharge fell to 2.1 L/min. By this time the soil upslope of the weir (i.e. towards the hillcrest) was approaching saturation and would take some considerable time to drain. Put simply, the pore space in the soil matrix was substantially filled with water from the antecedent moisture absorbed by the 30.5 mm which fell at Costerfield the previous day as well as that from the present storm. | Rainfall, tunnelflow & suspended settlement flow: 12 May 1992 |
Suspended sediment flux of tunnelflow: 12 May 1992 | Suspended sediment is material that is carried by flow (e.g. dispersed clay) as opposed to that which is dissolved (i.e. solutes). The suspended sediment concentration is expressed in the unit 'kg/m3'. The first sample of suspended sediment was taken at the time of the first discharge peak and from then on the curve followed that of the falling limb of the hydrograph until discharge fell to 6.3 L/min and the sediment concentration remained at approximately 2 kg/m3. The total mass of suspended sediment was approximately 1.4 kg and its distribution over time followed that of the hydrograph. |
Particle size class | Diameter range (µm) |
Sand | Diameter ≥ 60 |
Silt | 2 ≤ Diameter < 60 |
Clay | Diameter < 2 |
Particle size class | Diameter range (µm) |
Fine sand and/or coarser | 2 ≤ Diameter < 6 |
Coarse silt | 20 ≤ Diameter < 60 |
Medium silt | 6 ≤ Diameter < 20 |
Fine silt | 2 ≤ Diameter < 6 |
Coarse clay | 1 ≤ Diameter < 2 |
Colloids | Diameter < 1 |
Particle size distribution flux of tunnelflow with respect to time: May 1992 | The 'fine sand and/or coarser' class was at its maximum percentage 10 minutes after the first discharge peak. The percentage of fine silt was consistently high over time, and the values for fine silt and medium silt were generally much higher than the coarse clay and colloid curves for most of the runoff period. The trends of medium silt and coarse silt were alike in that they decreased at a similar rate with time, although the former class had considerably higher values throughout. This pattern also bore an inverse relation to the clay fraction percentages with both subdivisions rising steadily over time, particularly in relation to decreasing discharge. |