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Ecological erosion control on coal-spoil banks: an evaluation
Ecological Engineering 18 (2002) 371– 377
www.elsevier.com/locate/ecoleng
Short communication
Ecological erosion control on coal-spoil banks: an evaluation Martin J. Haigh a,*, Sv. Gentcheva-Kostadinova b a
Department of Geography, Oxford Brookes Uni6ersity, Oxford OX3 0BP, UK b Department of Ecology, Uni6ersity of Forestry, 1156 Sofia, Bulgaria
Received 18 July 2000; received in revised form 3 May 2001; accepted 5 June 2001
Abstract The relative merits of slope protection by forestation and by mechanical protection with contour wattles are evaluated for a steep, 17–18°, embankment of coal-briquette spoils, where drought and spoil properties prevent the establishment of ground vegetation. Ground loss, monitored for 6 years by erosion pins, shows that both treatments reduce the development of surface flow pathways. However, inter-rill soil losses are not significantly smaller on the wattle-protected than on the unprotected control slope. By contrast, ground losses under forest (1.9 mm year − 1) are significantly smaller than on either the unprotected slope (7.0 mm year − 1) or that protected by wattles (6.2 mm year − 1). Ground losses from sites with \20% leaf litter cover average 0.54 mm year − 1 and never exceed 3.5 mm year − 1. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Erosion rates; Rills; Road embankment; Coal spoils; Forest reclamation; Contour wattles; Leaf litter accumulation; Pernik; Bulgaria
1. Introduction This study evaluates the role of forestation in erosion control on reclaimed coal-land in Bulgaria. Here, coal spoil-banks cover around 16,000 ha, including 12,700 ha not yet successfully reclaimed (Malakov, 1993). Erosion is a widespread problem. Near 60% of Bulgaria’s ‘low productiv* Corresponding author. Tel.: + 44-1865-483-750; fax: + 44-1865-483-937. E-mail address: [email protected] (M.J. Haigh).
ity’ land is counted ‘severely degraded’ (Onchev, 1988). The steeper slopes of reclaimed coal-spoils are especially vulnerable.
2. The study area
2.1. Pernik mining basin Pernik lies in western Bulgaria at 750 m above msl (42° 36% N, 23° 03% E). The coalfield produces some of Bulgaria’s better quality brown coals
0925-8574/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 8 5 7 4 ( 0 1 ) 0 0 0 8 7 - 8
372
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
(Ivanov, 1973). Production peaked in the 1960s and has since declined. The region has become an unemployment black spot and is singled out for EU-PHARE investment.
2.2. Coal briquette spoil experimental site Until the mid-1970s, some of Pernik’s coals were enhanced by conversion to briquettes. This generated a large volume of dark coloured, infertile, ash-spoils. Adjacent to the northern peripheral road of Pernik, these are formed into a 17– 18° slope, south (TB170°) facing, roadside embankment. The briquette spoils resist natural vegetative colonisation and support no surface ground cover. However, in 1978–1979, parts of the embankment were successfully forested with Betula pendula and Pinus nigra, planted at a 2- by 1.5-m spacing. Another part of the same slope was given mechanical protection. Typical, contour wattles, 0.3– 0.5m-high woven-wood fences, were constructed along the contour at 4– 6-m intervals (Stys and Helesicova, 1992). The site grants an unusual opportunity to examine the effectiveness of trees in erosion control—in the absence of undergrowth.
2.3. Climatic and microclimatic context Pernik’s climate is temperate continental. The mean annual temperature is 8– 10 °C and mean annual precipitation 550– 650 mm. Nationally, rainfall erosivity is low to medium— R factor: 48 – 130 (Onchev and Kolchakov, 1988). During late summer, surface temperatures on the darkcoloured spoils (2.5YR.3/2 – 2.5/0) peak at 50– 70 °C. Between April and September, there is almost no plant-available moisture in the 0– 20 cm layer. Water exists at depths of \ 0.4 m, but poor soil structure restricts capillary rise. Deep rooting trees exploit such reserves and thrive where ground surface vegetation fails (Donov et al., 1978). VSB2.4. Human interference Human disturbance is typical of the urban fringe and this site lies across the road from Pernik’s
housing (Stys and Helesicova, 1992). The test site is not fenced and is crossed by human and animal traffic moving from the city to open land beyond. A footpath crosses the crest of the embankment but the steep main slope suffers less trafficking. Since 1990, Pernik’s economic crisis has encouraged increased disturbance. In the winter of 1993/1994, firewood collectors lopped the site’s birch trees to about 1 m. The following winter, all remaining trees and wattles were cut to ground level.
3. Methods At the outset, this site provided a model natural laboratory for an evaluation of the value of trees (without undergrowth) in erosion control. Here were three adjacent slope segments (angle: 17–18°), created at identical times, from identical materials, by identical means and all entirely devoid of ground surface vegetation. The three test zones differed only in that while one had not undergone any treatment, another had been defended with contour wattles, and a third with trees. On 9 September, 1988, three test profiles were marked out in each of the three test zones. Microtopographic changes were monitored with erosion pins in these nine tests. Data was collected annually, each September, and supplemented with records collected in April. Sadly, the turbulence of the times affected the experiment. Two records were lost, one (September 1993) through theft from a field vehicle and another (September 1992) through logistic problems. These have had to be replaced with records from April visits. This study was terminated by the complete destruction of the erosion control works in 1994/1995.
3.1. Ground surface ele6ation monitoring: erosion pin technique An erosion pin is simply a metal rod (length \600 mm, diameter B 7 mm) hammered into the spoil to a depth where it is beyond disturbance by frost and incidental trampling (Haigh, 1977). A small length, ca. 25 mm, is left exposed. The pin provides a benchmark against which changes in
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
the elevation of surrounding surfaces are registered. Each erosion pin record combines two measurements, one to the left and one to the right of the pin, and can be replicated to within 1 mm of the original (Haigh, 1977). For this study, pins were set at 5-m intervals down the slope profile on both forest and control plots with pins located on the same contour in each of the three replicate profiles. The contour wattled slope had, effectively, been broken into a series of small slope segments. So, here, pins were set on one typical, mid-slope terrace at 1m intervals between the wattle-defended risers. ‘Erosion’ pins measure changes in ground surface elevation, ground loss not erosion. Ground loss is a compound measure that combines elevation changes due to erosion and deposition with those due to changes in soil packing, especially those due to seasonal factors such as frost or organic activity and accumulation (Schumm and Lusby, 1963). So, erosion pin records better indicate true soil loss when year-on-year records are compared. Textbooks suggest that soil loss data provided by erosion pins are less accurate than those from slope-foot collectors (Hudson, 1995). However, pins are more effective geoecological tools. They alone indicate the complex patterns of ground loss and gain within a slope profile (Haigh, 1978). Further, they detect long-term positive changes in soil depth due to organic accumulation and de-compaction, where slope foot collectors can not. The soil conservation profession, as a whole, is steering away from its traditional obsession with soil loss — the erosion pin technique has the benefit that it indicates soil growth as easily as soil removal (Shaxson, 1995).
3.2. Ancillary measurements Supplementary measures recorded at each erosion pin included: slope angle—recorded 0.2 m up and down slope of each pin with a slope pantometer (Young, 1972), distance of each pin from the slope crest, and ground cover by leaf
373
litter—in a 1 m2 quadrat centred on each pin. Rill incision affects some pins and where one passes within 0.5 m of a pin, its depth (cm) is recorded. The pins used are too long to be trodden in by animals or easily moved by humans. However, when they gain a substantial exposure, they can be bent. Such disturbance is quantified by a simple index: no disturbance: 0, visible disturbance: 1, severe disturbance: 2.
4. Results After losses due to theft, vandalism, trampling and burial, 134 (85%) erosion pins survived the full 6 years providing combined data sets of size n= 360 on the control plot, n= 216 on the terraced plot, and n= 204 on the forested plot. Just 27% of the pins, all from the forest plot, had leaf litter scores and 31% had rills within 0.5 m. Trafficking affected 51% of the surviving pins, 27% severely. The average slope at each erosion pin was 32.6% (S.D. 14.8), i.e. 17.9° (S.D. 7.62). Taken as a whole, the raw data show that, on average, the exposure of each pin increased by 5.86 (S.D. 5.89) mm year. Mean annual ground loss on the control plot was 6.91 mm (S.D. 6.14), forest plot 2.50 mm (S.D. 3.55) and terrace 7.29 mm (S.D. 6.13). However, these raw data distort the relativity of ground losses because each test has lost pins from part of the profile and, in the forest slope, from the entire slope foot zone. The test’s embankment has a slope-foot drainage ditch, which inhibits deposition, but there remains a weakly developed slope-foot concavity where erosion scores might be expected to be small (Haigh, 1978). Similarly, the sample terrace occupies a mid-slope site where, on the control plot, ground loss rates are close to the maximum. So, the raw results distort the impact of both the contour wattles and forestation. Analysis proceeds by comparing ground loss rates on data standardised by slope angle. Pins with slope angles that lie within 1 S.D. of the site mean, (i.e. 1898°), are selected. Table 1 summarises mean annual ground losses. The greatest
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
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Table 1 Mean annual ground losses by treatment (mm) Test plot
Mean annual ground loss (mm) (9 S.E., n)
Range (max, min)
Control
7.01 9 0.93 (360)
Forest
1.86 9 0.61 (204)
Sample terrace
6.17 9 1.26 (214)
23.17 (−21.67, +1.50) 10.83 (−7.67, +3.33) 21.83 (−18.67, +3.17)
difference divides forest from the other two tests. Results from the mechanically protected slope differ little from those on the control. The ratio of ground loss on control (7.01 mm year − 1) to forest plot (1.86 mm year − 1) is 3.77:1.
5. Statistical analysis
5.1. Introduction Kolmogorov–Smirnov tests show that, despite some negative skew, data distributions are not significantly different from normal (P B0.05), so with the exception of the trafficking index, parametric tests may be employed. Several tasks emerge. First is to link the changes measured by the erosion pins to erosion rather than another influence on surface elevation. Second is to identify any ancillary factors affecting ground loss. Third, the degree to which erosion is affected by the plot’s three treatments (forest recultivation,
mechanical protection, no treatment) must be quantified and confirmed by statistical analysis. Finally, the impact of increased human disturbance in the later years of the project must be evaluated.
5.2. Ground loss: due to erosion? Traditional soil conservation relates rill and inter-rill erosion to six controls: rainfall erosivity, soil erodibility, distance from slope crest, slope angle, vegetation cover and mechanical protection of the soil surface (Hudson, 1995). Here, while vegetation cover and mechanical protection differentiate between tests, slope angle and distance from the crest vary systematically across the site and should be major controls of within-site variations in erosion. By contrast, there is no obvious reason why the other controls of ground loss, mainly changes in the bulk density of the mine spoils due to soil moisture, freeze–thaw activity, trampling, vegetation growth and time, should correlate with either slope angle or distance from the slope crest. Analysis confirms strong significant correlations between these two soil erosion indicators and mean annual ground loss. Neither leaf litter cover nor the trampling index makes significant correlations with either slope angle or distance from the crest. The measure of rill incision made significant correlations with both variables but not with mean annual ground loss. In sum, ground loss is mainly due to inter-rill erosion.
Table 2 Correlations between mean annual ground loss and test-plot parameters (correlation coefficients are Pearson’s except for Trafficking–Spearman’s) Variable
Forest
Control
Terrace
Slope angle (degrees) Distance from slope crest (m) Rill incision (cm) Trafficking Leaf litter (m2/m2)
0.518 (P= 0.001) 0.608 (PB0.0005) No rill incision 0.361 (P= 0.015) −0.553 (PB0.0005)
0.541 (PB0.0005) 0.347 (P= 0.003) 0.406 (P =0.001) 0.138 NS (P = 0.150) No leaf litter
0.047 NS (P= 0.349) 0.067 NS (P = 0.668) 0.184 NS (P= 0.282) 0.330 (P= 0.025) No leaf litter
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
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Fig. 1. Cumulative ground loss 1988 –1994 on a 17 –18° coal-spoil embankment (with no ground vegetation) under three treatments: forest, mechanical protection with contour wattles, and no treatment (control). Heavy lines show measured ground losses. Light lines are linear regressions. (Deviations are mainly due to increased trafficking across the site after 1992).
5.3. Ground loss: correlations with slope angle, distance from the slope crest, depth of rill incision, leaf litter co6er, or the trampling index Table 2 displays the significant correlations between site parameters and mean annual ground losses on the three test plots. On the control plot and forested plot, increased ground loss is positively associated with increased slope angle and increased distance from the slope crest. Ground loss is positively associated with rill incision on the control plot and negatively associated with leaf litter cover on the forest plot. None of these correlations are significant on the sample wattle terrace. However, on both this plot and the forest plot, where evidence of trafficking is restricted, there is a positive association between trafficking and ground loss. On the control, where most of the plot suffered trafficking, the association is positive but not significant. In sum, all the variables show the expected direction of correlation with mean annual ground loss and, in many cases, this is statistically significant (P B 0.05).
and on the forest and terrace plot (P= 0.004), but no significant difference between the control and terrace plots (P= 0.59). In sum, forestation significantly reduces ground losses but slope protection by contour wattles makes no significant difference to inter-rill ground loss rates. However, there is significantly less rill activity on the contour-wattled terraced plot (P= 0.01) than on the untreated control plot, where the mean is four times greater.
5.5. Changing rates of ground loss Fig. 1 shows the trend of annual ground loss records and also linear regression trend lines for the same three data sets. Differences between the actual data and the trend lines highlight the acceleration of erosion through the 1990s, especially after 1992, when trafficking and firewood lopping increased across the site. After 1993, the wattle system began to break down. As this happens, rates of ground loss decline on the terrace plot as pins begin to receive sediments formerly held behind barriers up-slope.
5.4. Ground loss: reduced by forest reculti6ation or mechanical protection with wattles? 6. Discussion and conclusion T-test comparison confirms that there is a significant difference between mean annual ground loss on the control and forest plot (P B 0.0005),
The correlations that link ground loss scores to slope angle and distance from the slope crest, two
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M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
major controls of rainwater erosion, confirm that ground losses are due to erosion. The strength of the correlation between slope angle and distance from the crest, however, is much higher on the unvegetated parts of the site (r= 0.799, P B 0.0005), than under forest (r= 0.448, P = 0.003), which suggests other factors intervene. This is supported by an examination of the distribution of the lowest ground loss records on site, the 35 records where total ground loss is less than 1 mm. On the control plot, low ground loss sites are found at the slope foot and at the slope crest. On the forested plot, they are widely distributed across the slope and located on sites with a higher than plot-average litter cover (51%) but no trafficking or rill incision. These differences with the control plot are all highly significant (P B 0.001). Moving ahead, comparing ground loss rates on sites where there is a greater than 20% cover of leaf litter (0.54 mm year − 1, S.D. 2.62) with those where there is less than 20% cover (7.55 mm year − 1, S.D. 5.72), finds differences that are very highly significant. The final ratio of non-forest to forest ground loss is 14:1 (Wiersum, 1985). Moreover, as leaf litter expands above 20% surface cover, so ground loss rates remain below 3.0– 3.5 mm year − 1. Rickson (1995) confirms that leaf mulches become effective at very low applications, especially in adverse conditions. The leaf-litter layer reduces rainsplash detachment, increases water absorption, reduces runoff and hence erosion. Here, the litter mulch depresses erosion to low levels, encourages soil organic activity and hence the unpacking and growth of the soil, ground gain rather than ground loss (Filcheva et al., 2000).
6.1. Effect of erosion control treatments When, in 1995, the test plots were cleared of trees and wattles, a major impact of the soil conservation treatments became visible. While inter-rill ground loss rates remain similar on the wattle protected and unprotected control plots, the treatments made a major impact on rill development. The wattle-protected area had many fewer surface flow pathways and in this respect,
more resembled the forest-protected plot. Hence, while the forest tests had significantly fewer rills than either the terrace or control plot (PB 0.0005), the terraced plot also had significantly fewer rills than the unprotected control plot (P= 0.01) In sum, through 6 years of study, ground loss on the unvegetated control plot was 3.77 times that on the forest plot but only 1.14 times that on the sample terrace. The difference is significant in the first case and not in the second. Sites that experience ground gain that is not due to sediment transport, i.e. soil growth, are those where leaf litter accumulates. The rising ground loss graph in the post-1992 data suggests that litter accumulation and soil growth demand an absence of soil disturbance. When the effect of leaf-litter accumulation (\ 20% cover) is considered, the ratio of ground losses on the control and forest sites increases to 14:1. The conclusion is that forestation, with leaf litter mulching of the surface, leads to a massive reduction in inter-rill ground loss while contour wattle terracing does not. However, both forestation and terraces suppress rill evolution.
Acknowledgements Thanks go to the British Council (Sofia) for their sustained sponsorship of this work since 1986 and to the Earthwatch Institute, who assisted during 2 years. The research team would like to thank the following colleagues for their personal contribution: Dr Elena Zheleva, Mrs Tanya Minkovska, Mr L. Blake, Mrs M. Grigorova, Dr R. Milanov and Dr Tsvetan Yordanov.
References Donov, V., Gentcheva, Sv., Zheleva, E., 1978. Recultivatisiya na Promishleni Nasipi. Zemizdat, Sofia (in Bulgarian). Filcheva, E., Noustorova, M., Gentcheva-Kostadinova, Sv., Haigh, M.J., 2000. Organic accumulation and microbial action in surface coal-mine spoils, Pernik, Bulgaria. Ecol. Eng. 15, 1 – 15. Haigh, M.J., 1977. Use of erosion pins in the study of slope evolution. Br. Geomorphol. Res. Group Tech. Bull. 18, 31 – 49.
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377 Haigh, M.J., 1978. Evolution of slopes on artificial landforms, Blaenavon, UK. University of Chicago, Chicago, Geography Research Paper 183. Hudson, N.W., 1995. Soil conservation 3e. Batsford, London. Ivanov, D.I., 1973. 80 Gorodini Durdjavna Mina Georgi Dimitrov, Pernik. Durdjavno Izdatelstvo Tehnika, Sofia (in Bulgarian). Malakov, P., 1993. Country Report: Bulgarian Delegation. United Nations Economic Commission for Europe, Working Party on Coal, Workshop on Environmental Regulations in Opencast Mining Under Market Conditions (Most, Czech Republic) November 9 –11, 1993, pp. 1 – 14 ENERGY/WP.1/R.31. Onchev, N., Kolchakov, I., 1988. Predicting erosion degradation of some soils in Bulgaria. In: Bulgarian National Committee IHP, UNESCO International Symposium on Water Erosion, Proceedings, Sofia, pp. 309 –314. Onchev, N., 1988. State and problems of use of the eroded lands in Bulgaria. In: Bulgarian National Committee IHP, UN-
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ESCO International Symposium on Water Erosion, Proceedings, Sofia, pp. 295 – 301. Rickson, R.J., 1995. Simulated vegetation and geotextiles. In: Morgan, R.P.C., Rickson, R.J. (Eds.), Slope stabilization and erosion control: A bioengineering approach. E&F.N. Spon, London, pp. 96 – 111. Schumm, S.A., Lusby, G.C., 1963. Seasonal variation of infiltration capacity and runoff on hillslopes in western Colorado. J. Geophys. Res. 68, 3655 – 3666. Shaxson, T.F., 1995. Principles of good land husbandry. Enable 5, 3 – 13. Stys, S., Helesicova, L., 1992. Promeny Mesicni Krajiny. Nakladatelstvi Bily slon, Praha (in Czech with English Summary). Wiersum, K.F., 1985. Effects of various vegetation layers in an Acacia auriculiformis forest plantation on surface erosion in Java, Indonesia. In: El-Swaify, S.A., Moldenhauer, W.C., Lo, A. (Eds.), Soil erosion and conservation. Soil Conservation Society of America, Ankeny, IA, pp. 79 – 89. Young, A., 1972. Slopes. Oliver and Boyd, Edinburgh.
www.elsevier.com/locate/ecoleng
Short communication
Ecological erosion control on coal-spoil banks: an evaluation Martin J. Haigh a,*, Sv. Gentcheva-Kostadinova b a
Department of Geography, Oxford Brookes Uni6ersity, Oxford OX3 0BP, UK b Department of Ecology, Uni6ersity of Forestry, 1156 Sofia, Bulgaria
Received 18 July 2000; received in revised form 3 May 2001; accepted 5 June 2001
Abstract The relative merits of slope protection by forestation and by mechanical protection with contour wattles are evaluated for a steep, 17–18°, embankment of coal-briquette spoils, where drought and spoil properties prevent the establishment of ground vegetation. Ground loss, monitored for 6 years by erosion pins, shows that both treatments reduce the development of surface flow pathways. However, inter-rill soil losses are not significantly smaller on the wattle-protected than on the unprotected control slope. By contrast, ground losses under forest (1.9 mm year − 1) are significantly smaller than on either the unprotected slope (7.0 mm year − 1) or that protected by wattles (6.2 mm year − 1). Ground losses from sites with \20% leaf litter cover average 0.54 mm year − 1 and never exceed 3.5 mm year − 1. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Erosion rates; Rills; Road embankment; Coal spoils; Forest reclamation; Contour wattles; Leaf litter accumulation; Pernik; Bulgaria
1. Introduction This study evaluates the role of forestation in erosion control on reclaimed coal-land in Bulgaria. Here, coal spoil-banks cover around 16,000 ha, including 12,700 ha not yet successfully reclaimed (Malakov, 1993). Erosion is a widespread problem. Near 60% of Bulgaria’s ‘low productiv* Corresponding author. Tel.: + 44-1865-483-750; fax: + 44-1865-483-937. E-mail address: [email protected] (M.J. Haigh).
ity’ land is counted ‘severely degraded’ (Onchev, 1988). The steeper slopes of reclaimed coal-spoils are especially vulnerable.
2. The study area
2.1. Pernik mining basin Pernik lies in western Bulgaria at 750 m above msl (42° 36% N, 23° 03% E). The coalfield produces some of Bulgaria’s better quality brown coals
0925-8574/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 8 5 7 4 ( 0 1 ) 0 0 0 8 7 - 8
372
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
(Ivanov, 1973). Production peaked in the 1960s and has since declined. The region has become an unemployment black spot and is singled out for EU-PHARE investment.
2.2. Coal briquette spoil experimental site Until the mid-1970s, some of Pernik’s coals were enhanced by conversion to briquettes. This generated a large volume of dark coloured, infertile, ash-spoils. Adjacent to the northern peripheral road of Pernik, these are formed into a 17– 18° slope, south (TB170°) facing, roadside embankment. The briquette spoils resist natural vegetative colonisation and support no surface ground cover. However, in 1978–1979, parts of the embankment were successfully forested with Betula pendula and Pinus nigra, planted at a 2- by 1.5-m spacing. Another part of the same slope was given mechanical protection. Typical, contour wattles, 0.3– 0.5m-high woven-wood fences, were constructed along the contour at 4– 6-m intervals (Stys and Helesicova, 1992). The site grants an unusual opportunity to examine the effectiveness of trees in erosion control—in the absence of undergrowth.
2.3. Climatic and microclimatic context Pernik’s climate is temperate continental. The mean annual temperature is 8– 10 °C and mean annual precipitation 550– 650 mm. Nationally, rainfall erosivity is low to medium— R factor: 48 – 130 (Onchev and Kolchakov, 1988). During late summer, surface temperatures on the darkcoloured spoils (2.5YR.3/2 – 2.5/0) peak at 50– 70 °C. Between April and September, there is almost no plant-available moisture in the 0– 20 cm layer. Water exists at depths of \ 0.4 m, but poor soil structure restricts capillary rise. Deep rooting trees exploit such reserves and thrive where ground surface vegetation fails (Donov et al., 1978). VSB2.4. Human interference Human disturbance is typical of the urban fringe and this site lies across the road from Pernik’s
housing (Stys and Helesicova, 1992). The test site is not fenced and is crossed by human and animal traffic moving from the city to open land beyond. A footpath crosses the crest of the embankment but the steep main slope suffers less trafficking. Since 1990, Pernik’s economic crisis has encouraged increased disturbance. In the winter of 1993/1994, firewood collectors lopped the site’s birch trees to about 1 m. The following winter, all remaining trees and wattles were cut to ground level.
3. Methods At the outset, this site provided a model natural laboratory for an evaluation of the value of trees (without undergrowth) in erosion control. Here were three adjacent slope segments (angle: 17–18°), created at identical times, from identical materials, by identical means and all entirely devoid of ground surface vegetation. The three test zones differed only in that while one had not undergone any treatment, another had been defended with contour wattles, and a third with trees. On 9 September, 1988, three test profiles were marked out in each of the three test zones. Microtopographic changes were monitored with erosion pins in these nine tests. Data was collected annually, each September, and supplemented with records collected in April. Sadly, the turbulence of the times affected the experiment. Two records were lost, one (September 1993) through theft from a field vehicle and another (September 1992) through logistic problems. These have had to be replaced with records from April visits. This study was terminated by the complete destruction of the erosion control works in 1994/1995.
3.1. Ground surface ele6ation monitoring: erosion pin technique An erosion pin is simply a metal rod (length \600 mm, diameter B 7 mm) hammered into the spoil to a depth where it is beyond disturbance by frost and incidental trampling (Haigh, 1977). A small length, ca. 25 mm, is left exposed. The pin provides a benchmark against which changes in
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
the elevation of surrounding surfaces are registered. Each erosion pin record combines two measurements, one to the left and one to the right of the pin, and can be replicated to within 1 mm of the original (Haigh, 1977). For this study, pins were set at 5-m intervals down the slope profile on both forest and control plots with pins located on the same contour in each of the three replicate profiles. The contour wattled slope had, effectively, been broken into a series of small slope segments. So, here, pins were set on one typical, mid-slope terrace at 1m intervals between the wattle-defended risers. ‘Erosion’ pins measure changes in ground surface elevation, ground loss not erosion. Ground loss is a compound measure that combines elevation changes due to erosion and deposition with those due to changes in soil packing, especially those due to seasonal factors such as frost or organic activity and accumulation (Schumm and Lusby, 1963). So, erosion pin records better indicate true soil loss when year-on-year records are compared. Textbooks suggest that soil loss data provided by erosion pins are less accurate than those from slope-foot collectors (Hudson, 1995). However, pins are more effective geoecological tools. They alone indicate the complex patterns of ground loss and gain within a slope profile (Haigh, 1978). Further, they detect long-term positive changes in soil depth due to organic accumulation and de-compaction, where slope foot collectors can not. The soil conservation profession, as a whole, is steering away from its traditional obsession with soil loss — the erosion pin technique has the benefit that it indicates soil growth as easily as soil removal (Shaxson, 1995).
3.2. Ancillary measurements Supplementary measures recorded at each erosion pin included: slope angle—recorded 0.2 m up and down slope of each pin with a slope pantometer (Young, 1972), distance of each pin from the slope crest, and ground cover by leaf
373
litter—in a 1 m2 quadrat centred on each pin. Rill incision affects some pins and where one passes within 0.5 m of a pin, its depth (cm) is recorded. The pins used are too long to be trodden in by animals or easily moved by humans. However, when they gain a substantial exposure, they can be bent. Such disturbance is quantified by a simple index: no disturbance: 0, visible disturbance: 1, severe disturbance: 2.
4. Results After losses due to theft, vandalism, trampling and burial, 134 (85%) erosion pins survived the full 6 years providing combined data sets of size n= 360 on the control plot, n= 216 on the terraced plot, and n= 204 on the forested plot. Just 27% of the pins, all from the forest plot, had leaf litter scores and 31% had rills within 0.5 m. Trafficking affected 51% of the surviving pins, 27% severely. The average slope at each erosion pin was 32.6% (S.D. 14.8), i.e. 17.9° (S.D. 7.62). Taken as a whole, the raw data show that, on average, the exposure of each pin increased by 5.86 (S.D. 5.89) mm year. Mean annual ground loss on the control plot was 6.91 mm (S.D. 6.14), forest plot 2.50 mm (S.D. 3.55) and terrace 7.29 mm (S.D. 6.13). However, these raw data distort the relativity of ground losses because each test has lost pins from part of the profile and, in the forest slope, from the entire slope foot zone. The test’s embankment has a slope-foot drainage ditch, which inhibits deposition, but there remains a weakly developed slope-foot concavity where erosion scores might be expected to be small (Haigh, 1978). Similarly, the sample terrace occupies a mid-slope site where, on the control plot, ground loss rates are close to the maximum. So, the raw results distort the impact of both the contour wattles and forestation. Analysis proceeds by comparing ground loss rates on data standardised by slope angle. Pins with slope angles that lie within 1 S.D. of the site mean, (i.e. 1898°), are selected. Table 1 summarises mean annual ground losses. The greatest
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
374
Table 1 Mean annual ground losses by treatment (mm) Test plot
Mean annual ground loss (mm) (9 S.E., n)
Range (max, min)
Control
7.01 9 0.93 (360)
Forest
1.86 9 0.61 (204)
Sample terrace
6.17 9 1.26 (214)
23.17 (−21.67, +1.50) 10.83 (−7.67, +3.33) 21.83 (−18.67, +3.17)
difference divides forest from the other two tests. Results from the mechanically protected slope differ little from those on the control. The ratio of ground loss on control (7.01 mm year − 1) to forest plot (1.86 mm year − 1) is 3.77:1.
5. Statistical analysis
5.1. Introduction Kolmogorov–Smirnov tests show that, despite some negative skew, data distributions are not significantly different from normal (P B0.05), so with the exception of the trafficking index, parametric tests may be employed. Several tasks emerge. First is to link the changes measured by the erosion pins to erosion rather than another influence on surface elevation. Second is to identify any ancillary factors affecting ground loss. Third, the degree to which erosion is affected by the plot’s three treatments (forest recultivation,
mechanical protection, no treatment) must be quantified and confirmed by statistical analysis. Finally, the impact of increased human disturbance in the later years of the project must be evaluated.
5.2. Ground loss: due to erosion? Traditional soil conservation relates rill and inter-rill erosion to six controls: rainfall erosivity, soil erodibility, distance from slope crest, slope angle, vegetation cover and mechanical protection of the soil surface (Hudson, 1995). Here, while vegetation cover and mechanical protection differentiate between tests, slope angle and distance from the crest vary systematically across the site and should be major controls of within-site variations in erosion. By contrast, there is no obvious reason why the other controls of ground loss, mainly changes in the bulk density of the mine spoils due to soil moisture, freeze–thaw activity, trampling, vegetation growth and time, should correlate with either slope angle or distance from the slope crest. Analysis confirms strong significant correlations between these two soil erosion indicators and mean annual ground loss. Neither leaf litter cover nor the trampling index makes significant correlations with either slope angle or distance from the crest. The measure of rill incision made significant correlations with both variables but not with mean annual ground loss. In sum, ground loss is mainly due to inter-rill erosion.
Table 2 Correlations between mean annual ground loss and test-plot parameters (correlation coefficients are Pearson’s except for Trafficking–Spearman’s) Variable
Forest
Control
Terrace
Slope angle (degrees) Distance from slope crest (m) Rill incision (cm) Trafficking Leaf litter (m2/m2)
0.518 (P= 0.001) 0.608 (PB0.0005) No rill incision 0.361 (P= 0.015) −0.553 (PB0.0005)
0.541 (PB0.0005) 0.347 (P= 0.003) 0.406 (P =0.001) 0.138 NS (P = 0.150) No leaf litter
0.047 NS (P= 0.349) 0.067 NS (P = 0.668) 0.184 NS (P= 0.282) 0.330 (P= 0.025) No leaf litter
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377
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Fig. 1. Cumulative ground loss 1988 –1994 on a 17 –18° coal-spoil embankment (with no ground vegetation) under three treatments: forest, mechanical protection with contour wattles, and no treatment (control). Heavy lines show measured ground losses. Light lines are linear regressions. (Deviations are mainly due to increased trafficking across the site after 1992).
5.3. Ground loss: correlations with slope angle, distance from the slope crest, depth of rill incision, leaf litter co6er, or the trampling index Table 2 displays the significant correlations between site parameters and mean annual ground losses on the three test plots. On the control plot and forested plot, increased ground loss is positively associated with increased slope angle and increased distance from the slope crest. Ground loss is positively associated with rill incision on the control plot and negatively associated with leaf litter cover on the forest plot. None of these correlations are significant on the sample wattle terrace. However, on both this plot and the forest plot, where evidence of trafficking is restricted, there is a positive association between trafficking and ground loss. On the control, where most of the plot suffered trafficking, the association is positive but not significant. In sum, all the variables show the expected direction of correlation with mean annual ground loss and, in many cases, this is statistically significant (P B 0.05).
and on the forest and terrace plot (P= 0.004), but no significant difference between the control and terrace plots (P= 0.59). In sum, forestation significantly reduces ground losses but slope protection by contour wattles makes no significant difference to inter-rill ground loss rates. However, there is significantly less rill activity on the contour-wattled terraced plot (P= 0.01) than on the untreated control plot, where the mean is four times greater.
5.5. Changing rates of ground loss Fig. 1 shows the trend of annual ground loss records and also linear regression trend lines for the same three data sets. Differences between the actual data and the trend lines highlight the acceleration of erosion through the 1990s, especially after 1992, when trafficking and firewood lopping increased across the site. After 1993, the wattle system began to break down. As this happens, rates of ground loss decline on the terrace plot as pins begin to receive sediments formerly held behind barriers up-slope.
5.4. Ground loss: reduced by forest reculti6ation or mechanical protection with wattles? 6. Discussion and conclusion T-test comparison confirms that there is a significant difference between mean annual ground loss on the control and forest plot (P B 0.0005),
The correlations that link ground loss scores to slope angle and distance from the slope crest, two
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major controls of rainwater erosion, confirm that ground losses are due to erosion. The strength of the correlation between slope angle and distance from the crest, however, is much higher on the unvegetated parts of the site (r= 0.799, P B 0.0005), than under forest (r= 0.448, P = 0.003), which suggests other factors intervene. This is supported by an examination of the distribution of the lowest ground loss records on site, the 35 records where total ground loss is less than 1 mm. On the control plot, low ground loss sites are found at the slope foot and at the slope crest. On the forested plot, they are widely distributed across the slope and located on sites with a higher than plot-average litter cover (51%) but no trafficking or rill incision. These differences with the control plot are all highly significant (P B 0.001). Moving ahead, comparing ground loss rates on sites where there is a greater than 20% cover of leaf litter (0.54 mm year − 1, S.D. 2.62) with those where there is less than 20% cover (7.55 mm year − 1, S.D. 5.72), finds differences that are very highly significant. The final ratio of non-forest to forest ground loss is 14:1 (Wiersum, 1985). Moreover, as leaf litter expands above 20% surface cover, so ground loss rates remain below 3.0– 3.5 mm year − 1. Rickson (1995) confirms that leaf mulches become effective at very low applications, especially in adverse conditions. The leaf-litter layer reduces rainsplash detachment, increases water absorption, reduces runoff and hence erosion. Here, the litter mulch depresses erosion to low levels, encourages soil organic activity and hence the unpacking and growth of the soil, ground gain rather than ground loss (Filcheva et al., 2000).
6.1. Effect of erosion control treatments When, in 1995, the test plots were cleared of trees and wattles, a major impact of the soil conservation treatments became visible. While inter-rill ground loss rates remain similar on the wattle protected and unprotected control plots, the treatments made a major impact on rill development. The wattle-protected area had many fewer surface flow pathways and in this respect,
more resembled the forest-protected plot. Hence, while the forest tests had significantly fewer rills than either the terrace or control plot (PB 0.0005), the terraced plot also had significantly fewer rills than the unprotected control plot (P= 0.01) In sum, through 6 years of study, ground loss on the unvegetated control plot was 3.77 times that on the forest plot but only 1.14 times that on the sample terrace. The difference is significant in the first case and not in the second. Sites that experience ground gain that is not due to sediment transport, i.e. soil growth, are those where leaf litter accumulates. The rising ground loss graph in the post-1992 data suggests that litter accumulation and soil growth demand an absence of soil disturbance. When the effect of leaf-litter accumulation (\ 20% cover) is considered, the ratio of ground losses on the control and forest sites increases to 14:1. The conclusion is that forestation, with leaf litter mulching of the surface, leads to a massive reduction in inter-rill ground loss while contour wattle terracing does not. However, both forestation and terraces suppress rill evolution.
Acknowledgements Thanks go to the British Council (Sofia) for their sustained sponsorship of this work since 1986 and to the Earthwatch Institute, who assisted during 2 years. The research team would like to thank the following colleagues for their personal contribution: Dr Elena Zheleva, Mrs Tanya Minkovska, Mr L. Blake, Mrs M. Grigorova, Dr R. Milanov and Dr Tsvetan Yordanov.
References Donov, V., Gentcheva, Sv., Zheleva, E., 1978. Recultivatisiya na Promishleni Nasipi. Zemizdat, Sofia (in Bulgarian). Filcheva, E., Noustorova, M., Gentcheva-Kostadinova, Sv., Haigh, M.J., 2000. Organic accumulation and microbial action in surface coal-mine spoils, Pernik, Bulgaria. Ecol. Eng. 15, 1 – 15. Haigh, M.J., 1977. Use of erosion pins in the study of slope evolution. Br. Geomorphol. Res. Group Tech. Bull. 18, 31 – 49.
M.J. Haigh, S. Gentche6a-Kostadino6a / Ecological Engineering 18 (2002) 371–377 Haigh, M.J., 1978. Evolution of slopes on artificial landforms, Blaenavon, UK. University of Chicago, Chicago, Geography Research Paper 183. Hudson, N.W., 1995. Soil conservation 3e. Batsford, London. Ivanov, D.I., 1973. 80 Gorodini Durdjavna Mina Georgi Dimitrov, Pernik. Durdjavno Izdatelstvo Tehnika, Sofia (in Bulgarian). Malakov, P., 1993. Country Report: Bulgarian Delegation. United Nations Economic Commission for Europe, Working Party on Coal, Workshop on Environmental Regulations in Opencast Mining Under Market Conditions (Most, Czech Republic) November 9 –11, 1993, pp. 1 – 14 ENERGY/WP.1/R.31. Onchev, N., Kolchakov, I., 1988. Predicting erosion degradation of some soils in Bulgaria. In: Bulgarian National Committee IHP, UNESCO International Symposium on Water Erosion, Proceedings, Sofia, pp. 309 –314. Onchev, N., 1988. State and problems of use of the eroded lands in Bulgaria. In: Bulgarian National Committee IHP, UN-
377
ESCO International Symposium on Water Erosion, Proceedings, Sofia, pp. 295 – 301. Rickson, R.J., 1995. Simulated vegetation and geotextiles. In: Morgan, R.P.C., Rickson, R.J. (Eds.), Slope stabilization and erosion control: A bioengineering approach. E&F.N. Spon, London, pp. 96 – 111. Schumm, S.A., Lusby, G.C., 1963. Seasonal variation of infiltration capacity and runoff on hillslopes in western Colorado. J. Geophys. Res. 68, 3655 – 3666. Shaxson, T.F., 1995. Principles of good land husbandry. Enable 5, 3 – 13. Stys, S., Helesicova, L., 1992. Promeny Mesicni Krajiny. Nakladatelstvi Bily slon, Praha (in Czech with English Summary). Wiersum, K.F., 1985. Effects of various vegetation layers in an Acacia auriculiformis forest plantation on surface erosion in Java, Indonesia. In: El-Swaify, S.A., Moldenhauer, W.C., Lo, A. (Eds.), Soil erosion and conservation. Soil Conservation Society of America, Ankeny, IA, pp. 79 – 89. Young, A., 1972. Slopes. Oliver and Boyd, Edinburgh.