Miroslav Hríb1 and Pavol Dvorscák2
1 Assistant Professor, Faculty of Forestry, of Technical University in Zvolen, Department of Forest Constructions and Meliorations, T.G. Masaryka 24, 960 53 Zvolen, Slovak Republik.2 Associate Professor; address as above.
Abstract
Erosion has destroyed parts of the forest road network, which has lost its function and, consequently, sanitation or erosion control methods should be applied. In two localities in the Zvolen basin (Vel'ká Stráz and the Arboretum Borová Hora. 300 m a.s.l.) we measured actual soil loss from volume and sediment concentrations of runoff from elementary runoff microplots (area 1 m2, slopes 100% and 66%), on which erosion control blankets, nets and hydroseeding were applied.
Surface runoff was measured during one year's rainfall and separately utilizing a rainfall simulator with an intensity of 3.84 mm/min-1 a duration of 20 minutes.
Erosion control blankets and nets have an immediate soil protection effect and reduce absolute soil losses compared with slope areas with unprotected bare soil surface (nets over 92 percent, blankets over 95 percent).
Erosion control blankets that contain coconut fibre have surface drainage qualities and raise the volume of surface flow from 10 to 40 percent compared to straw blankets.
Measurement of actual soil loss is 84 percent which shares part of the surface volume runoff. Erosion control blankets containing straw effectively support the regime of surface flow from concentrated flow to non-concentration flow. We achieved a similar effect, when we sowed and covered a prepared surface of slopes on forest skidding roads with straw layers of 4-6 cm thickness. They were fixed with branches and wooden pegs in three other research areas in the Slovak Republic.
Keywords: Forest road, slope, erosion control method, blanket, net, rainfall, simulator, soil loss, surface runoff, sediment concentration.
Introduction
The excessive occurrence of alluvia resulting in silting up and depreciation of water resources, loss of the productive capacity of forest land and degradation of the forest land resource, force us to solve the problem of erosion control and sanitation of forest roads. This refers mainly to the skidding roads which do not comply with basic technical requirements and which cannot be simply converted into hauling roads. The problem of forest road sanitation has become very urgent in some gravitational units, especially in protected landscape areas. This fact can be proved by some articles and partially also by research results. Generally it is possible to state that each gravitational unit includes a certain amount of skidding roads and hauling roads with slopes requiring protection works and sanitation. Leaving these roads in their original state would result in their deterioration, requiring later more complicated measures and increased costs for maintenance, erosion control methods and conservation.
The modern special erosion control materials produced as erosion control blankets and nets are highly efficient with immediately effects on slopes. If the basic materials are from natural materials such as jute, coconut, straw or cotton, then the result of soil protection measures are attractive because erosion control vegetation can be supported by original natural seeding of grasses, shrubs and trees.
As erosion control blankets and nets are rather expensive, we research was carried out on their effect on microplots and then we designed similar cheaper natural methods for forest road slopes.
Methods
The aims were:
a) to quantify and to analyse the influence of all types of erosion control materials for surface runoff and soil losses from elementary runoff microplots in two localities, Vel'ká Stráz and Arboretum Borova Hora;b) to design simple natural erosion control methods on cutting (45°) and filling (33.7°) slopes, the beds of skidding roads in three different localities, Polana, Zboj and Zelezná Breznica.
In two localities (Vel'ká Stráz and the Arboretum Borová Hora, 300 m a.s.l.) in the Zvolen basin we determined actual soil loss method from volume and sediment concentrations of runoff from elementary runoff microplots (area 1 m2 = 0.5 x 2 m, cutting slopes 45° and filling 34°) by the ambulant deluometrical method. Surface runoff was measured during one year's rainfall (1993-1994) and separately utilizing a rainfall simulator with an intensity of 3.84 mm/min-1 for a duration of 20 minutes.
Table 1. Chosen mechanical and physical specifications of slope soil in depth 0-20 cm in the localities Velká Stráz (VST) and Arboretum Borová Hora (ABH)
|
Soil specifications |
Localities |
||
|
Velká Stráz |
Arboretum |
||
|
Content (%): gravel |
48.6 |
28.3 |
|
|
|
sand |
34.0 |
36.7 |
|
|
powder |
13.9 |
27.1 |
|
|
clay |
3.5 |
7.9 |
|
De by Kramer in mm |
8.93 |
1.58 |
|
|
d10 |
0.01 |
0.0031 |
|
|
d30 |
0.31 |
0.033 |
|
|
d50 |
1.85 |
0.375 |
|
|
Share of sand fraction |
12.02 |
13.62 |
|
|
Humus content (%) |
0.52 |
2.09 |
|
|
Volume weight (kg/m-3) |
1441 |
1023 |
|
|
Liquid limit of Attenberg (%) |
23.2 |
49.8 |
|
|
Plastic limit (%) |
24.3 |
32.4 |
|
|
Plastic index |
6.9 |
17.4 |
|
|
Erodibility factor by Ambroz |
0.065 |
0.183 |
|
The following erosion control materials were applied on the microplots:
a) blankets Aquasol:BTK (coconut)
BTS (straw)
BTSK (coconut and straw)b) nets Aquasol:
KGW 400 (coconut)
KGW 700 (coconut)
SS 500 (jute)c) biogeotextile Retex (cotton)
d) hydroseeding
e) bare soil
The units with herbaceous and grass mixture were sown with the following compositions: Phleum pratense 22 percent, Festuca pratensis 17.7 percent, Festuca rubra 18.7 percent, Poa pratensis 9.4 percent, Dactylis glomerata 10 percent, Lolium perenne 10.2 percent, Trifolium repens 13 percent on all microplots and research sections at densities of 6-7 g/m2.
Research results
The type of runoff regime on slopes of forest roads is quite different from that under trees, because of absence a humus.
The highest volume of surface runoff was on the microplots with material RETEX (cotton) 93.6 I/a (slopes 45°, locality VST). It represents 20.7 percent of natural rainfall. On the slopes of 34°, the same material had the volume of surface runoff of 69.6 I/a (15.4% of atmospheric rainfall).
Erosion control blankets that contained coconut fibre have surface drainage qualities and raise the volume of surface flow from 10 to 40 percent compared to straw blankets.
Erosion control blankets and nets have an immediate soil protection effect. They reduce noticeably absolute soil losses compared to slope areas with unprotected bare soil surface (nets over 92%, blankets over 95%). We can show the following averages of measured reductions in percent:
|
|
45° slope |
34° slope |
|
Erosion control blankets (BTS, BTK, BTSK) |
99.51 |
99.45 |
|
Erosion control nets (KGW 400, KGW 700) |
99.47 |
88.70 |
|
Erosion control jute net (SS 500) |
99.73 |
95.22 |
|
Biogeotextile from cotton (RETEX) |
99.79 |
96.68 |
|
Hydroseeding |
99.71 |
- |
|
Bare soil |
0 |
0 |
As natural rainfall has a high variability, we decided to apply separately a rainfall simulator with an intensity of 3.84 mm/min-1 for a duration of 20 minutes. From the statistical analyses the results were:
b) measurement of actual soil loss is 84 percent which includes part of the surface volume runoff;c) erosion control blankets containing straw effectively support the change of the surface flow regime from concentrated to non-concentrated.
Table 2. Absolute soil losses are dependent on volume surface runoff and statistical significance of exponential model Y=exp(a+b.x) using Rainfall simulator on microplots
|
Type of materials |
Ik |
Ik (%) |
P |
A |
B |
|
RETEX |
0.934 |
87.5 |
0.008 |
- 2.69703 |
0.922013 |
|
BTK-blankets |
0.982 |
96.5 |
0.0001 |
- 2.08222 |
0.166108 |
|
BTSK |
0.789 |
62.3 |
0.0066 |
- 4.79623 |
13.0661 |
|
BTS |
0.759 |
57.6 |
0.0108 |
- 3.45448 |
3.27164 |
|
KWG 700-nets |
0.918 |
84.4 |
0.0001 |
- 2.41631 |
0.459843 |
|
KWG 400 |
0.953 |
90.9 |
0.0001 |
- 0.8125 |
0.222926 |
|
SS 500 |
0.753 |
56.8 |
0.018 |
- 2.1083 |
0.566769 |
|
Bare soil |
0.680 |
46.3 |
0.0302 |
0.84197 |
0.440105 |
We reached a similar effect when we sowed and covered a prepared surface of slopes or soil beds of forest skid roads with straw layers of 4-6 cm thickness. They were fixed with branches and wooden pegs in three others different research areas in the Slovak Republic.
Figure 2. Design of natural erosion control methods for steep slopes of forest roads.
Jute or coconut nets with straw layers of 4-6 cm thickness are recognized as natural erosion control methods for steeper slopes. When we placed a hay layer instead of a straw layer, no sowing with a grass mixture was carried out. This method is suitable for protected landscape areas and for areas destroyed by erosion around some important botanical localities. When the original hay was not available we applied erosion control measures without sowing. Only a net (jute or coconut) was put on the surface of steep slopes or as an alternative to straw layers. These measures:
b) have sufficient effect to reduce immediately absolute soil erosion loss;
c) favour microclimate conditions to establish the natural vegetation cover;
d) permit easy growth of natural seeding of the original vegetation.
Figure 3. Design of nets for natural erosion control methods on slopes with sowing of original hay layers for protected landscape areas
Figure 4. Design of nets for natural erosion control methods on slopes without sowing for destroyed areas in protected landscape areas
The special advantages of geonet materials and procedures are:
b) use of natural materials;
c) slope arrangement regardless of the season;
d) minimum transport weight;
e) high capacity of water absorption;
f) biological decomposition without residues;
g) immediately efficient erosion control effect.
It is suggested to apply later the following erosion control material:
h) ditches and furrow trenches -jute net SS 500;i) embankment slopes of roads in contact with a watercourse, river beds, banks of water reservoirs - SKH KGW 700
Conclusions
In recent years blankets and nets have become increasingly important in the process of landscape transformation. Coconut fabrics which, in comparison with other textiles, have a very long life service and which are mechanically more stable are very convenient for engineering - biological methods of sanitation and erosion control protection.
Blankets and nets can be used successfully on newly created slopes originating in the construction of forest roads. Such slopes are greatly exposed to climatic influences of weather and only effective vegetation in combination with blankets can prevent soil erosion. However, young plants need sufficient protection at the beginning of the growing season to be able to protect the slope against destruction.
The use of anti-erosive nets and blankets reduces soil erosion immediately and effectively even without undersowing. A sufficient amount of seeds of autochthonous vegetation or turf is the key problem in the realization of anti-erosive projects on protected territories and in the vicinity of botanically valuable localities. The microrelief and microclimate of the slope surfaces with anti-erosive net (alternatively also with a straw bed) enables catching and implantation of seeds of autochthonous vegetation and their protection on the soil surface.
References
Aquasol - Erosionsschutzsysteme: Produktbeschreibung Technische Daten. Wien, 4 pp.
Carvell, K.L. & Ashworth, S.T. 1985. Rhizomes for vegetating and stabilizing steep forest road banks. Tree-Planters' Notes 3 6(1): 10-13.
Cook, M.J. & King, J.G. 1983. Construction cost and erosion control effectiveness of filter windrows on fill slopes. Research Note, Intermountain Forest and Range Experiment Station. USDA Forest Service. No. INT-335. 5 pp.
Dub, O., Nemec, J. et al. 1969. Hydrologie.Technický pr vodce. SNTL Praha 380 pp.
Dvoršcák, P., Hríb, M. & Juško, V. 1994. Zásady nápravných a asanacných opatrení v záujme odstránenia negatívnych a rušivých zasahov do prírody spôsobených nevhodne budovanou a nevyuzitel 'nou lesnou dopravnou siet 'ou. Referencná úloha MP SR. Technická univerzita Zvolen, , 65 pp.
Hríb, M. 1995. Umelé zadazd'ovanie a meranie povrchového odtoku na protierózne zabezpecených svahoch. In: Zborník z konferencie "Stav a rozvoj výucby a výskumu pedológie, geológie a bioklimatológie ". Technická univerzita Zvolen.
Koreñ, J. 1992. Klasifikations methoden der Waldterrains mit Ökologisation von Holzerntetechnologien. In: Zbor. ref. 26. Int. Symposion - Mechanisierung der Waldarbeit. Boku, Wien. pp. 103-112
Megahan, W.F., Monsen, S.B., & Wilson, M.D. 1991. Probability of sediment yields from surface erosion on granitic roadfills in Idaho. Journal of Environmental Quality 20(1): 53-60.
Midriak, R. 1986. K metódam merania povrchového odtoku a eróznych pôdnych strát v lesných porastoch a nad hranicou lesa. Vodohosp. Cas. 34(6): 653-657
North American Green. 1990. Erosion control blankets. NAG. Evansville. pp. 18
Pobedinskij, A. V. 1984. Sochranit'i usilivat' vodoochrannozašcitnyje funkcii lesa. Lesn. Choz. 56(10): 38-42.
Retschek, W. 1989. Naturnahe Böshungssicherungsmethoden mit Geotextilien aus Kokos-Naturfasern. ÖIAZ, 134. Jg. Heft 9. pp. 482-485
Scott-Beasley, R., Granillo, A.B. & Beasley, R.S. 1985. Soil protection by natural vegetation on clearcut forest land in Arkansas. Journal of Soil and Water Conservation 40(4): 379-382
Šach, F. 1990. Vliv obnovních spúsobú a tezebné- dopravních technologií na erozi púdy a odtokové poméry na obnovních secích. (Výskumná správa). VÚLHM, Opocno, pp. 134
