Erosion Control in Mountainous Areas
by MASATAKA OHMASA
Director, Government Forest Experimental Station, Meguro, Tokyo
A constant struggle is waged in Japan against erosion and landslides on hill slopes laid waste by man or devastated by exceptionally heavy rains. The general aim is to re-establish a vegetative cover as rapidly as possible and if this can include useful tree species, so much the better. There is no exact standard method laid down for carrying out operations because the topography, soil and local climate with which the forester has to contend vary from place to place. But the following practices are those most commonly employed in Japan.1
1An extract from a contribution to a forthcoming FAO Forestry Development Paper dealing with "Tree. Planting Practices for Temperate Asia".
The site to he re-clothed with vegetation is prepared in two stages. The surface of the eroded land is first evened off; then it is stabilized. But in the case of a hillside with a slope of 25 degrees or less and with a fairly regular surface, special preparation of the site is regarded as unnecessary. However, wherever there is a tendency for the debris accumulated on the lower slopes of a hill to become sodden from rain or melting snow and threaten a landslide, preventive measures are taken as soon as the accumulation has reached a depth of 3 meters or more, stone or wicker revetments or cylinders of wire being embedded in the ground (hence being called "buried works").
Restoring the slope of a hill which has been cut up by erosion, torrents or by a landslide, is a necessary preliminary to the surfing or terracing operations which will follow. Material obtained from levelling the surface is used to fill the depressions and fissures cut in the hillside, the work starting at the top of the hill and proceeding down to the foot. Care is taken not to interfere with any patches still carrying natural vegetation.
Site preparation and levelling are carried out by manual labor, using picks and shovels. Sometimes use is made of explosives, and mechanization of the operations, employing rock-drills and other equipment, is now being studied.
Once the surface of the hillside has been satisfactorily levelled, it is terraced by much the same methods as are used in other countries. The hillsides that require treatment in Japan are, however, usually fairly steep, so that the terracing takes on the appearance of a series of ascending steps. The terraces are made stable either by surfing, by simple terrace works, or by masonry or wicker revetments. (Figure 1).
FIGURE 1. A variety of terracing methods, using masonry fascines and wicker work.
FIGURE 2. Turfing of steps, with seedlings planted on the terrace.
FIGURE 3. Simple terracing.
FIGURE 4. Blanketing a whole slope.
This method is usually employed where there is a slope of 30 degrees and the ground is fairly hard. The horizontal steps vary from 1.5 to 3 meters in height and 0.6 to 1 meter wide, according to the slope of the hillside.
Turfs are placed at the base, on the face and on the top of the outer edge of the step, and made compact with earth taken from the inner edge, as shown in Figure 2. To prevent the turfs on the face of the step from slipping, they are usually braced by shoots of Shinoarundinaria sp. and stakes of Salix spp., driven some 15 centimeters or more into the ground as supports.
The turfs are, as a rule, cut from natural grass cover in a forest area, and are 20 to 27 centimeters long, 30 to 40 centimeters wide and 10 centimeters thick. - They survive quite well provided the ground into which they are transplanted has a fair moisture content. Results are poor in places where there is much seepage or considerable dampness.
Simple terrace works
These are only effective where there is a reasonable depth of soil and the slope is moderately gentle. Steps are made in the hillside about 0.5 meters wide at intervals of about 1 meter. The top of the steps is hoed to a depth of some 15 centimeters to make a filth. Along the outer edge of the step Torreya nucifera or Lespedeza bicolor are planted in parallel strips and behind these tree seedlings are planted. (See Figure 3.)
Various materials are used to protect the front face of the step: turf, straw bundles, fascines, stone, logs or poplar stakes. Frequently some parts of a hillside; otherwise requiring surfing, can be treated by simple terracing.
Wicker work and masonry
This is employed on slopes that are too steep for surfing to hold. Wicker work prevents the soil from slipping and holds the slope firm. It is usually about 20 centimeters high, although in rare cases it can be as much as 50 centimeters. It is held by stakes that are driven into the ground at an appropriate angle.
Masonry means dry-built stone walls constructed against the hillside, and is used where the ground is very damp or where evening-off is difficult to perform owing to rough hard ground unsuitable for terracing. Masonry revetments are also constructed to prevent earth filling or spoil from sliding down the slope. Masonry is apt to be expensive, however, unless there is a plentiful supply of stone available in the immediate neighborhood. Walls are sloped at a gradient of between 3:10 and 5:10, and vary between 0.7 to 2 meters in height.
In places where the ground is too loose owing to weathering or because it is frost-bound, terracing is usually unsuccessful and it is necessary to blanket the whole hillside. The slope is covered with fascines, straw, Torreya nucifera, etc. or with a network of corded bundles of the same materials, held down with stakes. (See Figure 4.)
Depressions and gullies on hillsides where water flows down are very prone to erosion and have to be specially protected. Channels are constructed which may be either of a trapezoid or arc cross-section. The former type of channel in stone is used where there is a great deal of water and an arc of turfs where the flow is less. Both kinds of channel are normally roofed with stone or turfs. The bed of the lower parts of the channels is usually lined with solid stone work, similar to dam construction.
FIGURE 5. Hillside drainage channels.
Main channels are mostly provided with stone benches at suitable intervals, of a sufficient height to control the flow of water. As a general rule, a fanwise pattern of branch channels, connected by transverse terraces, runs into the main channel. (See Figure 5.)
These channels serve to drain off the surface water, but sometimes it is also necessary to drain off the subsoil water. In this case, the usual practice is to bury closed conduits at an appropriate depth in the ground. A ditch is dug and one or two cylinders of wire laid in it and covered over with gravel until the ditch is filled. The cylinders are secured by wooden stakes driven in at intervals of about 2 meters. If it is not too costly, the ditch is sunk deep enough to reach rock level, otherwise conduits which are merely ditches filled with buried fascines or gravel must often suffice.
Amongst the tree and grass species favored for erosion control in Japan, the pines Pinus densiflora, P. thunbergii and P. rigida have all been used from ancient times. They are unsuited for the thin, dry soil of the upper parts of hill slopes, but are useful for the lower slopes and valleys. Alders (Alnus japonica, A. hirsuta, A. firma and A. multinervis) are widely used, especially to act as nurse trees to the pines. Robinia pseudoacacia is also widely planted at the foot of slopes, beside a stream or river, or on embankments. The trees grow rapidly and provide a very good check against soil slip, while they produce useful firewood and wood for charcoal. Oaks, particularly Quercus dentata, have deep roots and therefore hold the soil excellently, but they are of slow growth. They are usually put in under a cover of pines and alders.
Populus spp. and Juniperus spp. grow quickly and are commonly used to protect river banks and stream beds. Prunus serrulata takes root well and grows even on sharply sloping - bare ground. Castanea crenata grows rapidly and retains the soil very well. Lespedeza bicolor is effective where it is difficult to establish cover by ordinary planting. Other useful species are Amorpha fruticosa and Acacia spp.
Among the grasses used are Miscanthus sinensis, Spodiopogon cotulifer, Artemisia vulgaris, Indigofera pseudotinctoria, Microles pedeza striata, and more recently Festuca elatior var. arumdinacea and Eragrostis curvula.
Actual planting is usually carried out by setting out seedlings on the terraces, direct seeding being employed only where planting is likely to prove a failure,
Planting time varies with locality and with the general progress made in site preparations, but it is generally done in the spring, from February to May, or, in localities where autumn planting is possible, after the leaves have fallen.
The planting hole is made 25 to 30 centimeters wide and deep, and the seedling is planted at a fair depth. The number of seedlings planted per hectare is at least 5,000 to 6,000 and more often 10,000 to 12,000, giving 15 to 20 trees per 10 meters of terrace, although the spacing varies with the species and the particular site. Two-year-old seedlings, not transplanted, are used in the case of the pines, usually mixed with Alnus firma. For the latter and Robinia, one-year seedlings are used. When vegetative cover has to be restored on denuded land as rapidly as possible, woody shrubs are closely planted between the trees and sometimes grasses are sown or planted as well. The slopes between terraces may also be planted, and the outer edges of the terraces often protected by planting Torreya nucifera.
As fertilizer, ammonium sulphate, calcium super-phosphate, straw ashes, wood ashes and, recently, solid or granular chemical fertilizers are commonly used. The usual dosage is 30 to 60 grams per tree. Good results have been obtained by laying compost and solid manure, the effectiveness of which is long lasting, in ditches 20 to 30 centimeters wide.
Direct seeding on the terraces is carried out with a mixture of seed, calcium superphosphate, ammonium sulphate and soil mulch spread evenly and then covered with earth. To prevent the ground from becoming too dry, a thin layer of straw is spread over a seeded surface.
To seed a terrace with Lespedeza bicolor, a mixture of seed (treated to hasten germination), ashes and calcium superphosphate is broadcast on the terracing behind the Torreya nucifera.2
2A mixture of 1.35 kilograms of seed, 7.5 kilograms of ash and 7.5 kilograms calcium superphosphate is sufficient for 180 meters of terrace. deep, cut one per square meter, and again covered with straw.
Seed of perennials and shrubs is also frequently sown on the slopes between terraces. In this case, a similar mixture is scattered in raked lines or spots, and the seeded portions are then covered with straw. Where the slope is gentle, the seed mixture may also be deposited in holes 20 centimeters in diameter and 5 centimeters
Where the ground is very poor or has been greatly denuded, no attempt is made to plant up idrectly with seedlings. The method followed is to try to establish a natural succession of vegetation, to create an environment in which forest trees can later develop. The method is as follows:
Firstly, broken and loose earth is scraped off the slope into depressions in the hillside or down to the valley, leaving a firm smooth surface for seeding. At key points, surfed channels and stone revetments are constructed. Then, horizontal steps 20 to 30 centimeters wide are cut in the slope and spaced at intervals a little less than the length of the straw sheaves which will be used to cover the slope. These steps afford a foothold for the planters and serve as the points where the ends of the straw covers meet and can be tightly pressed down with earth to hold surface soil and check rainfall run-off. On the surface thus prepared, numerous small holes are made with a hoe, or, where the slope is very steep, shallow horizontal lines are raked into the slope to hold the seed mixture which is to be used.
This mixture has seeds of hardy and less tolerant plant species and plants of spreading and erect growth habits mixed together, with forest tree seed added. The following is a common mixture for the latter: Podocarpus nagi, Arundinella hirta, Cypbopogon goeringii, Lespedeza cuneata, Lespedeza bicolor, Alnus hirsuta, Pinus densifora, Pinus thunbergii, and Quercus variabilis. These tree seeds are first put into a cloth bag and soaked in water to force germination.
The whole seed mixture is mixed with wood ashes of equal or double quantity and then further mixed with a combined fertilizer (calcium superphosphate, ammonium sulphate, and wood ashes) and sandy loam, exact proportions having been worked out for the various components and for the quantities required per surface area to be seeded.
The whole mixture is eventually scattered on the slope and pressed down with a tamping board. Immediately after an area has been seeded, specially selected forest tree seed, such as Quercus variabilis and Q. acutissima, may be sown in 2 centimeter holes at a spacing of one seed per square meter. Then, thin layers of straw are laid over the surface, the ends being packed down securely with earth.
If forest tree seed is not sown directly with the first plant mixture, the tree species are introduced by planting in the year following the establishment of a vegetative cover.
An ingenious method of combining the advantages of both planting and seeding has been devised by an officer of the Maebashi Regional Forestry Office. It involves the use of "vegetation blocks" which can be set out either on a terrace or on a slope, and result in the rapid establishment of a vegetative cover. (See Figure 6.)
FIGURE: 6. "Vegetation blocks" set out on a elope.
To make the "block", a composite mixture is kneaded together and then compressed in molds to produce a "brick" 33 centimeters long, 20 centimeters wide and 2 centimeters deep. The mixture is composed of earth (graded sand and soil with a considerable amount of A-horizon), fertilizer (compost, ammonium sulphate, calcium superphosphate, fiber of straw and dried grass), clay and water.
Five holes, 2 centimeters in diameter, penetrate the block (made at the time of molding) to allow stakes to be driven through the block to anchor it to the ground. At the same time the upper surface of the block is pitted with 300 small holes (top diameter, 6 millimeters; lower diameter, 4 millimeters; depth, 5 millimeters).
A seed mixture with wood ashes, prepared in previously determined proportions suited to the particular locality where work is to be carried out, is spread over the top surface of the block, thus filling up the many small pits, and then the block is lightly pasted over with a clay and water mix.
Finally, the individual blocks are set out on the hill slope and staked down with stakes of Dentzia crenata and poplar which themselves in due course set root and sprout.