Erosion control in South China

Erosion control in South China

¢ATENA ELSEVIER Catena 29 (1997) 211-221 Erosion control in South China 1 Jian-an Sheng a,*, An-zhong Liao b a Deqing County Water and Soil Conserv...

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¢ATENA ELSEVIER

Catena 29 (1997) 211-221

Erosion control in South China 1 Jian-an Sheng a,*, An-zhong Liao b a

Deqing County Water and Soil Conservation Office, Deqing, Guangdong, People's Republic of China b Guangdong Province Water and Electric Bureau, Deqing, Guangdong, People's Republic of China

Abstract

Severe soil erosion is common in South China where torrential rainfall erodes the deeply weathered and denuded granitic hills. Locally developed control measures have been applied successfully to reduce erosion, including the benching of steep slopes, the construction of check dams and reforestation. Of particular importance is the treatment of large gullies, locally called broken hills. These have led to the disintegration of hillslopes and the deposition of colluvium and fines on the agricultural land. Gully treatment includes the stabilization of the headwalls, the containment of sediments in the gully floor zone and the reclamation of flatland below the gully mouth. Re-vegetation of the slopes is essential and the planting of economic crops provides incentive to the farmers to preserve the vegetation cover. Keywords: Erosion control; South China; Gully treatment; Re-vegetation; Granitic hills

1. Introduction

In South China where the deeply weathered granitic hills have undergone periods of deforestation followed by intense erosion, various measures have been used to control erosion and to ameliorate its undesirable effects. Very often, the eroded hills flank the flatland where agricultural activities occur and where villages are sited. The deposition of eroded materials has ruined hectares of farmlands, silted up the streams and sometimes led to farm abandonment (DCSCT, 1983a). Erosion control therefore is closely tied to land management practices. Although many erosion control methods have been routinely applied throughout South China, they have not been described sufficiently in literature outside China. This paper presents a summary of the treatment measures employed commonly in the area. It

* Corresponding author. i This paper, originally written in Chinese, was translated and edited by Ming-ko Woo. 0341-8162/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. Pll S 0 3 4 1 - 8 1 6 2 ( 9 6 ) 0 0 0 5 7 - 4

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is not our intent to compare these methods with those employed elsewhere in the world; that is beyond the scope of this special issue.

2. Characteristics of the area

2.1. Geology

In granitic areas, tropical weathering often produces a deep weathering profile sometimes exceeding 50 m (Ruxton and Berry, 1957). The regolith consists of quartz grains and felsic clay minerals. The upper weathering zones are very friable. The soil developed in granite areas is acidic, often with a hard pan present. The weathered materials are easily erodible. Many joints inherited from the bedrock also facilitate slope failure. Thus, mass wasting and erosion by raindrop and running water are the major agents of soil erosion in the granitic areas when the protective vegetation cover is removed (Luk et al., 1997a this issue). 2.2. Climate

Of primary importance to soil erosion in South China is the frequent occurrence of heavy rainfall. Many stone pedestals attest to the effectiveness of raindrop erosion. Rainfall intensity often exceeds the infiltration capacity to generate surface flow which rapidly channels into rill runoff. Both sheet erosion and rill erosion are highly active during intense storms. Prolonged rainfall, saturating the weathered materials, frequently leads to mass wasting on the steepened slopes. The rainfall characteristics have been presented in another paper (Woo et al., 1997a, this issue). Monsoonal climate determines that erosion is highly seasonal. 2.3. Vegetation

The natural vegetation cover is luxuriant forests with a variety of tree species and a rich undergrowth of fern, grass and shrubs. Fires, cutting for firewood and reaping the fern for fuel have depleted the original vegetation. Once deprived of the cover, the granitic hills are eroded quickly. The humus is washed away while rills and gullies are created. 2.4. Topography

The granitic hills form rolling topography, with the hillslopes meeting the flat valley bottoms at abrupt angles. Eroded materials divulged from the hills are rapidly deposited at this junction. Thus, the farmlands adjacent to the hills are prone to the accumulation of nutrient-deficient and poorly sorted materials from the gullies and the rills (DCSCT, 1983b). Statistically, there is a preferred occurrence of gullies on south-facing slopes. For example, a basin in Deqing County shows that 56% of the 221 gullies are concentrated

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on south slopes while only 2% are developed on north slopes. This phenomenon has been attributed to larger radiation receipt on the southerly aspect which leads to more intense drying of the soil and a consequent restriction of the vegetation growth (Munro and Huang, 1997, this issue). Slope steepness is another factor affecting erosion and sediment production. Observations made in Zhaoqing area indicated that erosion was relatively weak on slopes < 6 °. For a 20 ° slope, the sediment yield was 338 t per (kin 2 a), and for a 24 ° slope, the yield reached 485 t per (km 2 a).

3. Erosion control methods

Erosion control is practised to reduce sediment production and runoff generation, and to prevent water and sediments from reaching farmland. The guiding principles are to employ engineering techniques to arrest the immediate loss of soil, and to develop re-vegetation programmes for long-term stabilization of the slopes. In South China, three types of erosional activities are recognized and each is treated by a different set of control measures. Sheet erosion occurs most extensively on bare or thinly vegetated surfaces and on ploughed fields in steep terrain. Rain splash and sheet wash are the chief agents of erosion. Rill erosion often follows deforestation. Single rills may deepen, broaden and coalesce to form distinct channels on the hillslopes (Woo et al., 1997b, this issue). The effects of rill erosion are more severe than sheet erosion. Gully erosion is the most devastating form of soil loss. In South China, these gullies are known as 'broken hill' because they break up the hillslopes into serrated ridges separated by deep gashes (Fig. 1). During rain events, large quantities of water and sediment are discharged from the narrow gully mouths (diCenzo and Luk, 1997. this issue).

3.1. Treatment of surface (sheet) erosion An experimental study in Deqing found that the presence of a vegetation cover (grass or fern) can increase infiltration, reduce surface runoff and hence retard sheet erosion significantly (Woo and Luk, 1990). For most denuded hillslopes in South China, reforestation is often the major effective measure to reduce erosion in soils which may have already lost their A and B horizons. Pine is usually planted first, eventually mixed with some broad-leaved species. The trees and the leaf litter will reduce rain-drop erosion and retard surface runoff (Hudson, 1981). The decomposition of the leaves will return nutrients to the soil to bind the soil particles, plant residue and living organisms together to form aggregates (Smith and Elliott, 1990). This results in increased infiltration and decreased surface runoff (Smika and Unger, 1986). Where plant growth conditions are more favourable, such as on footslopes or slope concavities, drought-resistant fruit trees can be grown to augment farm income.

3.2. Treatment of rills Narrow rills are revegetated with a variety of ground cover including fern. For rills wider than 1 m, revegetation is accompanied by cutting benches along contours to

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reduce flow concentration, and the construction of small dams along the rill bed. The main purpose is to stabilize the bed and to prevent the rill from being enlarged or extended. These practices can be effective in retarding soil erosion (Arledge, 1988; Woo et al., 1997b, this issue).

3.3. Treatment of gullies Gullies, the most serious form of erosion, have received the greatest attention and treatment in South China. Three zones can be distinguished in a gully (or broken hill) system, each with its erosional or depositional environment (Fig. 1): (1) the headwall zone, formed in the weathered granite, retreats through mass wasting and rill erosion,

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Fig. 1. Erosional and depositional zones of a gully system, consisting of (1) a headwall zone where mass wasting, rill erosion and sheet erosion are active; (2) the gully floor where sediment transport by running water is significant; and (3) the zone below the gully mouth where sediments are deposited and sorted, forming a colluvial fan and a saturated belt further downslope.

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leading to an enlargement of the gully; (2) the gully floor partly covered by materials transported from the backwall zone and partly consisting of the exposed weathered granite, is subject to downcutting and sediment removal by running water; (3) the zone below the mouth of the gully undergoes sediment deposition and sorting, with the coarse materials forming a colluvial fan at the proximal end, and the fines washed further onto the flatland to form an impervious layer for the development of wetlands and ponds. Successful erosion control often requires an integrated treatment of the gully. This involves stabilization of the gully walls, reduction of erosion and sediment movement on the gully floor and improvement of soil conditions below the gully mouth.

3.3.1. Stabilization of the gully walls Channels in a soil conservation system can be used to convey water away at non-erosive velocities (Hudson, 1981; Sheng, 1988). Above the backwall of a gully, trenches may be dug on the slope to divert the slope runoff away from the backwall. A typical trench consists of a ditch dug into the original slope, and a berm to prevent the water in the trench from spilling downslope (Fig. 2). Within the headwall zone, the steeper parts of the slopes may be cut to form benches. Slope benching is an effective way to stabilize the steep headwalls (Jiang et al., 1981; Morgan, 1986). Although this method is very labour-intensive, it should be applied to the very active headwalls or where backward retreat is a serious threat. Considerations have to be given to the angle of repose of the slope materials, and the improvement of drainage and soil moisture conditions, after benching. The amount of excavation should be minimized to reduce cost. Benches are constructed from the upper to the lower parts of the steep headwall (Fig. 3). Each bench consists of a lip that is 0.3-0.4 m high and 0.2-0.3 m wide. The bench itself has a width of 0.5-1.0 m and the elevational difference between each bench is 0.8-1.0 m. The frontal slope of each bench can range from 1:0.3 to 1:0.5, but for loose materials, the range should be 1:0.7 to 1:1. It is recommended that the bench width should increase downslope, but the elevational difference between each bench should decrease downslope. Good drainage has to be ensured between benches so as to prevent erosion. Benches constructed on loose materials may be covered by a layer of straw, twigs or fern and compacted with a layer of earth for strengthening and stabilization.

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Fig. 3. Schematic representation of benching along the slope of a gully, Jinji Hill, Deqing County.

The construction of the trenches above the gully and the benches on the headwall should be accompanied by revegetation. One tree species commonly planted on the headwall zone and on the hillslope is the South China pine (Pinus massoniana). Besides being highly adapted to this subtropical, acidic soil environment, the plant produces resin that is of commercial value. This offers economic incentive to the local farmers to tap the trees, rather than to cut them for fuel.

3.3.2. Sediment containment on the gully floor Check dams are commonly used for gully reclamation and erosion control in many parts of the world (Arledge, 1988; Finkle, 1986; Hudson, 1981; Hurni, 1988; Morgan, 1986). In South China, the gully floor is initially left untreated but a check-dam is built across the gully mouth to stop the sediment from spreading to the flatland. Usually, stone masonry dams are used to block off gullies with narrow and incised outlets (Fig. 4), and earth-dams are built for gullies with wide mouths. A stone-lined spillway is also added to each dam to accommodate overflow (Fig. 5). Sometimes, the dam may be raised when the area behind is filled. However, there is a limit to the height which the dam can be raised. In some cases, a second or a third dam is constructed along the floor to retain the sediments, until the amount delivered from upslope (the headwall zone) is reduced by the re-establishment of a vegetation cover. When the area behind the dam is mostly infilled, the gully floor may be vegetated. This, coupled with stabilization of the gully wall, will prolong the effective life of the dam and offer more time for erosion upslope to be curtailed. However, if reforestation does not arrest erosion after the area behind the dam is filled, the dam may be breached or overtopped and the treatment effect is nullified. 3.3.3. Reclamation of sediment-covered flatland Pressure to utilize the flatland for agriculture requires that the marginal land below the gully be reclaimed. To do so, both the physical and the chemical properties of the soils have to be considered. The highly permeable sediment comprising the colluvial fan

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Fig. 4. Stone-masonry dams built across the mouths of two gullies. (Photo: M.K. Woo). does not retain moisture and any water and nutrients will leach rapidly from the surface zone. To reduce infiltration, sometimes earth is dug from the hillslopes and added to the fan before m a n u r e or chemicals are spread. M u c h fertilizer has to be applied for plant growth.

Fig. 5. An earth dam with a spillway built across the outlet of a wide-mouthed gully. (Photo: M.K. Woo).

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Fig. 6. Several gullies carved on the hillslopes have deposited coarse materials at the foothill zone and fine materials on the floor of Lichong valley. The latter forms wetlands with a high water table. A large ditch is dug along the valley for drainage before crop growth can begin. (Photo: M.K. Woo). At the distal end, the accumulation of fines renders the soil waterlogged. Cultivation of the wetland is possible after ditches are dug to lower the water table (Fig. 6). Sometimes, the wetland or shallow ponds may be dredged to form a fish pond (also used for duck raising). Even so, the pond has to be drained and its bottom limed once every several years. The cleaning of the pond prevents excessive accumulation of iron and other chemicals harmful to the fish.

3.3.4. Erosion control at Jinji H i l l - - a n example Jinji Hill in Deqing County was a round-topped hill underlain by weathered granite. It covered an area of 15 ha and suffered severe erosion, particularly by gullying on its southeastern slopes. The gullies occupied 4 ha and there was little vegetation in these areas (Fig. 7a), F r o m the winter of 1958 when the W a t e r and Soil Conservation Station was established, until 1964, erosion control measures were applied to the eroded areas. Extensive benching was made, involving the transfer of 2.5 × 105 m 3 of earth (Fig. 7b). Before the treatment, soil loss from Jinji Hill averaged 989 m 3 a -1. One year after treatment, this dropped to 87 m 3 a -1. After 4 years o f reforestation, the slopes were stabilized (Fig. 7c), and the areas did not suffer much soil loss even under torrential

Fig. 7. History of erosion control, Jinji Hill, Deqing County. (a) The hill was intensely dissected by gullies. Some check dams were already built across some gully mouths (1959). (b) Construction of benches on the slopes and check dams across the gully mouths (1964). (c) Erosion was controlled and forests established on the hill (1988).

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rainstorms of > 100 mm. This example illustrates the effectiveness of erosion control practices.

4. Discussion and conclusions Erosion control in South China utilizes locally available resources. A bulldozer may be used to build the earth dam; otherwise, human labour is employed in the preparation of the control measures. All the methods take advantage of the weathered materials which can be excavated easily without the use of heavy machinery. These methods can be adapted to these places where the labour cost is low relative to the cost of construction materials. The control measures have evolved over a period of years and have been applied to various locations in South China. While they have proven to be effective when coupled with revegetation, failures are common if they are not inspected regularly for necessary repair. Until a vegetation cover is established to reduce erosion, all the measures offered are intermediate solutions. On the other hand, vegetation grows quickly under a subtropical climate and it may take only a decade or so for an effective cover to be established. Population pressure remains very strong and the woodland is a potential target for fuel collection. Several periods of deforestation in the recent past (notably 1968, 1978 and 1982) have caused devastating effects on the granitic uplands. To be effective, the engineering measures of erosion control should be accompanied by a policy which protects the vegetation cover on hillsides. The policy of 'closing the hills and nurturing the forests' permits some areas to be partially or totally closed to firewood gathering and logging, depending on the condition and maturity of the tree cover. Villagers who violate the rule are fined. In addition, the villagers should be educated on the importance of erosion prevention and control. Where possible, the planting of trees that yield economic returns is encouraged. This will provide cash return to the rural area and an incentive to reduce indiscriminate destruction of the vegetation cover.

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Jiang, D., Qi, L. and Tan, J., 1981. Soil erosion and conservation in the Wuding River valley, China. In: R.P.C. Morgan (Editor). Soil Conservation: Problems and Prospects. John Wiley, Chichester, pp. 461-479. Luk, S.H., diCenzo, P.D. and Liu, X.Z., 1997a. Water and sediment yield from a small catchment in the hilly granitic region, South China. Catena, 29: 177-189. Morgan, R.P.C., 1986. Soil Erosion and Conservation. Longman, Harlow, 298 pp. Munro, D.S. and Huang, L.J., 1997. Rainfall, evaporation and runoff responses to hillslope aspect in the Shenchong basin. Catena, 29: 131-144. Ruxton, B.P. and Berry, L., 1957. Weathering of granite and associated erosional features in Hong Kong. Bull. Geol. Soc. Am., 68: 1263-1292. Sheng, T.C., 1988. Demonstrating conservation practices on steep lands in Jamaica. In: W.C. Moldenhauer and N.W. Hudson (Editor). Conservation Fanning on Steep Slopes. Soil and Water Conservation Society, World Association of Soil and Water Conservation, Ankeny, IO, pp. 207-214. Smika, D.E. and Unger, P.W., 1986. Effect of surface residues on soil water storage. Adv. Soil Sci., 5: 111-138.

Smith, J.L. and Elliott, L.F., 1990. Tillage and residue management effects on soil organic matter dynamics in semiarid regions. In: R.P. Singh, J.F. Parr and B.A. Steward (Editors). Dryland Agriculture: Strategies for Sustainability. Adv. Soil Sci., 13: 69-88. Woo, M.K., Huang, S.M., Zhang, J.Q. and Li, Y., 1997a. Rainfall in Guangdong Province, South China. Catena. 29:115-129. Woo, M.K., Fang, G.X. and diCenzo, P.D., 1997b. The role of vegetation in the retardation of rill erosion. Catena, 29: 145-159. Woo, M.K. and Luk, S.H., 1990. Vegetation effects on soil and water losses on weathered granitic hillslopes, South China. Phys. Geogr., 11: 1-16.