Benggang erosion: the influencing factors

CATENA ELSEVIER

Catena 27 (1996) 249-263

Benggang erosion: the influencing factors XuJiongxin Institute of Geography Chinese Academy of Sciences, Beijing 100101, China

Received 2 May 1995; accepted 25 March 1996

Abstract In the tropical and southern subtropic granite areas of South China, a special kind of small-scale erosional landform is widely distributed, called "benggang" by the local people. Many natural factors are responsible for the development of benggang, such as lithology, precipitation, relief, vegetation etc., which are discussed in detail in this paper. Study shows that the development of benggangs is the outcome of a joint operation of natural factors and human activities. Lithology, especially the characteristics of the granite weathering mantle, provides material in which benggangs can develop, while intense rainstorms in a tropical monsoon climate act as a powerful force to cause benggang erosion. The slope characteristics, such as steepness and exposure, control the spatial difference of the distribution of benggangs. The development of benggangs results from the reverse succession of vegetation induced by human activities in this region, and the density of benggangs is closely related to forest cover and unit timber growing stock. Keywords: benggang; erosion; tropic and subtropic zones; granite weathering mantle; South China

1. Introduction

In the granite areas of tropical and subtropical South China, there is a widely distributed erosional phenomenon and the associated small scale erosional landform, called " b e n g g a n g " by the local people. It seems that there is not such a word which expresses the same meaning as " b e n g g a n g " in Chinese, and we adopt this Chinese word directly (Xu and Zeng, 1992). Meaning " a collapsing h i l l " in Chinese, benggang can be defined as an erosional landform with a very high rate of sediment transfer, which is formed in the hillslopes with the cover of thick granite weathering mantle and caused by the joint operation of mass wasting and flowing water erosion, with the former dominant (Xu and Zeng, 1992). 0341-8162/96/$15.00 Copyright © 1996 Elsevicr Science B.V. All rights reserved. Pll S0341-8162(96)00014-8

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Fig. l a is a sketch m a p s h o w i n g the p l a n f o r m o f a b e n g g a n g , a n d Fig. l b g i v e s a v i e w o f typical b e n g g a n g in G u a n g d o n g P r o v i n c e , C h i n a . M o r p h o l o g i c a l l y , a b e n g g a n g c a n be d i v i d e d as t h r e e units: the u p p e r c a t c h m e n t , the unit w i t h a c t i v e m a s s w a s t i n g

(a)

20

30 m

(b)

Fig. 1. (a) A sketch map of a benggang. 1. Upper catchment. 2. Benggang wall where active slides and slumps occur. 3. Benggang slope where wasted material is deposited. 4. Channel, usnally deep and narrow. 5. Fan formed by sediment transported out of the benggang. The units 2. 3 and 4 together are called "bengkou" by local Chinese people. (b) A view of a typical benggang in Wuhua County, Guangdong Province. Note that three benggangs eat their way against each other and finally become merged and form a multiple benggang. Out of the benggang (in front) is a big fan built by benggang-derived material.

J. XuJiongxin / Catena 27 (1996) 249 263

251

called "bengkou" by local Chinese people, and the fan formed by the material transported out of the bengkou. There is a channel system in the benggang to drain runoff and sediment, and the main channel usually cuts deeply into the massmovement-derived material in the bengkou. The maximum dimension of a single benggang does not exceed 300 m in general, and its planform varies from elongated, to bottle-shaped, dish-like or anabranched. In some sense, a benggang is similar to a gully, and some researchers in China regard a benggang as the head of gully where mass-movement is very active. The main difference between a benggang and a gully, in our opinion, is that the development of the former is dominated by the action of mass-wasting; the development of the latter, by the action of flowing water. Benggang is also different from general slope failure, because after a slope failure occurs, the resulting material will be deposited at the foot of the slope which will stabilize the slope over a considerable period of time. However, within a benggang, after the mass-movement occurs, the resulting material will be transported by flowing water in a rather short time, and then the head wall of the benggang loses stability again, and mass-movement recurs. In a word, the close coupling of mass-wasting and flowing water transportation makes benggang erosion a quasi-continual process, at least during wet seasons. Benggang may exist as single landforms, but when many benggangs develop on a hillslope and eat their way against each other and then become merged, a badland landscape may appear. At this stage, the erosional landscape is similar to those occurring in other humid tropical and subtropical areas of the world, such as dongas in Swaziland (Goudie, 1995) and lavaka in Madagascar (Wells, 1990). Benggang erosion generally results in extremely rapid water and soil loss. According to our measurements in a runoff and erosion plot of active benggang in Wupihe Catchment in Wuhua County of Guangdong Province, the sediment yield from the benggang ranges from 500 to 1000 tonnes per hectare per year. The near-by farm fields may be buried by eroded material from the benggang, leading to sandification of land and a marked decline in land productivity. Therefore, controlling benggang erosion is an urgent task for water and soil conservation which we are facing in South China. As a small scale landform, benggang has a very high rate of sediment transfer, and it develops very rapidly, - - a whole cycle containing initiation, youth, mature and senile stages as described by Xu and Zeng (1992) may be completed in 100-150 years. This will be discussed later. Thus, the benggang also provides a very good example and a natural experimental site for studying the regularity of landform evolution. In the present paper, we will study the factors influencing benggang erosion, and a more comprehensive elucidation of formative mechanism of benggang will be reported in another paper.

2. Lithology The existence of a tropical granite weathering mantle is the necessary condition for the development of benggang. In South China with a humid tropical climate, granite is widely distributed, and thus a thick weathering mantle is well developed, with the thickness ranging from 40 to 60 meters, and the thickest up to 100 meters. This provides

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the necessary material for benggang development, and the genetic character and thickness of the weathering mantle can be regarded as two of the most important factors influencing benggang erosion. 2.1. Characteristics of the granite weathering mantle According to the difference in degree of weathering, a typical granite weathering profile can be classified as several horizons from top to bottom: 1. residual soil, 2. completely weathered, 3. highly weathered, 4. moderately weathered, 5. slightly weathered, 6. faintly weathered and 7. fresh rock (Geological Society, 1990). In this sequence, the grain size becomes coarser, and the content of clay declines. Since the land surface has suffered from a long-term denudation since the thick weathering mantle formed, the material exposed to erosion may be any one of the foregoing horizons, so the material for benggang development is different in different locations. Because of a high content of clay, the residual soil has a relatively high swelling-shrinking index, and after several wetting-drying cycles, deep and wide cracks may be easily formed, giving rise to an increased possibility of mass wasting. The study of Zhang et al. (1993) indicates that the soil mechanic properties of residual soil is different from that of other horizons, as shown in Fig. 2. With the increase in soil moisture, the residual material's soil mechanical indices such as cohesion and shearing resistance angle decline sharply, but for completely and highly weathered horizons, such a decline is much slower. This means that when the soil moisture is increased, the decline in the shearing strength of the former is much more than that of the latter, so in the residual soil the critical condition for slope failure may be reached more easily, leading to an easier occurrence of slides and slumps. This is one of the reasons why most of the benggangs appear where residual soil horizon is exposed to erosion. Moreover, due to a fine texture and compact structure, residual soil has a high resistance to erosion by flowing water, while the underlying horizon is coarse textured and loose, so its resistance to water erosion is much lower. In cases where the water flow cuts through the residual soil and into the underlying more erodible material, the erosional rate may increase rapidly. However, if the downcutting goes further and reaches the moderately weathered horizon, a erosion resistant armoring layer may be formed gradually, leading the downcutting to decline or even stopping it. After that, the base-level of the benggang becomes stable, and the benggang becomes stable also. This process can be generalized as Fig. 3, which assumes an idealized condition in which the erosion starts in the residual soil and then cuts down to all the underlying horizons successively. 2.2. Thickness of the weathering mantle Study shows that there exists a critical thickness of weathering mantle for the occurrence of benggang, below which benggang can hardly be formed. This is because, in general, the thickness of weathering mantle is the limit of the gully's downcutting, the height of slope wall must reach a threshold to allow occurrence of slope failure. The investigation in Guangdong Province indicates that benggang cannot be seen where the

J. XuJiongxin / Catena 27 (1996) 249-263

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Fig. 2. The cohesion c and shear resistance angle q; varying with soil moisture o>, based on data from the residual soil horizon (a) and the complete and highly weathered horizons (b) of the granite weathering mantle (after Zhang et al., 1993).

2 .~

3

6 Time Fig. 3. A diagram showing the downcutting rate of a benggang varying with time. 1. Residual soil. 2. Completely weathered. 3. Highly weathered. 4. Moderately weathered. 5. Slightly + faintly weathered. 6. Fresh rock.

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254

thickness of weathering mantle is less than 5 m, and among the total number of benggangs investigated, 10% are found where the weathering mantle thickness is 5 - 1 0 m, 30% are found where the thickness is 10-20 m, and 60% are found where this thickness is 20-50 m (Liao and Zeng, 1992). The thickness of the weathering mantle determines the size of the benggang. The thicker the weathering mantle, the more space a benggang can get to develop itself fully, thus the larger the size of the benggang. To envisage the influence of weathering mantle thickness on benggang development, we have measured the thickness of weathering mantle in 4 locations in the Wupihe catchment, Wuhuan County, and related this to the dimensions of benggang represented by the area of the largest benggang found in this location. This is because the dimension of a benggang depends on the stage of its development, and small benggangs may also be found in areas with thick weathering mantle. So the largest benggang found in an area should closely reflect the possibility of the maximum size of benggang which can be developed under a given thickness of weathering mantle. This relation is given in Fig. 4, which shows a close direct correlation between weathering mantle thickness and maximum bengkou (mass wasting) area. Based on this correlation, a quantitative relation has been established, as shown in Fig. 4. The investigation in Guangdong Province also shows that the distribution of benggangs varies with altitude. In Guangdong Province, benggangs are most developed where the altitude ranges from 50 to 250 m above the sea, they are less developed where the altitude is 250-300 m, and the altitude of 500 m above the sea is the upper limit of benggang distribution (Yao and Luk, 1989). In our opinion, this characteristic distribution is caused by the variation of weathering mantle thickness with altitude. Alter the thick granite weathering mantle formed, the study area has undergone a neo-tectonic uplift, with different amplitude of uplift in different places. Stronger erosion occurs where the amplitude of uplift is larger, leading to the association of higher altitude with smaller thickness of weathering mantle, and vice visa. This is why benggangs decrease

~"

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Y= 15.07)~.57 r=0.99

10000

X < 2~

1000

10 100 T H I C K N E S S OF W E A T H E R I N G M A N T L E (m)

Fig. 4. The maximum bengkou area varying with the thickness of weathering mantle (based on measurements from the largest benggangs found in each of 4 sample regions).

J. XuJiongxin / Catena 27 (1996) 249-263

255

in number with altitude and finally they are thought to disappear. However, more field investigation is needed to test this hypothesis.

3. Topography 3.1. Slope steepness In areas with gentle slope steepness, for example, less than 15°, gullies cannot cut down to a sufficiently large depth, and slides and slumps are not likely to occur. So the occurrence of benggangs is very limited. On the other hand, in areas with very high slope steepness, due to a limited rainfall accepting area ( = A cos 0, where A is the area of slope and 0 is the slope angle), - - for simplicity, assuming the vertical fall of raindrops, - - the runoff production is also limited, a factor not conducive to the benggang development. Logically, on slopes with medium steepness, the frequency of benggangs should be the highest. This is evidenced by the distribution of benggang frequency with slope steepness based on 606 benggangs in Yongchun County, Fujian Province, as shown in Fig. 5. It can be seen when the slope steepness is less than 45 ° or more than 20 °, the benggang frequency is the highest. When the slope steepness is more than 60 ° or less than 15°, benggangs hardly occur.

3.2. Slope shape The generally accepted classification of slope shape in China takes only the longitudinal shape of slope into account, and generalizes the slope as a longitudinal profile, then classifies slopes as linear, concave, and convex. In fact, the slope shape is not two dimensional, but three dimensional, as classified by Ruhe (1975). He represents the longitudinal and lateral slope shapes by slope length and slope width, respectively, each being classified as linear (L), concave (C) and convex (V). Their combinations produce nine types of geometrical forms of hillslope (Fig. 6).

60N =606

>. 0 Z

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I..1_

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20 30 40 5'0 60 S L O P E S T E E P N E S S (degree)

70

Fig. 5. Distribution of benggang frequency with slope angles.

256

J. XuJiongxin / Catena 27 (1996) 249-263

IH

If

Fig. 6. Geometric forms of hillslopes. Slope length is down the form; slope width is across the form. L, means linear; V, convex; C, concave. The simplest form (1) is co-linear (LL). Group III forms, the most complex, are double curved. Group II forms are linear in one dimension and curved in. The other (after Ruhe, 1975, Fig. 6.1). As to slope length, on c o n v e x slopes, the probability for the sediment c o m i n g f r o m upslope to deposit is very low, and an increase in erosion intensity can often be observed. On c o n c a v e slopes, due to a decline in water f l o w velocity, both seepage o f r u n o f f and deposition of s e d i m e n t can be e x p e c t e d to occur. As to the slope width, the c o n c a v e slope leads to a concentration o f runoff, while the c o n v e x slope leads to a d i v e r g e n c e of runoff, both exerting a strong influence on re-distribution of runoff, and therefore, on soil erosion. W h e r e the r u n o f f is concentrated, gullies will be well d e v e l o p e d and cut d o w n strongly, finally leading to the formation of benggangs. W h e r e the r u n o f f is divergent, sheet and rill erosion are dominant, and the opportunity for a rill to d e v e l o p to a gully is very limited.

Table l Relation of slope shape to benggang development Slope geometry (for meaning, see Figs. 1 and 5)

Number of benggangs

Concave-Concave (CC) Convex-Concave (VC) Linear-Convex (LV) Concave-Convex (CV) Convex Convex (VV) Linear-Convex (LV) Concave-Linear (CL) Convex-Linear (VL) Linear (LL)

!5 1 7 0 3 l 4 1 7

Total

39

Percentage of the total 38.4 2.6 17.9 0 7.7 2.6 10.3 2.6 17.9 100

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Table 2 Benggang developmentin relation to the longitudinal and lateral slope shape Slope shape No. of benggangs % of total

In slope width

In slope length

Concave

Convex

Linear

Concave

Convex

Linear

23 59

4 10.3

12 30.7

19 48.7

5 12.3

15 38.5

We have studied the influence of slope shape on benggang development in the Wupihe catchment in Wuhua County, Guangdong Province. Thirty nine benggangs in this river basin have been investigated in the field, and the shape of the hillslope where each benggang is located was identified, with the results given in Table 2. From the tables, the following can be learned: 1. As to slope width, the concave slope is most favorable to benggang development, the linear being the next, and the convex is unfavorable to benggang development. This is because concave slope causes concentration of runoff, enhancing the water flow's downcutting ability and also increasing the water infiltration along the cracks: both conducive to the downcutting of gullies and occurrence of a benggang. The inverse is true for convex slopes. 2. As to slope length, the concave is most favorable to benggang development, the linear being the next, and the convex is unfavorable to benggang development. The reason for this is just the same as in the case of slope width. However, if the concavity of the slope is very high, then sediment deposition will likely occur on the lower part of the slope, a factor unfavorable to gully downcutting. This is why the percentage of benggangs in this case, 19%, is smaller than those in the case of laterally concave slopes. 3. The combination of the two slope parameters makes concave-concave slope (CC in Fig. 6) the one where the benggang percentage is the highest, 38.4%. 3.3. The exposure o f slope

It is well-known that insolation is different on hilislopes with different exposures (aspect). When the orientation of a hillslope meets the direction of summer monsoon rainfall at a certain angle, the actual rainfall it receives varies with the angle. These variations lead to differences in soils, vegetation, and the conditions for runoff generation and concentration, giving rise to the variation in benggang development on hillslopes with different exposures. Many researchers have already observed these differences (for example, Yao and Luk, 1989). We have measured the long axis orientation of 796 benggangs on the 1:10000 map of the Wupihe River basin and the neighboring areas, and plotted the histogram of benggang frequency appearing in 16 orientations (Fig. 7). It can be seen that, on north-facing slopes with the orientation angle of 0°, the frequency of benggang is very low, and it increases with the increased orientation angle, reaching the maximum on SSE-SSW facing slopes, and then declines. On the south- and near south-facing slope,

J. XuJiongxin / Catena 27 (1996) 249-263

258

o~"

12l

0

N=796

0

N

45

90

135

180 225 27'0 315

E S W SLOPE EXPOSURE (degree)

360

N

Fig. 7. The distribution of benggang frequency with hillslope exposure (aspect), based on data from Wupihe River basin, Wuhua County, Guangdong Province.

the input of insolation is high, evaporation is strong, hence the soil moisture during dry seasons is low, and the growth of vegetation is relatively poor. During wet seasons, these slopes accept more precipitation than others, for they face against the summer monsoon, so the runoff intensity is high, and gullies are easy to form, deepen and enlarge. On the south-facing slope, soil moisture is high in the wet season and low in the dry season, and the resulting strong drying-wetting alternations of soils will lead to occurrence of deep and wide cracks, along which rain water and runoff percolation occur. Thus slope failure occurs frequently, and benggangs form. For the north-facing slope, the case is just inverse. It should be pointed out that this figure also shows a broad distribution of high values of benggang frequency from SSE to E-facing slopes, suggesting that other factors may also be important in determining the spatial distribution, such as the dominant direction of joints in the granite. More field work is needed to verify this.

4. Climate

It is by no means accidental that benggangs occur mainly in South China, because the benggang development is closely related to the hot and humid climate zones in tropical and subtropical southern China. The humid tropical climate itself favors the formation of thick granite weathering mantle. In Guangdong Province, mean annual precipitation varies from 1500 to 2500 mm, the number of rain days is 140 or more, and the maximum daily rainfall is 200-300 mm, and even 400 mm in some places. The maximum hourly rainfall is more than 40 ram, and in most places more than 60 mm. The 10 minutes maximum rainfall is 25 mm in most places. These precipitation features provide important dynamic conditions for the benggang development. High-intensity rainstorms are the major driving force for the benggang development. The occurrence of slides and slumps on the benggang's wall can be closely related to the

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259

rainstorms. For example, on July 19, during a rainstorm of 109 mm, a slump occurred in the No. 2 benggang, a monitored benggang in the Wupihe catchment. After that, we made measurements of the slump body, and its volume was estimated as 158 m s. The type of rainstorm also exerts an influence on the benggang erosion. According to the observation in the Shenchong River basin of Deqing County in 1988 (Clarke and Luk, 1988), the total rainfall in the form of typhoon rainstorm and frontal rain were 367 mm and 319 mm respectively; almost the same, but the resulting erosion amount expressed in the retreat of the benggang wall was 137 mm and 60 mm respectively, showing a pronounced difference. This is because that the intensity of typhoon rainstorms is often much higher than that of the frontal rains. When a typhoon storm occurs, the associated high-intensity rainfall often causes the initiation of some new benggangs. For example, according to the investigation by the Water and Soil Conservation Office of Wuhua County, during the No. 7 Typhoon in 1986, 10430 benggangs were formed, with a total area of 2794 ha. On the other hand, long-duration low intensity rains may sometimes lead to the rejuvenation of benggangs of senile age. During these rainstorms, rain water has sufficient time to infiltrate into the loose material previously deposited in the bengkou, and finally makes it saturated. When a underlying less permeable layer exists, the soil moisture will exceed the liquid limit. If the slope is steep enough, the saturated soils will start flowing downslope slowly. Among the rejuvenated benggangs, a majority falls into this category.

5. Vegetation In natural condition, the land surface in southern China is covered by dense evergreen, broad leaf forests with well developed undergrowing shrub and grass storeys, which protect the land surface from erosion effectively. Although the rainfall erosivity is rather high, the erosion intensity is very low. The erosion type is dominated by sheet and rill erosion, and very few rills can develop to the gully stage. Therefore, the benggang developed from a gully can hardly be observed. During the low-frequency high-magnitude rainstorms, shallow slides may occur in the forest covered hillslopes, providing a starting point for the benggang development. However, in natural conditions, the secondary vegetation will grow rapidly in the slide-caused hollow and thus retard the trend towards benggang formation. For this reason, the frequency of occurrence of benggangs of this type is also very low. Basically, strong benggang erosion is a phenomenon associated with human activities. In China, the earliest historical record about benggang appeared in Deqingxianzhi, a book on geography, culture and history in Deqing County, Guangdong Province, written by local scholars in 1838 in the Qing Dynasty. Because benggang erosion destroyed woodland and farmland and even the houses of local people, some records may be left and the information may be passed from one generation to another. So, by visiting the local people, the initiation of benggangs now of senile age may be roughly dated, and then the timespan could be estimated. According to our investigation in Wuhua County, most of benggangs of senile age were initiated 100-150 years ago, indicating a whole

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cycle of benggang development can be completed in 100-150 years. In Guangdong Province, the large-scale destruction of forest by man was also started from about 150 years ago, so the history of benggang erosion is coincidental with the history of man-induced destruction of forests in this region. 5.1. The reverse succession of vegetation in relation to the benggang development The climax plant community in tropical and subtropical southern China is evergreen broad-leaf forest, in which the elemental cycle is very strong. However, due to abundant rainfall, very strong leaching occurs in soils, leading to a low content of Ca, P and organic N. In such an ecosystem, the maintenance of high primary productivity results from the processes of rapid decomposition of dead plants and the resultant mineral elements enter the cycle quickly. The cutting of forest disrupts the closed elemental cycle in the ecosystem, leading to a rapid loss of nutrients, then the soil may tend to be increasingly infertile. Thus some nutrient deficiency tolerant tree species such as Masson pines (Pinus massoniana) may take over and the secondary vegetation becomes a mixed conifer and broad leaf forest or conifer forest, usually dominated by Masson pines. If the secondary forest is destroyed again by man, the vegetation degradation will be continued towards a tropical shrub and steppe and even a totally bare land which looks like a tropical desert. This process can be regarded as a reverse succession of the plant community caused by man's repeated destruction of vegetation. Obviously, the reverse succession means a sharp decline in the land surface bio-mass, which in its turn leads to a decrease in infiltration and an increase in surface runoff. The enhanced erosivity intensifies sheet and rill erosion, and rills may easily develop to gullies. Without effective vegetation cover, the soil moisture of the surface material of the weathering mantle undergoes a high seasonal and annual variation, and strong drying-wetting cycles of clay-rich soils may cause some deep cracks in the soil body. Rainwater and runoff flow into these cracks, leading to a marked decline in the shear resistance of the soil body. In the meantime, with the increase in the gully depth, the driving-force acting on the soil blocks dissected by deep cracks, also increases. This in turn increases the possibility for the occurrence of shallow slips and slides, and thus the gully may develop into a benggang. This process is described by a threshold model (Xu and Zeng, 1992), which is modified from Statham (1977). 5.2. Relation of benggang density to the vegetation factor To express the degree of benggang development, the index of benggang density is introduced (Xu and Zeng, 1992), defined as the ratio of benggang number in a given region to the area of this region. This index can be used to reflect the frequency of benggang occurrence in space. Based on data from Wuhua County, Guangdong Province, the benggang density is plotted against the forest cover percentage and the timber growing stock per km 2 in Fig. 8a and b respectively. The data for forest cover and unit timber growing stock come from the forest resource inventory conducted by the Forestry Bureau of Wuhua County, and the benggang density is calculated based on the number of benggangs in different areas of this county, obtained by the Water and Soil

J. XuJiongxin / Catena 27 (1996) 249-263

261

100 <

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0

10

15

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FOREST COVER

35

40

45

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(%)

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01 100

1000

10000

UNIT TIMBER GROWING STOCK (m ^ 3/ha) Fig. 8. Benggang density varying with the forest cover percentage (a) and the unit timber growing stock (b), based on data from Wuhua County, Guangdong.

Conservation Office of Wuhua County. It can be seen that, with the decline in forest cover and its unit timber growing stock, the benggang density increases sharply. The correlation between benggang density and these two vegetation indices are significant at a level of 0,005, on which two regression equations have been based respectively, as shown in Fig. 8.

5.3. The scale of benggang in relation to L~egetationfactors The field investigation shows that, on the hillslopes covered by dense forest, the benggang density is very low, but when benggangs occur, their size may be larger. Study shows that the area of bengkou, the unit of benggang where active mass movement occurs, increases with the increased unit timber growing stock (Fig. 9). This may be explained as follows.

J. XuJiongxin / Catena 27 (1996) 249-263

262

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lOOO 1O0 1000 10000 UNIT TIMBER GROWING STOCK (m ^ 3/ha) Fig. 9. The average area of the b e n g g a n g ' s bengkou varying with unit timber growing stock, based on data from Wuhua County, Guangdong Province.

The denser the forest and the higher its unit timber growing stock, the longer and stronger the roots of trees. Therefore, the shear resistance of soil body resulting from the tree roots effect is very strong. Hence, a slope failure can only occur when the driving force is also very strong. Such a condition is usually associated with a high-magnitude typhoon event hitting the hillslope. Under certain conditions, the cracks deeply cutting the soil body may thread together, forming a large sliding plane. A shallow slump or slide may occur during a high-magnitude typhoon rainstorm, and then a benggang may develop based on the slump-induced depression. Obviously, the benggang of this kind may have a larger size. On the other hand, when the unit timber growing stock is large, the weight of the forest itself will increases the driving force, but this effect is usually a secondary one compared with the root-induced increase in shear resistance.

6. Conclusions

The benggang landform is a widely distributed erosional phenomenon in the tropical and subtropical granite region of South China. It is formed by mass wasting and water erosion, characterized by very high rate of sediment transfer. The existence of a thick granite weathering mantle is a necessary condition for benggang development, and relief controls the distribution of benggangs in space. Benggangs are observed to be most developed on hillslopes with a medium steepness, or with a south-facing exposure, or with a laterally or longitudinally concave slope shape in study area. High-intensity rainstorms are the most important dynamic factor for benggang development, while long-duration low-intensity rainfall may cause the rejuvenation of old benggangs. Study shows that the frequent occurrence and wide distribution of benggang erosion developed during the last recent 150 years, coincidental with the history of forest destruction by man in this region, thus the man-induced forest destruction can be regarded as a major factor for the benggang erosion. Benggang erosion occurs as a result

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263

o f m a n - i n d u c e d reverse vegetation succession, and a close correlation exists b e t w e e n b e n g g a n g density and v e g e t a t i o n indices such as forest c o v e r and unit timber g r o w i n g stock. So the restoration o f vegetation can be taken as a possible way to control b e n g g a n g erosion in this region.

Acknowledgements T h e support f r o m the National Natural S c i e n c e F o u n d a t i o n o f C h i n a is gratefully a c k n o w l e d g e d . I wish to express m y deep gratitude to Prof. M. T h o m a s , Prof. A. Schick and Dr. A . K e r e n y i , w h o s e c o m m e n t s are highly valuable in i m p r o v i n g the paper. Thanks are also due to the W a t e r and Soil C o n s e r v a t i o n O f f i c e and Forestry Bureau, W u h u a County, G u a n g d o n g Province, for access to s o m e data used in this study.

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