Fan Hengwen Xiao Honglang Liu Xinmin
(Institute of Desert Research, Chinese Academy of Sciences and State Forestry Administration
260 Donggang West Road, Lanzhou 730000 , P.R. China)
Chinese traditional techniques for stablizing shifting-sand are characterized by many advantages, such as, convinience of establishment, soundly to environment, safety to growth of plants, low cost for technical support, easier adaptation and extention, and remarkable result. Traditionally, local straw and hay materials are widely used for stablizing sands. Materials are manifold and multiplicity, dispense with fine processing, low and cheep cost, for example sand barrier made by straw, branches of tree and shrub, clay and gravel. Shapotou Station of Desert Experiment and Research, Institute of Desert Research, Chinese Academy of Sciences and the State Forestry Administration began to study various sandbreaks and their protective function combining with the measures of revegetation of sand dunes from 1956. Some main sand barriers, such as grass mantle, strip grass mantle, grid grass mantle, checkerboard barriers, grid semi-cover straw barriers (for short straw checkerboards) and rang semi-cover barriers, were tested at the initial time. Theory and practice, after then, have proved that straw checkerboard barriers have the best fixing-sand capability and the longest survival period. Straw checkerboards were selected as main mechanical measures to stabilize shifting sands in Shapotou area.
Revegetation on moving sand dunes are usually eroded by wind and covered by sands. The close combination of biological fixation with artificial barriers and checkerboards plays effective role in stablizing sands. In the areas with conditions to fix mobile dunes with plants, artificial barriers and straw checkerboards should be established at beginning to reduce wind velocity near the dune surface and control the movement of sands; meanwhile, sand barriers should be set up to obstruct drift sands inside the range of fixing-sand area.
The combination of straw checkerboards with sand barriers could promote the improvement of environment and strengthen the growth of plants. Shapotou area is characterized by temperate desert climate, and as consequence, vegetation growths is relatively slow. It is an essential measure to apply the mechanical barriers and enclosive fences to stablize shifting sands in the revegetated areas.
1 Principles of
Straw Grid Sand-barriers
In order to make sound effects of the straw checkerboards and sand barriers, eight kinds of barriers in various design have been tested and popularized one after another in the field from1957 to 1958 (table 1). Best design of the barrier was 15-20 cm in height and it was estimated that safety distance is 1.5 m. The smooth sand surface was greatly changed by the barriers and the roughness (Z0) of dune surface was improved to reduce wind velocity above the ground. The roughness was increased from 0.0025 cm on shifting-sand to 1.5 cm in the revegetated sqaures of 1m1m straw checkerboards. Wind velocity (V2) at a height of 2 m and (V0.5) at a 0.5 m height were weakened separately from V2=9.6 m/s and V0.5=8.4m/s on shifting-sand to V2=7.1 m/s V0.5=5.0m/s in the straw checkerboards. Sand flow (Q) in the straw checkerboards is only 1% of that on the shifting sands (Table 2). So straw checkerboards and barriers are the available solution for fixing shifting sands, which was already applied widely through extension at grass-root level.
Straw checkerboards assisted by proper plants have better protective benefits, such as the roughness of old straw checkerboards with plants of 20-40 cm high and 20% coverage are separately 2.398 in 1m1m size and 1.886 in 2m2 m size (SSDRIDR,1991).
|
size（m） |
11 |
11.5 |
12 |
13 |
1.52 |
22 |
23 |
33 |
|
depth（cm） |
0 |
7.0 |
8.0 |
7.9 |
12.0 |
13.5 |
14.4 |
25.3 |
Table 2. Protective
effects of 1m1 m straw checkerboards
|
Type |
East wind |
|
Northwest wind |
||||||
|
V0.5 |
V2 |
Sand flow (g/cm m) |
Roughness (cm) |
|
V0.5 |
V2 |
Sand flow (g/cm m) |
Rough-ness (cm) |
|
|
Shifting Sand |
8.4 |
9.6 |
1.411 |
0.0025 |
|
6.8 |
7.3 |
1.374 |
0.0025* |
|
Straw barrier |
5.0 |
7.1 |
0.003 |
1.5000 |
|
6.4 |
7.3 |
0.013 |
1.0000 |
*Observation under no wind-sand flow
Influenced by straw barriers, the flowing field near the surface was seperated into four sub-fields, or two slowdown areas at the front and the back of barriers as the drifting flow was blocked by barriers and it was compelled to cross over the barriers, at the same time, a disorder flow will cross the enclosed fences and barriers（Liu Xianwan,1991）. These four smaller flow fields will weaken air current, reduce ability of sand transport and change function of transport rate of sands at the different height.
Both the straw checkerboards and sand barriers have the same functions on the shifting sand surface with multi-direction of wind. The establishment of the straw checkerboards should be separately treated on different surfaces of sand dunes according to the wind-sand disasters. A normal size of straw checkerboard is 1m1 m, which is less cost and easier to be accepted. The size of 2 sq. metres in 1m2m design is suitable to the flat and low-lying areas or the windward side of shifting dunes. Wheat straw and rice hay could be used as the protective materials as they are soft, flexible and durable for constructing the checkerboards and barriers. In the area with single wind direction, strip-shaped barriers could be established at the windward side to slow down and accumulate the sands.
Wheat or rice straws are costless and convenient for planting the checkerboards and sand barriers. There are rich sources of such materials in Shapotou area. It is estimated that 5-6 tons of straw materials is needed for planting the checkerboards in size of 11 m per ha. 50-60 man powers are needed for making one hectare of straw checkerboards. Total investment, including collecting and transporting is about 900-1,200 RMB Yuan per ha. In consideration of material source, dried reeds were widely used as main materials for establishing the checkerboards in prevention of highway to transport raw petroleum in the Taklimakan Sand Desert. Straw checkerboards in size of 1m1m play estimated role in controlling sand movement and in reducing the erosion. The wide application of straw checkerboards shows that they could be kept relatively for a longer time and the design of 1m1m size should be taken as suitable arrangement. The design of 1m2 m size could be adopted only at the non-eroded position of sand dunes, and bigger size are ineffective in fixing sand surface on a large scale as they are easy to be blown and they need to be replaced frequently after a wind or a sand disaster.
The technique to establish the setting straw checkerboards is simple. Firstly, the prepared straw materials are mulched in a thickness of 2-3 cm in strips on the sand surface, and each checkerboard is one sq. metre in size; then plant the straws into the sands for a depth of 15-20 cm with spades, and remain 15-20 cm above the surface; finally the checkerboards should be recovered with sands. Main strips of straw barriers should be vertical to main wind direction on the windward slopes of sand dunes. The main strip-shaped barriers should be firstly planted while establishing straw checkerboards; then subsidiary strips on the windward slopes have to be planted. Main strips of barriers should be set from the bottom to the top of a sand dune as the essential step, then subsidiary strips of barriers should be constructed on the slop form the top to the bottom at the leeward side. These methods could play a role of preventing the barriers from tramples and protecting the already made-straw checkerboards stabilizing shifting sands.
If straw checkerboards end on the windward slop or the top of sand dunes, the straw barriers should be extended or connected with that at the leeward slope of sand dunes. This is aimed to avoid the break of barriers on the section strongly eroded by wind blowout and sand accumulation. Meanwhile, the first row of vertical barrier should be set up on the top of sand dunes at front edges of protective systems. When planting checkerboards in large scale, straw paved walkside for carrying straws should be made along ridges of sand dunes.
Straw materials will lose gradually flexibility as well as break down partly after planting of 1-2 year, but they still have effects to anti-erosion of wind. After two to three years, the straw materials of the checkerboards will be turned into humus and weathered under the impacts of sand accumulation and sunshine. In general, lodged straws are gradually eroded by wind or covered by shifting-sand after 4-5 years, so that the barriers will lose their function to control erosion, and decline the protective ability for stabilizing shifting sands where revegetation has not yet grown up. In order to pledge integrality of barriers, the weathered checkerboards should be replaced and even re-established in time until plant community grown up and sand surface was fixed. Otherwise, it will need longer time and high cost to replace the straw checkerboards with biological mulching in the process of sand stabilization. Plant community and lichen crust will be formed when the straw checkerboards were replaced by biological approaches.
Shifting sands from outside of protective area will move continuously and bury straw checkerboards. Moving speed of sand dune is estimated about 10 to 20 m per year, and for some sections they may move forward to 50 m annually. Though straw checkerboards were replaced for several times, shifting sands will continuously invade and be accumulated inside the revegetated areas if they were managed without attention. In order to control the movement of shifting sands, sand barriers and enclosed fences have to be made and fixed sand mounds have to be artificially formed. Sand barriers and enclosed fence have good stability and anti-collapses, but they are easily to be deflated at the foot of the barriers or fences.
The effect of the barriers to accumulate shifting sands is normally influenced by the height, the density and the position of the barriers and sand fences. Lower barriers and sand fences are much easier to be covered by sands. Higher barriers and sand fencce will endure the pressure of strong wind, and they need quality materials and are discommodious for planting. An optimum height of barriers and sand fences should be about one meter in Shapotou area. Sand fences need to be maintained one time a year. 120-nip angle is good for flexing line fences. Sand accumulation is not same at different sections for topographical undulation, so sand mounds are different in heights. In order to increase width and height of sand accumulation, more than one row of the barrier and sand fence should be planted.
The study on the similarity in the wind tunnel shows that accumulative rate of barrier and sand fences is averagely 70-80%, maximum is 96.5% and minimum 44.6%. With increase of density, sand accumulation becomes longer on the leeward slopes, and volume of sand accumulation will be enlarged. Fences of 30% hole rate have a largest amount of amassing sand. As far as fences of 40% hole rate, the width of amassing sand on the both sides of fences is 18-20 times of fence height, sand flux is 6.2 m3/m.a, the rate of obstructing sand is 90%, and height of amassing sand is 1.1 m. The branch fences of 50% hole rate were eroded at the their bottoms initially and have amassing sand far away fences mainly on the leeward position. For these reasons, fences of 30-40% hole rate are widely used and have good benefits obstructing sand (Zhu Zhenda et al., 1998; Lin Yuquan et al.,1991).
The fences are made of vitex twigs, poplar branches, used crossties, maize stems and bamboo materials, among which maize stems fence is costless. Maiz stem fences require 30-50% density and 1.5m2.0m size. Usually 20-30 cm of fence is planted into sand layer, and supported with a 2 m long post, which is buried into sand layer at a depth of 0.75 m in a distance of every other 2 m. The ends of fences should be reinforced with triangle posts. The connecting parts of the fences should be overlapped by 10 cm. In order to prevent the fences from wind erosion at the bottom, 1-4 rows of straw checkerboards should be set up on the both sides of fences.
In the experiment area of drifting sand fixation of Shapotou station, the plantation was initiated since 1956 and expanded in1964 and 1965. According to the observation in May 1973, there were five pioneer species of plant collocations, the vegetative coverage of these collocation could be usually as dense as 30% and some could be even as high as 40% (table 3). Collocation of the fixing-sand plantation is designed in an inter-planting belts, namely a belt of two rows of shrubs, a belt of one or two rows of semi-shrubs. Space between rows was 11 m for shrubs and 0.5m1 m for semi-shrubs. The empty space between the belts is as wide as three metres beyond the belt composed of two rows of plant. The empty space behind the belt composed of one row of plant should be at least two meters.
Planting density of revegetation for fixing dunes should be considered at begining. Low density does not play the function to reduce wind velocity and to stabilize sands. High density will do consume more soil moisture and impact the growth of revegetation. Rational density should be stressed on the basis of root, water balance and field tests. For this reason, optimum density is 2,500-3,300 individuals of shrubs should be planted per hectare in Shapotou area (Table 4).
Secondly, affirmance of the density should be determined by the water balance in soils and plants. Water conditions can be divided into three periods. Firstly the period of water consumption from May to July, when rainfall is sparse and plants grow in bloom, which is a key period plants survive or withering. Secondly the period of water supply in rain season from July to Sept. Thirdly the period of water stability in which rainfall is sparse and plants need little water in dormancy. Long-term observation of soil/water dynamic in the plantation indicates that soil moisture is lower in the planted area than that in drifting sand area in the depth of 0-200 m. The position growing Hedysarum scopairum is the driest. It is obvious that water depletion of Hedysarum scopairum is highest. Hedysarum scopairum was selected as main species and observation was carried out through the period of water consumption from May to July. On the basis of mensuration of water consumption and saving and according to 75% of available moisture used by vegetation, plants densities are estimated that the density of Hedysarum scopairum is 2,240 shrubs per hectare and its coverage could be 12%; the density of Salix gordejevii is 1120 individual per ha and that of Artemisia sphaerocephala is 6,744 individual per ha (Table 5).
|
Collocation |
Plant |
Survive rate(%) |
Height (cm) |
Crown (cm) |
Cover (%) |
|
2 rows of Hedysarum scopairu -2 blank spaces behind 1 row of Caragana korshinskii + 1 row of Artemisia Ordosica -2 blank spaces behind |
Hedysarum scopairum |
88.3 |
97.2 |
103 97 |
29.5 |
|
|
Caragana korshinskii |
73.3 |
82.2 |
6760 |
|
|
|
Artemisia Ordosica |
80.0 |
49.4 |
7470 |
|
|
2 rows of Amorpha
fruticosa -1 blank space 1 row of Hedysarum scopairum -2 blank space 2 row of Artemisia Ordosica +1 row of Caragana korshinskii +2 row of Hedysarum
scopairum -2 blank space |
Hedysarum scopairum |
100 |
71.7 |
9891 |
26.8 |
|
|
Caragana korshinskii |
100 |
49.5 |
4035 |
|
|
|
Amorpha fruticosa |
50 |
29.2 |
2221 |
|
|
|
Artemisia Ordosica |
90 |
42.0 |
6161 |
|
|
2 row of Hedysarum
scopairum -2 blank spaces 1 row of Caragana korshinskii -2 blank spaces |
Hedysarum scopairum |
100 |
162.8 |
11891 |
32.5 |
|
|
Caragana korshinskii |
40 |
163.0 |
121107 |
|
|
1 row of Hedysarum scopairum +2 row Caragana
korshinskii -2 blank spaces |
Hedysarum scopairum |
66.7 |
121.8 |
114 111 |
30.8 |
|
|
Caragana korshinskii |
66.9 |
111.7 |
9796 |
|
|
2 row of Artemisia ordosica -1 blank space 2 row of Hedysarum scopairum -1 blank space 2 row of Artemisia ordosica -1 blank space 2 row of Caragana
korshinskii -1 blank space |
Hedysarum scopairum |
80.1 |
133.9 |
124103 |
43.7 |
|
|
Caragana korshinskii |