Chen Guangting
(Institute of Desert Research, Chinese
Academy of Sciences, Lanzhou, 730000)
China is one of the countries that
covered a vast desert area with serious sandy desertification. The whole
national area of desert, Gobi, and the area of sandy desertification is about
1,530,000 km2 (According to the data published by the national
conference of control sandy disasters on Set, 29, 1991), it is about 15.9
percent of the national area. Being open to traffic, the railway lines of
Baotou-Lanzhou set a precedent for constructing roads in the sandy area. For
forties of years, the railway lines of Jining-Erlianhaote, Gantang-Wuwei,
Lanzhou-Wulumqi, Baotou-Shengfu and so on had been constructed that cross
desert and Gobi area. The lines of Kuerle-Kashi, Baotou-Yan'an are being
constructed at present, the lengths of railway lines cross the sandy area are
about 1,200 km, with wind-sandy disasters taking place in an area of 200 km in
width and 1,000 km in length. Up to now, being taken into effectiveness of
those is about halves, the others are being brought under control for the time
being. Many of the national highways cross the sandy area, the major lines including:
the highway lines of Xilinhaote-Zhangjiakou, which cross the sandy area of
Hunsantake, that of Baotou-Yulin cross the Kubuqi and Maowusu desert, and the
desert highway of Tarim desert. According to the information we acquired, the
first desert highway had been constructed with prevention systems is the line
of Yinshuiqiao-Yanhu which lies in the Tenggeli Desert in Zhongwei County,
Ningxia Autonomous Reigon, the whole length is about 22 km. Being open to
traffic in 1995, the desert highway of Taklimakan Desert crossing the sandy
area with the lengths of 447 km had been constructed the prevention systems
along the highway with the system on a large scale along desert highways, which
is rarely found in other countries of the world.
The combinations of trees, shrubs, herbs,
and the established biological prevention belts is the methods of solution,
however, because of the natural conditions, it is difficult to construct the
biological prevention systems in the arid area. Even in the locations with
better natural conditions, it will take a long time to rebuild or construct the
man-made vegetation. Therefore, before the construction of biological
prevention systems, it has to be combined with the engineering methods. In the
extremely arid area, it is more depended on the engineering sand prevention
systems that need to be kept for a long time. The prevention systems had been
constructed along the railway lines of Yinshuiqiao-Gantang for about forties
years, the frontier prevention fences have to be safeguarded, and the straw
checkboard belts have to be renewed constantly.
Traditionally, the measures of preventing
sands had been divided into three types: that of vegetation, engineering, and
the chemical methods. The author considers that it can not cause chemical
reactions through the uses of natural or synthetic solidified chemical liquid
among the particles of sands, so, the chemical measures of solidifying sands
can not be in parallel with the measures of engineering. The chemical measures
of solidifying sands through the binding of sand particles by chemical liquid
is the merely way of engineering prevention sands. There are great natural
difference between the east and the west of the nation, though, most of the
regions adopted the synthetic measures that combined with of engineering and
biological along the arterial highways and railways, but, they lay particular
emphasis on some of the measures. In east, the biological measures played a
leading role, and the engineering measures are the supplementary ones. While in
the extremely arid area of the west, the engineering measures are adapted
extensively in the middle of the nation. Especially in the area of annual
precipitation at less than 200 mm, it is more important to combine the measures
of engineering with biological for a long time.
1 Sandy disasters prevention and control measures
in wandering dune area----A case study of the prevention systems along the
railway lines of Baotou---Lanzhou in Shapotou area
The item "The constructions of the
prevention systems in the line of Baotou-Lanzhou in Shapotou" acquired the
special national prize on progress in science and technology in 1986. As the
initial desert railway lines which cross the southeast edge of the Denggeli
Desert in the location of Yingshuiqiao-Gantang, the prevention systems had the
pioneering qualities in the location of Shapotou. It is the typical prevention
systems that under the measures of fixation taking the first place, combing
fixation with prevention, as well as the efficiency of transportation and
conduction, it is the synthetical prevention systems under the combination with
the measures of engineering and vegetation.
In Shapotou area, grid dunes dominate
among the types of aeolian sand landforms and the relatively height is 15-20m,
annual average precipitation is about 185mm, groundwater level is about under
several decades of metres, so, it can not be utilized by the vegetation. On the
surface of the dunes, the dry lay of sands is 3-20cm, under it, the stable
moisture content is about 2-3%, the vegetation cover of Agriophyllum squarrosum, Pugionium cornutum, Stilpnolepis centiflora,
Artemisia sphaerocephala, Hedysarum scoparium is only 1-2 percent.
The prevention systems of Shapotou have
two fundamental belts: that of fixation and prevention. It can be divided into
four belts by the disposition. The first, the belt of preventing sands on the
edge, with different materials, whose height is about one metre, and as the
sand-fences to prevent the sands. The second, the belt of fixing sands with no
irrigation. Under the prevention of the straw checkboards with the standards of
1m1m,
are the main parts of the prevention systems (mixed with the shrubs and herbs).
The third, the belt of trees and shrubs with irrigation. The fourth, the belt
of transporting sands with pebble platforms. Practically, the lengths of the
integrated fundamental belts are about 2 km, if including the sections of the
second to the fourth belt, the lengths are about 9 km, and the rest is only
composed of the belts of fixation and transportation. In the range of the
inspected (k701-715), the average lengths of prevention belts are about
235-580m with the average of 335.5m and the fixation belt is 150-480m with the
average of 241m. Naturally, the belt's width of trees and shrubs is 17-90m with
the average of 67.7m. In practical conditions, it is wider a little bit. The
width of the synthetical prevention systems with reasonable construction is
enough at about 130m. However, it is extremely necessary with the efficient
preventing belts in practice. It can be set several rows of fences to prevent
sands when necessary, with those, most of sands be prevented under the fences,
and so that the sand can be stablished efficiently.
The experiments are carried out by
introducing varieties, which are selected by sifting the seeds of Artemisia ordosica, Caragana Korshinskii,
Caragana intermedia, C. microphylla, Hedysaru mlaeve. But, with the growing
of the plants, the consumption of water resources increased with underground
water decreased, and with artificial vegetation evolved gradually, the cover of
the vegetation is decreased. In the mean time, on the surface of the sands
appeared many of inferior vegetation such as mossflora, phycophyta and so on. Artificial vegetation degeneration
is expressed as follows: Density of cover decreased sharply. It had begun to
construct the vegetation sand stabilization belts since 1956, with the
expansion in the middle of 60s, according to the data of 1973, the density of cover
reached about 30 percent generally, and some reached about 43.7 percent.
However, with the surveillance in 1976, the total density of cover decreased at
about 30 percent. Further more, some vegetation of the density of cover is only
22.5 percent.
Construction varieties of the vegetation
population. Through the introduced cultures as the vegetation of sand
stabilization, even if the locally sand stabilization vegetation with better
characteristics, because of the changes of ecological conditions (mainly
because of the sand stabilization, and the lost of the conditions of being
buried by sands), plant do not bloom or only bloom but do not bear fruits. As a
result, all vegetation but artinisia
ordosica cannot be regenerated naturally, therefore, the communities will
be succeeded with the only plant of Artemisia
ordosica.
As mentioned previously, the reduction of
the density cover will not impart influences on the sand stabilization because
of the consolidated soil in the surface of the dunes. After the artificial
vegetation had been constructed in 3-4 years, the surface of dunes appeared the
consolidated soil with the width of 0.1-0.2cm. After 8 years, it may be
developed to the width of about 0.5cm, and 1.2cm in 25 years. The consolidated
soil is formed firstly with friable soil naturally, beneath the grounds are
sand soils which are similar to the sands, when time goes by, the consolidated
soil will be stabilized naturally. According to the data surveyed in the field,
after about 24 years, with the artificial vegetation being formed, the color of
the consolidated soil is changed into brown. So, the abilities against erosion
is strengthened, and appeared the lay of brown soil whose thickness is about
3-4cm. Firstly under the affection of vegetation and the stabilization of sand
surface, the consolidated soil can be formed. Being taken part in the process
of the consolidated soil forming, the mossflora such as bacteria, fungus, blue
algae and so on are important. According to the analytical results, nitrifying
bacteria, ammonifying bacteria is doubled than that in shifting sands, even
appeared the kinds such as azotobacter and so on; the kinds of blue algae
including: microcoleus vainatus,
oscillatoria pseudogeminata, phormidium amblguum, schizothrix rupicola. The
kinds of terra cariosa included: navicula
minima var atomoides, hantzschia amphioxys. The kinds of mossflora
included: barbula ditrichoides and bryum argenteum and so on. They
propagated in a large number on the mantle soil.
With the growth of the consolidated soil
and the soil forming process, the herbs such as bassia dasyphylla, corispermum, eragrostispoaeoides propagated
sharply, herbs such as salsola ruthenica,
setararia viridis, echinops gmelinii, allium monolicum and scozonera divaricata
begin to invade. The more herbs amass, the thicker the mantle soil is, and
the dunes will be more stabilized.
Decades of plants, such as semi-shrubs,
shrubs, and trees had been cultured since 1956. However, many of them can not
be adapted with the bad conditions, even the kinds of trees in location,
because of the scarce of water and nutrient within dunes and will be in a state
of poor growth. The trees with strong adaptabilities, for example, locally elaegnus angustifolia can form the shape
of shrubs. Trees and shrubs were established with irrigation water of Huanghe
river in 1956 with the used tree species such as robinia pseudoacacia, populus simonii, P.gansuensis, pinus sylvestris
var mongolica and so on and the used the shrubs such as Salix ordejevii, amorpha fruticosa and
so on. With the healthy growth of trees the function of preventing wind is
obtained. However, forest culturing in the arid area, especially culturing the
trees must acquire the preconditions that have resources of water for
irrigation, so, it is difficult to apply and disseminate.
2 Sandy Prevention and
Control Measures along the Railway Lines in Gobi Area----A Case Study of
Prevention Systems in Yumen
Many of the national railway lines cross
the Gobi area, because of the strong wind and scarce of sands, and produced
unsaturated sand flows. In Gobi area, the action of sand flows can be taken as
erosion. The 60 percent of lines of Lanzhou-Xingjiang cross the Gobi area and
the Yumen sections (Hesan Lake-Wangdong, Sanshilijing-Gongchan River, Junken-Erdaogou)
lies in the north of Qilian Mountain. "Construction of prevention systems
along the railway lines of Lanzhou-Xingjiang in Yumen area" acquired the
first national prize on progress in science and technology in 1954, even more,
it can be taken as the typical engineering means against the disasters of the
sand flows in Gobi area.
In addition to that both sides of railway
lines pebbles (grain size distribution at 2-20mm) cover high platforms,
scattered the sand fields, scrub dunes and yardang. Yearly precipitation is
only 61.3-85.3mm and varies dramatically with only 25-41.5mm in some of the
years, in contrast to that high-water year, it is at 165mm. The amount of Sept.
rainfall occupies one fourth of the whole year's precipitation, in contrast to
the days of no rainfall about 140 days in spring and winter. In Gobi area, wind
is very strong, the average wind velocity is about 3.3-4.7m/s in spring and
winter. The days of having gale is 42 annually. All of this is disadvantageous
to the growth of plants, however, the water resources on the surface of grounds
of Changma River, Chijing River, Baiyang River and Shiyou River is advantageous
conditions to construct the belts of forest with irrigation.
In accordance with the strong wind,
scarce of sandy resources and the erosion abilities of sand flows, main
measures were adapted, including "reduce the wind velocity and shut off
sandy resources" and "combination of prevention and protection"
parallel with or nearly parallel with the railway lines with auxiliary
engineering or biological measures. Therefore, the prevention systems mainly
composed of the forest belts. At the edge of the railway lines are the higher
sand barriers which include shrub branches and other materials. With those
materials, the fences height are about 1.5m at the backWards of fences, there
comes a 10-15m wide forest belts, which, according to the situations of wind
and sandy resources, can be acted on the principles "Set protection
measures according to the degree of disasters, and protection is more important
than bring it under control". Generally it can be classified into three
modes: (1) Average protection mode. On the upwind side, set one or two rows of
forest belts, in front of the railway lines, set the first rows of the forest
belts with the width of 20-30m. On the side which toward the direction of the
wind, disposed two rows of shrubs, the other side disposed three-five rows of
trees, every 50m set the second protection forest belts, whose width is about
20m, and mixed with trees and shrubs. The forest belts' edge keep a distance
about 3.5m from the railway lines, the total width of the prevention is
130-195m. (2) Secondary protection mode. Generally on the upwind side have two
rows of forest shelterbelts, on the basis of the general protection mode,
expanded the width of the prevention belts on the upwind side, the width is
about 40-50m, row spacing is 5m still. In the front of the systems, disposed
three-four rows of shrubs, the other side six-seven rows of trees. Also, according
to different situations, expand the width of forest belts on the secondary side
which toward the direction of the wind, the width is about 20-25m, the total
width 210-220m. (3) Focal protection and prevention mode. On the basis of the
secondary protection and prevention mode formerly expressed, on the frontier of
upwind side, added another forest belt, the width is 40-50m, in the frontier,
set shrubs, behind those are trees. The intervals between two belts are 100m,
on the secondary side which toward the direction of the wind, the forest belts
are widened to 25--30m, on the sections with serious sand disasters the total
width is 370m, outside the prevention systems, the natural vegetation can be
protected with irrigation. In addition to those measures, utilizing the scrap
material such as black liquids of oil refinery and paper mill as the materials
to cover the blown sands are used on both sides of railway lines in Yumen.
According to the different goals of
preventing sands and against wind, different trees are selected. When the
shelterbelts are afforested that those shrubs covered a large scale of grounds
or branch roots flourishing, against sand covering, cutting, hitting and
adventitious roots soon be grown up after sand covering, adventitious buds soon
be sprouted after erosion, can endure the physiological drought, having
abilities of regeneration, be easy to propagate and be selected. Shelterbelts
require these plants which have tall and big canopies, growing up and closing
quickly. These trees with root systems developing quickly and with strong
abilities against wind, enduring drought and infertility, should be selected.
Especially trees are dominated. Through selected by sifting of seeds, the trees
adapted to both local natural environment and those conditions formerly
expressed, including those species of Poplus
gansueusis, P.alba, P.albla var pyramidalis, P.hopeiensis, P.opera, P.nigra, E.laeagnus angustifolia, Hippophae rhamnoides, Caragana korshinskii, hedysarnm scoparinm, endronhaloxylon ammod and so on.
In Gobi area, the pebbles are far more
than soil with fertilizer washed away constantly. Therefore the results of low
survival rate appears or the trees do not bloom, and are difficult to form the
forest. In the process of afforestation along the railway lines, according to
both laws that sand flows deposited when encounter the barriers or
topographical relief and the experiences, methods of digging trenches to plant
trees are tried. With trench loose deposited soil, within them containing dead
litter, the soil are somehow some fertile and has water-holding capacities so
as to offer the better beds for trees growth (especially on second growth) and
increased survival rate of trees.
The work has begun to investigate, study,
and design in terms of protection and prevention engineering measures since
May, 1966. After 20 years' construction and study, by the end of 1994, on both
sides of railway lines Junken-Erdaogou, Sanshilijing-Gongchanghe River, 96 km
of shelterbelts of prevention have been established and the area of forestation
is about 560 ha, so as to form the green barriers on both sides of railway
lines and control of the invasion of sand flows to the railway lines, to assure
the safeties of driving vehicles, and to accumulate billions of Chinese
currency RMB in terms of economic benefit.
3
Prevention and Control Measures along the Highway in the Extremely Arid Area----a
Case Study of the Engineering Measure Prevention System along the Desert
Highway in Taklimakan Desert
As the largest desert in China, and the
second active desert in the world, Taklimakan desert are occupied by 85 percent
of the area of the wandering dunes, having the strongest fluxion among the
deserts in the world. 92 percent of lines cross mobile sand area, the desert
are composed of all kinds of mobile dunes with different shapes and heights. In
the hinterland, the relative height of crescent dunes is about 1m, and moving
velocity is 3-5m, yearly transporting sands qualities is 1,000-2,300 cubes for
every one kilometre section. Under bad enviroment like this, it is difficult to
build roads, and even more difficult to prevent sand disasters and assure the
highway unlocked.
Yearly precipitation is only 32-48mm
along the desert highway, while evaporation capacities is above 3,200-3,400mm,
and yearly average relative humidity is only 45 percent, especially in the end
of spring and the begining of summer even is only 10 percent. The highest
temperature of the sand beds in summer is about 74℃,
in winter -23℃,
the thickness of dry sand lays is 20-40cm, while underground water level is 8m
deep in many of the hill lands. Further more, in addition, fewer of
palaeochannel of river, have fresh water, the underwater in most of places
whose mineralization degrees are above 4.5g/l, even 25g/l. All of these
conditions caused that preventing sands by biological methods is very difficult
to practice. So, assuring the routes "Based on the measures of machinery
prevention, and assured the roads unlocked, targets are aimed at constructing
and rebuilding the ecological balance by the measures of biological prevention
with chemical stabilization sands as the auxiliary measures". The first is
to construct the engineering prevention systems by constructing fence barriers
and laying straw checkboards, the second, creating conditions and utilizing
irrigation with salt water, and constructing "green asiles".
Among the engineering measures,
prevention, stabilization, transportation, conduction, and control are used synthetically.
Building highway in desert marked the study of the measures on control sands.
To sand flows, principles of "transporting and conducting, taking the
measures of prevention and stabilization only when necessary" are taken.
The designers also took part in designing of road beds, especially on the wind
gap or sand platforms, designed the section for transporting sands, and cleared
the barriers on both sides of the lines, therefore, benefiting the sand flows
cross the roads and with no depositing on the road beds. In the sections where
the direction of wind is single, divided the fences into sections with tangle
angles, so, formed the feather rows to conduct sands.
Most of the prevention systems are
sand-fences and straw checkboards along the desert highway in Taklimakan
Desert, experimental materials such as macromolecular compounds, emulsifying
pitches, salt liquids are used on the experimental sections. The materials
woven fences mainly took the materials such as the reeds, in the later, in order
to construct quickly, most of fences had been superseded by nylon webs, the
materials to weave the straw checkboards take the material such as reeds which
are crushed. Widths of shelterbelts are mainly according to the positions of
aeolian sand landforms and are adapted flexiblely. Usually on the upwind side,
the width is about 50-80m, narrowly 30m, broadly 40m, the other side is 30m,
the lengths of the sand-fences in all the highways are 893km, and the straw
checkboards are about 535,200,000m2.
Through experimental stages of selecting
seeds and irrigating with salt water along the desert highway, a dozen of shrub
and herb seeds are selected as the vegetation of stabilization sands. For the
time being, the experimental section is being constructed, and more than
10,000,000 nursery stocks are prepared. Therefore, the material preparation is
aimed at biological prevention.
The prevention method with engineering
measure along the desert highway in Taklimakan Desert assured the highway
unlocked, and accelerated steps of oil expendition and development in
hinterland. Being open to traffic for only a year, the desert oil field is
uncovered with proved reserves acquired at above one hundred million tons. In
the Tazhong area, the yearly production of about 20,000,000 tons on the second
year, the desert highway saved transportation expenses at about two hundred
millions. The item of desert highway had been appraised for the tenth national
achievements in science and technology in 1995, and acquired national first prize
on progress in science and technology in 1996.
All of illustrations formerly mentioned
showing: Sandy disaster prevention and control measures of highways and
railways must be based on both the characteristics of regional sand features
and the analysis of the activities of sand and wind, and comply with the
fundamentals of wind-sand physics. Only through application of the biological
and engineering measures synthetically, can such as the measures of prevention,
stabilization, transportation and conduction, achieve good results.