Prevention and management of wind-blown sand damage along Qinghai-Tibet Railway in Cuonahu Lake area

YinHai Yang , BenZhen Zhu, FuQiang Jiang, XiLai Wang, Yong Li

Northwest Research Institute Co., Ltd. of China Railway Engineering Corporation, Lanzhou, Gansu 730000, China

Prevention and management of wind-blown sand damage along Qinghai-Tibet Railway in Cuonahu Lake area

YinHai Yang*, BenZhen Zhu, FuQiang Jiang, XiLai Wang, Yong Li

Northwest Research Institute Co., Ltd. of China Railway Engineering Corporation, Lanzhou, Gansu 730000, China

This paper analyzes the characteristics of climate, geology and geomorphology, vegetation, and sand dune distribution in the Cuonahu Lake area beside the Qinghai-Tibet Railway. The types and causes of railway blown-sand hazards are discussed, and the effectiveness of various sand-controlling measures is assessed. From the perspective of integrated management, a sand-controlling system that combines several engineering measures, including nylon net sand barriers, concrete sand barriers, movable-board sand barriers, sand interception ditches, gravel/rock cover, film sandbags, and permanent vegetation is most beneficial.

Qinghai-Tibet Railway; Cuonahu Lake; wind-blown-sand disaster; prevention measures

1. Introduction

Cuonahu Lake is a tectonic lake lying in a mountain rift basin of a southern margin offshoot of Tanggula Mountain.The lake area is about 400 km2surrounded by about 131 km2of desertified grassland (Figure 1). The lake is 4,650 m a.s.l. and is the largest freshwater lake in the northern part of the Qinghai-Tibetan Plateau.

Cuonahu Lake is one of the most famous scenic spots along the Qinghai-Tibet Railway and is known by Tibetans as "ShenHu" (Divine Lake). The local arid climate and anthropogenic destruction have accelerated the process of desertification near Cuonahu Lake in recent years. This heightens serious security risks to the operation and maintenance of the railway, including aeolian accumulation, aeolian erosion, and abrasion of locomotives, vehicles, and communications signal equipment.

During the construction of the Qinghai-Tibet Railway,riprap slopes frequently became sand-buried and the porosity of the outer layers of riprap was blocked by drifting sand,affecting the ventilation of the riprap embankments. It required significant manpower and financial resources to prevent this during construction, and these factors continue to affect the normal operation and maintenance of the railway because cement hardening is polluted by blown sand, although a protection system was set up.

In winter and spring, drifting sand erodes the railway because large areas of lake bank are exposed, despite many engineering measures that were implemented along the railway in recent years. Sand disasters to the railway along Cuonahu Lake are especially severe in the strong wind season (October to April). For example, accumulated sand has encroached on the beam under the Basuoqute Bridge (18-32 m), which has seriously affected the safety of the railway(Zhuet al., 1994; Zhang and Gao, 2007).

In order to eliminate the blown-sand hazards near Cuonahu Lake and ensure the safe operation of the railway, here we offer some comprehensive control measures based on the characteristics of blown-sand hazards along the lake, and describe experiences with blown-sand hazards in other regions.

Figure 1 Geographical location of the Cuonahu Lake

2. Natural conditions in the Cuonahu Lake area

2.1. Climatic characteristics

Cuonahu Lake is located in the southwest region of Gansu, in the eastern part of the Qinghai-Tibetan Plateau. At 4,500 m a.s.l., it has a sub-frigid, semi-humid monsoon climate. The annual mean air temperature is about -1.3 °C; the hottest month of July average temperature is 8.0 °C with an absolute maximum of 24.2 °C, and the coldest month of January average temperature is -12.9 °C with an absolute minimum of -41.2 °C (Li SCet al., 2007). The normal annual precipitation is 411 mm, the annual maximum precipitation 583.9 mm, and the mean annual pan-evaporation is 1,690.7 mm, which is almost three times as larger as the annual precipitation. Relative humidity is 50%. Herbaceous plants and cushion plants are the dominant species in the study area, covering up to 30%.

Strong winds (≥8 m/s) mainly occur in the October to April period (Table 1), and the ≥4.5 m/s monthly mean wind speed is mainly distributed in the February to April period(Figure 2). In 2008 there were 157 days of strong winds (≥8 m/s), mainly in winter and spring, while the yearly mean wind speed was 4.12 m/s; moreover, in January 2008 there were 19 days of strong wind, with wind gusts up to 38 m/s,all of which provided powerful transportation conditions for drifting sand. The dominant wind direction is WNW, being same as the axial direction of local barchan dunes (Figure 3).

Table 1 Wind speed data at Cuonahu Lake during 2008

2.2. Geology and geomorphology

Low mountains and hills surround the basin of Cuonahu Lake, but there are piedmont plain and degraded lake beach on the northwest side. The surrounding terrain is flat and open, consisting of quaternary lake deposits, alluvium deposits, pluvial deposits, aeolian deposits, and talus deposits on the surface, and exposed bedrock is granitic gneiss of the Upper Paleozoic in the mountain area.

The formation lithology is as follows: (1) Fine sand is distributed from the surface to 8 m under the ground surface, which ranges from thick alluvium to moderate density and from moist to saturated; the natural moisture content ranges from 4.3% to 6.8% to a 2-m depth from the surface, and the mineral elements consist of quartz and feldspar. (2) Medium sand and coarse sand are distributed from 8 m to 30 m under the ground, which is moderate density and ranges from moist to saturated. (3) Gravelly soil is distributed from 30 m to 40 m under the ground, and

Figure 2 Monthly average wind speed rose map

Table 2 presents the trend of size fraction on the ground surface of Cuonahu Lake. It can be seen that the surface composition is mainly fine sand (0.315-0.160 mm and is slightly dense and saturated. (4) Granitic gneiss is distributed 40 m under the ground, and ranges from highly weathered to completely weathered.＜0.160 mm), which accounted for the overall were 75% and 93%, respectively. The particle distribution is uniform and the fine sand is aeolian sand.

Figure 3 Wind direction rose map

Table 2 Size fraction (%) on the ground surface of Cuonahu Lake

2.3. Vegetation

The dominant vegetation types at Cuonahu Lake are alpine grass and alpine meadow. The main specie isKobresia pygmaeaand the companion species areK. humilis,K.royleana,C. ivanoviac,Festuca ovina,Oxytropis glacialis,Astragalus confertus,Artragalus selago, andStipa purpurea.The alpine grass is clearly desertified and seriously degraded due to livestock overgrazing (Yanget al., 2007).

2.4. Sand dunes distribution

Mobile dunes are distributed widely, in a fan shape, on the east bank of Cuonahu Lake, and are mostly barchan dunes. The formation of dunes is closely related to the topography and geological conditions around Cuonahu Lake.There are different degrees of mobile dunes on the aggraded flood plain on the east bank of the lake, which is a sloping surface to the west of the nearby low mountainous region and the Basuoqu River valley. Drifting sand land is distributed in an area of about 3 km2on the east bank; the barchan dunes take up an area of about 2 km2from the east bank of the lake to the foot of the nearby low mountainous region and the river bed and both banks of the Basuoqu River (Wu,1987; Duan, 2002).

The barchan dunes are formed by the single dominant wind axial direction at the study area, which is generally N60°W. Their length is 10-30 m, width is 5-20 m, and height is 1-5 m; the windward sides are convex and shallow(grade 8°-15°), and leeward sides are concave and steep(grade 30°-40°) (Duan, 2002).

Because a mountain pass exists at the northwest side of the lake, grit from the lakeside zone is transported by the threshold wind along the direction of S60°E in the windy season; this grit is deposited and forms the barchan dunes and also the accumulating sand areas in the piedmont plain,the Basuoqu River valley, and the slope zone. The obstruction of the mountain body at the east side of railway and the influence of the Basuoqu River change the wind direction and reduce the wind speed, thus lowering the sand-carrying capacity of the wind. The wind-blown sand in the Basuoqu River valley and the slope zone is the source of sand carried by surface runoff, river erosion, and wind transportation, and redeposited on the east bank of Cuonahu Lake.

3. The types and causes of railway blown-sand hazards

3.1. The types of railway blown-sand hazards

The main types of blown-sand hazards are aeolian accumulation, aeolian erosion, and abrasion along the railway in the Cuonahu Lake area.

3.1.1 Aeolian accumulation

Fine sand is transported by wind action from west to east and leads to sand deposition within a certain range. In the flood season the wind-blown sand on the river bed is eroded and transported by the river, and the river bed and both banks develop high scarp. Due to sand burial, the Basuoqu River bed caused a swing phenomenon on the east bank of Cuonahu Lake (Duan, 2002). Sand burial is the most damaging and dangerous effect of aeolian sand accumulation.Small quantities of sand can fill the railway ballast and cover the slope surface, while large accumulations of sand can bury the rails, damage the track, halt the operation of trains,and even cause traffic accidents on the nearby highway of Qinghai-Tibet.

3.1.2 Aeolian erosion

Wind force causes intense vegetation denudation and transportation of fine sand on the ground surface. Because local evaporation is much greater than precipitation, the windy and dusty area at Cuonahu Lake is nearly always dry.Duan (2002) showed that the dry state exists from the sandy surface to a 0.2-m depth, because of continuous evaporation.Therefore, fine sand frequently moves on the mobile dune surfaces and the blown-sand flow caused by strong wind enhances wind erosion (Li WYet al., 2007). Areas that have no appropriate protection will be eroded, including the roadbed body and the surface of both railway roads.

3.1.3 Abrasion

Wind-blown sand has high hardness because it has many quartz grains, which can impact the locomotives, rolling stock, and communications signal equipment. Sand can increase wear of the rails, fastenings, and railcar wheels, and generally shorten the service life of the railroad. Sand filling the ballast bed can cause many railway problems, can increase the maintenance workload, and can shorten the major overhaul cycle. Dust can hinder the vision of the locomotive driver and can endanger the health and safety of workers and visitors (Di and Zhang, 1998). Sand damage not only endangers the traffic safety, but also damages the quality of the railway. Rails and fastenings that are buried in sand seriously corrode; the life of rails is significantly shortened because flaking rust blocks have been shown to be up to 100 mm long,80 mm wide, and as much as 4 mm thick (average thickness 2 mm) (Lin, 2007). Sand on the railway seriously impairs flexibility and drainage, resulting in other railway problems.The railway shoulder becomes eroded, lowering the track bed and exposing the sleeper heads, thus making it difficult to maintain of the railway gauge, level, and direction.

Table 3 provides data on the catastrophic railway sand damage near Cuonahu Lake in 2008. The damage occurred mainly on the right side of the embankment. A total of 3,430 m of track were affected, the eroded area covered a total of nearly 45,000 m2, and more than 99,000 m3of blown sand was deposited.

Table 3 Data on the Qinghai-Tibet Railway sand calamity at Cuonahu Lake during 2008

3.2. The causes of railway blown-sand hazards

As described in Section 2.1 and in Table 1, during the October-April windy season there is very little precipitation,the air is dry, and the river is generally low. These factors lead to most of the river bed being exposed, providing a rich sand source for wind transport. In addition, during the melting period from freeze to thaw at the end of cold season and the beginning of the warm season, the sand surface is loose and can be easily blown, transported, and deposited by strong winds (Wuet al., 2007). The unique terrain of the valley is a natural wind tunnel, and its east-west orientation is nearly perpendicular to the north-south railway. The railway is an obstacle to wind-blown sand flow, causing the sand to be deposited and frequently leading to sand disasters under the bridge and on both sides of the embankment.

4. Assessment of the effects of implemented sand-controlling measures

Table 4 summarizes the distribution and types of engineered treatments of the sandy road bed at Cuonahu Lake in 2008, affecting 8,333 m of road bed. In this section, some treatments are adopted to solve the problem of sand-damage.Just like stone-checker is adopted to protect the semi-fixed sand dune and sand barrier is used to control the serious parts around 200-300 m of the road bed.

Table 4 The distribution of engineered control measures on aeolian sandy subgrade at Cuonahu Lake during 2008

4.1. Evaluation of existing sand-resistance measures

Sand-resistance measures were designed and implemented based on the saturation rule of erosion and deposition of wind-blown sand. Artificial obstacles (high-banded concrete sand barriers) were set up at appropriate distances on the both sides of the railway, which led to wind-carried sand being accumulated behind them (Huanget al., 2000).Concrete walls were laid out at the forefront of sand flow actions, which resulted in accumulation of sand around the sand barriers, limiting the movement of sand. Sand barriers were installed at the windward and leeward sides of the railway. Each barrier height is 1.7 m, wall thickness is 10 cm, width is 10 m, and length is about 8 km, and they were installed 200 m, 100 m, and 50 m away from the railway.The sand barriers can prevent and delay the forward movement of sand and can lead to the accumulation of sand around the barriers, preventing burial of the railway.

After two years of observation, sand deposition was found around the concrete walls, which effectively captured considerable amounts of the wind-blown sand.However, the design did not fully consider the local dominant wind direction, which led to the concrete sand barrier not being exactly perpendicular to the dominant wind direction. Its built angle actually reduced the effect of sand resistance. Because it is not easy to judge the dominant wind direction, the sand-resistance measures were weak at the windward side and strong at the leeward side; the sand that was captured has become a new sand source under the shifting winds. In addition, the barriers were placed too close to the railway, which also formed new sand sources that threaten the safety of the railway.

4.2. Evaluation of existing sand-fixing measures

Within a certain range on both sides of the embankment,sand-fixing protection measures were adopted to prevent wind-blown sand damage. The purpose of fixing sand is to solidify or cover the sand surface so as to avoid wind erosion to prevent a new sand source. The two primary approaches to fixing sand are, first, to cut off contact between wind and the sand surface, and second, to increase the surface roughness to reduce the wind speed at the surface(Chen, 2004). The specific methods are as follows.

(1) Pebbles and macadam covering the sand surface.Pebbles, macadam, and other materials cover the surface to a thickness of up 2-3 cm. In one test section, the leeward side of the embankment was covered to a width of 10 m and length of 8 km, at a distance of about 200 m from the railway. This design has been shown to be rugged, durable,and able to resist wind and heat, and retain water. After two years of observation, the windbreak and sand-fixation ef-fect has been good; sand has been effectively prevented from leaving the land surface. However, not enough stone was used, due to the increased cost of transportation.Therefore, the thin cover layer of pebbles and macadam was blown away by strong wind-blown sand and the original sandy surface was re-exposed, becoming a new sand source.

(2) Checkered stone sand barrier. A semi-concealed sand barrier with a grid-like surface was filled with rubble.This was installed in a test section on the leeward side of the embankment, with a width of 10 m and length of about 8 m, 150 m from the railway. It has been shown to effectively reduce the surface wind speed in different directions,fix the in situ sand surface, and also prevent sand from other places depositing around the barrier. Thus, this barrier plays dual role of sand resistance and sand-fixing (Di and Zhang, 1998). After two years of observation, considerable shifting sand accumulated inside the box and a stable concavity of erosion and deposition was formed, effectively increasing the surface roughness, increasing air resistance, reducing the surface wind speed, and reducing the amount of wind-carried sand (Zhang and Zou, 1990). However, the concavity of erosion and deposition that formed after about one year has in some places sustained serious wind-blown sand damage and cannot continue to prevent sand movement. It needs to be renovated at significant cost,so the life of this type of barrier is evidently short.

5. Recommendations to prevent and control wind-blown sand damage

Railway protection systems consist of one or several protection zones along and parallel to the railway. As more serious sand damage occurs, wider and more frequent zones will be established and more types of protective measures will be utilized. The two most effective sand protection measures are sand-fixing and sand resistance; sand-fixing is implemented farther away from the railway and sand resistance is implemented nearer to the railway (Zhang and Zou,1990). Given the variations in dominant wind direction,Because of the complicated and variable topography and geomorphology in this wind direction, the treatment of transportation, which could cause sand damage along the railway, should not be adopted. During the construction of sand control measures, removing sand near the railway is the recommended first step, as well as re-engineering any sand barriers that failed to control wind-blown sand, and ensuring that barriers are perpendicular to the dominant wind direction. Based on the characteristics of the wind-blown sand damage, intensity, sediment size, and terrain, the barrier designs must factor the distance between barriers, height, and density.

5.1. Recommended sand resistance measures

Sand-resistance belts are mainly located at the outer edge of the sand-fixing belt(s), at an appropriate distance from the railway. These measures include nylon net sand barriers,concrete sand barriers, upright sand-resistance barriers, and sand interception ditches at the periphery of sand-fixing belts, all placed on the primary and secondary windward side of the embankment.

5.1.1 Upright sand resistance fence barriers

Wind-blown sand movement can be intercepted by upright sand-resistance fence barriers, which leads to the sand being deposited around the fence barriers rather than burying the railway. The principle of a sand-resistance fence barrier is that the fence can change the local-scale airflow direction and speed, leading to reduced airflow speed after the fence; the wind-carried sand is deposited near the fence.When the fence is tightly structured, the range of deposited sand on the windward side is usually 2 to 3 times the height of the fence, and about 4 to 5 times the height of the fence on the leeward side. According to Donget al. (1989),if the fence porosity is 25%, less sand is deposited in front of the fence but the range of deposited sand on the leeward side is usually 7 to 8 times the height of the fence. If the fence porosity is 50%, the range of deposited sand on the leeward side is usually 12 to 14 times the height of the fence.

(1) Nylon net sand barriers. Sand damage surveys in recent years showed that local sand sources include sandy loam soil that is exposed when the lake water level drops in the winter season, and sand sliding across the frozen lake surface. To control those sand sources, nylon net barriers can be set up near the railway at the outer edge of the lake, which can immobilize sand on the bank of the lake.(After the water level rises in summer, the sand on the bank lake is transported by water movement to the center of lake, which can effectively reduce the amount of movable sand.) In addition, this measure may also play a role in environmental protection by effectively blocking the garbage of the lake bank, which can preserve the ecological environment of Cuonahu Lake.

It is recommended that the nylon sand barriers should be up to 2 m high along the lake bank; a porosity of 30% is appropriate. The appropriate angle between a sand barrier and the main wind direction is 80°. The intervals between the concrete columns should be no more than 5 m, their height should be 2.5 m, the burial depth should be 50 cm(leaving 2 m exposed), and the diameter should be 20 cm. At every 50 cm of height, two symmetric hooks should be set in the concrete columns on which to hang the nylon netting.

(2) Upright concrete sand barriers. To windward side,concrete barriers, which are 2 m high and vertical to dominant wind direction, are set 20 m from the nylon net barriers. The concrete barriers which are compact structure should be repaired in time and covered with gravel when they will fail.

In the secondary wind direction, upright concrete barriers should be set about 50-100 m from the railway. The other characteristics of sand barrier placement are similar to those in the primary wind direction.

(3) Movable-board sand barriers. Movable-board sand barriers are upright structures that can absorb wind energy,reduce the strength of wind-blown sand flow, and can effectively prevent sand movement. These structures can be built of reused concrete boards that are installed upright in sand or hung up. This measure is cost-effective and has a long life.

Board barriers should be set up about 10 m away from upright concrete barriers, perpendicular to the dominant wind direction. The height should be 2 m and the porosity should be 40%.

Board barriers set at the second wind direction should also be 10 m away from upright concrete barriers, perpendicular to the dominant wind direction, with a height 1.5 m and a porosity of 30%.

5.1.2 Sand interception ditches

The bottom surface of Cuonahu Lake is sandy loam soil and has considerable vegetation that can fix sand, resulting in relatively little sand movement. In this situation sand resistance can be accomplished by sand interception ditches and sediment barriers. Ditching and embankments are located 150 m from both sides of the railway; the ditch width is 2 m and the depth is 2-2.5 m. A trapezoidal dike made of excavated soil, cement, and gravel was built at the windward side and the ditch was dug perpendicular to windward. All four sides of the ditch were sprayed with cement to prevent wind erosion. The ditch will fill with sand after 2-4 years, at which time it can be covered by gravel. Then a new ditch will be opened about 30-50 m away. This measure can prevent considerable sand movement. It is low-cost and easy to engineer.

5.2. Recommended sand-fixing measures

Sand-fixing immobilizes sand near the railway and leads to formation of a crust on the surface of shifting sand, lessening its threat to the railway. Moreover, sand-fixing measures can block transitory quicksand. The most common sand-fixing measures are gravel/rock coverage and sandbag barriers. The sandbag material should be UV- and weather-resistant, and the sandbags are usually set near the railway embankment, which can reduce transportation costs. Moreover, based on the local terrain and wind strength, the placement and distance between the sandbags can be adjusted. Li (1980) reported that sandbags can function as barriers when they are buried. The width of this engineered protection should be based on local wind direction, wind speed, sand sources, wind-blown sand strength, and other factors.

5.2.1 Gravel/rock cover

A semi-concealed, grid-like barrier alternating with gravel/rock cover, set in a wind-blown sand area, not only can fix the sand surface but also can accumulate sand from other places. This type of sand barrier plays a dual role of sand-fixing and sand resistance, and its prevention effect is remarkable. Placed at the foot of embankments, the ranges of sand-fixing can be 200 m wide at windward and 150 m wide at leeward. Gravel/rock cover and grid-like barriers are placed at 50-m intervals. The gravel covering on the sand surface should always be 5-8 cm. If stone is not available,slag covering can be used just as effectively, at less material and labor cost.

5.2.2 Film sandbag barriers

In this approach, UV- and weather-resistant film sandbags form a semi-concealed, grid-like sand barrier. The distance between the barrier and the railway is 200 m in the main wind direction wind, and 150 m in the secondary wind direction. The sand barrier sits 20 cm above the ground and the range of sand-fixing is 50 m at the windward side and sub-wind side. Moreover, the sand barrier is vertical to the prevailing wind direction, and these barriers are parallel to each other and are generally 1-2 m apart. The top of the next barrier line should be 5 cm higher than the bottom of the previous barrier line. When the grid-like box and sandbag eventually become buried and no longer prevent sand movement, they can be reset and continue to be effective.These measures can last up to 5 years, are low-cost, and are easy to transport.

5.3. Vegetation measures

In some areas around Cuonahu Lake there is sufficient irrigation water and suitable climate for permanent vegetative protection measures. Plants can be an excellent improvement to the local microclimate, and desert plants can consolidate the surrounding sand with their extensive root systems, and thus offer the benefits of controlling desertification and reducing the damage of wind-blown sand.

According to Gaoet al.(2004), local dominant plant species should be chosen, and planted at intervals among stone boxes, film sandbag barriers, movable-board sand barriers, and leeward of upright sand barriers. They should be planted in the spring, when those barriers are first installed. With reasonable maintenance, plants can survive well and play the roles of sand-fixing, sand resistance, improving the local climate, and preserving the ecological environment.

6. Conclusions

Sand protection measures at Cuonahu Lake must consider the primary and secondary wind directions, sand sources, topography, and other natural conditions, and should be based on sound engineering principles and cost/benefit ratios. Nylon net sand barriers, concrete sand barriers, movable-board sand barriers, sand interception ditches, gravel/rock cover, film sandbags, and permanent vegetation measures have all been shown to be more or less effective and, collectively, can protect the safe operation of the Qinghai-Tibet Railway and can enhance the local environment.

The work was supported by the China National Natural Science Foundation (Gant No. 50908152), and the Special Funds from Scientific Research Institutes Technology Development and Study Projects (2008EG123206 and NCSTE-2007-JKZX-209).

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10.3724/SP.J.1226.2012.00132

*Correspondence to: YinHai Yang, Engineer of Northwest Research Institute Co., Ltd. of China Railway Engineering Corporation,Lanzhou, Gansu 730000, China. Tel: +86-931-4952725; Email: yyh1_2_3@163.com

May 12, 2011 Accepted: August 21, 2011