(Shaanxi Research Institute for Sand Control, Yulin 719000)
1 Introduction
Flattening sand dunes with flood water is a way of transporting sand to the desired position by conducting or pumping water from water sources, rivers, lakes or reservoirs in the sand area to flatten sand dunes. Reclaiming desert land in such a way is called flattening sand dunes with flood water and reclaiming desert land in free season, which is one of the comprehensive measures to control wind sand and also a major method of building farmlands.
Flattening sand dunes with flood water is a great beginning of the masses of the sand area. Early in Ming Dynasty, the masses of Leilongwan Village in Hengshan County of Yulin Prefecture tried to find out a tentative method of flattening sand dunes with floodwater, but it was done independently with no significant result. According to the local history of Jingbian County, Zhang Chongcheng, a villager of Yangqiaopan improved the method in 1942, and he was the first to flatten the sand dunes 2.4 km2 around Yangqiaopan. As a model worker, Zhang Chongcheng was awarded the title of Hero for Water Conservancy, and attended the gatherings of heroes in the Shaanxi-Gansu-Ningxia Border-Area in 1944. Now the irrigated land of 267 km2 in Yangqiaopan has been reclaimed with the method since then. After the founding of the People’s Republic of China, scientific researchers and the masses in the sand area have improved the method and spread the experience constantly. Farmlands have been reclaimed extensively by using the technique of flattening sand dunes with floodwater. By the end of 1996, the reclaimed desert lands reached 95000 km2 in Yulin Prefecture, Shaanxi Province, which pushed desert control to a new phase. The sand areas have good harvests in succession due to the change of environment. The per unit yield of corn, rice and potatoes ranks first in the province. Per capita consumption of grain is 100.5 kg higher than the prefecture level. Credit must go to the flattening of sand dunes with floodwater and reclaiming desert land.
The method of flattening sand dunes with flood water and reclaiming desert land in free season should be adopted in sand areas where there are water sources.
The basic requirement for the method is water sources. Yulin sand area has a vast territory and very rich water resources. There are six big rivers such as the Kuye River, the Tuwei River, the Yuxi River, the Qingshui River, the Hongliu River and the Luhe River, with a drainage area of 14,700 km2. Besides there are big reservoirs such as Xinqiao, Jiucheng, Hekou and so on, and main irrigation canals of Yudong, Honghai, Xisha, and Dinghui. The sand area has a surface runoff of 1.71 billion m3, underground water resources of 1.73 billion m3, and with an amount of replication of 1.03 billion m3 subtracted, the total amount of water resources is 2.41 billion m3. There are also more than 200 lakes with various sizes, among which the Hongjiannao Lake is the biggest, with an area of 54 km2 and a storage of 500 million m3. There are a lot of oases, with underground water lying hidden in the earth very shallow. The adequate water sources are very favourable conditions for flattening sand dunes with flood water and reclaiming desert land extensively.
3.1 The preparation for the project
3.1.1 Forming a leading group
In the process of construction, a temporary construction leading group should be formed of the leaders from the township government, the water supervision station and the administrative village to be in charge of the construction. Natural village is a basic unit in rural areas.
3.1.2 Making a plan
After the leading group is formed, a detailed construction plan should be made, including the sequence and schedule of constructing canals and roads, the calculation of the demand of water yield, labour, materials and equipment, and the budget for various expenses, and also the detailed rules and regulations and the safety codes of construction.
3.1.3 Raising funds
The project should follow the principle of taking self-raised funds as the dominant factor and the government support as the auxiliary.
Except for those large diversion works invested by the state, small projects are mostly constructed by the villagers.
The incentive policy is that he who invests and throws oneself into the work will be benefited.
3.1.4 Planning and overall
arrangement
On the basis of a rough plan, further planning should be made for water sources and farmlands. In order to meet the needs of construction and the water yield for irrigation, adequate water sources such as rivers, reservoirs and lakes should be chosen. For the places where there are inadequate water sources, dams should be built to store water.
The reclaimed desert land should be planned at open terrain where construction installations such as headrace, scouring ditch, batardeau, and tailrace can be built. The elevation of land should be lower than that of water sources. The land should be of uniform texture and in a friable state, which will scatter and collapse when it is flooded by water.
After the land is reclaimed, a unified planning should be made of soil amelioration, network of forests, irrigation and drainage, crop plantation landscaping, and the layout of farmlands, canals, wells, roads and forests. The reclamation of land should be carried out by stages. The order of building the land should be in accordance with the layout of the headrace, in which farther and higher places prior to nearer and lower places, to ensure there to be outlets for water and sand, so as to silt up the low-lying land.
3.2 Diversion works
Diversion works is the construction project built between the water intake and the drainage ditch, which differs according to different water sources, terrain and scales of flattening sand dunes and reclaiming desert land. It generally includes: headrace, water storage basin, scouring ditch, batardeau and tailrace etc.. For those places where water is pumped directly from rivers, reservoirs or lakes, headrace and water storage basin are not necessary. In short, flexible overall arrangement should be made according to local conditions. The general principle is less investment, higher efficiency; well-layout farmland convenient for cultivation.
(1) Headrace: Conducting water from the water source to the sand dunes where transporting of sand is intended, with a gradient of 0.5-1.0%. The fracture surface is decided by the water yield conducted. The ladder -shaped fracture surface, with a side slope of 1: (0.5-1.1), differs according to different building materials.
(2) Water storage basin: A temporary water storage installation from headrace to scouring ditch. The elevation of the basin should be higher than that of the sand dunes to be flattened. To store water in sand reach or to build batardeau according to the terrain and the local conditions.
(3) Scouring ditches: They are dug on the planned sand dunes, the position and style of which vary in accordance with the form of the sand dunes and the scouring process. The gradient of the ditch should be over 1%. Ditches can be dug in various positions or in various ways. For smaller round sand dunes, ditches can be dug on the top, halfway, or on the bottom of the sand dunes respectively in accordance with the elevation of the water storage basin. For those irregular and especially huge sand dunes, ditches can be dug on the left and right sides, or on all sides, or circuitously respectively.
(4) Batardeau: Built on the four sides of the planned desert land, it is used to block water and sand washed away from sand dunes to silt up the low-lying land between sand dunes. The plane figure of it should be rectangle or square, so that the reclaimed land is convenient for use.
The height of the batardeau is 0.5-0.8m initially, which will be heightened as the land is silted up. The top width of the ladder-shaped fracture surface is 0.3-0.5m, and inside and outside slope ratio is 1:1.
(5) Tailrace: Tailrace is built at the lowest point of batardeau to discharge the unnecessary water in batardeau after silting up the land.
The elevation and the position of the tailrace are changed along with the silting up of the land, keeping the tailrace a little higher than the mud flow but lower than the batardeau. It should be temporarily built with straws or bricks. And drainage ditch should also be built to avoid washing away of the land.
4 Major Methods of
Flattening Sand Dunes with Flood Water and Reclaiming Desert Land
4.1 Grabbing sand top
This method can be adopted when the water level of the canal is higher than or parallel to the round sand dunes or the top of sand dunes, or when water in sand reach can be pumped. Conducting water to scour the top of the sand dune in half so that it can be washed away easily. The method is adopted most often due to large drop height, strong impulse force, high efficiency, and lower labour intensity. The concrete steps are Scouring ditch is dug on top of a sand dune with water conducted in.
The ditch is deepened unceasingly by hydraulic force, with the two sides collapsed constantly, thus an increasing “V”-type gully is formed from top to bottom, and the sand dune will be flattened successively. Meanwhile, the gradient at the lower reaches will be lowered. When bulk of sand collapsed, water flow will be obstructed and transient gathering water puddles are thus formed. When they are full, the sediment-laden flow will scour out. This can happen periodically. As time goes on, the “V”-type gully will be expanded to “U”-type gully which will be increased from lower reaches to upper reaches, and thus the sand dune is flattened gradually.
The wide application of water pumps is a very favourable condition for the project. By putting the discharging tube at the top of a sand dune, the projects of building headrace, water storage basin and scouring ditch can be omitted. This method is not restricted to terrain or water level and can be applied to different types of sand dunes. The method is often adopted, as it is easy to operate and is highly efficient.
4.2 Scouring sand dune side
This method is adopted when water can only be conducted half way up and dunes. It requires large water yields and great labour intensity. The concrete steps are scouring ditch is dug at the side of a sand dune, and hydraulic power is used to scour the sides and hence the dune was flattened. The method requires people to shovel sand into water in order to increase the sand content in the sediment-laden flow, meanwhile the outside of the bank should be controlled to avoid collapse.
4.3 Splitting the border of a dune
This method is suitable for using low level water to scour largedunes when water cannot be conducted to the side or top of a dune. When water yield is sufficient, there will be better result, less labour intensity. But, there will be bulk of sand collapse that is a great threat to the workers’ safety. Close attention should be paid to the mobilization and safety of the project. The concrete steps are: a scouring ditch is streched at the border side of a dune and hays used to control the direction of water, splitting sand dune into water from outer side to inner side progressively, and the whole dune will be flattened at the end. For a huge isolated sand dune, when water yield is sufficient, a ditch can be streched at the other side of the dune to scour from both sides to quicken the steps of splitting.
4.4 “Horse mane” method
As the scouring ditches divide on the left and right sides of a dune, just like a horse’s mane, so it is named. This method is adopted when the water level is a little lower or parallel to the huge barchan or barchan chain, and sand can also be transported to either side of the dune simultaneously. The method requires less labour intensity, but achieves better result. The concrete steps are conducting water to one side of a dune near the water source, and a catch is dug inclined to one side. When part of the dune is flattened, another ditch is dug inclined to another side, then move to either side again and again from the distant to the nearer part. Finally it will be flattened.
4.5 Quincunx method
This method is adopted when there are several small dunes lower than or parallel to the water level of canals, scatter around the waterstorage basin, which is suitable for widespread drifting sand dunes when there is adequate water yield. The concrete steps are: Batardeaus are built between the adjacent sand dunes, and a water storage basin is formed at the center, then scouring ditch is dug along the top of each dune and water is conducted simultaneously to flatten the dunes. Another quincunx method is to build batardeau to store water at the top of a huge barchan or round dune, then four or five scouring ditches are dug and water is conducted simultaneously to four sides to flatten the dunes. Sometimes, after grabbing the top of a dune, the sand on either side has not been flattened, now the water gap is blocked, ditches are dug on four sides to scour sand.
4.6 Zigzag method
After the sand dunes are flattened initially, there are still some terraces with little inclination. Scouring ditch of zigzag form is dug from the upper land to lower land, then water is conducted to flatten the terraced dunes by the impulse of water. This method can also be used for dunes with gentle slopes.
4.7 “Sparrow warfare”
This method is similar to sparrow warfare (as a form of guerrilla warfare) which crushes one by one. It is most often adopted in the winding up of the project to eliminate the remaining sandpiles. When huge sand dunes have been flattened initially, there are still many sand mounds 1-2m high scattered around. Now workers are separated and assigned to each sandpile to conduct water to flatten it. The method can also be used to level land when huge dunes have been flattened initially but there is still great slope. The concrete steps are a cob wall is built at the lowest point of the slope, and ditches with a depth of 0.7-1.0m and an interval of 6-8m are dug downhill. Workers stand at either side of the upper ditches to poke sand with shovels constantly until the slope is flattened with flood water.
The techniques of flattening sand dunes with flood water and reclaiming desert land in free season not only can reclaim land but also can bring about great economic, ecological and social benefit. It is determined through years of research by our institute that about 7.4m3 of sand can be transported daily using a two-wheeled vehicle drawn by man, 95.2m3 of sand by an earth mover per shift, but 74-124m3 each workday using this technique which is 2-15 times more efficient than man-made machine.
The economic benefit of this technique is calculated with the static conversion method according to the assessment coefficient of irrigation benefits in various places put forward by The Economic Research Association of Water Conservancy of Shaanxi Province. Taking Honghai Canal Experimental Area in Yulin City as an example: The area has 144 km2 of reclaimed desert land by flattening sand dunes with flood water and has achieved obvious benefit in agriculture, forestry, animal husbandry, and side-line production. The net gain is 1.1555 million RMB Yuan. The total investment was 1.0626 million RMB Yuan which can be recouped within a year.
The ecological environment has been improved. With large stretches of barren sand afforested, the afforested area takes up 80% of the areas suitable for forests and grass. And the biological species has been increasing year by year. Turning barren sand into cropping land is very significant when the arable land is being reduced sharply. It has created conditions for production and has expanded the land resources, and the per-capita income in farming has increased 1.4 times.
Through years of practice, it can be concluded that flattening sand dunes with flood water is a great beginning of the masses in sand areas, and it is a great contribution to the economic and social development made by comprehensive control of sand by hydraulic power. This technique requires less investment, fewer labour, with faster speed, higher efficiency and better result. The problem of desertification and the deterioration of ecological environment can be controlled as long as it is carried out scientifically. The resources of sandy land are of great potentiality. The undertaking of flattening sand dunes with flood water and reclaiming desert land is of great importance to land and environment protection, and also economic development. It is an essential measure for people in the sand area to shake off poverty and become prosperous and it is a correct choice to utilize water and land resources in sand areas.
1. Jiao Juren, Integrated Techniques and Practices of Sands Rebuilding by Hydraulic Power, Shaanxi Science and Technology Press, June 1996.
2. Yang Zhongxin, Flatten sand dunes with flood water and reclaim desert land in free season, and the amelioration of newly- built desert land. China Water Conservanc. 11,1984.