Study On Rural Settlement Development And Soil Erosion In The Much And Coarse Silt Area In North Shaanxi Loess Plateau

Much and coarse silt (sediment) area in North Shaanxi Loess Plateau covers an area of 4.35 thousand (4.35 × 10~4) km~2—that is, 55.4% of that in middle and lower reaches of the Yellow River. It is the key area to produce the silt of the Yellow River, producing 310 million tons(3.1 × 10~8t) of silt every year. It faces the serious problem of soil erosion. Since the reform and open policy, economy in North Shaanxi Loess Plateau has developed and the population has grown. That has brought about a great advance in small-scaled urbanization. A large number of habitants built new houses now and then, and deserted the old ones. In this way, rural settlements have developed rapidly. But at the same time, people in the countryside didn't quite realize the importance of water and soil conservation and paid little attention to it. Deserting soil was thrown anywhere during settlement and road construction. Serious soil erosion comes to exist. A large quantity of old caves is deserted and is easy to collapse, speeding soil erosion. And more serious water erosion exists in using rural settlements. Along with the government's policy of Grain for Green, erosion in slopes and gullies will reduce and the ecological environment will be improved. The problem of soil erosion in rural settlements, however, seems more serous, and asks for urgent solution. For the past few years, many scientists have paid much attention to the problem of soil erosion in urban settlements and have done some research works. But no research has been done into soil erosion in rural settlements. It is merely involved in the article about other contents. Therefore the research work into soil erosion in rural settlements in North Shaanxi Loess Plateau may not only enrich the soil erosion study and enlarge the research field, but also add some new research achievements to this field. The research work can also provide scientific proof and reference for preventing soil erosion, reducing the amount of silt into the Yellow River, constructing ecological rural settlements and serving for water and soil conservation. Besides, it is used for reference to other areas in the Loess Plateau.The paper is based on the field research work in 19 counties in the much andcoarse silt area of North Shaanxi Loess Plateau. By means of field-spot observation, typical field section investigation and quantitative analysis and with the aid of a great deal of data, the development of rural settlements and its soil erosion is systematically studied. It is indicated that the soil erosion of rural settlements in North Shaaxi Loess Plateau be very serious and have been the "new increase point" of water and soil loss in this area. The paper has studied and achieved as follows:1. Complete analysis is made to the development history of rural settlements in North Shaanxi Loess Plateau, and the history of cave settlements is divided for the first time. It is pointed as follows: North Shaaxi Loess Plateau has a long history in rural settlements. Cave settlements came to exist as early as Yangshao Culture Period 5000 years ago. And the earliest written record about cave settlements 3000 years ago was discovered. The history of cave settlements' development is divided into four periods as follows: cave-scattered period, technology-perfect period, cave settlement-steady period, and settlement-extending and migration period.2. On the basis of the field research, quantitative analysis is made on type, distribution, land use features and development trend of rural settlement. The conclusion has been drawn as follows: Because of the influence of geology, landform, climate, water source, traffic and other social economic conditions, the rural settlements have developed in forms of 4 types—river plain type, slope piedmont-mesa type, small gully type and Liang Mao slope type. Hereinto the distribution density of the former two types is larger obviously than that of the last two. The scale and density of settlements is small—average 0.47one/km~2. The distribution is uneven and becomes sparse gradually along the river tree system, with the characteristics of approaching towards the sun, towards the gully and towards the road. As far as the land use type is concerned, yard covers the largest area that is 41.2 % of the whole settlement area; cave the second largest area that is 25.2 %; open space between houses 23.9 %; and roads between houses 9.7%. Meanwhile the size of the settlement area is different in regional distribution. In Yuyang District, Suide County and Baota District, the average size for each family is 484m~2, 404m~2 and 476m~2. The actual size is 15.8-80.7% larger than statistics figures. Besides, rural settlements are increasing in size, migrating in the manner of peristalsis migration, expansion migration, scatter migration, and great-distance migration.3. It is the first time to employ field-spot observation in Yulin, Suide and Yan'anand systematically analyze the basic features of water erosion of runoff, silt and silt granularity in rural settlements. The critical value of runoff uprush per rainfall has been provided. It is pointed that runoff is easy to form in rural settlements, causing the loss of water. Its average annual runoff modulus is 91622.3m~3/km~2, with the average runoff coefficient 0.335—far more than that of the area between Yulin and Yan'an (0.1-0.2). Runoff amount increases with the increase of the amount of per rainfall, and the runoff amount in roads (between houses), yards, and open space (between houses) increases rapidly when precipitation per rainfall reach 17.5, 18, 28.3mm (critical values). Plenty of runoff causes water erosion inside rural settlements; its average erosion intensity is 5434.3t/km~2. The average erosion intensity in roads, open space and yards is 7324t/km~2. 6873.2t/km~2 and 2081.7t/km~2 individually, reaching the very serous level of erosion except yard erosion, It is urgent for prevention. Silt granularity of water erosion is in the major of semi-coarse silt (10~50um) and coarse silt (50~100um), occupying 43% and 15% individually. The coarse silt contents are smaller obviously than that of slope outside the settlements. The silt granularity becomes thin from south to north. The water erosion coefficient of coarse silt from loess in yards, roads and open space is 0.43, 0.48 and 0.66 individually, with the average 0.52.4. On the basis of field research, for the first time, quantitative analysis is made to soil erosion by cave collapse in rural settlements in the much and coarse area of North Shaanxi Loess Plateau. It has the following conclusions: Caves collapses because of many human factors such as deserting caves, unreasonable selecting of location, slope cutting, caves' construction materials and construction skills. Half of the deserted caves collapsed. In the whole area, the largest erosion amount for each cave is 231.5m~3/a. The average erosion amount for each cave is 46.3m~3/a, with the average loss coefficient 0.08. Cave collapse occurs in complicated forms. It differs in different area, mainly happens in July to September and January to March. As to position, cave gate collapses frequently. As to type, loess cave and loess cave with stone gate are easier to collapse.5. On the basis of sampling investigation, for the first time, quantitative analysis is made to soil erosion during the cave (house) construction in rural settlements. It has the following achievements: Serious soil erosion is caused by plenty of digging surface during the cave construction in much and coarse silt area in North Shaanxi Loess Plateau. Generally, digging soil amount for constructing a new cave is between 69.2and 240 m~3, with the average digging soil 125.8m~3 per cave. A major of digging soil is deserted, and the deserting soil is between 11.6 and 149m~3 per cave, with the average deserting soil 64.2m~3 per cave and the average coefficient of deserting soil 55%. Furthermore, deserting soil is piled anywhere or thrown into river or gully bed directly. On average, during building a new cave, soil is lost at 11.69m~3/a, with an average loss coefficient 0.175. This must be paid more attention. Reasonably using digging soil and controlling deserting soil efficiently may reduce the loss of soil greatly, and control or reduce the erosion intensity in rural settlements effectively also.6. On basis of the investigation in Nianzhuanggou river basin (in Yan'an city), for the first time, estimate calculation is made to the total soil erosion of the rural settlements of small river basin. It has the following achievements: In 54.2 km~2 area of Nianzhuanggou river basin, the rural settlements (deserted settlements included) cover 1.08%. But the total annual soil erosion reaches 43173t, coving 7.96% of the total annual soil erosion amount of Niazhanggou river basin. The annual erosion modulus is up to 73426t/km~2. For a further estimation by this rate, the total soil erosion of the whole rural settlements in the much and coarse area of North Shaanxi may reach 32 million(3.2×10~7) t/a, which covers 10.3% of that of the whole much and coarse area of North Shaanxi. So the related departments have to take it seriously. As to the main erosion manners, the annual soil erosion amount caused by newly-built caves, cave collapse and settlements' water erosion is individually 23465t, 15524t and 4136.7t, covering the percentage of 54.3%, 36%, 9.7% individually. The total soil erosion caused by caves' collapse and caves' construction is up to 90 percent of the total soil erosion in the rural settlements. They have turned to be the major manners of soil erosion.7. The counter relations between rural settlements' development and soil erosion is analyzed and summarized systematically. It is indicated that the development of rural settlements causes serious soil erosion in the forms of water erosion, cave collapsing erosion and deserted soil erosion. Meanwhile soil erosion in rural settlements affects settlements' environment and does harm to the safe of settlement, enforcing people to improve house structure continuously, and causing rural settlements to be deserted and migrated.8. The countermeasures to prevent and harness soil erosion of rural settlements are put forward in the paper. The work to prevent and harness soil erosion of rural...