تحلیلی بر نقش سیاست‌های زمین شهری بر رشد پراکنده شهری(مطالعه موردی: سبزوار)

Introduction
Dust phenomenon is the One of the most important hazards in the dry and semi-arid region. The occurrence of this phenomenon is accompanied by many physical and financial losses. Iran is exposed to numerous local and trans-regional dust systems due to being in the arid and semi-arid belt of the world. Observations of recent years indicate an increase of this phenomenon in the country. Surface properties of dust harvesting zones affect the amount, intensity, and type of dust particles entering the atmosphere. Consequently, it is important to know the geomorphological and land use characteristics of the dust harvesting areas. Land use changes associated with global climate change have a significant relationship with desertification and increasing dust storms. Changes in river floods and the level of temporary and permanent lakes in dry and semi-arid environments cause significant fluctuations in the amount of dust in these areas. The purpose of this research is to determine the role of each geomorphological feature and land use in dust source regions in Khorasan Razavi province.
Materials and Methods
In the present descriptive-analytical research, the required data were collected mainly through documents from various organizations and analyzed using quantitative models like Holdren and Entropy methods. In addition, maps for physical expansion and growth in different previous periods were prepared to demonstrate the expansion of the city in graphical form using existing maps and aerial photos or satellite images via Arc GIS software.
Discussion and Results
The city of Sabzevar in the last half century under the influence of natural population growth and migration of villagers with varying degrees of intensity in different periods has engaged a dramatic growth of the population, so that the population of Sabzevar has been 7.98 from 1956 to 2016. In contrast, the area of the city has been growing faster than its population, so that its area has been equal to 18.15 in the same period. This imbalance between population growth and the area in each period indicates the predominance of horizontal growth and urban sprawl. Comparison of increasing the urban area and the state land assigned areas show that a total of 3705 hectares added to Sabzevar between 1956 and 2016 about 31.55% was because of the land assignment policy and 19.13% the result of informal settlements and villages annexed to the city. The most important thing about this area is that almost 100% of these lands are in areas that have been assigned directly by the government, or have been prepared by the government, and then made available to the residents. Thus, the greatest rate of urban sprawl can be seen in the directions that the government has had a direct role in its establishment. Therefore, the land assignment policy by allocating about 40% of the area added to the city limitations has acted as the most important factor in the physical expansion of the city of Sabzevar.
The results of the calculations of the Holdren model demonstrated that population growth from 1956 to 1986 had a larger share in the urban area changes, so in the period from 1976 to 1986, the share of population changes was 99% and the share of per capita change was 0.01. However, this trend has been shifted in two subsequent periods, from 1986 to 1996 and from 1996 to 2016, and the share of per capita change has surpassed the share of population changes. In the period of 1986 to 1996, per capita share was 61% versus 39% of population changes and in the period of 1996 to 2016 was 38.47% of the land per capita versus 61.53% of the population. This is due to the impact of land attached to the city through the lands allocated in the framework of land preparation project in Tohidshar and the annexation of peripheral villages, including Kelate Sifer, Saleh Abad and Ghalee No Dehraz.
Conclusions
The investigation of the process of changes and patterns of urban physical expansion, and the identification and the analysis of the effective factors are important, because firstly urban scattered and uncontrolled horizontal growth leave the inevitable and sometimes undesirable consequences on the economic, social and ecological aspects of the city. And, secondly, analyzing the process of physical changes in the past of the city helps us to have more precise planning for systematic control of development. Among the factors affecting the horizontal growth of the city of Sabzevar in the past few decades, land and housing policies have had a significant role in the city's physical expansion in certain directions. Thus, about 40% of the area added to the city in the last half century has been caused by the state land assignment policy. Discontinuous tissue formation in Tohid town was one of the obvious consequences of the policy that took place followed by the land assignment in the preparation project.
The assignment of these lands that took place in relatively large segmentations with an area of 250 square meters on average affected the physical expansion and construction of the city in the past two decades in several ways.
Firstly, this policy accelerated the city’s horizontal expansion than the population growth. Secondly, due to the fact that these lands have been usually fragmented to regular segmentation and with predicted access network, urban expansion on these lands has a regular and chess-like form. Thirdly, the land assignment policies, particularly in the preparation project, led to the establishment of a new pattern of physical expansion in the form of urban discontinuous expansion, which can identify outlines for future expansion of the city.
In addition to the factors mentioned, the addition of villages and illegal settlements to the urban area in recent decades has had a significant impact on the direction and extent of physical growth of Sabzevar.