Extended Abstract

Introduction

Urban expansion is a major spatial outcome of socio-economic change and a critical challenge for sustainable urban development, particularly in arid and semi-arid regions. In such environments, physical urban growth is strongly influenced by natural constraints, and geomorphological factors play a decisive role in shaping the direction, intensity, and spatial pattern of expansion. Variables such as elevation, slope, geological formations, drainage conditions, and tectonic structures may either facilitate or restrict urban development. Ignoring these constraints can increase environmental vulnerability and exposure to natural hazards. Consequently, understanding the interaction between urban expansion and geomorphological factors is essential for risk-aware urban planning.

In Iran, rapid urban growth over recent decades has been driven by population increase, administrative restructuring, and service concentration. While many studies have focused on metropolitan areas, medium-sized cities located in dry and environmentally constrained settings have received comparatively less attention. Birjand, the capital of South Khorasan Province in eastern Iran, represents a typical medium-sized city that has experienced notable physical expansion during the past two decades. At the same time, the city is situated in a geomorphologically sensitive environment characterized by variable topography, heterogeneous geological formations, active fault systems, limited water resources, and seasonal drainage networks. These conditions make Birjand a suitable case for examining the influence of geomorphological factors on urban expansion. This study aims to analyze the spatio-temporal pattern of urban expansion in Birjand from 2000 to 2024 and to quantitatively assess its relationship with selected geomorphological variables.



Methodology

This study adopts a quantitative and applied approach integrating remote sensing, geographic information systems, and statistical analysis. Multi-temporal Landsat images acquired by the TM and OLI sensors for the years 2000, 2010, 2015, 2020, and 2024 were obtained from the United States Geological Survey. Standard radiometric, atmospheric, and geometric corrections were applied to ensure comparability across time periods.

Urban built-up areas were extracted using an object-based image analysis approach combined with the Support Vector Machine classification algorithm. Classification accuracy was evaluated using stratified random sampling and an error matrix, and the results indicated acceptable overall accuracy and Kappa coefficients for all periods. Urban expansion intensity was quantified using the Urban Expansion Intensity Index, which measures the proportion of newly developed built-up land relative to the total study area within each time interval. To analyze directional growth patterns, the study area was divided into eight equal directional sectors based on the city center, and UEII values were calculated for each sector and each time period.

Geomorphological variables, including elevation, slope, aspect, curvature, topographic position index, distance from faults, distance from rivers, and geological formations, were derived from a digital elevation model and thematic spatial datasets. Pearson correlation analysis was applied to examine the relationships between urban expansion intensity and geomorphological factors across different time periods.



Results and Discussion

The results indicate a continuous increase in built-up areas in Birjand during the 2000–2024 period, with the most pronounced expansion occurring between 2015 and 2020. Urban growth did not follow a uniform spatial pattern but instead exhibited a clear directional tendency. Among the eight directional sectors, the southwestern sector showed the highest cumulative urban expansion intensity, followed by the northeastern and northern sectors, indicating a persistent spatial preference in urban development.

Correlation analysis revealed that geomorphological factors significantly influenced the spatial pattern of urban expansion. Elevation showed a strong and stable positive correlation with urban expansion intensity across all periods, indicating concentration of development within specific elevation ranges. Geological formations also exhibited notable relationships with expansion intensity, suggesting preferential development on lithological units with higher construction suitability. In addition, distance from faults displayed a negative correlation, indicating that a considerable portion of urban expansion has occurred near fault zones, which may increase seismic risk.

Slope showed a weak negative correlation with urban expansion, confirming the preference for low-slope terrains, while distance from rivers exhibited a weak positive relationship, suggesting relative avoidance of flood-prone areas. Other variables, including curvature, aspect, and topographic position index, showed negligible correlations and did not play a significant role in directing urban growth. Temporal analysis of correlation coefficients indicates that the influence of geomorphological factors intensified up to the 2015–2020 period and declined thereafter, reflecting a gradual shift toward a development pattern increasingly shaped by human and managerial factors.



Conclusion

This study demonstrates that urban expansion in Birjand over the past two decades has been strongly influenced by geomorphological constraints, particularly elevation, geological formations, and proximity to fault systems. Urban growth followed a persistent directional pattern, with expansion concentrated mainly toward the southwest, northeast, and north. While natural factors played a dominant role in earlier stages of development, their relative influence has decreased in recent years, indicating a transition from a nature-controlled expansion pattern toward a combined natural–managerial model. The findings highlight the importance of incorporating geomorphological considerations into urban planning and land-use policies, especially in environmentally sensitive and tectonically active regions. The analytical framework applied in this study can be used in other medium-sized cities in arid and semi-arid environments to support more sustainable and risk-aware urban development.