*4.2. Process of Built-Up Expansion*

In the present study, we introduced the process of built-up expansion through the fragmentation and frequency approach. The steps of built-up expansion over time have been identified using the six indices of the landscape fragmentation approach. The results showed that the perforated and patch category areas were 6.24 km<sup>2</sup> and 13.42 km<sup>2</sup> in 2001, respectively, and have risen to 13.23 km<sup>2</sup> and 17.86 km<sup>2</sup> in 2021 (Figure 6). These new and isolated built-up nodes were originally rural neighbourhoods. The study area has experienced an economic and infrastructure progression because of the rise of MSME and governmental schemes that has accelerated the urbanisation process. As a result, some of the perforated and patched portions have expanded outwards through time, eventually connecting with small and medium cores. Replacing the edges and patches in the study period has increased the core categories. The increase in the percentage of the core can be attributed to the increasing built-up density and the conversion of intervening spaces into the built-up and the emergence of a new infrastructure [46]. The emergence of new patches and edge is widespread, signifying the spatial expansion of built-up areas by replacing agricultural land. Additionally, the results showed that, in 2001, the large core of the built-up area encompassed 2.92 km2, which was amplified by 7.42 km<sup>2</sup> in 2021. As shown in this scenario, the large core of the built-up area, defined in 2001, was fixed three times but gained extra area in 2011 and 2021. Many studies reported that agricultural land is continuously declining in English Bazar over time [11,14,46]. Additionally, the frequency approach mimics fragmentation results, in which the permanent built-up area (frequency 3) is present in the core areas. In contrast, frequency 1 was fresh and isolated built-up nodes, while frequency 2 was the built-up transition area of small and medium cores. In this way, we showed the process of built-up expansion at the spatial scale. However, the research on the process of built-up expansion has not been explored ye<sup>t</sup> using remote sensing as per the authors' knowledge; most of the research has concentrated on the overall builtup expansion mapping and pattern analysis [63,64]. As shown by the current study, the area under build up has been increasing gradually over time, which can result in several socioecological problems, such as UHI [65], pollution [66], urban flooding, etc., in English Bazar and its contiguous areas in recent decades.
