Comparing hillside urbanizations of Beijing-Tianjin-Hebei, Yangtze River Delta and Guangdong-Hong Kong-Macau greater Bay area urban agglomerations in China

Urban agglomeration is the most obvious regions in the Chinese rapid urban land expansion. The developed urban agglomerations in China (i.e., Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Guangdong-Hong Kong-Macau Greater Bay Area (GBA)) have entered a suburban urbanization period; however, it is not clear whether the urbanization on low-slope hilly regions (hillside urbanization) exist in these urban agglomerations. In this study, we proposed a quantification framework to detect hillside urbanization with multiple earth observation data and socio-economic data and further compared their spatiotemporal patterns from 2007 to 2017 in these three urban agglomerations. The results showed: (1) the urban area of BTH, YRD and GBA has expanded by 1.82, 2.37 and 1.53 times, respectively; (2) widespread hillside urbanization regions were found in BTH (475.82 km(2)), YRD (440.41 km(2)) and GBA (298.14 km(2)); (3) GBA had the largest hillside urbanization rate (10.55%), followed by BTH (6.33%) and YRD (3.18%); (4) the hillside urbanization of BTH, YRD and GBA provided accommodation and workplaces for about 1.05, 0.97 and 1.37 million people, respectively; and (5) the minimum and maximum high environmental cost (HEC) hillside urbanization rates were found in BTH (0.53%) and GBA (2.92%), respectively. Our findings may provide some new insights into urban sustainability.