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 km2), YRD (440.41 km2) and GBA (298.14 km2); (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.