Underground space as an urban indicator: Measuring use of subsurface

Use of Urban Underground Space (UUS) has been growing significantly in the world's biggest and wealthiest cities. UUS has been long acknowledged to be important to the urban development agenda: sustainability, resilience, livability, and creating a better urban environment in particular. These issues are traditionally monitored using urban indicators, however UUS has not been properly included and considered in urban indicator lists (sets or systems) yet - the gap this paper is aiming to bridge. The paper reviews existing approaches to the composition of urban indicator lists, highlighting indicator types, challenges related to data collection, and agencies that are concerned with the issue. Further the paper has identified the importance of UUS inclusion in the lists that give integrated assessment and monitor urban sustainability, resilience, climate change adaptation and mitigation, as well as progress towards smart, livable, and compact cities. Existing global quantitative data on UUS have been examined in 8 cities; and three key indicators (descriptors) were suggested to monitor UUS use: Developed UUS volume (m(3)); UUS use density (m(3)/m(2)); and Developed UUS volume per person (m(3)/person). Current average UUS use densities in cities are identified as up to about 0.05 (m(3)/m(2)) (which can be interpreted as a virtual depth of UUS use of 5 cm), and the developed UUS volume per person is up to about 10 m(3)/person; while city central areas (central business districts) can have a virtual depth of developed UUS of several metres (m(3)/m(2)). Compatibility, comparability, uniformity, and sustained monitoring of urban indicators data (including UUS indicators) found to be posing significant challenges to the research across geographies, and industry/economic sectors. (C) 2015 The Author. Published by Elsevier Ltd.