Use of SAR images for urban settlements development analysis and green roofs potential retrofit estimates in hydro-geological risk prone area

Extreme modification of the natural landscape through rapid urbanization has repercussions on atmospheric, land and water systems. Indeed, the increase of human settlements in urban areas has great impacts on local urban heat island (UHI) effect, air quality and surface runoff. In particular, the UHI effect is exacerbated, the ambient air quality standards worsen while the runoff production increases. The urban warming has implications on the thermal comfort level of the people, the air pollution causes a wide range of adverse long-term human health problems while the extra runoff production causes hazardous flooding events with consequent loss of human life and property. Since the flood damage are the most severe and measurable, in time, many urban planners and engineers, in order to minimize the hydrological impacts of uncontrolled conversion of rural land into urban impervious surfaces, have studied and investigated promising technologies like the Low Impact Development (LID) practices. LID measures have proven to be a very effective tool for mitigating flood impacts. They include a number of techniques like green roofs, blue roofs, bioswales, bioretention cells and pervious pavements able to lower peak discharges and volumes during critical hydrological events. Among these, green roof (GR) is one the most flexible system in attenuating flood flows in urban areas. In this context, the remote sensing technology represents a valid source of information for the analysis of urban settlements development and green spaces since it allows the build-up areas extraction and the quantitative estimation of city’s green area potential gain on rooftops. The aim of the present work is to quantitatively estimate potential green cover at rooftop level using data obtained by SAR sensors in Sarno river basin, which is a hydrogeological risk prone area. At first, remote sensing imagery has been used to detect the land use/cover change transitions between 1995 and 2016 in the studied catchment area. Sarno watershed has experienced a rapid urbanization during the two considered decades, indeed the built-up area has doubled moving from about 7% in 1995 to about 12% in 2016. SAR technique allows for a quantitative estimation of potential area on rooftops for green retrofit. The GRs retrofit potential in the case study area amounts to about 20% of the total impervious surfaces. The effectiveness of GRs in mitigating the flooding events for the proposed greening scenario at basin scale, has been tested using Storm Water Management Model (USEPA). Simulations have been run in order to reproduce the GR hydrological response to storm events with different rainfall intensities. The results show that, on the one hand, at basin scale, the GR provides low attenuation of the flooding events so in order to pursue the desired goal, it should be used in combination with different types of LIDs. On the other, the proposed approach, demonstrates that remote sensing imagery provides powerful tools able to provide operative information to urban planners and decision-makers about the built-up expansion and the potential of existing buildings for green roof retrofitting