Fig 1.
Schematic representation of the methodology used in this work to select the mitigation strategy to implement (GR or RWH, as a function of roof slope) and the analysed scenarios.
The selection of the mitigation solution to install is done at “Building scale”, i.e. evaluating each building separately. The analysis of the discharge reduction for the different scenarios is developed at “City scale”, i.e. investigating the effects for an extended urban area, which can be a city or a large neighbourhood.
Fig 2.
Geographical location of the 9 selected case studies.
(a) Location of the 9 cities on a world map. (L1-L9) Boundaries of the study cases with investigated rooftops. Flat roofs are highlighted in green, while sloped roofs are represented in blue. Cities are ordered based on their geographic location from west to east. Maps have been developed with the help of QGIS 3.4 [37], using layers downloaded from the different geoportals listed in the S1 Table and satellite images derived from the Landsat website (http://landsat.visibleearth.nasa.gov/).
Fig 3.
Average lag time between rainfall and runoff .
Six potential rainfall durations corresponding to 1, 3, 6, 12,18 and 24 h have been investigated and plotted for extensive (a) and intensive (b) GRs and for RWH (c) tanks. Different symbols and colours represent the nine selected locations.
Table 1.
Cost-efficiency summary for each location.
Maximum discharge reduction (as a percentage of initial runoff), total cost of the installation of the different solutions in the entire city and effectiveness (ratio between total costs and maximum discharge reduction).
Fig 4.
Outflow reduction ΔQ at daily scale as a function of the discharge in unaltered conditions.
Moving average outflow reduction ΔQ at daily scale is plotted as a function of the discharge in unaltered conditions at logarithmic scale, for different scenarios in the selected 9 locations. Only events with discharge in unaltered conditions above the 95% quantile are plotted.