Plot Scale Evaluation of the Hydrologic and Water Quality Benefits of Blue Roofs Compared to Green and Conventional Roofs

Abstract

Highlights There were no significant differences (a = 0.05) in rainfall retention between blue and green roofs. Blue roofs appear to be a viable option to green roofs when the priority is stormwater management. Blue and green roofs need to be studied on larger scales to gain more confidence in the treatment provided. Abstract. Impervious surfaces caused by urbanization alter hydrologic conditions. In ultra-urban areas (75% or more impervious), rooftop stormwater control measures (SCMs), such as green roofs, exploit underutilized rooftop space. Green roofs reliably attenuate rainwater, but associated water quality treatment results are not as consistent. This study explored the viability of blue roofs, an emerging technology, as an alternative to green roofs. Nine adjacent plots consisting of three each of conventional (control), green, and blue roofs were constructed in Raleigh, NC. As a part of hydrology monitoring, on-site rainfall was measured using a tipping bucket and verified using a manual gauge. Water quantity data were collected using water level loggers in storage bins placed under the roof plots to record outflow. Measured rainfall retention was similar for blue and green roofs, and both systems were significantly (a = 0.05) and substantially (50%) more retentive than control roofs. A comparison of evaporation rates suggested blue roofs may be more effective at regenerating storage capacity than green roofs. Thirteen water quality events were sampled, and the mean total nitrogen (TN) concentration from the green and blue roofs was 5.83 and 2.18 mg/L, respectively. Similarly, the mean total phosphorus (TP) concentration from the green and blue roofs was 2.75 and 0.25 mg/L, respectively. Total suspended solids (TSS) data were collected, but seven of 13 samples were below the practical quantitation limit (2.5 mg/L), and the data were not statistically analyzed. Total kjeldahl nitrogen (TKN), TP, and orthophosphate (O-PO4 3-) concentrations from green roof treatments were significantly (a = 0.05) larger than concentrations from blue roofs. Except for nitrate/nitrite nitrogen (NO2,3-N), there were no significant differences between blue and control roof pollutant concentrations. However, green roofs often discharged substantially higher pollutant loads than control roofs for every pollutant other than NO2,3-N and total ammonical nitrogen (NH3-N). While blue roofs may lack the aesthetic appeal of green roofs, this study suggests blue roofs are a relatively cost-effective option, likely preferable, when solely focused on the benefits of stormwater management from rooftops. Keywords: Blue roofs, Conventional roofs, Green roofs, Hydrology, Monitoring, Stormwater control measures, Water quality.