Going Green (Roof)

by Danny Croghan

What is the meaning of life? What came before the big bang? And how do green roofs mitigate water quality? Truly among life’s great questions. Today I will take you through my attempts to answer the third.

Green roofs for those unaware are essentially roofs with vegetation planted on them. Though a simple concept, the benefits of green roofs in urban areas can be wide ranging. Green roofs can help combat the urban heat island effect by increasing evapotranspiration, reduce flooding by reducing stormwater run-off, and increase Carbon sequestration. As a further potential benefit, green roofs should also alter stormwater runoff quality, however this has been much less tested.

To investigate the impacts of green roofs on water quality I was shipped to Northwestern University to develop an experiment at the Chicago Botanical Gardens which has two green roofs. The green roofs comprise an unmanaged green roof (Figure 1) – planted with drought resistant species that require maintenance, and a managed green roof (Figure 2) – planted with a range of species and regularly tended to. To compare against these we also instrumented a control roof – an asphalt roof which represented standard roofs.

To enable the collection of water samples we designed an automatic pump sampler (Figure 3) that would be triggered by flow from the green roof drains (Figure 4) and collect water samples throughout storms. Water was routed from pipes into buckets and once flow rose above the threshold to trigger the pump samplers, collection started. In addition to this we also instrumented three of the green roof drains with flow meters to get an idea of the flow coming from each of the roofs.

The main water quality metrics of interest were anions, absorbance, fluorescence, and DOC, and water temperature. We collected data throughout July, August, and September 2019 aiming to collect as many storms as possible covering a range of storm types. After an uncharacteristically dry July we were able to collect 12 storms across this period – and the automatic pump sampler worked surprisingly well, showing potential for future use in high frequency data collection.

While results analysis is still underway, results do show that substantial differences exist between control roofs and green roofs. Green roofs unsurprisingly lead to far higher export of DOC than control roofs, however they also appear to moderate stormwater temperature surges that are commonplace in cities like Chicago which experience regular convective storms. Interestingly, substantial differences in water quality also existed between the unmanaged and managed green roof – the unmanaged roof tends to have increased DOC exports but reduced stormwater surges and reduced amounts of water quality parameters indicating pollution. Thus, not only do green roofs appear to be a useful tool to mitigate water quality, but the precise design can have a dramatic effect on their remediation abilities.

Green roofs still comprise a small amount of the total green infrastructure in urban areas, but their use as a means to remediate stormwater quality, reduce stormwater discharge, and combat the urban heat island effect suggests they may be one of the best tool available in urban areas as a means of combatting the negative effects of urbanization.