Green Roofs Assist in Flood Prevention

Yesterday Toronto got more rain in two hours than it normally does in a month which meant some serious flooding happened. This got me thinking of a program that Toronto (alleged crackhead) Mayor (busted for DUI) Rob (loves pollution) Ford (reads while driving) cancelled. The cancelled program promoted green roofs to help with flood control while lessening wear on existing infrastructure.

So the ineptitude of the current Toronto mayor got me thinking of how things could have been different with forethought of climate change. It’s worth noting that Rob Ford spent the flood idling in his SUV:

I gathered some example of other cities and areas that are using green roofs (and similar) to curb their flooding problems.

In Singapore they have combined recreation with flood prevention:

The barrage is part of a comprehensive system of flood control to decrease flooding in the low-lying areas in the busy quarters of the city. During the heavy rains, a series of nine crest gates activate to release excess storm water into the sea when the tide is low. When high tide comes in, giant pumps drain excess storm water at at a rate of one Olympic-size swimming pool per minute.

In New York they are looking into a variety of solutions, which we looked at before.

In Rotterdam, the city’s green roof initiative has proven to be effective in flood alleviation.

Although large areas of green roofs have many benefits for cities, such as reducing air pollution and helping to combat the heat island effect, Rotterdam’s priority was for water retention, since the city has a shortage of areas where water can be stored following heavy rainfall. Water management has always been a major concern in the Netherlands, since approximately 60% of the country lies below sea level. The analysis of the potential of green roofs in Rotterdam that preceded the introduction of the subsidies focused heavily on their capacity for water storage in order to reduce peak water discharge following a rain storm and help prevent flooding.

Over in the UK, the Green Roof Centre has quite a lot of information on how green roofs can help flood management:

Once established a green roof can significantly reduce both peak flow rates and total runoff volume of rainwater from the roof compared to a conventional roof. Green roofs store rainwater in the plants and substrate and release water back into the atmosphere through evapotranspiration.

The amount of water that is stored on a green roof, and then evapotranspired into the atmosphere, is dependent on the depth and type of growing medium, type of drainage layer, vegetation used and regional weather. The FLL Guidelines should be followed to ensure that actual runoff will be in accordance with calculated runoff.

A green roof can easily be designed to prevent runoff from all rainfall events of up to 5 mm and as part of a SuDS strategy, should reduce the volume of surface or underground attenuation required at the site boundary. In summer, green roofs can retain 70–80% of rainfall and in winter they retain 10–35% depending on their build-up (Green roofs benefits and cost implications, Livingroofs.org In association with ecologyconsultancy, March 2004). The difference is due to a combination of more winter rainfall and less evapotranspiration by the plants because growth is not as vigorous during the winter months.

Edit:
I like this tweet from Toronto’s chief planner Jennifer Keesmat as a good conclusion to this post: