Why Does Boston Not Use Green Roofs to Combat Flooding?

Boston is in trouble, and they know it.

Massive tidal shifts of 10-14 feet in the Boston Harbor mean that every time a North Eastern storm coincides with high tide, the excess stormwater cannot readily flow into the harbor. The result is flooding. Boston expects this to happen every year as tidewater is expected to rise to street level in the next 15 years.

The resulting economic damages climb into billions of dollars lost. AI simulator software technology maps these impacts, as cities struggle to understand flooding complexity to see what happens how, to whom, when, and at what cost. This software deploys a diverse collection of stormwater solutions ……but not green roofs!!!!

What the flood?!? Why not? If roofs cover 50% of downtown Boston, why are green roofs not considered part of the solution?

Green Roof Diagnostics (GRD), is a $400.000 green roof laboratory built by a consortium of Sempergreen, Knauf, Hydrotech, and GRSP. We confirmed that stormwater retention based solutions are indeed awesome and even civil engineers can agree that green roofs are great as volume reduction tools, but at the peak of the storm… does the civil engineer see a green roof a primary peak flow reduction tool? Aaaahhhh.......not so much.

As an industry, we proudly state annual retention rates of 40-50 per cent. As a result, the size of ‘retention cups or sponges’ are our collective bragging rights. My cup (or sponge) is bigger than yours! Yet, civil engineers don’t care. Let me re-phrase because this statement causes industry riots. Retention is great…. unless the cup/sponge was filled with yesterday’s rain.

Retention combined with True Detention capacity is the ultimate value potential of a green roof.

Few green roofs can deliver this. Charlie Miller’s unique award winning green roof designs provided both. He has been saying this out loud for 20+ years. If it were not for Charlie, civil engineers Joshua Robinson and Brad Garner from Green Roof Diagnostics we would never have solved that problem for our clients. Brad rebuilt the GRD laboratory in 2017 to emphasize True Detention based testing. He also built the Evapotranspiration calculator in 2018 that clearly show that most storms come in clumps of 2-3 days in a row (see https://www.purple-roof.com/model) Simply put, on the first and or second day, the rain fills up the retention capacity of the green roof, and then it releases water as fast as it rains on the 3rd day. The re-charge rate of a retention based green roof is simply too slow (7-21 days), and because of this, civil engineers prefer alternatives that re-charge quickly. ….any solution with a cup or sponge that does not re-charge within 24 hours …does not qualify as True Detention.

True Detention in green roofs should be the focus

Detention is focused on peak flow reduction, and peak flow delay. If we want cities like Boston to use green roofs as primary stormwater management tools, and True Detention is what civil engineers are looking for, this is what we as an industry need to provide.

Yours truly,

Oscar Warmerdam
CEO Sempergreen USA
CEO Purple-Roof