Green Roofs & Hurricane Winds - Reviewing UF Wind Tests

With two tropical depressions in the Atlantic and several cyclonical systems developing this August 2012 hurricane season, the October 2010 video of green roof hurricane tests in UF's Civil Engineering facility is reposted along with the original narrative.

As food becomes scarcer and more Urban Ag activities spring up across our cities, material science will play an important role in designing secure plant growing systems for patios, walls and roofs.

Though the video is quite long, Urban Green roof and hurricane design issues require comprehensive attention to potential safety issues, just as any other roofing material or system would demand.

October 2010:

We started off today referring to the Green Roof Wind Uplift Test duration as "until failure".

At least it looked as though the test would proceed until the mat failed.

After all, the MetroVerde Green Roof had been sitting in the hot Florida sun, non-irrigated, 1" thick layer of engineered soil for 18 months at the University of Florida.

We are going into our fifth week of zero precipitation (no rain).

The plants looked brown.

The panel was fixed at a 3/12 slope just feet away from the large hurricane simulator.

The plants appeared vulnerable, the engineered soil just waiting to be blown off the mat along with vegetation.

The large diesel engines fired up and the turbines spun.

50 MPH for one minute -  dust blew off the roof.

70 MPH for a minute plus - not much happened except the plants whipped back and forth.

90 MPH and the dust around the base of the testing platform flew and the plants bent backwards - almost parallel to the roof slope.  Shingles on a shed 300' away began flapping.

We were all amazed, having seen other green roofs under hurricane tests blow away, soil and plants...

We took a break and looked at the panel.  A small amount of the engineered soil had blown off the mat.

The 120 MPH for over three minutes.

The dead, brown material blew off the plants - like a good pruning.  Even the large, tender Echeveria was still there, albeit leaning a little.

The nodding garlic - Allium canadense was beautiful.

The plant roots were so intertwined in the mat that 80% of the engineered soil remained.

Successful.  The first Florida Designed Green Roof Panel to pass the 120 MPH wind uplift test.




Lots learned -

Will be working with UF more in the future!

Enjoy the video - I'll post the link as soon as YouTube finishes processing it - in the meantime here are a couple pics and happy Green Roofing!

MetroVerde Green Roof Passes 120 MPH Hurricane Testing

120 MPH Winds on Green Roof - Hurricane Testing

Wind, wind & wind across Green Roofs in Arid Areas

The central states along the Mississippi have been receiving more than their fair share of rain recently as evidenced by the US Army Corps of Engineers blasting of levees to alleviate flooding.

MetroVerde Florida Green Roof, Breaking Ground

Here in Jacksonville we've had little if any rainfall for weeks now.  The saying April showers brings May flowers does not hold true for the geographic climate here.  As weather fronts push down and across the US they loose momentum and dissipate just before reaching Jacksonville.  All the dry weather is rough on green roof plants.

However, being situated on the Atlantic Ocean we are constantly exposed to high winds.  When the weather fronts come through they drop all their moisture west of Jacksonville typically but retain enough wind turbulence to stress unprotected green roof plants.

Up on the Breaking Ground Green Roof for most of yesterday I watched in amazement at the amount of water the dry winds were stealing from the plants as breezes whipped across the flat roof at 4 or 5 meters per second.

Of course the CAM plants were fine and this is why we suggest planting a belt of CAM plants around unprotected green roof perimeters.  CAM plants generally keep their stomata closed during the day.

C4 plants like the Poaceae hold their own, as long as they are established.  The corn, lemon grass and native grasses don't seem to mind the desiccating winds though I am sure after time they too would suffer without some rainfall.  I love the wave action native grasses produce in the wind.

The Asteraceae too do well, again as long as they are established and again, most are considered C4 photosynthesis possessing plants.

But the C3 plants can dry out so quickly.

Because we are doing an install I water the plants to help them settle into their new home.

Yesterday the wind was so drying and strong I could hold the hose to where the stream of water was flowing vertically up and evaporating or being stolen by the wind before the droplets could ever make it back down to the plants.

Watching the C3 plant leaves move in the wind I could see the same occurrence happening, water being quickly vaporized out of the leaves just as the droplets from the hose where also 'gone with the wind'.

Sitting there with the hose I witnessed some of the more tender vegetables quickly wilt even with the hose  water directed at their roots.  The plants vascular system could not keep up with the wind induced evaporation of leaf moisture.

Without water in the leaves plants can not conduct photosynthesis.  Without photosynthesis plants die,

Wind impacts are so significant across green roofs.

In my opinion wind exposure is, with light, availability the most important consideration for green roof design.

We've posted numerous articles here on how to use CAM plants as wind breaks.  Understanding wind impacts on green roofs is critical for long term green roof design success.

Wind and Green Roof Plants

Included here are two photos of the same black-eye pea plants.  One is on a wind protected green roof, the other is on a wind exposed green roof.  Note the wind burn on the wind exposed roof plants.

Both looked the same prior to the 48 hour wind storm just leaving (killed 40 persons in the southeast US during tornados).

We recorded between 10 and 20 Km/Hr winds on a continual basis for the 48 hours with almost non-existent water vapor or air humidity.

Florida Green Roof, Wind Protected Green Roof Plants

Florida Green Roof, Wind Exposed Green Roof Plants

Wind can burn or kill green roof plants quickly, taking an otherwise beautiful planting and turning into a brown mess within a matter of days.  Even with more than adequate irrigation water applied - simply because the vascular system of the plants cannot keep up with the demand for water in the leaves.

CAM plants and those plants with stomata remaining closed or closing under lack of water conditions must be used as perimeter wind break plants on green roofs with out parapets or other wind protection if the green roof is not otherwise sheltered.

Understand the different types of photosynthesis green roof plants have.  For a primer, check out the many articles we have posted before on CAM, C4 and C3 plants.

Other helpful sites are included here;

Great Irish Gardening blog article on wind damage in Ireland to garden plants.

North Carolina State University has another brief yet informative note on wind desiccation of plants.

Very interesting and informative Permaculture Wind Break Site.

Remember, your green roof site may receive plenty or precipitation or irrigation water, but if it is constantly exposed to desiccating winds the plants will experience the effects of wind damage.

Green Roof Dead Zones - Why Wind Breaks are Necessary

Green Roof, Extensive - Nature Irrigation via Dew

Green Roof, Extensive - Wind Damage

 Wind is the silent killer of Green Roofs.  Wind can cause more acute damage to green roof plants than a lack of light.  On a chronic basis, light will eventually contribute to stress or death - light is required for photosynthesis.  However strong, desiccating winds can cause plant vascular failure and dehydration within a matter of hours.

The above photos show two edges of a small trial extensive green roof.

The top photo portrays the amount of available water vapor in the form of dew.  As is usual for Jacksonville, we experience prolonged droughts.  It has not rained for a week and we are well behind in our 52 week rainfall accumulation.  However water vapor usually offers a steady supply of condensation on a daily basis.  Click on the top photo for an enlarged view of the green roof plants and the dew condensed on the leaf blades.

The bottom photo shows the northeast corner edge of the 50mm thick green roof.  This edge takes the initial wind impact of most daily winds as the other corners are more protected.  Winds at speeds of only 2 meters per second can evaporate plant leaf water stores faster than the plants vascular system can resupply the leaf with moisture.  Without water, CO2 cannot be absorbed and photosynthesis may not occur.

Additionally, desiccating winds can collapse leaf structure, rupturing vacuoles and other cells causing permanent and possibly fatal damage to the plant.

Surprisingly, many designers do not look to wind as the cause of green roof plant problems.  Yet the wind phenomena is not only restricted to green roofs.  I was speaking to a group of professionals last week and a landscape architect from Orlando mentioned how on busy highways where automobile induced wind currents continually hit landscape plants placed in the median areas, those plants are difficult to keep alive.

Parapets are one answer but how do you prevent the wind induced green roof 'dead zones' from occurring on an exposed roof?

The answer lies in proper integration of wind resistant plants, such as the CAM plants we've discussed.  CAM plants can serve as wind break plants for C4 and C3 interior placed green roof plants.  Remember, for best biodiversity results follow the 10/20/30 rule.  Of course one can design a green roof with wind resistant plants from only one Genus, such as only Sedum, but biodiversity opportunities suffer greatly when rooftop monocultures are used.

All wind exposed green roofs will have perimeter dead zones.

Depending on the strength and water vapor content of the wind, this 'dead zone' can be from 50mm to 50cm in width or more.

Understanding how the wind travels across the roof, and the combined interactions with available light are crucial to placement of CAM plants as wind breaks.

Finally, pumping additional water and adding fertilizer will not solve wind issues.  Know your prevailing wind directions across the green roof.  Design properly with wind resistant plants as wind breaks and you can minimize the effects of winds across green roofs.